. 6
( 12)


704 Interamnia The sixth-largest asteroid known. It was discovered in 1910 by
Vincenzo Cerulli, who gave it the Latin name for Teramo, the Italian city where
he worked. Its diameter is 338 km (210 miles).
interferometer An instrument that brings together two or more separate beams of
electromagnetic radiation from a celestial object so that they interfere and
produce a characteristic pattern. In basic interferometry, telescopes are used in
pairs and the resulting interference patterns are analyzed by computer. By using
more than two mirrors, or radio antennas, it is possible to produce high-
resolution maps or images. This technique is often known as ¤ aperture synthesis.
Interferometry has been used in radio astronomy for decades. More
recently it has been applied to the infrared and to visible light. Instruments for
optical interferometry have been constructed, such as the Cambridge Optical
Aperture Synthesis Telescope in the UK and the Navy Prototype Optical
Interferometer in the USA. In addition, several very large telescopes have been
designed so they can be used for optical interferometry. They include the

International Astronomical Union

An artist™s impression of the International Gamma-Ray Astrophysics Laboratory

¤ Keck Observatory, the ¤ Very Large Telescope and the ¤ Large Binocular Telescope.
¤ radio interferometer, very-long-baseline interferometry.
International Astronomical Union (IAU) An organization formed in 1919 for
fostering international cooperation in astronomy. Its members include both
national organizations and individuals.
The history of the IAU goes back to the international cooperation
established for the ¤ Carte du Ciel project. Today, the IAU is recognized as the
international authority on astronomical matters requiring cooperation and
standardization, such as the of¬cial naming of astronomical bodies and
features on them. The Central Bureau for Astronomical Telegrams and the
Minor Planet Center located at the ¤ Smithsonian Astrophysical Observatory
operate under its auspices. The IAU is also concerned with the promotion of
astronomy in developing countries.
International Gamma-Ray Astrophysics Laboratory (INTEGRAL) An
international orbiting observatory for ¤ gamma-ray astronomy. The project is led
by ESA, in cooperation with Russia and the USA. The observatory was launched
into Earth orbit on October 17, 2002. There are four instruments “ an imager, a
spectrometer, an X-ray monitor and a camera for visible light “ all of which
observe the same region of sky simultaneously. The nominal mission of two
years was extended until 2008.
International Space Station (ISS) An orbiting space station, which is a joint
project between the space agencies of the USA, Russia, Japan, Canada and
Europe. Construction began in 1998 and is expected to be complete in 2010.

interplanetary dust

The International Space Station under construction in orbit in 2006.

The ISS is being assembled in orbit from a variety of modules and elements
transported by the Space Shuttle or by Russian launch vehicles. The size and
scope of the ISS has been reduced compared with the original plan but the
¬nished space station will still have 10 main pressurized modules. It is in low
Earth orbit, at an altitude of 360 km (220 miles) and makes one circuit of Earth
in 92 minutes. The ¬rst crew arrived at the ISS in November 2000 and there has
never been fewer than two astronauts or cosmonauts on board since.
The completion date was to have been 2005, but the project has been
subject to delay for several reasons, including budget problems and the
destruction of the ¤ Space Shuttle Columbia in an accident in 2003, which led to
the grounding of all Space Shuttles for more than two years.
International Ultraviolet Explorer (IUE) An ultraviolet astronomical telescope
with a 45-cm (18-inch) primary mirror, which was launched into Earth orbit in
1978. A joint NASA“ESA“UK project, it continued to observe successfully for 18
years, finally ceasing operations in September 1996.
interplanetary dust Small dust particles in the space between the planets.
The particles are thought to come from collisions between asteroids in the
¤ asteroid belt and from the gradual break-up of comets. Fragments from
comets initially form ¤ meteor streams, but disperse over long periods. The dust
cloud in the plane of the solar system extends out from the Sun for at least
600 million km (370 million miles). It shows up in the sky as the ¤ zodiacal light.

interplanetary magnetic field

Its density is very low, equivalent to one grain in a cube hundreds of meters
across at the Earth™s distance from the Sun.
interplanetary magnetic field (IMF) The general magnetic field in the solar
system. Its origin is the ¤ solar wind, which carries the solar magnetic field with
it as it blows outwards. The Sun™s rotation makes its shape a spiral. The field
varies considerably from place to place and over time, depending on the
behavior of the solar wind. Its strength declines with distance from the Sun.
interplanetary medium The material in space between the planets of the solar
system, which is composed of ¤ interplanetary dust, gas from the ¤ interstellar
medium, and electrons, protons and helium nuclei (alpha particles) streaming
outwards from the Sun.
interstellar dust Small particles in the ¤ interstellar medium. Interstellar dust
grains range in size from 0.005 mm to 1 mm and are generally mixed in with gas.
Though accounting for less than one percent of the mass of the typical
interstellar medium, the dust absorbs far more light and emits far more
infrared radiation than the gas. It causes ¤ interstellar extinction and ¤ interstellar
reddening. Starlight scattered from dust particles creates ¤ reflection nebulae.
Energy the dust absorbs from starlight raises its temperature to a few tens
of degrees above absolute zero. Even at such a low temperature, the dust emits
some radiation, mainly in the infrared. If dust is heated above about 1500 K, it
is destroyed.
It is unlikely that all interstellar dust has the same composition. Graphite
(a common form of carbon) and silicates of iron, aluminum, calcium and
magnesium are thought to be some of the commonest materials. Some of the
particles, if not all, are not spherical.
Most interstellar dust is thought to come from dust shells that form
around cool ¤ red giant stars. Solid particles can condense in the cool gas of
their outer layers. Grains of material may also condense within ¤ molecular
interstellar extinction The dimming of light from distant stars because of
absorption and scattering by interstellar dust. Red light is dimmed less than
blue light, which results in ¤ interstellar reddening. The blue light from a star
near the center of the Galaxy is reduced in brightness by 25 magnitudes by the
interstellar material along our line of sight.
interstellar medium (ISM) The material in the space between stars in a galaxy.
The interstellar medium in our own Galaxy has at least one-tenth the total
mass of the stars. In general, spiral galaxies have substantial amounts of
interstellar material and elliptical galaxies little or none.
The interstellar medium is not uniform and is made up of a number of
components: dark clouds of gas and dust, regions of ¤ ionized hydrogen and
¤ neutral hydrogen, ¤ molecular clouds, ¤ globules, very hot tenuous gas and


high-energy ¤ cosmic ray particles. Its chemical composition is enriched by
material blown off ¤ supernovae and other stars. On a scale of thousands of light
years, the structure of the interstellar medium is probably dominated by
merging supernova remnants. ¤ astration, Local Bubble.
interstellar molecules Molecules in the ¤ interstellar medium, especially in
¤ molecular clouds. They are destroyed by the ultraviolet radiation from hot stars
and so are found chiefly in dense clouds where they are shielded. Before 1963,
CH (methylidyne), CHþ and CN (cyanogen) were the only interstellar molecules
known. In 1963, radio emission at a wavelength of 18 cm was recognized as
being from hydroxyl (OH). Around 100 different molecules have been
identified since 1968, mainly by their spectra at millimeter wavelengths. Most
are simple organic molecules.
interstellar reddening The apparent reddening of light from distant stars by
¤ interstellar dust. The degree to which light is scattered and absorbed in
the ¤ interstellar medium depends strongly on its color: blue light is dimmed
more than red light. As a result, the colors of stars viewed through interstellar
material are altered and appear redder.
intracluster medium (ICM) The material between the galaxies in a ¤ cluster of
galaxies. The ICM is made up of several components. Tenuous hot gas emits
X-rays. Diffuse radio emission is produced by high-energy particles. Stars that
have been torn from interacting galaxies are also found in intergalactic space.
These known components of the ICM, and the galaxies themselves, only
account for 20 percent of the total mass of a typical cluster. Some of the unseen
mass is almost certainly associated with the galaxies but the motion of galaxies
in clusters suggests that there is also ¤ dark matter in the ICM.
Io The third largest of the moons of Jupiter and one of the four discovered by
¤ Galileo in 1610. Its diameter is 3643 km (2264 miles). It is the most
volcanically active body in the solar system and its surface is brightly colored “
much of it a greenish yellow dappled with patches of orange and white.
Volcanism had been predicted on the basis that strong tides caused by
Jupiter would stir up and melt rock in the interior of Io. Eight active eruptions
were identi¬ed in images returned by the ¤ Voyager 1 spacecraft. Monitoring
by ground-based observatories, by the Hubble Space Telescope, and the
¤ Galileo spacecraft have con¬rmed the continuous high level of eruption
activity on Io. Over 100 centers of volcanic eruptions are now known. Many are
surrounded by roughly circular halos of ejected material.
There are a variety of features associated with the volcanism. These
include calderas, lava ¬‚ows and lava lakes where molten material gushes out
from beneath the surface, and immense fountain-like plumes of gas and dust.
The colored crust is made of sulfur and solid sulfur dioxide. No impact craters
are seen; any that were formed in Io™s early history have long since been


A close-up of the volcanic surface of Io from the Galileo spacecraft.

covered. Mountains rise up to 16 km (52 000 feet) high, but they are not
volcanoes. Io has a thin atmosphere of sulfur dioxide and a ring of electrically
charged particles. A ring of such particles also surrounds Jupiter, enclosing
the orbit of Io. Data from the Galileo spacecraft suggest that Io has a
metallic core.
Iocaste A small outer moon of Jupiter discovered in 2000. Its diameter is about
5 km (3 miles).
ion An atom that has gained or lost one or more electrons and so is electrically
ion drive A popular name for electric propulsion, a method of propelling a
spacecraft which uses an electrically generated jet rather than the gases
produced by burning chemical fuel.

Ion drive. An artist™s impression of the
ion engine powering the SMART-1

ion tail

ionization The removal of electrons from atoms or molecules. Atoms and
molecules can be ionized by collisions or by absorbing radiation.
ionized hydrogen Hydrogen in which the atoms have been split to make separate
electrons and protons. (A neutral hydrogen atom has a single proton as its
nucleus and one electron.)
The main way hydrogen clouds in interstellar space are ionized is by
absorbing ultraviolet radiation. Ionized hydrogen is the chief constituent of
hot clouds known as H II (or Hþ) regions. They are roughly spherical and can be
up to 600 light years across. Intense ultraviolet radiation from young hot stars
embedded in the clouds ionizes the gas. There is also ionized hydrogen in
¤ supernova remnants and the shells of ¤ planetary nebulae.
ionosphere An ionized layer in the atmosphere of a planet. Earth™s ionosphere lies
between heights of about 50 and 600 km (30 and 350 miles), though its extent
varies considerably with time, season and ¤ solar activity. It is created by the
effects of ultraviolet radiation and X-rays from the Sun. There are four different
layers with different characteristics known as the D, E, F1 and F2 layers in order
of height. The E and F1 regions, between 90 and 230 km up, form the main part
of the ionosphere.
ion tail (type I tail) One of the two types of tail ¤ comets develop when they get
near the Sun. Ion tails, also known as the gas tails or plasma tails, consist of
ionized atoms and molecules that glow after absorbing sunlight. The ion tail
forms in the plane of a comet™s orbit. It streams nearly straight away from the

The irregular
galaxy NGC


Sun but curves by a few degrees. It is ˜˜blown™™ by the ¤ solar wind and affected
by the Sun™s magnetic field. ¤ disconnection event.
IRAS Abbreviation for ¤ Infrared Astronomical Satellite.
7 Iris One of the larger asteroids in the main asteroid belt, with a diameter of
208 km (129 miles). It was discovered from London by John Russell Hind in 1847.
iron meteorite A type of ¤ meteorite composed almost entirely of iron and nickel.
irregular galaxy Any galaxy that is not obviously an ¤ elliptical galaxy or a ¤ spiral
galaxy. About a quarter of known galaxies are irregular. Many appear to have
star-forming regions, and are dominated by regions of luminous gas and bright
young stars. Radio observations of the hydrogen gas in irregular galaxies often
reveal that there is a rotating disk of gas; in this respect, and in their star
content, they resemble spiral galaxies.
irregular variable A pulsating variable star that changes in brightness slowly and
in an irregular way.
Isaac Newton Telescope (INT) A British 2.5-m (98-inch) reflecting telescope at the
¤ Observatorio del Roque de los Muchachos, La Palma, Canary Islands. The telescope
was originally in the UK. It was rebuilt and provided with a new primary mirror
on removal to La Palma, where it came into operation in 1984.
Ishtar Terra One of the major upland areas on the planet ¤ Venus, comparable in
size with the continent of Australia. It includes the highest mountain peaks on
Venus, Maxwell Montes.
ISO Abbreviation for ¤ Infrared Space Observatory.
Isonoe A small outer moon of Jupiter discovered in 2000. Its diameter is about
4 km (2.5 miles).
ISS Abbreviation for ¤ International Space Station.

An image of the asteroid Itokawa
returned by the spacecraft Hayabusa.


25143 Itokawa A small asteroid visited by the Japanese spacecraft ¤ Hayabusa.
Images returned by Hayabusa show it to be elongated, measure 535 · 209
meters (1755 · 685 feet). It seems to be a loose collection of fragments,
probably debris left over when a larger asteroid was shattered by an impact.
The surface is littered with many rocks and boulders but there are no impact
craters. Itokawa™s path around the Sun crosses the orbits of both Earth
and Mars.
IUE Abbreviation for ¤ International Ultraviolet Explorer.

James Clerk Maxwell Telescope A submillimeter-wave telescope at the ¤ Mauna
Kea Observatories on the island of Hawaii operated by the UK, the Netherlands
and Canada. Its 15-m (49-foot) re¬‚ector is on an ¤ altazimuth mount. The dish
is constructed from 276 lightweight panels and is designed so its shape does
not change wherever it is pointed.
James Webb Space Telescope ( JWST) The space telescope under development as
the successor to the ¤ Hubble Space Telescope. The projected launch date is no
earlier than 2013 and the telescope™s anticipated lifetime is 5“10 years. The
JWST will have a primary mirror about 6.5 m in diameter and will operate in
the wavelength region between the red end of the visible spectrum and the
It will be placed in orbit around the Sun at a point 1.5 million km
(1 million miles) from Earth in the direction opposite the Sun, rather than
in orbit around Earth. This has the advantage of simplifying operations by
minimizing stray light and temperature changes.
The project was initially known as the Next Generation Space Telescope
but was later named in honor of a former NASA administrator.

The James Clerk Maxwell Telescope.

Jet Propulsion Laboratory

An artist™s impression of the James Webb Space Telescope.

Jansky, Karl (1905“1950) Jansky was an American radio engineer, who in 1931,
while he was investigating interference with long-distance communications,
became the ¬rst person to detect cosmic radio emission. He surmized,
correctly, that the radio waves came from interstellar gas in the ¤ Milky Way.
The unit in which radio astronomers measure the strength of radio emission
from cosmic sources is called the jansky in his honor.
Janus A small moon of Saturn, discovered by Audouin Dollfus in 1966 when the
rings were edge-on as seen from Earth. Janus orbits just beyond the outer edge
of the ring system at a distance of 151 422 km (94 089 miles) and its orbit is
shared by another small moon, Epimetheus. The two may be fragments of a
larger object that was shattered by an impact. Janus is irregular in shape,
measuring 194 · 190 · 154 km (121 · 118 · 96 miles).
JD Abbreviation for ¤ Julian date.
Jeans, Sir James Hopwood (1877“1946) The British astronomer and physicist Sir
James Jeans is remembered as a proli¬c author of popular books and textbooks
that did much to promote astronomy to the general public. He read applied
mathematics at Cambridge University, graduating in 1900. He applied his
talent to a range of theoretical problems, including the dynamical theory of
gases and the origin of the solar system. From 1923 to 1944 he was on the staff
of the ¤ Mount Wilson Observatory in California.
Jet Propulsion Laboratory ( JPL) An institution in Pasadena, California, operated
by the California Institute of Technology (Caltech) in support of programs of

Jet Propulsion Laboratory

Karl Jansky.

A Cassini spacecraft image of Janus.


The Jewel Box star cluster.

the US ¤ National Aeronautics and Space Administration (NASA) and of other
agencies. It is the principal center in the USA for the development and
operation of interplanetary spacecraft.
Jewel Box (NGC 4755) An open star cluster in the constellation Crux. Its brightest
member is a sixth-magnitude, blue, supergiant star, Kappa Crucis. It includes
several blue and red supergiants and its appearance in a small telescope is
impressive. The contrasting colors of the stars is said to be why John ¤ Herschel
described the cluster as the ˜˜Jewel Box.™™ The cluster™s distance is 7800 light
Jodrell Bank Observatory The astronomy facility of the University of Manchester
in the United Kingdom. It incorporates a radio astronomy observatory where
the main instrument is a 76-m (250-foot) fully steerable dish, which was ¬rst
opened in 1957. It has been known as the Lovell Telescope since 1987 in honor
of the astronomer Sir Bernard Lovell, who was the driving force behind its
conception and construction.
Johnson Space Center ( JSC) A large space technology complex at Houston Texas,
which is the headquarters for the astronauts of the US National Aeronautics
and Space Administration (NASA). It was named after the late US President
Lyndon B. Johnson, who took a strong interest in the space program.
jovian Pertaining to the planet Jupiter.
jovian planets The giant planets Jupiter, Saturn, Uranus and Neptune.
JPL Abbreviation for ¤ Jet Propulsion Laboratory.


Jodrell Bank Observatory. The control room for the Lovell Telescope
(visible through the window) and MERLIN.

JSC Abbreviation for ¤ Johnson Space Center.
Julian calendar A calendar brought into use in the Roman Empire by Julius
Caesar from 46 bc. Three years of 365 days were followed by one of 366 days,
making the average length of a year 365.25 days. Since this is 11 minutes 14
seconds longer than a true ¤ tropical year, the seasons gradually shifted relative
to the civil year. To correct for this, the ¤ Gregorian calendar was introduced
from 1582.
Julian date ( JD) The interval of time in days since noon at Greenwich on January 1,
4713 bc.
Juliet A small moon of Uranus, about 94 km (58 miles) in diameter, discovered
by ¤ Voyager 2 in 1986.
3 Juno The third asteroid to be discovered. Its diameter is 248 km (154 miles) and it
was found by Karl L. Harding in 1804.
Jupiter The largest planet in the solar system and the ¬fth in order from the Sun.
After Venus, it is the second-brightest planet as seen from Earth. It travels
around the Sun in a slightly elliptical orbit at an average distance of 5.20 AU,
taking 11.86 years to complete one revolution.
With a diameter of 143 082 km (88 911 miles) at its equator, Jupiter is over
11 times larger than Earth and about one-tenth the size of the Sun. Its mass is
318 times Earth™s, and about 0.1 percent that of the Sun, making it more
massive than all the other planets put together.
Jupiter™s composition (by number of molecules) is very similar to the Sun™s:
90 percent hydrogen and 10 percent helium. The most signi¬cant trace gases
in its atmosphere are water vapor, methane and ammonia. There is no solid
surface beneath the cloud layer. Instead, there is a gradual transition from gas


Jupiter. An image taken by the Cassini spacecraft.

to liquid as the pressure increases with depth below the outermost layers. At a
particular depth, the nature of the liquid changes abruptly. Below that level,
the hydrogen has the properties of a metallic liquid, in which the atoms are
stripped of their electrons. At the very center of Jupiter there may be a small
core of rock and perhaps ice. Some heat remains inside Jupiter from when
it ¬rst formed, causing it to radiate between 1.5 times and twice as much heat
as it currently absorbs from the Sun.
Observed by eye, the disk of Jupiter appears crossed by alternating light
zones and dark belts. There are ¬ve or six in each hemisphere, corresponding
to wind currents. Results from four spacecraft that passed by Jupiter between
1973 and 1981 (¤ Pioneers 10 and 11, ¤ Voyagers 1 and 2), and from the more
recent ¤ Galileo mission, have revealed how complex the patterns created by
the ¬‚ow of gases are within these bands.
White and colored ovals appear and are relatively long-lived features. The
best-known and most conspicuous of all is the Great Red Spot, which has been
observed for around 300 years and is as wide as Earth. Its origin is uncertain
but it is essentially a huge anticyclone.
The colored clouds are in the highest layers, which take up only 0.1“0.3
percent of Jupiter™s radius. The origin of their coloration remains a mystery,
though it must stem from trace constituents of the atmosphere. The cloud
colors correlate with depth in the atmosphere. Blue features are the deepest,
followed by brown, then white, with red being the highest.


A probe released by the Galileo spacecraft in 1995 parachuted through
Jupiter™s upper atmosphere and returned data on the composition and physical
conditions. Ground-based observations of the entry site indicated that it may
have been a relatively cloud-free spot, explaining why hardly any evidence was
found for the expected three layers of cloud consisting of ammonia crystals at
the highest level, ammonium hydrosul¬de in the middle, with water and ice
crystals below. Winds up to 530 km/h (330 mph) were even faster than
anticipated. The abundance of helium was only about half that expected. A
likely explanation is that the helium is more concentrated towards the center
of the planet. The probe also discovered an intense radiation belt.
The existence of a faint ring around Jupiter was ¬rst suggested by results
from Pioneer 11 in 1974 and was con¬rmed by Voyager images. The main part
lies between 1.72 and 1.81 Jupiter radii from the center of the planet. The ring
consists of tiny particles only micrometers in size, which must constantly
be replenished.
There are at least 62 moons orbiting Jupiter, most of them small. The
four large Galilean satellites “ Io, Europa, Ganymede and Callisto “ are easily
visible with a small telescope or binoculars.
Radio emission from Jupiter was discovered in 1955. It was the ¬rst
indication of Jupiter™s strong magnetic ¬eld, which is 4000 times greater than
Earth™s. The radio emission is caused by electrons spiraling around in the
magnetic ¬eld.

K Abbreviation for ¤ kelvin, a unit in which temperature is measured. One kelvin
corresponds to an interval of one degree on the Celsius scale. The kelvin
temperature scale starts from absolute zero, which is À273.16  C.
Kale A small outer moon of Jupiter discovered in 2001. Its diameter is about 2 km
(1 mile).
Kallichore A small outer moon of Jupiter discovered in 2003. Its diameter is about
2 km (1 mile).
Kalyke A small outer moon of Jupiter discovered in 2000. Its diameter is about
5 km (3 miles).
Kamiokande ¤ Super-Kamiokande.
Kappa Crucis The brightest star in the open cluster NGC 3324, popularly known
as the ¤ Jewel Box.
Kaus Australis (Epsilon Sagittarii) The brightest star in the constellation
Sagittarius. With Kaus Meridionalis (Delta) and Kaus Borealis (Lambda), it
marks the Archer™s bow It is a ¤ B star of magnitude 1.9 and 145 light
years away.
Keck Observatory Two 10- m (400-inch) re¬‚ecting telescopes belonging to the
California Institute of Technology (Caltech) and the University of California.
They are at the ¤ Mauna Kea Observatories on the island of Hawaii and have been

The twin domes of the Keck Observatory.

Keeler, James Edward (1857“1900)

funded by the W. M. Keck Foundation. The ¬rst telescope was completed in 1992
and the second in 1996. The primary mirrors each consist of 36 hexagonal
segments. The precise shape of the mirrors is maintained by specially designed
supports under computer control. The use of ¤ adaptive optics makes it possible
to produce images with a resolution of 0.04 arcseconds at a wavelength of 2 mm.
The two telescopes can be used together as an ¤ interferometer. Because
Keck I and Keck II are about 85 m (nearly 280 feet) apart, the interferometer
has a resolution equivalent to a telescope with an 85- m mirror, which is only
0.005 arc seconds at a wavelength of 2 mm.
Keeler, James Edward (1857“1900) Keeler was an American pioneer in
astronomical spectroscopy. On graduation from Johns Hopkins University in
1881 he became an assistant at the Allegheny Observatory, Pittsburgh. In
1891 he was appointed the director there. Using spectroscopy he found that
Saturn™s rings are not solid. He worked at the Lick Observatory, California in
1898 and 1900, where he photographed 120 000 galaxies. This achievement
established the superiority of large re¬‚ecting telescopes over refractors.
Keeler Gap A narrow gap towards the outer edge of the bright A ring of ¤ Saturn.
¤ ring system.
Kennedy Space Center (KSC) A NASA space launch center at Cape Canaveral on the
Atlantic coast of Florida. It is the only place from which the ¤ Space Shuttle has been
launched, and is one of the few facilities where the Shuttle can land. In the past,
all the manned launches in the Mercury, Gemini, Apollo and Skylab programs
were from KSC. It was named after the late US President, John F. Kennedy.
Kepler (1) A lunar crater, 32 km (20 miles) in diameter, in the Oceanus
Procellarum. It has terraced walls and a central peak, and is the center of a
large, bright ray system.
Kepler (2) A NASA space observatory designed to search for ¤ extrasolar planets. It is
scheduled for launch in June 2008 and will be placed in an orbit around the
Sun, trailing behind Earth. The 0.95- m telescope will monitor the brightness of
thousands of stars, looking for minute dips when a planet passes in front of a
star. It will be able to detect planets as small as Earth.
Kepler, Johannes (1571“1630) Kepler was one of the most important scientists of
his time and made contributions in a number of areas. He is remembered
especially for the laws of planetary motion he formulated (¤ Kepler™s Laws) and
he believed in a mathematical harmony underlying the universe.
Kepler was born in Wurtemburg, Germany, the son of poor parents. He
graduated from the University of Tubingen in 1591, where he studied for the
Lutheran ministry. However, he took up mathematics instead and acquired a
deep interest in the ¤ Copernican system.
The endless religious turmoil and numerous epidemics of his times
considerably affected his career. In 1598, all Lutherans were expelled from

Kepler, Johannes (1571“1630)

A ¬sheye view of the Space Shuttle Atlantis leaving launch pad 39B at the Kennedy Space
Center on 9 July 2006.

Graz, Austria, where he had a teaching position. Fortunately Tycho ¤ Brahe, by
that time Imperial Mathematician in Prague, invited Kepler to join him in 1600
to work on 20 years of observations of Mars. It took him eight years to ¬nd the
reason for the puzzling motion of Mars: that its orbit is an ellipse. Brahe™s
precise data enabled Kepler to become the ¬rst person to explain planetary
motion correctly. On Tycho™s death in 1601, Kepler took over his position but
in 1611, the Emperor fell from power in a civil war and Kepler moved to Linz.
He stayed in Linz for 14 years before taking up his ¬nal appointment in Silesia.
There his patron provided him with the facilities he needed to write and study
in return for producing horoscopes.

Kepler™s laws

Johannes Kepler.

In optics, Kepler was the ¬rst to explain the formation of images and the
principles of how a telescope works. He was also the ¬rst to explain how the
Moon causes tides. He invented the word satellite to describe a small body
orbiting a much larger one.
Kepler™s laws Three fundamental rules about planetary motion, discovered by
Johannes ¤ Kepler from his study of detailed observations of the planets made
by Tycho ¤ Brahe:

1. The orbit of each planet is an ellipse with the Sun at one of the foci.
2. Each planet travels around the Sun such that the line connecting the planet
to the Sun sweeps out equal areas in equal times.
3. The squares of the sidereal periods of the planets are proportional to the
cubes of their mean distances from the Sun.

Kitt Peak

A composite image of the remnant of
Kepler™s Supernova constructed with
data from the Hubble Space Telescope,
the Spitzer Space Telescope, and the
Chandra X-ray Observatory.

The ¬rst two were published in 1609 and the third in 1619. The physical
basis for the laws was not understood until ¤ Isaac Newton formulated his law
of gravity.
Kepler™s Supernova (Kepler™s Star) A ¤ supernova that appeared in the
constellation Ophiuchus in October 1604. ¤ Johannes Kepler observed it and
determined its position. At its brightest it reached a magnitude of about À2.5
and the light curve shows that it was a Type I supernova. The remnant is a
source of radio emission, and there is also a faint visible nebula.
Keyhole Nebula (NGC 3372) A dark dust nebula near the center of the ¤ Carina
Nebula. The circular part of the keyhole shape is a bubble expanding at 40 km/s.
Keystone The four stars Epsilon, Zeta, Eta and Pi in the constellation ¤ Hercules.
Kids A group of three stars, Epsilon, Zeta and Eta, in the constellation Auriga.
Their name comes from the fact that Alpha Aurigae is named Capella, meaning
˜˜the little she-goat.™™
kiloparsec (symbol kpc) A unit of distance equal to one thousand ¤ parsecs, which
is 3261.61 light years.
Kirkwood gaps Gaps in the ¤ asteroid belt due to the in¬‚uence of Jupiter. There are
notable gaps where the orbital periods of the asteroids would be in the
proportion to that of Jupiter by factors of 4:1, 3:1, 5:2, 7:3 and 2:1. Asteroids
cannot remain in such orbits because of regular gravitational interactions with
Jupiter. The explanation was ¬rst given by Daniel Kirkwood in 1857.
Kitt Peak A mountain-top observatory site near Tucson, Arizona, which is home to
one of the largest collection of astronomical research instruments in the
world. These include the ¤ Kitt Peak National Observatory, facilities of the US
¤ National Solar Observatory, and the ¤ WIYN Telescope operated by the US
¤ National Optical Astronomy Observatories. A number of universities and other
research organizations also lease space on Kitt Peak for astronomical work.

Kitt Peak National Observatory

An infrared image of the Keyhole Nebula.

Telescopes on Kitt Peak, Arizona.

Kitt Peak National Observatory (KPNO) A facility of the US ¤ National Optical
Astronomy Observatories located at ¤ Kitt Peak in Arizona. The largest telescope is
the 4- m (160-inch) ¤ Mayall Telescope.
Kiviuk A small outer moon of Saturn in a very elliptical orbit. It was discovered in
2000 and is about 14 km (9 miles) across.

Kuiper, Gerard Peter (1905“1973)

Kleinmann“Low Nebula An extended source of infrared radiation in the ¤ Orion
Nebula. It is a region of star formation lying behind the luminous gas.
216 Kleopatra An asteroid discovered by Johann Palisa in 1880. Radar
observations reported in 2000 showed Kleopatra™s shape to be very unusual. It
resembles a distorted dumbbell or ˜˜dog-bone™™ measuring 217 · 94 · 81 km
(135 · 58 · 50 miles) and its composition is metallic.
Kohoutek, Comet A comet discovered in 1973 when it was still near the orbit of
Jupiter. Predictions that it would prove spectacular when it got nearer to Earth
turned out to be incorrect. However, it was the subject to an extensive,
coordinated observing program by professional astronomers and much new
information about comets was obtained as a result.
Koronis family One of the ¤ Hirayama families of asteroids, at a mean distance of
2.88 AU from the Sun. The members have very similar compositions so
probably come from the break-up of a single parent body. The largest member
is 208 Lacrimosa, which is about 45 km (28 miles) in diameter. The family is
named after 158 Koronis, which has a diameter of 35 km (22 miles) and was
discovered in 1876.
Kreutz group ¤ sungrazer.
Kruger 60 A faint binary star in the constellation Cepheus. The two members of
the system, magnitudes 10 and 11, orbit each other in a period of 44 years.
Since their orbits are face-on to us, and the two stars are easily resolved, their
relative motion can easily be followed over a period of decades. Both stars are
dwarf ¤ M stars and the fainter component is a ¤ ¬‚are star. At a distance of 13
light years, it is one of the nearest stars to the solar system.
KSC Abbreviation for ¤ Kennedy Space Center.
K star A star of ¤ spectral type K. K stars have surface temperatures in the range
3500“4900 K and are orange in color. Absorption lines of neutral and ionized
calcium are strong in their spectra. There are also numerous absorptions
due to neutral metals and molecules, particularly in the spectra of cooler K
stars. ¤ Arcturus and ¤ Aldebaran are examples of K stars.
Kuiper Airborne Observatory (KAO) A 0.915- m (36-inch) Cassegrain re¬‚ecting
telescope mounted in a Lockheed C141 Starlifter jet transport aircraft, which
was operated between 1975 and 1996 by NASA as a national facility in the USA.
Important discoveries made with the observatory include the ring system
around the planet Uranus.
Kuiper, Gerard Peter (1905“1973) Kuiper was a pioneering Dutch“American
planetary scientist. Born and educated in the Netherlands, he emigrated to the
United States, where we worked for the rest of his life. He held appointments
at the Lick Observatory, Harvard University, the Yerkes Observatory and the
McDonald Observatory. From 1960 until his death he was the head of the Lunar
and Planetary laboratory of the University of Arizona.

Kuiper Belt

Gerard Kuiper.

Kuiper discovered Uranus™s moon ¤ Miranda in 1948 and ¤ Nereid a new
moon of Neptune, in1949. He also discovered that Saturn™s moon ¤ Titan has
an atmosphere containing methane, and identi¬ed carbon dioxide in the
martian atmosphere. In 1951 he predicted the existence of the belt of small, icy
planetary bodies beyond Neptune, which is known as the ¤ Kuiper Belt.
Kuiper Belt A population of small, icy, solar-system bodies beyond Neptune, most
of which are similar in size to ¤ asteroids, though some are larger than Pluto.
They occupy a ring-shaped region in the plane of the solar system extending
from the orbit of Neptune (30 AU from the Sun) out to 100 or 150 AU. This
population is believed to be the source of ¤ short-period comets. The name of
Gerard ¤ Kuiper, a distinguished Dutch“American planetary scientist, is
attached to the belt because he predicted its existence in 1951. However, an
Irish writer and theorist, Kenneth E. Edgeworth, had published a similar idea
in 1943 and 1949. In recognition of his contribution, some astronomers call it
the Edgeworth“Kuiper Belt.
The ¬rst observational evidence for the existence of the Kuiper Belt was
the discovery in 1992 of a faint object known as 1992 QB1 in a near circular
orbit about 50 AU from the Sun. Hundreds more members have been found
since. ¤Pluto is one of the largest known member of the Kuiper Belt. The
discovery of ¤ Eris, which is slightly larger than Pluto, was announced in 2005.
Kuiper Belt object (KBO) A body belonging to the ¤ Kuiper Belt.

Lacerta (The Lizard) A small, inconspicuous constellation between Cygnus and
Andromeda. It was introduced by Johannes Hevelius in the late seventeenth
century and contains only one star brighter than fourth magnitude.
Lagoon Nebula (M8; NGC 6523) A luminous nebula in the constellation
Sagittarius. It is a complex region of ¤ ionized hydrogen, gas and dust with hot,
recently formed stars. A star cluster, NGC 6530, lies near the center of the
nebula. The light from two naked-eye stars in the cluster, 7 and 9 Sagittarii, is
responsible for ionizing the gas. The nebula is estimated to lie at a distance of
4500 light years.
Lagrangian points Equilibrium points in the orbital plane of two large bodies
circling about each other, where a much smaller object could remain. There
are ¬ve Lagrangian points for two bodies in circular orbits around each other,
called L1 to L5. They are named for the great French mathematician, Joseph
Louis Lagrange (1736“1813), who discovered that they exist.
L4 and L5, in the orbit of the less massive of the two large bodies and 60
either side of it, are stable as long as the ratio of the masses of the two large
bodies exceeds 24.96. The ¤ Trojan asteroids, which share the orbit of Jupiter,
are examples of objects trapped at L4 and L5 in the Sun“Jupiter system.
Objects at the other three Lagrangian points are not stable and are easily
displaced by small perturbations. Nevertheless, L1 and L2 in the Sun“Earth
system are important locations for stationing spacecraft, because they remain
at ¬xed distances from Earth as it orbits the Sun. Regular course corrections
are needed to keep spacecraft at these locations. In practice, a spacecraft

The Lagrangian points.

Laplace, Marquis Pierre Simon de (1749“1827)

Pierre-Simon Laplace.

normally follows a circular ˜˜halo orbit™™ around the Lagrangian point.
¤ Roche lobe.
Laplace, Marquis Pierre Simon de (1749“1827) Laplace was a French applied
mathematician who extended the work of Isaac ¤ Newton on motion in the
solar system. His greatest work was his Celestial Mechanics, published in ¬ve
volumes between 1799 and 1825. In it he proved that the solar system is
dynamically stable, which Newton had not been able to do. Laplace also argued
in favor of the so-called ˜˜nebular hypothesis™™ for the origin of the solar system,
according to which the planets formed from the same rotating mass of gas as
the Sun.
Large Binocular Telescope (LBT) A telescope consisting of two 8.4-m mirrors on a
single mount, located at the ¤ Mount Graham International Observatory in
Arizona. The project is a collaboration between the Italian Istituto Nazionale
de Astro¬sica, a consortium of US universities and German astronomical
research institutes. The binocular arrangement gives the telescope a
light-gathering power equivalent to a single 11.8-m mirror, and a resolution
corresponding to a 23-m telescope. The telescope began operating in 2006.
Large Magellanic Cloud (LMC) ¤ Magellanic Clouds.
Larissa A small satellite of Neptune discovered during the ¬‚yby of ¤ Voyager 2 in
August 1989. It measures about 208 · 178 km (129 · 111 miles).
Las Campanas Observatory An observatory in the Sierra del Condor, Chile, at a
height of 2300 m (7500 feet). It is operated by the Carnegie Institution of
Washington and is the site of the ¤ Magellan Telescopes. The other main
instruments are the Irenee du Pont 2.5-m (100-inch) and Henrietta Swope 1-m
(40-inch) re¬‚ectors.

Leavitt, Henrietta Swan (1868“1921)

The Large Binocular Telescope in 2005 when the ¬rst of its two mirrors
came into operation.

Laser Interferometer Gravitational-Wave Observatory (LIGO) Two
¤ gravitational wave detectors constructed by a consortium led by the California
Institute of Technology and the Massachusetts Institute of Technology. The two
widely separated detectors are located in Washington state and Louisiana.
Scienti¬c operation began in 2001.
La Silla Observatory One of the two sites of the ¤ European Southern Observatory. It
is located in the southern part of the Atacama desert, about 600 km (370 miles)
north of Santiago de Chile, at an altitude of 2400 m (7900 feet). The
instruments include a 3.6-m (142-inch) telescope, the 3.5-m (138-inch) ¤ New
Technology Telescope and the 15-m (50-foot) Swedish/ESO Submillimeter
last quarter The ¤ phase of the Moon when it is waning and half of its disk is bright.
Last quarter is formally de¬ned as the time when the Moon™s celestial ¤ longitude
is 270 greater than the Sun™s. It occurs about seven days after full Moon.
latitude Angular distance measured north or south of an equator. In celestial
¤ equatorial coordinates, ˜˜latitude™™ is known as ¤ declination.
leap second ¤ Universal Time.
Leavitt, Henrietta Swan (1868“1921) Leavitt was an American astronomer best
remembered for discovering that there is a relationship between the
luminosity of ¤ Cepheid variable stars and the periods with which they vary. She
graduated from Radcliffe College in 1892. Three years later she became an
unpaid assistant at Harvard College Observatory, receiving a salary only from
1902. She devoted herself to the Observatory™s program of measuring the


La Silla Observatory.

magnitudes of stars from photographs. She discovered about 2400 variable
stars. By 1912 she could shown how dimmer Cepheid variables have long
periods of variability. This relationship proved to be of immense importance
for determining the distances to galaxies.
Leda A small moon of Jupiter discovered by Charles Kowal in 1974. It is about
20 km (12 miles) across. With Elara, Himalia and Lysithea it belongs to a family
of four moons with closely spaced orbits. Their average distances from Jupiter
are between 11.1 and 11.7 million km (6.9 and 7.3 million miles).
lenticular galaxy A galaxy intermediate in shape between an elliptical and a
spiral. Lenticular galaxies are so-called because their shape resembles a convex
lens. In the ¤ Hubble classi¬cation of galaxies they are type S0.
Leo (The Lion) One of the 12 constellations of the traditional ¤ zodiac. The pattern
made by the brightest stars of this large and conspicuous constellation
resemble the shape of a lion in pro¬le. The asterism outlining the head is
known as ˜˜the Sickle.™™ There are 10 stars brighter than fourth magnitude, the
brightest being ¤ Regulus and ¤ Denebola. Leo also contains numerous galaxies.
Leo Minor (The Little Lion) A small and very inconspicuous constellation between
Leo and Ursa Major. It was introduced by Johannes ¤ Hevelius in the late

Leverrier, Urbain Jean Joseph (1811“1877)

The lenticular galaxy NGC 5866, which is in the constellation Draco, and lies at a distance
of 44 million light years.

seventeenth century and contains only one star brighter than fourth
Leonids An annual ¤ meteor shower that radiates from ˜˜the Sickle™™ in the
constellation ¤ Leo. It lasts about four days, peaking on November 17. Though a
small number of meteors are detected each year, spectacular displays are seen
occasionally. For example, in 1966, observers in the USA saw up to 40 meteors
a second. The Leonids are associated with Comet 55P/Tempel“Tuttle, which
was ¬rst recorded in 1865 and has an orbital period of 33 years. The dust
stream giving rise to the meteor shower is concentrated near the comet rather
than being evenly spread around the comet™s orbit. Because of this, good
displays are possible only every 33 years, though they are not necessarily seen
even then if Earth does not happen to pass through the dust.
Lepus (The Hare) One of the 48 ancient constellations listed by ¤ Ptolemy. Lying
just south of Orion, it is small but distinctive and contains seven stars brighter
than fourth magnitude.
Leverrier, Urbain Jean Joseph (1811“1877) The French astronomer Leverrier
made the calculations that led to the discovery of the planet Neptune on

Lexell™s Comet

12h 11h 10h LYNX


µ κ

µ Alterf

Zosma +20º
+20º Algieba

θ Chertan
Denebola 3628
M65 M105
M96 M95 R
ι +10º
ρ ω


12h 11h 10h Alphard

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 Leo.

September 23, 1846, based on observations that Uranus was deviating from its
expected course. He further developed the work in celestial mechanics done by
Pierre Simon ¤ Laplace. In 1859 he announced his discovery that the
¤ perihelion point of the orbit of Mercury does not remain ¬xed but gradually
moves around in space. Leverrier wrongly believed this was due to the gravity
of an undiscovered planet between Mercury and the Sun, which he called
Vulcan. Sixty years later this phenomenon would be a key test of ¤ general
relativity, which provided the correct explanation. In 1854, Leverrier became
director of the Paris Observatory.
Lexell™s Comet A comet discovered by Charles ¤ Messier in 1770, but named after
Anders Lexell (1740“84) who investigated its orbit. He showed that a close
approach to Jupiter in 1767 caused a large change in the comet™s orbit. After
this, it came close enough to Earth to be visible. The comet passed within 1.2
million km (0.75 million miles) of Earth, the nearest a comet has ever been

Lick Observatory

Urbain-Jean-Joseph Leverrier.

known to come. However, another close approach to Jupiter in 1779 perturbed
its orbit so drastically that it was never seen again.
Libra (The Scales) One of the twelve constellations of the ¤ zodiac, in ancient times
known as the claws of the Scorpion. Libra is one of the least conspicuous
constellations in the zodiac, with just ¬ve stars brighter than fourth magnitude.
libration An effect that slightly alters which part of the Moon™s surface is visible
from Earth. Though the Moon very nearly keeps the same face towards the
Earth all the time, a total of 59 percent of the Moon™s surface can be viewed
from Earth at some time as a result of libration. There are several reasons for it.
Physical libration is a real irregularity in the Moon™s rotation; a greater effect is
geometrical libration, affecting both latitude and longitude. Libration in
latitude results from the Moon™s orbit being inclined to the ecliptic by an angle
of 5 90 . The elliptical shape of the Moon™s orbit means that its orbital velocity
is not constant, and this produces libration in longitude of 7 450 . Diurnal
libration is a small additional effect that results from observing the Moon at
different times of day.
Lick Observatory An observatory belonging to the University of California. The
observatory site is on Mount Hamilton in the Californian Diablo Range at a
height of 1300 m (4200 feet). The funds for the observatory were donated by a

light curve

The development of the light echo around a variable star, V838 Monocerotis, following an
outburst in 2002.

millionaire businessman, James Lick (1796“1876). The building and a 92-cm
(36-inch) refracting telescope were completed in 1888, twelve years after Lick™s
death. He is buried at the base of the telescope. The main research telescope is
the Shane 3-m (120-inch) re¬‚ector, in operation since 1959.
light curve A graph on which the brightness of a ¤ variable star (or other varying
astronomical object) is plotted against time.
light echo A re¬‚ection by neighboring interstellar clouds of a sudden burst of light
from a ¤ supernova, ¤ nova or other stellar outburst. The echo is a ring of
light surrounding the star, which expands over time.
light pollution The scattering of arti¬cial light into the night sky. Light pollution
increases the background brightness of the sky above its natural level and
interferes with astronomical observations. It is worst close to major centers of
habitation. Legislation has been enacted in parts of the USA to protect
important observatory sites from the damaging effects of unnecessary arti¬cial
lighting in nearby cities. However, the problem is a growing one and a matter
of worldwide concern for both amateur and professional astronomers.
light year (l.y.) The distance traveled through a vacuum by light (or any other form
of electromagnetic radiation) in one year. A light year is equivalent to
9.4607 · 1012 km, 63 240 astronomical units or 0.306 60 parsecs.
LIGO Abbreviation for ¤ Laser Interferometer Gravitational-Wave Observatory.
limb The extreme edge of the visible disk of a body such as the Sun, Moon or a planet.
LINEAR Abbreviation for Lincoln Near-Earth Asteroid Research, a project of the
Massachusetts Institute of Technology™s Lincoln Laboratory, funded by the
United States Air Force. The project makes use of a telescope on the White
Sands Missile Range in Socorro, New Mexico and began operations in 1998. It
employs technology originally developed for the surveillance of Earth
satellites, which has been adapted for detecting ¤ near-Earth objects.
Linne A small lunar crater, 2.4 km (1.5 miles) in diameter, situated in the Mare
Serenitatis. A claim made in the mid-nineteenth century that a neighboring
´ ´
crater called Linne B disappeared was never con¬rmed. Though small, Linne is

Local Bubble

The Little Dumbbell planetary nebula.

relatively conspicuous because it is surrounded by a bright area, probably
ejecta from the crater.
Little Dumbbell A popular name for M76 (NGC 650), a ¤ planetary nebula in
Perseus. It is the faintest object in the ¤ Messier Catalogue.
Little Dipper A popular North American name for the constellation ¤ Ursa Minor,
describing the ¬gure formed by its seven brightest stars.
Local Group The group of galaxies to which our own Milky Way ¤ Galaxy
belongs. It is dominated by the ¤ Andromeda Galaxy (M31), which is the largest
and most massive member, and our own Galaxy. Next in size are the spiral
galaxy in Triangulum, M33, which is a near companion of M31, and the Large
¤ Magellanic Cloud, near our Galaxy. The other members of the Local Group
are small elliptical and irregular galaxies plus a number of faint, dwarf
spheroidal galaxies, resembling isolated globular clusters. There is no
central concentration of galaxies, but two subgroups centered around the
two most massive galaxies. Four small elliptical galaxies (NGC 221, 205,
185 and 147) are satellites of M31; the Magellanic Clouds and various dwarf
galaxies are satellites of our own Galaxy.
The Local Group occupies a volume of space with a radius of about 3
million light years (1 megaparsec). The next nearest galaxies are two or three
time this distance away.
Local Bubble (Local Hot Bubble) An expanding bubble of low-density, hot gas in
which the Sun is located. Its boundary is nearest to the solar system about 90
light years away in a direction roughly towards the galactic center. The
furthest extent of the bubble is about 360 light years away. Its shape is like that
of an hourglass or peanut, because of pressure from another adjacent bubble.
Both these bubbles were blown out by shock waves from ¤ supernova

Local Interstellar Cloud

explosions that took place in our part of the Galaxy in the remote past and
radiate low-energy X-rays.
Local Interstellar Cloud (LIC) A small, diffuse cloud of interstellar material in
which the Sun lies. It is about 20“30 light years across and the Sun is currently
located towards one edge. The cloud is moving relative to the Sun and will pass
the Sun completely in the next few thousand years. The Local Interstellar
Cloud is within the ¤ Local Bubble.
Local Supercluster A ¤ supercluster of galaxies, centered on the ¤ Virgo Cluster. It is
more than a hundred million light years across and the ¤ Local Group is on its
Lockyer, Sir Joseph Norman (1836“1920) The British astronomer Lockyer is
particularly noted for his discovery and naming of the chemical element
helium, which he found in the atmosphere of the Sun through his study of the
Sun™s spectrum. His astronomical work was mainly concerned with the Sun
and spectroscopy and between 1870 and 1905 Lockyer organized eight eclipse
expeditions. He also founded the scienti¬c journal Nature and made signi¬cant
contributions to ¤ archeoastronomy.
Loki One of the most active volcanoes on Jupiter™s moon ¤ Io, and the most
powerful volcano in the solar system.
long-baseline interferometry A technique in radio astronomy in which two or
more radio telescopes separated by up to 1000 km or so are linked in real time
by signals transmitted via microwaves or cable in order to form a ¤ radio
interferometer. ¤ very-long-baseline interferometry.

The plume of the volcano Loki on an
image of Io taken by Voyager 1.

Lowell, Percival (1855“1916)

longitude Angular distance around an equator (or a circle parallel to the equator)
measured from an arbitrary place. In the celestial ¤ equatorial coordinate system,
the counterpart of longitude on Earth is ¤ right ascension.
long-period comet A ¤ comet with a period of revolution round the Sun greater
than 200 years. Some have periods of millions of years. ¤ short-period comet.
long-period variable A variable star with a period between about 100 and 1 000
days. The periods of long-period variables, and their change in brightness
(typically several magnitudes), both vary considerably from one cycle to
another. These variables are ¤ red giant stars. ¤ Mira is one of the best-known
Lost City Meteorite A meteorite that fell in Oklahoma in 1970. The ¤ ¬reball
observed as it passed through the atmosphere was photographed and the images
were used to locate the fallen meteorite, which was recovered a few days later.
Lovell Telescope ¤ Jodrell Bank Observatory.
Lowell, Percival (1855“1916) Lowell was an American astronomer who devoted
much of his personal fortune to searching for evidence of life on Mars. He
founded the ¤ Lowell Observatory in Flagstaff, Arizona, for that purpose. He
began work there in 1894 and spent 15 years observing Mars with an excellent
24-inch refractor. His interest had been stimulated by reports of channels or
˜˜canali™™ reportedly seen by the Italian astronomer ¤ Giovanni Schiaparelli.
Lowell claimed to see canals, oases, and seasonal vegetation, and became
convinced that Mars was inhabited. However, his claims were discredited
within decades. Lowell also became interested in searching for a planet beyond

Percival Lowell.

Lowell Observatory

Neptune. Though he was unsuccessful in his lifetime, the research program he
began led to the discovery of Pluto at the Lowell Observatory in 1930, some 14
years after his death.
Lowell Observatory A private observatory in Flagstaff, Arizona. It was founded in
1894 by Percival ¤ Lowell, who was particularly interested in the possibility of
intelligent life on Mars. ¤ Pluto was discovered there by Clyde ¤ Tombaugh in 1930.
There are ¬ve modest telescopes at the Flagstaff site together with the 33-cm
(13-inch) telescope used by Tombaugh to search for Pluto, restored and returned
to its original dome in 1996. In addition, the observatory operates three other
telescopes at Anderson Mesa, 24 km (15 miles) south-east of Flagstaff, including
the 1.8-m (72-inch) Perkins Telescope. The construction of the 4.2-m Discovery
Channel Telescope began in 2005 at a site 65 km (40 miles) south-east of Flagstaff.
luminous blue variable (LBV) A rare kind of star that is extremely massive and
luminous. The most massive stars known are of this type. Examples include
¤ Eta Carinae and ¤ P Cygni. LBVs undergo irregular eruptions during which
they lose material 10 or 100 times faster than they do in their quiescent state.
Though their visual magnitude changes, their total luminosity does not.
Rather, the change is in the wavelength of the energy they emit. Many are
surrounded by nebulosity created by gas they have ejected.
Luna A series of Soviet space missions to the Moon launched between 1963 and
1976. The ¬rst three were named ¤ Lunik. Luna 9 achieved the ¬rst soft landing
on the Moon, in the Oceanus Procellarum in January 1966. In March 1966, Luna
10 became the ¬rst lunar orbiting satellite. Luna 16 in September 1970, Luna
20 in 1972 and the last of the series, Luna 24 in August 1976, returned soil
samples. Lunas 17 and 21 landed the ¤ Lunokhod roving vehicles on the Moon.
Successes were also achieved with Lunas 11, 12 and 13 (1966), 14 (1968), 19
(1971) and 22 (1974).
lunar Pertaining to the Moon.
lunar eclipse ¤ eclipse.
Lunar Orbiter A series of American spacecraft launched in 1966 and 1967 with
the primary objective of mapping the Moon and locating suitable landing sites
for the manned ¤ Apollo program. They undertook the ¬rst systematic
exploration of the Moon™s surface and all ¬ve craft in the series were very
Lunar Prospector A NASA mission to orbit the Moon launched in November 1997.
It spent 18 months acquiring the most detailed and complete maps ever
obtained of the chemical composition of the lunar surface and the Moon™s
magnetic and gravity ¬elds. In July 1999 it was deliberately crashed near to the
Moon™s south pole in the hope that the signature of water might be detected in
material vaporized by the impact. In the event, nothing was detected.


Astronaut Dave Scott on the Apollo 15 Lunar Roving Vehicle.

Lunar Reconnaissance Orbiter A NASA mission to the Moon scheduled for
launch in October 2008. Its main purpose is to collect information needed in
planning for astronauts to travel to the Moon in the future. A second payload
will share the launch. The Lunar Crater Observation and Sensing Satellite will
travel independently of the orbiter and crash into the lunar surface in an
experiment to search for water ice.
Lunar Roving Vehicles (LRV) Battery-powered vehicles for traveling on the
Moon™s surface taken with the last three ¤ Apollo program missions (15, 16 and
17). The journeys made by the astronauts of Apollos 15, 16 and 17 were 28, 27
and 35 km (17.5, 16.5 and 22 miles), respectively.
lunar transient phenomenon (LTP) A suspected temporary appearance of
colored patches or obscuration on the surface of the Moon. (The term transient
lunar phenomenon, or TLP, is also used.) Reported observations are associated
particularly with the craters Aristarchus, Gassendi and Alphonsus. It remains
unclear whether or not real phenomena have been observed.
lunation A complete cycle of the phases of the Moon. One lunation lasts 29.530 59
days, which is also called a synodic month.


Lunik (alternatively Luna) 1 in 1959 became the ¬rst spacecraft to reach the Moon.

Lunik The name of the ¬rst three Moon missions launched by the Soviet Union in
January, September and October 1959. Lunik 1 missed the Moon by 5000 km
(3000 miles). Lunik 2 crashed near the crater Archimedes, but Lunik 3 returned
the ¬rst pictures of the lunar farside. Subsequent spacecraft in the series were
named ¤ Luna, starting with Luna 4.
Lunokhod An automated roving vehicle landed on the Moon during two
unmanned Soviet missions, Luna 17 and Luna 21. Lunokhod 1 was landed at a
site in the western part of Mare Imbrium by Luna 17 on November 17, 1970
and operated for 10 months. Lunokhod 2 was delivered on January 16, 1973, by
Luna 21 to the eastern part of Mare Serenitatis, where it worked for four
months. The total distances traveled were 10.5 km and 37 km (6.5 and 23
miles), respectively. Each eight-wheeled vehicle carried cameras, a
communication system, a laser re¬‚ector, a magnetometer, solar panels and a
cosmic ray detector.
Lupus (The Wolf) An ancient southern constellations between Scorpius and
Centaurus. It contains eight stars brighter than fourth magnitude.
l.y. Abbreviation for ¤ light year.


21h 20h 19h 18h 17h
+50 º


µ ±
θ Vega
Sulafat Sheliak
γ +30º
» Ring Nebula


20h h

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 Lyra.

Lynx An obscure northern constellation introduced in the late seventeenth
century by Johannes ¤ Hevelius to ¬ll a gap between Auriga and Ursa Major. It
contains only two stars brighter than fourth magnitude.
Lyra (The Lyre) A small but prominent constellation in the northern hemisphere.
Its brightest star, ¤ Vega, is zero magnitude and the ¬fth-brightest star in
the sky. There are three other stars brighter than fourth magnitude. Epsilon
Lyrae is a ˜˜double double,™™ consisting of a widely spaced pair of close double
stars. Lyra also has one of the best-known of all ¤ planetary nebulae, the
¤ Ring Nebula.
Lyrids An annual ¤ meteor shower, sometimes called the April Lyrids. Its radiant lies
on the border between the constellations Lyra and Hercules. The shower peaks
around April 22, and its normal limits are April 19“25. The meteor stream
responsible is associated with Comet Thatcher. Though usually sparse, this
shower has occasionally been good in the past. Records of it date back 2500 years.
Lysithea A small satellite of Jupiter, discovered by Seth B. Nicholson in 1938. It is
36 km (22 miles) across. With Elara, Leda and Himalia it belongs to a family
of four moons with closely spaced orbits. Their average distances from Jupiter
are between 11.1 and 11.7 million km (6.9 and 7.3 million miles).



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