. 9
( 12)


another neutron star as a companion, and another has two or three planet-
sized companions.
Pulsars are formed in ¤ supernova explosions, though only two “ the ¤ Crab
Pulsar and the ¤ Vela Pulsar “ are within currently observable supernova
remnants. ¤ magnetar.
pulsating star A ¤ variable star with an unstable internal structure that causes it to
pulsate in a regular way. ¤ Cepheid variable, RR Lyrae star.
Puppis (The Poop or Stern) A large southern constellation lying in rich starfields of
the ¤ Milky Way. It is the largest of the three sections into which the ancient
constellation Argo Navis was divided by Nicolas L. de Lacaille in the
mid-eighteenth century. It contains ten stars of the second and third
Pythagoras A large lunar crater near the north-west limb of the Moon. It is 129 km
(80 miles) in diameter and has high walls and a central peak.
Pyxis (The Compass) A small and insignificant southern constellation introduced
by Nicolas L. de Lacaille in the mid-eighteenth century. It contains only one star
that is brighter than fourth magnitude.

QSO Abbreviation for quasi-stellar object. ¤ quasar.
quadrant An instrument for measuring the altitudes of stars and the angular
separation between celestial objects. It consists of a quarter-circle marked in
degrees and a movable sighting arm. Prior to the invention of the telescope,
such instruments were the only ones available to astronomers for measuring
the positions of stars and planets.
Quadrantids An annual ¤ meteor shower between about January 1 and January 6,
peaking on about the 3rd. The radiant of the shower lies in the constellation
Bootes, near its border with Hercules and Draco. The name dates from the time
when this area of sky was identi¬ed as the constellation Quadrans Muralis,
now no longer used. The narrow stream of meteors responsible is not
associated with any known comet.
quadrature The position of the Moon or of a planet when its angular distance
from the Sun, as viewed from the Earth, is 90 .
50000 Quaoar One of the largest known ¤ Kuiper Belt objects, discovered in 2002. It
follows a near-circular orbit that sometimes brings it nearer than Pluto and
sometimes takes it farther away. Its diameter is approximately 1260 km
(780 miles).
quasar (quasi-stellar object, QSO) The most powerful type of ¤ active galactic
nucleus. Quasar is a contraction of ˜˜quasi-stellar radio source,™™ the phrase used
to describe these objects when they were ¬rst discovered in 1963. They

An X-ray image of the quasar 3C273
from the Chandra X-ray Observatory.
3C273 lies in the constellation Virgo
and is about 3 billion light years away.

quasi-stellar object

appeared to be star-like with very high ¤ redshifts. Although the quasars
discovered in the 1960s were all radio sources, most of the many thousands
now known are not strong radio sources.
The largest redshifts ever recorded have been measured for quasars and
they are the most distant objects observable in the universe. The highest
quasar redshift measured by 2005 was 6.4. This quasar is about 13 billion light
years away. The fact that we detect such remote objects means that they are
giving out far more light and energy than a normal galaxy. The source of their
energy is matter falling onto a supermassive ¤ black hole.
quasi-stellar object ¤ quasar.
quasi-stellar radio source ¤ quasar.
quiet Sun The Sun when it is at its minimum level of activity in the ¤ solar cycle,
with little evidence of ¤ solar activity.

RA Abbreviation for ¤ right ascension.
radar astronomy The use of radar for astronomy. Radar can be used to detect
¤ meteor showers, to measure distances to bodies in the solar system and
determine their size and shape, and to map the surfaces of planets and
moons. Radar signals transmitted by the 305-m (1000-foot) radio telescope at
¤ Arecibo Observatory have been used to map Venus and to characterize the size,
shape and structure of ¤ asteroids. The ¤ Magellan spacecraft, placed in orbit
around Venus, used ¤ synthetic aperture radar to map the planet™s surface, which
is concealed by opaque cloud, and the ¤ Cassini spacecraft used radar to
study the surface of ¤ Titan. Radar is of fundamental importance for making
precise measurements of distance within the solar system.
radial velocity The velocity of an object relative to an observer along the line of
radiant The point on the celestial sphere from which the trails of meteors
belonging to a particular ¤ meteor shower appear to radiate. Meteors entering
Earth™s atmosphere from a stream create trails that are almost parallel but
perspective makes them seem to diverge from a point in the sky.
radiation belt A ring-shaped region around a planet where electrically charged
particles (electrons and protons) are trapped and spiral around the direction of
the magnetic ¬eld of the planet. The radiation belts surrounding the Earth are
known as the ¤ Van Allen belts. Similar regions exist around other planets with
magnetic ¬elds, such as Jupiter.
radio astronomy The exploration of the universe by detecting radio emission
from celestial objects. The principal sources of cosmic radio emission are the
¤ Sun, ¤ Jupiter, interstellar hydrogen and ionized gas, ¤ pulsars, ¤ quasars, and
the ¤ cosmic background radiation of the universe itself. The radio frequencies
used for astronomy span a vast range, from 10 MHz to 300 GHz. Several
wavebands are protected internationally against interference, such as 1421
MHz (wavelength 21 cm), the natural frequency of atomic hydrogen.
¤ Radio telescopes are largely either single steerable dishes, up to 100 m in
diameter, or arrays of dishes linked to form ¤ radio interferometers. Single dishes
have poor angular resolution compared with optical telescopes, so they are
mainly used in investigations where positional accuracy is not vital, such as
the timing of pulsar signals. For structural details, for example making a map
of the emission from a radio galaxy, it is essential to use an interferometer.

radio galaxy

Radio astronomy: how the night sky would appear to someone with ˜˜radio eyes.™™

Since its inception in the 1940s, radio astronomy has been directly
responsible for the discovery of pulsars, quasars and the microwave
background. ¤ radio galaxy.
radio galaxy A ¤ galaxy that has an ¤ active galactic nucleus and is an intense source
of radio emission. About one galaxy in a million is a radio galaxy. The radio
emission comes from a pair of extended jets emanating from the galaxy and is
¤ synchrotron radiation, which is emitted by electrons traveling nearly as fast as
light. In ¤ Cygnus A, often regarded as the prototype radio galaxy, two huge
clouds of radio emission, stretching out symmetrically on each side of a
disturbed elliptical galaxy, span more than three million light years.
radioheliograph A radio telescope designed for mapping the distribution of radio
emission from the Sun.
radio interferometer A ¤ radio telescope in which two or more separate antennas
observe the same object simultaneously. The signals received by a pair of
antennas are fed into a receiver that multiplies the two voltages. The result
depends on the distribution of radio emission from the source being studied. A
single measurement of this kind gives little information but, if the spacing and
orientation of the interferometer are changed, the varying output can be
analyzed by computer to generate maps showing the distribution of radio
brightness on the sky. ¤ aperture synthesis, very-long-baseline interferometry.

radio telescope

The radio galaxy 3C433. Blue shows the distribution of stars from Hubble Space
Telescope observations and red shows the radio radiation as imaged by the Very Large

radiometer Any instrument for measuring the amount of electromagnetic
radiation received from an object.
radio source Any natural source of cosmic radio emission. In cosmology, it is
often used just to mean ¤ radio galaxies and ¤ quasars. ¤ radio astronomy.
radio telescope An instrument for collecting, detecting and analyzing radio
waves from any cosmic source. All radio telescopes consist of an antenna that
picks up the signals and feeds them ¬rst to an ampli¬er and then to a detector.
The large range of frequencies covered by radio astronomy means that radio
telescopes vary greatly because different techniques are used for different
parts of the spectrum.
A fundamental problem in radio astronomy is getting the best possible
resolving power. For resolution of half an arc second, comparable with a good
optical telescope, a single dish would need to be 100 km across for observing
21-cm radio waves! However, it is not possible to build fully steerable dishes
more than about 100 m across. Single steerable dishes are used mainly for


Ranger 7 took this image, the ¬rst
picture of the Moon by a US spacecraft,
on July, 31 1964, about 17 minutes
before crashing onto the lunar surface.
The large crater at center right is
Alphonsus. Above it is Ptolemaeus and
below it Arzachel.

studies of interstellar matter, through the ¤ twenty-one centimeter line, and
variable sources, such as pulsars. The higher resolution needed to map
structure in objects such as ¤ radio galaxies and ¤ quasars is obtained by linking
arrays or networks of telescopes to form a ¤ radio interferometer. ¤ aperture
synthesis, radio astronomy, very-long-baseline interferometry.
Ranger A series of nine American lunar spacecraft, launched between 1961 and
1965. Only the last three, Rangers 7, 8 and 9 were successful. Ranger 7,
launched in July 1964, returned 4000 images. Thousands more were obtained
with Rangers 8 and 9, launched in February and March 1965. They were ˜˜hard
landers,™™ designed to transmit images during their approach to the Moon until
they crash-landed. The three successful Rangers landed successively in the
¤ Fra Mauro region, the Mare Tranquillitatis and the crater ¤ Alphonsus.
Ras Algethi (Alpha Herculis) A bright variable star in the constellation Hercules. It
is a binary, consisting of a red supergiant ¤ M star with a sixth-magnitude
companion of ¤ spectral type F, which appears greenish in contrast. They are
400 light years away. The fainter star is itself a ¤ spectroscopic binary. The
primary star varies irregularly between magnitudes 3 and 4 in about 128 days.
The name Ras Algethi comes from Arabic and means ˜˜the kneeler™™s head.™™
2100 Ra-Shalom A small asteroid measuring 3 km (2 miles) across, discovered by
Eleanor Helin in 1978. It is the largest known member of the ¤ Aten group of
asteroids, whose orbits lie wholly within that of the Earth.
RATAN-600 A radio telescope at the ¤ Special Astrophysical Observatory of the
Russian Academy of Sciences, located at Zelenchukskaya in the Caucasus
Mountains. The name is an acronym from ˜˜Radio Astronomy Telescope of the
Academy of Sciences™™ in Russian. It consists of 900 parabolic plates forming a
circle 600 m (2000 feet) in diameter. It can be used as a whole, or each quarter
can operate as a self-contained unit.

re¬‚ection nebula

ray A light-colored, linear feature extending from a crater on a moon or planet. A
number of lunar craters are surrounded by extensive and conspicuous ray
systems. They show up particularly well at full Moon, when they can be seen
with the naked eye. Ray systems are associated with young craters, such as
Tycho and Copernicus. The rays are made of light-colored material blasted out
of the crater lying on top of darker terrain around the crater.
R Coronae Borealis The prototype of a class of peculiar variable stars that
suddenly and unpredictably dip in brightness by many magnitudes. R Coronae
Borealis (R CrB) is normally magnitude 5.8, but every few years dims by up to
nine magnitudes. This happens when it throws off a dust cloud. About 40 stars
of similar type are known. Typically they are ¤ supergiant stars of ¤ spectral type
F or G.
Reber, Grote (1911“2002) The American Grote Reber was one of the world™s ¬rst
radio astronomers. He trained as a radio engineer but was inspired to take up
¤ radio astronomy when he heard of the discoveries made by Karl ¤ Jansky. In
1937 he built the ¬rst radio telescope to use a parabolic dish. The 9.4-meter
(30-foot) dish was mounted on a tilting stand and he was able to detect a
number of intense cosmic sources of radio emission. He later constructed other
radio telescopes and made the ¬rst radio map of the sky in 1941.
red giant An evolved star that has expanded greatly and appears red.
A star becomes a red giant when the hydrogen fuel for nuclear fusion in its
central core is exhausted. The core collapses. This releases gravitational energy
and a new supply of heat that allows hydrogen fusion to restart, though in a
shell around the now inert core. The energy generated by shell burning of
hydrogen causes the great expansion of the star™s outer layers. As the gas
expands, it cools. Regardless of the star™s original ¤ spectral type, its surface
temperature drops until it reaches 4000 K. When the Sun becomes a red giant,
it will expand until its diameter is roughly the diameter of the Earth™s orbit.
Though the light emitted per square meter of a red giant™s surface is low
compared with hotter stars, they are very luminous over all because of their
huge size. All the bright red stars visible to the naked eye are giants or
supergiants, such as ¤ Aldebaran or ¤ Betelgeuse. ¤ stellar evolution.
Red Planet A popular name for the planet ¤ Mars, which looks distinctly red to
the naked eye.
redshift (symbol z) An increase in the wavelength () of electromagnetic radiation
between its emission and its reception. If the increase in wavelength is Á, the
redshift (z) is Á/. Three different mechanisms can cause redshifts to the
radiation from astronomical bodies: the ¤ Doppler effect, ¤ gravitational redshift,
and ¤ cosmological redshift.
reflection nebula A cool cloud of interstellar gas and dust that shines because the
dust scatters the light of stars near to it. A re¬‚ection nebula is not itself


A reflection nebula around stars in the constellation Chamaeleon.

luminous. The spectrum of the scattered light is the same as that of the
starlight, though blue light is scattered more strongly than red light.
regolith The loose, ¬ne-grained, soil-like material on the surface of the Moon or
any other planetary body.
Regulus (Alpha Leonis) The brightest star in the constellation Leo. It is a ¤ B star of
magnitude 1.4 with two companions of seventh and thirteenth magnitudes.
The name is Latin and means ˜˜little king.™™ Regulus is 77 light years away.
relativistic Traveling nearly as fast as light.
relativity ¤ general relativity, special relativity.
remote sensing The study of Earth or other planetary objects by observation
from a distance rather than direct contact and exploration. The term
is used particularly for the study of Earth, or other bodies, from orbiting

Rho Ophiuchi cloud

The lunar regolith, with a footpad of
the Apollo 11 lunar module and
astronauts™ footprints.

resolution The size of the smallest detail that can be distinguished with an
imaging instrument such as a telescope or spectrograph.
resolving power A telescope™s ability to distinguish detail. Theoretically, it is
limited by the size of the aperture (the diameter of the main light-collecting
lens or mirror). In practice, however, the resolving power of a ground-based
optical telescope is often limited by the quality of ¤ seeing rather than the
aperture, unless it is equipped with ¤ adaptive optics.
resonance A situation in which one orbiting body is subject to a regular periodic
disturbance by the gravity of another. Resonances occur between orbits that
have periods in whole-number ratios (e.g. 1:1, 2:1, 3:2). They are responsible for
phenomena such as the ¤ Kirkwood gaps in the ¤ asteroid belt and divisions in
Saturn™s rings.
Reticulum (The Net) A small southern constellation introduced by Nicolas L. de
Lacaille in the mid-eighteenth century. Its two brightest stars are of third
retrograde Motion of an object on the celestial sphere in the east“west direction,
or, for orbital motion or axial rotation in the solar system, motion that is
clockwise as observed from north of the ¤ ecliptic. The opposite of retrograde
motion is called ˜˜direct™™ motion.
Rhea The second-largest satellite of Saturn, discovered by Giovanni Cassini in
1672. ¤ Voyager 1 and ¤ Cassini images show that Rhea™s light-colored, icy
surface is saturated with craters and has bright wispy markings. Rhea™s low
density shows that it consists mainly of ice over a rocky core. Its diameter is
1528 km (949 miles) and its orbit is 527 100 km (427 500 miles) from Saturn.
Rho Ophiuchi cloud A large nebulous region near the star Rho Ophiuchi. It is a
mixture of ¤ re¬‚ection nebulae, ¤ emission nebulae, dark ¤ absorption nebulae and
¤ molecular clouds, and is relatively close, at a distance of about 700 light years.
Infrared observations reveal the presence of a cluster of at least 40 stars within
the dark cloud. This is a region of very active star formation containing many
¤ T Tauri stars and ¤ Herbig“Haro objects.


An image of Rhea from the Cassini

Rigel (Beta Orionis) The brightest star in the constellation Orion. At magnitude
0.1, it is slightly brighter than ¤ Betelgeuse, which is designated Alpha. Rigel is a
supergiant ¤ B star, with a seventh-magnitude companion lying 775 light
years away. The name Rigel is derived from Arabic and means ˜˜leg of the
right ascension (RA) One of the coordinates used to specify positions on the
¤ celestial sphere in the equatorial coordinate system. It is the equivalent of
longitude on Earth but is measured in hours, minutes and seconds of time
eastwards from the zero point, which is the intersection of the celestial
equator and the ¤ ecliptic, known as the ¤ First Point of Aries. One hour of right
ascension is equivalent to 15 degrees and is the angle through which the sky
appears to turn in one hour of ¤ sidereal time. ¤ declination.
Rigil Kentaurus The star ¤ Alpha Centauri. This Arabic name means ˜˜foot of the
rille (rill) A ¬ssure or channel in the lunar surface. Some are vertical faults. Others
are collapsed lava tubes, which tend to be more sinuous.
ring galaxy A rare kind of galaxy shaped like a ring. Ring galaxies are thought to
result from collisions between galaxies in which one passes right through
another. An expanding shock wave of interstellar gas triggers a burst of star
formation in a ring around the center of the ˜˜target™™ galaxy.
Ring Nebula (M57; NGC 6720) A bright ¤ planetary nebula in the constellation Lyra.
In a small telescope it looks like a luminous elliptical ring around a central
star, but the observational evidence suggests it has more the shape of a
dumbbell, and just happens to be viewed end-on. Its radius is one-third of a
light year, and the nebula is 2000 light years away. If it has always expanded at
its current rate of 19 km/s, it is about 5500 years old.

ring systems

The ring galaxy known as Hoag™s Object, imaged by the Hubble Space Telescope.

The Ring Nebula.

ring systems Ring structures, composed of numerous individual small bodies and
dust, surrounding the four largest outer planets “ Jupiter, Saturn, Uranus and
The rings of Saturn were discovered by ¤ Galileo when he ¬rst turned a
telescope on the sky in 1610. In 1857, James Clerk Maxwell demonstrated

ring systems

The structure of Jupiter™s ring system.

Saturn™s ring system. This simulated image depicts the observed ring structure. Purple
shows regions where there is a lack of particles under 5 cm (2 inches). Green and blue
shades indicate regions where there are particles smaller than 5 cm (2 inches) and 1 cm
(0.3 inch). White is a dense region for which accurate data could not be collected. From
other evidence, all ring regions appear to be populated by objects up to the size of
boulders, several to many meters across.

theoretically that the rings must be made up of many unconnected particles,
and this was later con¬rmed by spectroscopic observations showing that the
inner particles orbit more quickly than the outer ones. In 1977, nine narrow
rings around Uranus were detected when the planet occulted a star. In 1979,
¤ Voyager 1 discovered a faint band around Jupiter and, in the early 1980s,

ring systems

what seemed to be incomplete ring arcs were detected around Neptune, again
during an occultation. In 1989, ¤ Voyager 2 showed that the rings around
Neptune are complete but clumpy.
Virtually all the planetary rings lie within their ¤ Roche limits. In a disk of
debris around a newly formed planet, material beyond the Roche limit could
coalesce into moons while nearer to the planet, tidal forces would prevent
moons from forming.
The rings around Jupiter are faint and tenuous, and their re¬‚ection
qualities show that many of the particles can be no bigger than 1 or 2 m. Dust
of this size must be constantly replenished, perhaps by impacts on
boulder-sized objects in the ring.
Saturn™s rings are far more complex and extensive than any other system.
The rings easily visible from Earth were labeled A, B and C, C being the
faint inner Crepe ring. The A and B rings are separated by the Cassini Division
and there is also a narrow but conspicuous gap towards the outer edge of the
A ring, known as the Encke Division or Encke Gap. Voyager 1 detected
material inside the C ring, which was called the D ring. Beyond the A ring lie
more narrow, tenuous rings, known as the E, F and G rings. The ring particles
are thought to be a mixture of water ice and dust, though their composition
varies, and they range in size from a few micrometers up to a hundred
Voyager images showed that Saturn™s main rings consist of thousands of
narrow, closely spaced ˜˜ringlets.™™ Many of the observed structures are due to
the gravitational action of moons. For example, Pandora and Prometheus act as
˜˜shepherds,™™ con¬ning the F ring, and the Cassini Division lies where a
satellite with an orbital period half that of Mimas would lie. (This is an
example of a ¤ resonance phenomenon.) Observations of Saturn™s rings from
the ¤ Cassini spacecraft con¬rm their variety and complexity. Cassini™s
discoveries include the fact that the particles in the A ring form into clusters
several meters long that assemble and then get torn apart again.
The nine rings of Uranus found in 1977 are labeled, in order of increasing
distance from the planet: 6, 5, 4, ¬, , ,
,  and . Two more rings were found
by Voyager 2 in 1986 and also a pair of satellites, Ophelia and Cordelia,
shepherding the ring. In 2003, the Hubble Space Telescope found a further two
rings near the moons Portia and Mab. The nine main rings appear to be
composed of meter-sized boulders. But in backlighting, Voyager 2 also saw
many slender ringlets composed of dust.
Two main rings orbit Neptune (Leverrier and Adams rings), with a diffuse
sheet of material extending outwards from the inner of the two (Plateau).
There is also a tenuous third ring nearer the planet (Galle). A faint outward
extension to the Leverrier ring has been named Lassell and it is bounded at its

Roche limit

outer edge by the Arago ring. The outer ring, Adams, contains three bright arcs,
about 8 in extent, which seem to be dominated by dust-sized particles. These
have been called Liberty, Equality and Fraternity. It is thought that the
gravitational in¬‚uence of the moon Galatea, orbiting just inside the ring, acts
to con¬ne the arcs.
Roche limit The minimum distance from the center of a planet that a
hypothetical ¬‚uid moon can orbit and not be destroyed by tidal forces. If the
planet and satellite have equal densities, the Roche limit is 2.456 times the
radius of the planet.
Anywhere inside the Roche limit, a ¬‚uid moon would be torn apart. Solid
satellites can exist inside the Roche limit because the strength of the rock
holds them together. ¤ tides.
Roche lobes In a ¤ binary star system, an hourglass-shaped region between the two
stars. Its surface is where the gravitational force exerted by either star is the
same. Part way between the stars there is a unique location where the two
Roche lobes touch. This is called the ˜˜inner ¤ Lagrangian point.™™
When a star in a close binary system expands during the giant phase of its
evolution, it may completely ¬ll its Roche lobe. A system like that is said to be
˜˜semidetached.™™ Matter then streams through the inner Lagrangian point to
the other star. In a ¤ contact binary, both stars have completely ¬lled their
Roche lobes and gas can be transferred between them.
Rosalind A small moon of Uranus with a diameter of 72 km (45 miles). It was
discovered in 1986 by the ¤ Voyager 2 spacecraft.
ROSAT A German orbiting X-ray astronomy observatory, with participation by
NASA and the UK. It was launched in 1990 and operated until December 1998.
It carried a large X-ray imaging telescope and a wide-¬eld camera to observe in
the extreme ultraviolet.
Rosetta A European Space Agency space mission to a ¤ comet, consisting of an
orbiter and a lander. It was launched on March 2, 2004 and will reach its target,
Comet Churyumov“Gerasimenko, in 2014. Its journey sends it around Earth
three times (in 2005, 2007 and 2009), and around Mars once (in 2007) for
¤ gravity assist. The orbiter is equipped with 11 instruments and will release a
100-kg lander, which has nine instruments of its own. The orbiter will
ultimately be in an orbit 25 km above the comet™s nucleus, which is only about
4 km across. The lander will touch down slowly in November 2014 and ¬re an
anchoring harpoon into the comet™s surface immediately on impact. The
orbiter will stay with the comet as it gets nearer to the Sun and the mission is
due to end in December 2015.
Rosette Nebula (NGC 2237, 2238, 2239 and 2246) An ¤ emission nebula in the
constellation Monoceros, surrounding a young ¤ open cluster of stars
(NGC 2244). It is roughly circular in shape, with a central hole cleared of dust

Rosette Nebula

Artist™s impression of the Rosetta lander anchored to the surface of its target comet.

The Rosette

Rossi, Bruno Benedetti (1905“1993)

and gas by the radiation from the stars in the cluster. The nebula™s distance is
estimated to be 4500 light years.
Rossi, Bruno Benedetti (1905“1993) Rossi is most remembered as a pioneer in
the ¬eld of ¤ X-ray astronomy and he initiated the rocket experiment in 1962
that led to the ¬rst discovery of a cosmic source of X-rays, ¤ Scorpius X-1. In the
1940s and 1950s, he also advanced the study of ¤ cosmic rays at the
Massachusetts Institute of Technology, where he was a professor of physics
from 1946. He was born and educated in Italy, where he began his career as a
physicist, but he left for the United States in 1938. The ¤ Rossi X-ray Timing
Explorer was named in his honor.
Rossi X-ray Timing Explorer (RXTE) A NASA X-ray astronomy satellite
launched in 1995. It was named in honor of the X-ray astronomy pioneer,
Bruno B. Rossi, in 1996 after launch. The spacecraft was equipped with three
instruments. The Proportional Counter Array and the High-Energy X-ray
Timing Experiment (HEXTE) working together constituted the largest X-ray
telescope ¬‚own to date. The third instrument, the All Sky Monitor, was
designed to record the long-term behavior of X-ray sources and monitor
the sky.
Royal Observatory, Greenwich A UK observatory ¬rst established at Greenwich
near London in 1675 by King Charles II, to address the problem of ¬nding
longitude at sea. ¤ John Flamsteed, the ¬rst ¤ Astronomer Royal, was put in
charge. The Observatory subsequently played an important role in positional
astronomy and the meridian through Greenwich was adopted as the zero point
of longitude in 1884.
In the twentieth century, the emphasis changed to include more
astrophysics and researchers moved to Herstmonceux Castle, Sussex, in 1948.
The new establishment was known as the Royal Greenwich Observatory (RGO),
while the original observatory at Greenwich became part of the UK™s National
Maritime Museum. In 1990. The RGO™s headquarters moved to Cambridge in
1990 but it closed in 1998.
Royal Observatory, Edinburgh (ROE) A UK astronomy research establishment in
Edinburgh. Since 1998 it has comprised the UK™s Astronomy Technology
Centre and the Institute for Astronomy of the University of Edinburgh.
RR Lyrae star A category of pulsating ¤ variable star belonging to ¤ Population II
(relatively old stars). They are found particularly, but not exclusively, in
¤ globular clusters. Though similar to ¤ Cepheid variables, they are intrinsically
less luminous “ by as much as seven magnitudes. All have approximately the
same absolute magnitude (þ0.5) and this makes them useful distance
indicators. Their periods range between a few hours and just over a day, and
they vary by anything between 0.2 and 2 magnitudes. Their ¤ spectral type is A
or F. RR Lyrae, which varies over 0.567 days, is the best-known example.

Ryle, Sir Martin (1918“1984)

runaway star A young hot star traveling through space with an unusually high
velocity. It is thought that such stars could originally have been in binary or
multiple systems where a companion exploded as a ¤ supernova.
Three well-known examples are Mu Columbae, AE Aurigae and 53 Arietis.
From their speeds and directions of motion it has been calculated that all three
were ejected from the same region in the constellation Orion about three
million years ago.
Russell, Henry Norris (1877“1957) Russell™s greatest achievement was his
research on the luminosity of stars, which resulted in the ¤ Hertzsprung“Russell
diagram. He was a brilliant student at Princeton University where he gained his
doctorate, after which he spent three years at King™s College, Cambridge. At
Cambridge, he determined the distances to stars by measuring their ¤ parallax
and so discovered the relationships between their ¤ spectral types and their
luminosity. When describing his discovery, he introduced the terms ˜˜giant™™
and ˜˜dwarf™™ to describe stars for the ¬rst time. He spent most of his
professional life at Princeton, where he was director of the observatory from
1912 to 1947.
Ryle, Sir Martin (1918“1984) Ryle was a British radio astronomer who developed
revolutionary ¤ radio telescopes and detectors. Appointed a lecturer in physics at
Cambridge University in 1948, he completed the ¬rst map of the radio sky in
1950 and discovered some 50 radio sources. His Third Cambridge Catalogue, made
in 1959, led directly to the discovery of ¤ quasars when optical observers
tried to match starlike objects with the positions of radio sources Ryle had
measured. His third and fourth catalogs comprehensively disproved the
¤ steady-state theory advanced by the Cambridge theorist ¤ Fred Hoyle. Ryle
shared the Nobel Prize for physics in 1974 with a fellow Cambridge radio
astronomer, Anthony Hewish.

Sagan, Carl Edward (1934“1996) The American planetary scientist Carl Sagan
was one of the most well-known and in¬‚uential ¬gures in astronomy in the
USA during his lifetime. He was particularly known for his interest in
extraterrestial life and he strongly advocated searching for extraterrestrial
intelligence. As an advisor to NASA, he contributed to many of NASA™s
planetary exploration space missions. A gifted popularizer, he wrote several
bestselling books. His television series Cosmos, ¬rst shown in 1980, was an
enormous success. In it Sagan examined a wide range of issues in science,
particularly the origin of life in the universe. From 1968 he worked at Cornell
University and directed the Laboratory for Planetary Studies there.
Sagitta (The Arrow) The third-smallest constellation, but nevertheless a distinctive
little group of stars. The two brightest stars are third magnitude. It lies in a rich
part of the Milky Way, next to Aquila.
Sagittarius (The Archer) The southernmost constellation of the zodiac. The center
of the Galaxy (the Milky Way) lies behind the star clouds in Sagittarius. It is a
large constellation, with many bright stars. It also contains a large number of
star clusters and nebulae. The ¤ Messier Catalogue lists 15 objects in Sagittarius,
more than in any other individual constellation. They include the ¤ Lagoon
Nebula, the ¤ Tri¬d Nebula, the ¤ Omega Nebula and the third brightest ¤ globular
cluster in the sky, M22.
Sagittarius A The strongest radio source in the constellation Sagittarius. It is a
complex radio source associated with the galactic center. A bright, compact
and variable source embedded within it and known as Sagittarius A* is believed
to be the actual center of the Galaxy and to be powered by a supermassive
black hole there.
Sagittarius Arm One of the spiral arms of the ¤ Galaxy. It lies between the Sun
and the center of the Galaxy in the direction of the constellation Sagittarius.
saros The time for a sequence of lunar and solar eclipses before the cycle repeats
itself. A saros lasts 6585.32 days (about 18 years). After this period, the Earth,
Sun and Moon return to the same relative positions. Succeeding eclipses in a
particular ¤ saros series occur about 8 hours later and fall nearly 120 degrees of
longitude farther west. This was known to the ancient Babylonian and Mayan
astronomers and to the builders of ¤ Stonehenge.
saros series A series of lunar or solar eclipses occurring at intervals of one ¤saros.
Since there are up to seven eclipses every year, there are more than 80 saros

saros series

20h 19h 18h
“10º “10º

Omega Nebula

ρ1 M18
ρ2 Star Cloud
6716 M25 M24
π ξ2,1 µ
“20º ECLIPTIC “20º

ν2,1 M21
χ2 Trifid Nebula
χ1 ψ M22 M28
Nunki Lagoon
ω Kaus
„ Nebula
σ Borealis

Ascella Kaus Media W
δ γ Center
M55 M69
“30º “30º
θ2 Australis

“40º “40º
β2,1 Arkab

h h
19h 18h 17h

Magnitudes: 5 4 3 2 1 0 brighter than 0 Variable stars

Open clusters Globular clusters Planetary nebulae Bright nebulae Galaxies

A map of the constellation Sagittarius.

A radiograph of Sagittarius A, which
shows emission from gas withinin the
central 10 light years of our Galaxy.


Saturn imaged by the Hubble Space Telescope.

series running concurrently. The eclipses in a series return to the same
longitude after an interval of three saroses (54 years). However, each
succeeding eclipse in the series moves systematically in latitude (either north
or south), from one pole to the other until the series is completed.
satellite Any body in orbit around a larger parent body. Most planets in the solar
system have natural satellites, otherwise known as moons.
Saturn The sixth major planet of the solar system in order from the Sun, orbiting at an
average distance of 9.54 astronomical units. Saturn is one of the four giant planets,
second in size only to Jupiter. Its equatorial diameter is 9.4 times Earth™s and its
mass is 95 times greater. However, its average density is only 0.7 times that of
water. Hydrogen and helium make up the bulk of its mass. There is a central core,
10 or 15 times the mass of Earth, made of rock or a mixture of rock and ice. In the
high-pressure region surrounding the core, the hydrogen takes on the form of a
metal. The outer half of the planet consists of a deep atmosphere; the visible
features of the planet are cloud bands at the top of this atmosphere.
The cloud patterns on Saturn do not normally show much color contrast.
However, temporary storms sometimes occur. In late September 1990, a large
white spot developed, expanding over a period of weeks to encircle much of the
planet™s equatorial region. Similar large storms have been seen at intervals of
about 27“30 years, in 1876, 1903, 1933 and 1960, close to mid-summer on Saturn
in its northern hemisphere.
Images obtained by ¤ Voyagers 1 and 2 during their encounters in 1980 and
1981 reveal complex circulation currents, similar to those observed on Jupiter.
Saturn rotates rapidly, spinning once every 10 hours 32 minutes on average,
though the rate varies with latitude. This rapid spin makes Saturn bulge at the
equator. Its polar and equatorial diameters differ by 11 percent.

Schmidt“Cassegrain telescope

Saturn™s most striking feature is its spectacular ¤ ring system. The rings lie
in the planet™s equatorial plane, which is tilted at an angle of 27 to its orbit
around the Sun. They are easily visible in a small telescope. As the relative
positions of Earth and Saturn change, the rings are presented to us at differing
angles, sometimes appearing open, but at other times edge-on so that they
disappear from view. There are zones of differing brightness in the ring
system, separated by dark divisions. The most marked divisions are Cassini™s
and Encke™s. The Voyager images of the rings showed that they consist of many
thousands of narrow ringlets. They are only one kilometer thick and are made
up of a huge number of separate pieces of ice and dust, perhaps ranging in size
from a hundred meters down to a micrometer.
Before 1980, 10 moons of Saturn were known. By 2006 the number had
reached 56 and was increasing every year. ¤ Cassini“Huygens mission.
Saturn Nebula (NGC 7009) A ¤ planetary nebula in the constellation Aquarius.
Its unusual shape, with a faint partial outer ring, resembles the planet Saturn.
The double ring may be the remains of separate shells thrown off by the
central star.
Scheat (Beta Pegasi) The second-brightest star in the constellation Pegasus. It is
a ¤ supergiant ¤ M star 200 light years away and varies in brightness between
magnitudes 2.4 and 2.8. The name comes from Arabic and probably means
Schedar (Schedir; Alpha Cassiopeiae) The brightest star in the constellation
Cassiopeia. It is a ¤ supergiant ¤ K star 230 light years away with a magnitude
near 2.2, though it is slightly variable. The Arabic name means ˜˜breast.™™
Schiaparelli, Giovanni Virginio (1835“1910) Schiaparelli was director of the
Milan Observatory from 1860 to 1900. At the 1877 ¤ opposition of Mars, when
Earth and Mars were exceptionally close, he produced detailed maps of the
martian surface. He described several linear features that he designated as
˜˜canali,™™ meaning natural channels or grooves. ¤ Percival Lowell in the USA and
Camille Flammarion in France subsequently made extravagant claims that
they were arti¬cial canals. Schiaparelli also studied Venus and Mercury, and
discovered that the tracks in space of ¤ meteors follow the orbits of ¤ comets.
Schmidt camera A type of wide-¬eld astronomical telescope, designed to take
photographs. It was invented by Bernhard Schmidt in 1930 and has a specially
shaped glass corrector plate over the end of the telescope tube to ensure the
images are sharp. Schmidt cameras produce good images over very wide ¬elds
of view “ up to tens of degrees across.
Schmidt“Cassegrain telescope A design of optical telescope, incorporating
features of both a ¤ Schmidt camera and a ¤ Cassegrain re¬‚ector.
A Schmidt“Cassegrain has a corrector plate like a Schmidt camera but
there is a small secondary mirror at the center of the plate, which re¬‚ects the

Schroter™s Valley

Schroter™s Valley photographed by
Apollo 15 astronauts.

light back down the tube and through a hole in the primary mirror. The
resulting telescope is very compact, so the design is particularly suitable for
portable telescopes and is popular with amateurs.
Schroter™s Valley A winding valley in the Oceanus Procellarum on the Moon. It
starts in a small crater just outside the northern wall of the crater Herodotus,
and extends for about 200 km.
Schwarzschild, Karl (1873“1916) Schwarzschild, who was born in Frankfurt,
was both an observational astronomer and a noted theorist. In 1909 he became
director of the Potsdam Observatory. While serving with the German army in
Russia in 1916, he published the ¬rst exact solution to complex equations
¤ Einstein had published that year in his ¤ general relativity theory. He
introducing the concept of what is now termed the ¤ Schwarzschild radius,
which is important in the theory of ¤ black holes.
Schwarzschild radius A critical quantity that determines whether a body is a
¤ black hole, from which nothing can escape into the outside world. An object
that collapses inside its Schwarzschild radius becomes a black hole. The
Schwarzschild radius for an object the mass of the Sun is 3 km; for an object
the mass of the Earth it is 1 cm.
scintillation Twinkling “ rapid variations in the brightness of a star caused by
turbulence in Earth™s atmosphere. A similar phenomenon affects radio signals
from cosmic sources when they travel through Earth™s ionosphere.
Scorpio An alternative name for the constellation ¤ Scorpius, chie¬‚y used in
Scorpius (The Scorpion) A large, bright constellation of the southern part of the
zodiac. Its brightest star is the ¬rst-magnitude ¤ Antares.
Scorpius“Centaurus association A loose group of young hot stars about 400
light years away. It includes most of the brightest stars in the constellations
¤ Scorpius and ¤ Centaurus.
Scorpius X-1 The brightest X-ray source in the sky and the ¬rst to be discovered. It
is a low-mass ¤ X-ray binary star.

second contact

18h 17h 16h

ψ “10º

ν Graffias
ω 2,1
“20º M80
± σ

“30º 6383
Shaula »
Bug Nebula
… µ 2,1
G 6242
ι2 “40º
ι1 6231
· ζ 2,1
“40º AUSTRALIS 6178

18h 17h 16h 15h

Magnitudes: 5 4 3 2 1 0 brighter than 0 Variable stars

Open clusters Globular clusters Planetary nebulae Bright nebulae Galaxies

A map of the constellation Scorpius.

Sculptor (The Sculptor™s Workshop) An inconspicuous constellation of the
southern hemisphere introduced by Nicolas L. de Lacaille in the
mid-eighteenth century. Its four brightest stars are fourth magnitude.
Scutum (The Shield) A small constellation near the celestial equator,
introduced by Johannes ¤ Hevelius in the late seventeenth century with the
name Scutum Sobieskii in honor of his patron, King John Sobieski III. It has no
stars brighter than fourth magnitude but it contains the ¤ Wild Duck star
seasons The natural cycle of change in the prevailing environmental conditions
on a planet, over the course of its orbit around the Sun. A planet experiences
seasons if its rotation axis is not at 90 to the plane of its orbit. Both Earth and
Mars have marked seasons.
second contact In a total or annular ¤ eclipse of the Sun, the point when the edges
of the Moon™s disk and Sun™s ¤ photosphere are in contact at the start of totality
or the annular phase. In a lunar eclipse, second contact occurs when the Moon


just enters the full shadow (umbra) of the Earth completely. The same term can
also mean a similar stage in the progress of a ¤ transit or ¤ occultation.
secular Continuing, or changing in a non-periodic way, over a long period of time.
90377 Sedna One of the larger known ¤ Kuiper Belt objects. When discovered in
2003, it was the most distant known object in the solar system. It follows a very
elongated elliptical orbit and is estimated to be 1180“1800 km (730“1120
miles) in diameter.
seeing The effect of turbulence in the atmosphere on the quality of the image of
an astronomical object. In conditions of good seeing, images are sharp and
steady; in poor seeing, they are blurred and appear to be in constant motion.
selenography The study of the surface features and topography of the Moon.
selenology The study of lunar rocks and of the surface and interior of the Moon:
the lunar equivalent of geology.
semiregular variable A pulsating variable star with some regularity to its
variations but nevertheless unpredictable. Semiregular variables typically vary
by only one or two magnitudes over anything from a few days to several years.
A number of subgroups have been distinguished.
Serpens (The Serpent) The only constellation split into two parts, which are
known as Serpens Caput (the head) and Serpens Cauda (the tail). They lie either
side of the constellation Ophiuchus (The Serpent Bearer). The brightest stars in
Serpens are third and fourth magnitude.
Setebos A small outer moon of Uranus discovered in 1999. Its diameter is about
30 km (19 miles).
SETI Abbreviation for Search for Extraterrestrial Intelligence. It is used as a general
term rather than for any particular experiment. Most SETI projects have
involved the search for unnatural narrow-band radio signals. Optical SETI
looks for evidence of pulsed laser beams, which could be visible over distances
of hundreds of light years.
SETI Institute A private research establishment with headquarters in California
dedicated to research in astrobiology and the search for extraterrestrial
Seven Sisters A popular name for the ¤ Pleiades.
Sextans (The Sextant) A faint constellation of the southern hemisphere introduced
in the late seventeenth century by Johannes ¤ Hevelius, supposedly to
commemorate the instrument he used to make astronomical observations. Its
brightest star is magnitude 4.5.
Seyfert galaxy A type of galaxy with a brilliant point-like nucleus and
inconspicuous spiral arms, ¬rst described by Carl Seyfert in 1943.
Seyfert™s Sextet (NGC 6027) A group of galaxies in the constellation Serpens. It
consists of ¬ve galaxies, together with a large cloud of gas ejected by the
principal galaxy in the group. This galaxy, which is a spiral, and three

shadow bands

This Seyfert galaxy is in the southern
constellation Circinus and is 13 million
light years away. The image was taken
by the Hubble Space Telescope.

Seyfert™s Sextet imaged by the Hubble Space Telescope.

lenticular galaxies in the group all lie at a distance of 260 million light years
and are interacting with each other. The ¬fth galaxy is a spiral ¬ve times
farther away, coincidentally lying in the same part of the sky.
shadow bands A phenomenon sometimes observed brie¬‚y just before and just
after totality at a total solar eclipse. Irregular bands of shadow, a few


Saturn™s moons Pandora (right) and Prometheus (left), which act as shepherd satellites of the
F ring (the ring extending farthest right in this image from the Cassini spacecraft). The two
moons are separated by about 69 000 km (43 000 miles) in this view.

centimeters wide and up to a meter apart, move over the ground. What causes
them is not fully understood. They are seen only if the sky is very clear.
Shaula (Lambda Scorpii) The second-brightest star in the constellation Scorpius,
marking the Scorpion™s sting. It is a ¤ B star of magnitude 1.6 and is 700 light
years away.
shepherd satellites Moons, often found in pairs, that hold a planetary ring in
place and prevent it from dispersing through the action of their gravity. For
example, Prometheus and Pandora are the shepherd satellites for the F-ring in
Saturn™s ¤ ring system.
shield volcano A large volcano with gently sloping sides, built up by successive
lava ¬‚ows from a single vent. Individual layers may amount to only a few
meters but they can build up to create a very high mountain. Typically, the
slope of the sides is under 10 . At the top there is a large, shallow, ¬‚at-¬‚oored
crater called a caldera.
Shoemaker“Levy 9, Comet A comet that crashed into the planet Jupiter
in July 1994. When it was discovered photographically on March 25, 1993, by

The fragments of Comet Shoemaker“
Levy 9 crashing into Jupiter™s
atmosphere caused the huge dark dust
clouds visible in this ultraviolet image
from the Hubble Space Telescope.


Carolyn and Eugene Shoemaker and David Levy, it was in an elongated
two-year orbit around Jupiter and consisted of a string of about 20 separate
fragments. Calculations suggested it had been orbiting Jupiter for several
decades, but broke up during a close approach to Jupiter in July 1992. This
encounter also set the fragments on their collision course. They struck Jupiter
one after another between July 16 and 22, 1994. A number of the impacts
produced large dark clouds in Jupiter™s atmosphere, as well as powerful
¬reballs. The dark clouds were discernible for several months before they
shooting star A popular name for a ¤ meteor.
short-period comet A ¤ comet in an elliptical orbit, with a period of several years
or decades, comparable with the orbital periods of the planets. Short-period
comets have gone into their present orbits after close encounters with planets,
especially Jupiter. Two-thirds of all short-period comets are in orbits that
extend no more than one astronomical unit beyond the orbit of Jupiter. It is
suspected that they were originally in the ¤ Kuiper Belt.
Siarnaq A small outer moon of Saturn in a very elliptical orbit, discovered in 2000.
It is 32 km (19 miles) across.
Sickle An ¤ asterism formed by the stars Alpha, Eta, Gamma, Zeta, Mu and Epsilon
in the constellation ¤ Leo, named because of its shape.
sidereal Connected with the stars.
sidereal day The Earth™s rotation period with respect to the stars. The length of
the sidereal day is 23 hours 56 minutes and 4 seconds.
sidereal period The time taken by a planet or satellite to complete an orbit as
measured relative to the stars.
sidereal time Time measured by the rotation of the Earth with respect to the stars
rather than its rotation relative to the Sun, which is the basis for civil time. The
local sidereal time at a particular place is the ¤ right ascension on the meridian.
Observatories normally have a clock keeping sidereal time.
Siding Spring Observatory ¤ Mount Stromlo and Siding Spring Observatories.
Sikhote-Alin shower A major meteorite fall on February 12, 1947, in eastern
Siberia. The largest meteorite recovered weighed 1745 kg (3850 lb), but it has
been estimated that thousands of pieces fell, weighing up to a total of 100
tonnes. Much of it has never been found.
Sinope A small outer moon of Jupiter discovered in 1914 by Seth B. Nicholson. Its
diameter is 38 km (24 miles).
Sirius (Alpha Canis Majoris) The brightest star in the constellation Canis Major
and, at magnitude “1.46, the brightest star in the sky. It is a visual binary with
an orbital period of 50 years. The primary (A) is an ¤ A star and the secondary
(B) an eighth-magnitude ¤ white dwarf. Sirius B was ¬rst seen in 1862, and its
nature was deduced from its spectrum in 1925. Sirius lies at a distance of 8.7


A Chandra Observatory X-ray image of
Sirius A and B. The white dwarf, Sirius
B, is the brighter in X-rays.

light years and is the seventh-nearest star to the solar system. The name Sirius
is derived from Greek and means ˜˜scorching,™™ a reference to its brilliance.
Sirius is also known as ˜˜the Dog Star,™™ from the constellation in which it lies.
Sirrah An alternative name for the star ¤ Alpheratz.
1866 Sisyphus A small asteroid, diameter 7.6 km (4.7 miles), discovered in 1972 by
Paul Wild. It is in a comet-like orbit, inclined at 41 to the plane of the solar
system, which occasionally brings it relatively close to the Earth. It is the
largest known ¤ Apollo asteroid.
Skathi A small outer moon of Saturn in a very elliptical orbit. It was discovered in
2000 and its diameter is about 6 km (4 miles).
Skylab An American space station, launched into Earth orbit in May 1973. Three
crews, each of three men, were sent to the station for periods of several
weeks between 1973 and 1974. The station burnt up on re-entering the
atmosphere in 1979.
Slipher, Vesto Melvin (1875“1969) Slipher, an American astronomer, worked for
the whole of his career between 1901 and 1952 at the ¤ Lowell Observatory,

Skylab in orbit at the end of its

Small Astronomy Satellites

Vesto Slipher.

where he was made director in 1926. His skill was in spectroscopy. By 1917, he
found that 13 out of 15 spiral ˜˜nebulae™™ he was studying were moving away
from the Milky Way at velocities of hundreds of kilometers per second. Many
astronomers at the time questioned these ¬ndings, and it was not then known
that the ˜˜nebulae™™ were in fact galaxies beyond the Milky Way, but in
retrospect Slipher™s measurements were some of the earliest evidence that the
universe is expanding.
Sloan Digital Sky Survey A project to map one quarter of the sky, including
measurements of the positions and absolute magnitudes of over 100 million
celestial objects. One of its main objectives is to determine the distances of a
million of the nearest galaxies, and of 100 000 ¤ quasars. The 2.5-m (98-inch)
survey telescope, together with a 0.6-m (23.5-inch) support telescope opened in
1997 at the ¤Apache Point Observatory.
Small Astronomy Satellites (SAS) Three NASA spacecraft deployed in the 1970s
for X-ray and gamma-ray astronomy. SAS-1, launched in 1970, was the ¬rst
satellite dedicated to ¤ X-ray astronomy. After launch, it was given
the name Uhuru, which means ˜˜freedom™™ in Swahili, because the launch
date of December 12 coincided with the seventh anniversary of Kenya™s
independence. SAS-2, launched in 1972, was the ¬rst satellite to carry a

Small Magellanic Cloud

SMART-1 took this image of rugged
cratered terrain near the Moon™s north

gamma-ray detector. SAS-3, which followed in 1975, carried further X-ray
Small Magellanic Cloud ¤ Magellanic Clouds.
SMART-1 A European Space Agency mission launched into Earth orbit on
September 27, 2003. The main objective was to test technologies for use on
future missions. The spacecraft used an ¤ ion drive to slowly elongate its orbit
and increase its distance from Earth, until it could be transferred into an orbit
around the Moon on November 15, 2004. SMART-1 carried three instruments to
study the surface composition of the Moon. Its mission was scheduled to last
until August 2006.
Smithsonian Astrophysical Observatory (SAO) A research establishment
founded by the US Smithsonian Institution in Washington DC in 1890. In 1955,
under the directorship of Fred Whipple, its headquarters were moved to the
grounds of ¤ Harvard College Observatory (HCO). In 1967, an observatory was
established at Mount Hopkins in Arizona, now known as the ¤ Fred Lawrence
Whipple Observatory. In 1973, the Harvard“Smithsonian Center for Astrophysics
was formed by combining the SAO and HCO.
SN 1987A ¤ Supernova 1987A.
SNC meteorites Unusual ¤ meteorites, believed to have come from Mars. SNC are
the initials of the three different classes of these meteorites: shergottites,
nakhlites and chassignites.
SOFIA An airborne observatory consisting of a 2.5-m (100-inch) telescope mounted
in a modi¬ed Boeing 747 aircraft. It had been expected to begin operations in
about 2006 but the project has suffered delays. In 2006, NASA called into
question the continued funding of the project but it passed an initial review. Its
base will be NASA™s Ames Research Center in California. SOFIA is an acronym

Solar and Heliospheric Observatory

for ˜˜Stratospheric Observatory for Infrared Astronomy.™™ It is the successor to
the ¤ Kuiper Airborne Observatory (KAO).
Flying at between 41 000 and 45 000 feet, SOFIA will operate above 85
percent of Earth™s atmosphere and above 99 percent of the water vapor
in the atmosphere, which interferes with ¤ infrared astronomy. It will be
able to observe in the visible, infrared, sub-millimeter and microwave
regions of the spectrum. The project is a collaboration between the USA
and Germany.
soft gamma repeater A type of rare gamma-ray source that emits infrequent
bursts of radiation. Each burst lasts a maximum of a few seconds. Soft gamma
repeaters are thought to be a type of ¤ magnetar, young, rapidly spinning
¤ neutron stars. Two are known to be located within ¤ supernova remnants.
SOHO Abbreviation for ¤ Solar and Heliospheric Observatory.
Sojourner The name of the small roving vehicle carried by the NASA Mars mission
¤ Mars Path¬nder.
sol A martian ˜˜day,™™ which is 24 hours 37 minutes and 22.6 seconds in length.
solar Connected with the Sun.
solar activity A variety of phenomena on the Sun that involve disturbances and
outbursts of energy. Solar activity varies over time. The most obvious changes
in the ¤ solar cycle repeat after about 11 years. Phenomena such as ¤ coronal
mass ejections, ¤ ¬‚ares, ¤ sunspots, ¤ prominences and ¤ faculae are all
manifestations of solar activity.
Solar and Heliospheric Observatory (SOHO) A space observatory launched
by the ¤ European Space Agency on December 2, 1995. It was placed in orbit
around the Sun at the point, known as the L1 ¤ Lagrangian point, where
the gravitational forces of the Earth and Sun are equal. Its 12 instruments
were designed to investigate the solar atmosphere and how it is heated,
solar oscillations, how the Sun expels material into space, the structure

The change in solar activity from minimum to maximum is illustrated by this sequence of
three ultraviolet images of the Sun taken by the SOHO spacecraft.

solar constant

The solar cycle demonstrated by an 11-year sequence of X-ray images of the Sun from the
Japanese Yohkoh satellite.

of the Sun and processes operating within it. By 2005 over 1000 comets
had been discovered on SOHO images. The mission is due to end in 2009.
solar constant The solar power reaching the top of Earth™s atmosphere, at the
average Sun“Earth distance. Its value is about 1.35 kW/m2 but is not in fact
constant. It changes slightly over the course of a ¤ solar cycle “ a large sunspot
group decreases it by about 1 percent “ and there are also longer-term
solar cycle The periodic variation in the amount of ¤ solar activity, particularly the
number of ¤ sunspots. The cycle is about 11 years long, though it has been
closer to 10 years during the twentieth century.
At the start of a new cycle there are few, if any, spots on the Sun. The ¬rst
ones signalling the new cycle erupt around latitudes 35“45 north and south;
over the course of the cycle, subsequent spots appear closer to the equator,
¬nishing at around 7 north and south. On a graph, this pattern of appearance
creates a ¤ butter¬‚y diagram.
It is generally thought that the solar cycle is caused by an interaction between
the ˜˜dynamo™™ responsible for the Sun™s magnetism and the Sun™s rotation. The
Sun does not rotate as a solid body: the equatorial regions rotate fastest and this
ampli¬es the magnetic ¬eld, which eventually bursts into the photosphere,
causing sunspots. At the end of each cycle the overall magnetic ¬eld reverses.
solar nebula The cloud of interstellar gas and dust that condensed to form the Sun
and solar system about five billion years ago.

Solis Planum

The major planets of the solar system in order from the Sun (not to scale). From left to right:
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The four outer giant
planets are shown at about one quarter their real size compared with the inner planets.

solar system The Sun, together with the planets and moons, comets, asteroids,
meteoroid streams and interplanetary medium held captive by the Sun™s
gravity. The solar system is believed to have formed from a rotating disk of gas
and dust created around the Sun as it contracted to form a star, about five
billion years ago.
solar time Time based on the rotation of Earth. Earth™s rotation rate is not
precisely constant when checked against an atomic clock so solar time has
been superseded by International Atomic Time (TAI). ˜˜Leap seconds™™ are
occasionally introduced to keep TAI in step with Earth™s rotation.
Since Earth™s rotation axis is tilted and its orbit around the Sun is elliptical
rather than circular, the Sun™s apparent motion through the sky is not uniform
over the course of a year. Apparent solar time, as measured by a sundial, differs
from mean time by an amount that varies through the year and is known as
the ¤ equation of time.
solar tower A type of telescope used exclusively for observing the Sun. Close to
the ground, the heating effect of the Sun creates a layer of hot, turbulent air,
which makes images unsteady. To overcome this problem, the mirror used to
send an image of the Sun into the telescope is placed on a tall tower. ¤ vacuum
tower telescope.
solar wind A stream of particles, primarily protons and electrons, flowing
outwards from the Sun at up to 900 km/s. The solar wind is essentially the hot
solar ¤ corona expanding into interplanetary space.
Solis Planum (Solis Lacus) An ancient volcanic plain on Mars, lying to the south of
¤ Valles Marineris. To the visual observer, the area has a variable dark spot


The solar tower telescope at Mount
Wilson Observatory, California.

(the ˜˜lake™™) whose appearance has earned the feature the nickname of ˜˜the
eye of Mars.™™
solstices The two occasions in the year when the Sun reaches its most northerly
and most southerly position in the sky, or the Sun™s position at these times.
The solstices fall approximately midway between the spring and autumn
¤ equinoxes.
The solstices occur on about June 21 and December 21. At the summer
solstice, the Sun reaches its highest altitude in the sky and the hours of
daylight are at their maximum. At the winter solstice the Sun™s altitude at
noon is at its lowest and the period of daylight is a minimum. The summer
solstice in the northern hemisphere (June) is the winter solstice in the
southern hemisphere and vice versa.
Sombrero Galaxy (M104; NGC 4594) A spiral galaxy in the constellation Virgo that
we see very nearly edge-on. Its marked central bulge and conspicuous lane of
dark obscuring dust make it look rather like a wide-brimmed hat.
South African Astronomical Observatory (SAAO) A national optical astronomy
facility located at Sutherland, 220 miles from Cape Town, in South Africa. The
SAAO was formed in 1972 by merging the old Royal Observatory at Cape Town
and the Republic Observatory, Johannesburg. Some of their telescopes were
moved to the Sutherland site and, in 1974, a 1.9-m (75-inch) telescope was
purchased from the Radcliffe Observatory, Pretoria, and also moved. In 1999,
Sutherland was chosen as the site for the ¤ Southern African Large Telescope.

Space Shuttle

An image of the Sombrero Galaxy from the 8.2-m ANTU unit of the European Southern
Observatory™s Very Large Telescope.

Southern African Large Telescope (SALT) An 11-m telescope opened at
Sutherland in South Africa in 2005. The project involves 10 international
partners. Its design is closely based on that of the ¤ Hobby“Eberly Telescope. The
telescope™s angle of tilt is fixed and it rotates only at its base. Targets for
observing are selected by moving the instruments positioned above the main
South Atlantic anomaly A region over the South Atlantic Ocean where the lower
¤ Van Allen belt of electrically charged particles is particularly close to Earth™s
surface, creating a hazard for artificial satellites.
Southern Cross Popular name for the cross-shaped ¤ asterism in the constellation
¤ Crux, formed by the stars Alpha, Beta, Gamma and Delta.
Southern Pleiades Popular name for IC 2602, a large and bright ¤ open cluster of
stars in the constellation Carina. The star Theta Carinae (magnitude 2.8) is at
the center of the cluster and several other member stars are also visible to the
naked eye. The whole cluster is about one degree across on the sky.
Soyuz A Soviet spacecraft, used to carry up to three cosmonauts.
space The regions between the planets and stars, excluding their immediate
Space Shuttle A US reusable space vehicle that takes off like a rocket but lands
like an aircraft on a runway. A total of five have been built. The first flight was
made by Columbia on April 12, 1981. The second Shuttle, Challenger, was
destroyed in an explosion shortly after its tenth launch in 1986. It was replaced
by Endeavour, which was first launched in 1991. Discovery and Atlantis, the third

Space Shuttle

The stars of the Southern Cross (on the right), with the Milky Way and part of the
constellation Centaurus.


. 9
( 12)