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Astronomy Dictionary
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Click on an underlined word for more information on that subject.

L


LAGRANGE POINTS

Lagrange points (named for Josef Lagrange, the Italian-French mathematician who discovered them) are a set of five special points that occur between two large objects. At these points, a small orbiting object can orbit at a constant distance from both of the larger masses. This happens because at those five points, the gravitational force of the large objects is exactly equal to the centripetal force required to rotate with the objects. Three of the points are unstable (L1, L2, and L3) and two are stable(L4 and L5). Each of the stable Lagrange points forms an equilateral triangle with the two large masses.


LAIKA

Laika (meaning "barker" in Russian) was the first dog sent into orbit around the Earth. She rode on Sputnik 2, a Russian mission that blasted off on November 3, 1957 (Sputnik 2 was a metal sphere that weighed about 250 pounds = 113 kg). Laika was originally thought to have survived in Earth orbit for four days, dying in space when the batteries to the cabin over-heated. In 2002, it was revealed that Laika died roughly 5 to 7 hours into the flight, from overheating and stress. After orbiting the Earth 2,570 times, Sputnik 2 fell back to Earth on April 14, 1958, burning up during re-entry. Laika was a female mutt who was part Siberian Husky.


LAND, E. H.

Edwin Herbert Land (1909-1991) was an American physicist and inventor who developed the first modern light polarizers (which eliminate glare) and other optical devices, investigated the mechanisms of color perception, and developed instant photography film. Land established the Polaroid Corp. in 1937.


LARGE MAGELLANIC CLOUD

The Large Magellanic Cloud (LMC) is an irregular-shaped galaxy in the Local Group. The irregular shape may be the result of a disturbance, perhaps a collision of two galaxies. The Large Magellanic Cloud (LMC) is near the constellation Dorado, and is 163,000 light-years away.


LARISSA

Larissa is a tiny moon of Neptune. It was discovered using NASA's Voyager 2 mission in 1989. It orbits 73,550 km from the center of Neptune and is about 104 x 89 km.

LASSELL, WILLIAM

William Lassell was an amateur English astronomer (a brewer by trade) who discovered Triton, the largest moon of Neptune (in 1846) and Ariel, the brightest moon of Uranus in 1851. With W.C. Bond, he discovered Saturn's moon Hyperion in 1848.

LATITUDE

Latitude is the angular distance north or south of the equator of a celestial object.


LAVA

Lava is molten rock. It usually comes out of erupting volcanoes.

LEAP YEAR

Leap years are years with an extra day (February 29); this happens almost every four years. Generally , leap years are divisible by four, but century years are special, they must also be divisible by 400. The year 2000 was a leap year. The years 2001, 2002, and 2003 are not.


LEAVITT, HENRIETTA

Henrietta Swan Leavitt (1868-1921) was an American astronomer who first described the relationship between the period and the brightness (luminosity) of Cepheid variable stars. She also discovered 1,777 variable stars in the Magellanic Clouds.

LEDA

Leda is Jupiter's ninth and smallest moon. Leda is 9.9 miles (16 km) in diameter and orbits at an average of 6,900,000 miles (11,094,000 km) from Jupiter. Leda has a mass of 5.68 x 1015 kg. It orbits Jupiter in 238.72 (Earth) days. Very little is known about Leda. Leda was discovered by Charles Kowal in 1974.

LEHMANN, INGE

Inge Lehmann (May 13, 1888-1993) was a Danish geophysicist and mathematician. She realized the existence of a discontinuity between the inner and outer core (at about 250 km deep in the Earth); it is now called the Lehmann discontinuity.

LEMAITRE, GEORGES

Georges LeMaitre (1894-1966) was a Belgian mathematician who developed the Big Bang Theory of the formation of the universe.


LENTICULAR GALAXY

A lenticular galaxy is a lens-shaped galaxy. This enormous grouping of older stars has a smooth, even distribution and very little internal structure.


LEO

[Abbreviation: Leo] Leo (the lion) is a constellation of the zodiac that is shaped like a lion. Leo is seen along the Milky Way in the Northern Hemisphere along the ecliptic between Virgo and Cancer. The brightest star (Alpha Leo) in Leo is Regulus (meaning "little king"). The second-brightest star (Beta Leo) is Denebola (meaning "tail of the lion"), and the third-brightest star (Gamma 1 Leo) is Algieba (meaning "forehead"). The spiral galaxies M65, M66, M95, M96, and the elliptical galaxy M105 are nearby.


LEONID METEOR SHOWER

The Leonids are a meteor shower that occur each year from Nov. 14-20, with a maximum on Nov. 17-18. This meteor shower occurs each year as the Earth passes through the orbit of the comet Tempel-Tuttle, and icy remnants of the comet burn up as they enter the Earth's atmosphere. The meteors in this shower seem to emanate from the constellation Leo (but they do not).

LEPTON

Leptons are subatomic particles that are susceptible to the weak nuclear force but not the strong force (the force that binds an atomic nucleus together). There are six leptons: the electron, muon, tau, electron neutrino, muon neutrino, and tau neutrino.

LE VERRIER, URBAIN

Urbain Jean Joseph Le Verrier (1811-1877) was a French astronomer who predicted the existence and position of the planet Neptune using orbital calculations. John Couch Adams also predicted its existence, independently.


LIBRA

[Abbreviation: Lib] Libra (the balance) is a constellation of the zodiac. Libra is seen along the ecliptic between Virgo and Scorpius. The brightest star (Alpha 2 Lib) in Libra is Zuben Elgenubi, the second-brightest star (Beta Lib) is Zuben Elschemali, the third-brightest star (Gamma Lib) is Zuben Elakrab, and the fourth-brightest star (Delta Lib) is Zuben Elakribi.


LIBRATION

Libration is a rocking movement of the Moon. Librations cause us to view the Moon from different angles at different times, enabling us to see about 59 percent of the Moon's surface from Earth, even though the same side always faces us. There are librations due to variations in the rate of the Moon's orbital motion (longitudinal libration) and to the inclination of the Moon's equator with respect to its orbital plane (latitudinal libration). There is also an apparent libration due to an observer on Earth viewing the Moon from different angles as the Earth rotates (diurnal libration, which occurs each day).

LICK OBSERVATORY

The Lick Observatory is a University of California observatory located at Mount Hamilton, near San Jose, California. It has a 120 inch (3 m) reflecting telescope and a 36 inch (91 cm) refracting telescope

LIDAR

Lidar stands for Light Detection and Ranging. Lidar a type of radar that uses laser light (instead of microwaves) to provide information on the atmospheric winds on Earth.


LIGHT

Light is a type of energy (and the tiny part of the electromagnetic spectrum that we can see). The fastest that light can travel is 186,300 miles per second. Visible light has a wavelength from 10-7m to 10-8m.


LIGHT SPECTRUM

Light can be broken up into into its component colors (for example, by passing light through a prism) - this is a spectrum. Light from the Sun can be broken up into the colors of the rainbow (red, orange, yellow, green, blue, indigo, and violet). The different colors of light correspond to the different energies of the light waves: for example, blue light has more energy than red light. Different astronomical objects emit different electromagnetic spectra (spectra is the plural of spectrum), and these spectra are used to analyze the body.

LIGHT DISPERSION

Breaking light into its various wavelengths is light dispersion. Transparent substances disperse light because red light (longer wavelengths) is refracted less than blue light (shorter wavelengths).

LIGHT POLLUTION

Light pollution is the light that come from cities and make astronomical observing difficult.

LIGHT-YEAR

A light-year is the distance that light can travel in one year in a vacuum, which is about 5,880,000,000,000 miles or 63,240 AU or 9.46053 x 1012 kilometers.


LIMB

A limb is the edge of the apparent disk of a celestial body (like a planet, moon, or star). The "upper limb" of an object is the first part of it to become visible (or the last part to disappear).


LIMITING MAGNITUDE

The limiting magnitude is the magnitude of the dimmest star that you can see by the zenith (overhead). It is determined by weather and viewing techniques.


LINDBLAD, BERTIL

Bertil Lindblad (1895 - 1965) was a Swedish astronomer who theorized that the areas around the center of a galaxy revolves. Oort proved that this does indeed happen. He studied the structure and dynamics of star clusters, estimated the Milky Way's galactic mass, the period of our Sun's orbit, confirmed Harlow Shapley's direction and approximate distance to the center of the Galaxy, and developed spectroscopic means of distinguishing between giant and main sequence stars.

LINEAR

Comet LINEAR [C/1999 S4 (LINEAR)] was a comet that broke apart when it approached the Sun. The comet's icy core disintegrated as it passed close to the Sun (its perihelion) in July 2000. The Sun's intense heat had more to do with this comet's break-up than the Sun's gravitational forces.


LIPPERSHEY, HANS

Hans Lippershey (1570?-1619) was a German-born Dutch lens maker who demonstrated the first refracting telescope in 1608, made from two lenses; he applied for a patent for this optical refracting telescope (using 2 lenses) in 1608, intending it for use as a military device.

LIQUID

Liquid is a phase of matter in which the molecules are close together and can move around slowly.


LITHOSPHERE

The lithosphere is the solid, rocky, outer part of the Earth consisting of the crust and the upper mantle.


LITTLE DIPPER

The Little Dipper is a group of stars (an asterism) in the constellation Ursa minor (The Little Bear). This group of stars ends at Polaris, the pole star of the Northern Hemisphere.

LOCAL ARM

The Local Arm is the arm of the Milky Way Galaxy where our solar system is located. It is also called the Orion Arm.
Some Galaxies in the Local Group Diameter
light-years
Distance
light-years
Andromeda (M31) 150,000 2,200,000
Triangulum (M33) 40,000 2,400,000
Large Magellanic Cloud (LMC) 30,000 163,000
Small Magellanic Cloud (SMC) 20,000 190,000
NGC 6822 15,000 1,800,000
NGC 205 11,000 2,200,000

LOCAL GROUP

The Local Group is the cluster of galaxies to which we belong. It is a group of about 30 galaxies that is about 5 million light-years across. The largest of the galaxies are Andromeda galaxy, Triangulum, and our Milky Way. The Local Group also includes Fornax, the Large and Small Magellanic Clouds, M32, M33, M101, and 9 dwarf spheroidal galaxies. The center of the Local Group is roughly between the Milky Way and the Andromeda Galaxy, M31. The galaxies in the Local Group are moving in concert with each other, independent of the "Hubble flow" expansion.

LOCAL SIDEREAL TIME

Local sidereal time (abbreviated LST) is local time measured by the apparent motion of the stars (instead of the motion of the Sun). LST is measured by the right ascension that is currently at the observer's meridian of the local sky. Astronomers use LST to aim telescopes at astromonical objects.

LONGITUDE

Longitude is the angular distance east or west from the north-south line that passes through Greenwich, England, to a particular location. Greenwich, England, has a longitude of zero degrees. The farther east or west of Greenwich you are, the greater your longitude. Midway Islands (in the Pacific Ocean) have a longitude of 180 degrees (they are on the opposite side of the globe from Greenwich).


LONG PERIOD COMET

A long period comet has an orbital period over 200 years. This type of comet may originate from the Oort cloud.


LOW EARTH ORBIT

A satellite is in low Earth orbit (LEO) when it orbits near the top of the Earth's atmosphere, about 100-200 miles (160-320 km) up. These satellites orbit the Earth in about 90 minutes.

LOWELL, PERCIVAL

Percival Lowell (1855-1916) was an American astronomer and mathematician who founded the Lowell Observatory at Flagstaff, Arizona in 1894. Lowell studied Mars extensively, especially its surface markings, which he thought were canals. He also thought that the bright areas were deserts and the dark ones were areas containing vegetation (this was not true). Lowell published three books on Mars: Mars (1895), Mars and Its Canals (1906), and Mars As the Abode of Life (1908). Lowell also calculated that an unknown planet, dubbed Planet X, must orbit beyond Neptune. Percival Lowell calculated the rough location of Planet "X's" orbit, but died in 1916, before it was found. This planet was eventually found by the American astronomer Clyde W. Tombaugh in 1930 and named Pluto). Tombaugh did his observations at the Lowell Observatory.

LOWELL OBSERVATORY

The Lowell Observatory is the observaroty at which Clyde W. Tombaugh discovered Pluto in 1930. The rings of Uranus were also discovered there. Percival Lowell (1855-1916) founded the Lowell Observatory at Flagstaff, Arizona in 1894, with a 24-inch Alvan Clark refracting telescope. It is a National Historic Landmark. Lowell astronomers now use the newer Anderson Mesa Telescopes.


LOW TIDE

Low tide is the time of low water. Tides occur when the gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon.
Yerkes Luminosity Classes Star
Ia Very luminous supergiants
Ib Less luminous supergiants
II Luminous giants
IIIGiants
IV Subgiants
V Main sequence stars (dwarf stars)
VI Subdwarf
VII White Dwarf


LUMINOSITY

Luminosity is the total brightness of a star (or galaxy). Luminosity is the total amount of energy that a star radiates each second (including all wavelengths of electromagnetic radiation). The Sun is a as a G2V type star.


LUNA 1

Luna 1 or Mechta (meaning "dream" in Russian) was a sphere-shaped spacecraft that was the first to orbit the moon. It was launchedfrom Earth on January 2, 1959. After passing within 5995 kilometers of the moon, it went into orbit around the Sun, between the orbits of the Earth and Mars.


LUNAR ECLIPSE

A lunar eclipse occurs when the Earth's shadow falls on the moon. Lunar eclipses occur, on average, about every 6 months.


LUNAR HALO

A lunar halo is a luminous ring that is sometimes seen surrounding the moon. Some parts of the halo are very bright, others are not very bright. Sometimes, only a part of the ring is visible. The halo is produced as moonlight is reflected and refracted through tiny, flat ice crystals in the atmosphere. Halos always are at an angle of 22° away from the moon, due to the hexagonal structure of the ice crystals. The diameter of the halo is about an eighth of the sky.

LUNAR MODULE

The lunar module was a small spacecraft that was sent from the main Apollo spacecraft (in lunar orbit) down to the moon's surface and back again.

LUNAR MONTH

A lunar month (also called a synodic month) is the time between successive new or full moons. It lasts 29 days, 12 hours, and 44 minutes.


LUNAR PHASES

As the moon circles the Earth, the shape of the moon appears to change; this is because different amounts of the illuminated part of the moon are facing us. The shape varies from a full moon (when the Earth is between the Sun and the moon) to a new moon (when the moon is between the sun and the Earth). The phases of the Moon include: the new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, waning crescent, and new moon again.

LUNAR ROVER

The lunar rover (also known as the "moon buggy" or the Lunar Roving Vehicle) was a Jeep-like vehicle that the astronauts used to drive on the moon during the Apollo 15, 16, and 17 missions. It weighed 450 pounds, (or 75 pounds in the Moon's gravity). It could carry 1000 Earth-pounds. On the flight from the Earth to the moon, the lunar rover was folded up in the lunar module. There are three abandoned lunar rovers on the Moon.


LUPUS

Lupus (the wolf) is a constellation in the Southern Hemisphere. It is located between Centaurus to the west, Scorpius to the northeast. The open star cluster NGC 5822 is in Lupus.


LYRA

Lyra (the lyre) is a small, bright constellation in the northern hemisphere. The brightest star in Lyra is Vega (meaning "falling eagle") which has a white-pale blue color and is the fifth brightest star in the sky.

LYSITHEA

Lysithea is Jupiter's eleventh moon. Lysithea is 15 miles (24 km) in diameter and orbits 7,200,000 miles (11,720,000 km) from Jupiter. Lysithea has a mass of 8 x 1016kg. It orbits Jupiter in 259.22 (Earth) days. Very little is known about Lysithea. Lysithea was discovered by S. Nicholson in 1938.
Astronomy Dictionary
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

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