Astronomer = an Astronomer Is Someone Who Studies Astronomy

Astronomy Dictionary

Asteroid Belt = The asteroid belt is a doughnut-shaped concentration of asteroids orbiting the Sun between the orbits of Mars and Jupiter (2.2 - 3.3 A.U.). This belt may have once been a planet (in the same orbit) that was destroyed by a collision with a huge asteroid or comet.

Astronomer = An astronomer is someone who studies astronomy.

Astronomy = Astronomy is the scientific study of space, including the planets, stars, galaxies, comets, and nebulae.

Big Bang Theory = The big bang theory states that the universe began as a tiny but powerful explosion of space-time roughly 13.7 billion years ago (plus or minus 1 percent). In 1965, Arno Penzias and Robert Wilson used a horn antenna (in Crawford Hill, N.J.) and discovered the cosmic microwave background (CMB) radiation with a temperature of 2.7 Kelvin; this CMB was left-over from the early moments of the universe after the Big Bang (this was predicted by George Gamow and Ralph Alpher, in 1948). Contrast it with the Steady-State Theory.

Black Hole = A black hole is a massive object (or region) in space that is so dense that within a certain radius (the Schwarzschild radius, which determines the event horizon), its gravitational field does not let anything escape from it, not even light. It is thought that giant stars (those with a mass over 3 times the mass of the Sun) will evolve into red supergiants, then supernova , and then black holes. It is thought that the typical black holes has a mass of roughly 10 times that of the Sun, but the range must be huge. For a typical black hole with a mass 10 times that of the Sun, the Schwarzschild radius would be roughly 18.6 miles (30 km). The phrase 'black hole' was coined by the physicist John Archibald Wheeler; before Wheeler, black holes were called 'frozen stars.' Astronomers think that there may be a black hole at the center of each galaxy.

Ceres = Ceres is the largest of the asteroids. It was the first asteroid ever discovered (by Italian astronomer Giuseppe Piazzi on January 1, 1801). Its mass is equal to over one-third of the 2.3 x 1021 kg estimated total mass of all the 3,000 cataloged asteroids. It is about 578 miles (930 kilometers) in diameter.

Coma = A coma is the roughly spherical blob of gas that surrounds the nucleus of a comet. The coma is comprised of water vapor, carbon dioxide gas, and other neutral gases that have sublimed from the solid nucleus. The coma and the nucleus form the head of a comet.

Comet = A comet is a celestial body that orbits around the sun. It is made up of a nucleus (solid, frozen ice, gas and dust), a gaseous coma (water vapor, CO2, and other gases) and a tail (dust and ionized gases). Its long tail of gas and dust always points away from the sun, because of the force of the solar wind. The tail can be up to 250 million km long, and is most of what we see. Comets are only visible when they're near the sun in their highly eccentric orbits. NASA's Stardust Mission will visit Comet Wild 2 in 2004.

Constellation = A constellation is a group of stars that, when seen from Earth, form a pattern. There are 88 constellations, including the Andromeda

Coperncius = Nicolaus Copernicus (1473-1543) was an amateur Polish astronomer who developed the revolutionary Copernican system, a model of the solar system in which all the planets orbit the Sun. His ideas overturned the old Ptolemaic System. His seminal work was De Revolutionibus Orbium Coelestium ("On the Revolutions of the Celestial Orb"), published in 1543.

Corona = The corona is the top layer of the sun's atmosphere; it extends for millions of miles beyond the Sun's surface. The corona has very high temperatures (over a million K) and a very low density. During a complete solar eclipse (pictured above), only the ghostly corona is visible.

Crescent Moon = A crescent moon is part way between a half moon and a new moon, or between a new moon and a half moon.

Dirty Snowball = Comets are sometimes described as dirty snowballs.

Dwarf Star = Dwarf stars are relatively small stars, up to 20 times larger than our sun and up to 20,000 times brighter. Our sun is a dwarf star.

Full Moon = A full moon appears as an entire circle in the sky; it is illuminated by the Sun and from the Earth, we see its entire daytime side. A full moon occurs once each lunar month, when the Moon is on the opposite side of the Earth from the Sun.

Fusion = Nuclear fusion is an atomic reaction in which many nuclei (the centers of atoms) combine together to make a larger one (which is a different element). The result of this process is the release of a lot of energy (the resultant nucleus is smaller in mass than the sum of the ones that made it; the difference in mass is converted into energy by the equation E=mc2). Suns are powered by nuclear fusion, mostly converting hydrogen into helium. In this process, 4 hydrogen nuclei combine to form a single helium nucleus; about 0.3% of the original mass is converted into energy

Galaxy = A galaxy is a huge group of stars and other celestial bodies bound together by gravitational forces. There are spiral, elliptical, and irregularly shaped galaxies. Our Sun and solar system are a small part of the Milky Way Galaxy.

Galileo = Galileo Galilei (1564-1642) was an Italian mathematician, astronomer, and physicist. Galileo found that the speed at which bodies fall does not depend on their weight and did extensive experimentation with pendulums. In 1593 Galileo invented the thermometer. In 1609, Galileo was the first person to use a telescope to observe the skies (after hearing about Hans Lippershey's newly-invented telescope). Galileo discovered the rings of Saturn (1610), was the first person to see the four major moons of Jupiter (1610), observed the phases of Venus, studied sunspots, and discovered many other important phenomena.

Gas Giants = The gas giants are the large outer planets of our Solar System: Jupiter, Saturn, Uranus, and Neptune (but not tiny, rocky Pluto).

Gibbous Moon = A gibbous moon is between a full moon and a half moon, or between a half moon and a full moon.

Gravity = Gravity is a physical force that pulls objects together. Every bit of mass produces a gravitational force; this force attracts all other masses. The more massive an object, the stronger the gravitational force. Newton formulated the laws of gravity.

Halley’s Comet = Halley's comet is a periodic comet (made of frozen gas and dust), that orbits around the sun. Its earliest-recorded sighting was in 240 B.C. in China, but Edmund Halley was the first person to recognize that it was periodic. It was last seen in 1986 and will be seen next in the year 2061; its period is 76 years. When the Earth passes through Halley's comet's orbit (twice each year), its detritus causes the meteor showers the Eta Aquarids and the Orionids.

High Tide = High tide is the time of high water. High tides occur when the gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon.

Inner Planets = The inner planets are those planets that orbit close to the sun. They are: Mercury, Venus, Earth, and Mars. They are relatively small, composed mostly of rock, and have few or no moons.

Kirkwood Gap = The Kirkwood gaps are radial gaps in the asteroid belt. These gaps are orbital radii where the gravitational forces from Jupiter do not let asteroids orbit (they would be pulled into Jupiter). For example, an orbit in which an asteroid orbited the Sun exactly three times for each Jovian orbit would experience great gravitational forces each orbit, and would soon be pulled out of that orbit. There is a gaps at 3.28 AU (which corresponds to 1/2 of Jupiter's period), another at 2.50 AU (which corresponds to 1/3 of Jupiter's period), etc. The Kirkwood gaps are named for Daniel Kirkwood who discovered them in 1866.

Kuiper Belt = The Kuiper belt is a region beyond Neptune in which at least 70,000 small objects (KBO's) orbit, including Quaoar and Sedna. This belt is located from 30 to 50 (?) A.U.'s and was discovered in 1992. The Kuiper belt may be the source of the short-period comets (like Halley's comet). The Kuiper belt was named for the Dutch-American astronomer Gerard P. Kuiper, who predicted its existence in 1951.

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.

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.

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.

Meteor = A meteor is a meteoroid that has entered the Earth's atmosphere, usually making a fiery trail as it falls. It is sometimes called a shooting star. Most burn up before hitting the Earth.

Meteorite = A meteorite is a meteor that has fallen to Earth. Meteorites are either stone, iron, or stony-iron.

Meteoroid = Meteoroids are tiny stones or pieces of metal that travel through space.

Milky Way Galaxy = The Milky Way Galaxy is a spiral galaxy; our sun and solar system are a small part of it. Most of the stars that we can see are in the Milky Way Galaxy. The main plane of the Milky Way looks like a faint band of white in the night sky. The Milky Way is about 100,000 light-years in diameter and 1,000 light-years thick. There are about 2 x 10 11 stars in the Milky Way. This spiral galaxy formed about 14 billion years ago. It takes the sun roughly 250 million years to orbit once around the Milky Way. The Earth is about 26,000 light-years from the center of the Milky Way Galaxy. The major arms of the Milky Way galaxy are the Perseus Arm, Sagittarius Arm, Centaurus Arm, and Cygnus Arm; our Solar System is in a minor arm called the Orion Spur.

Minor Planet = A minor planet is another name for an asteroid.

Moon = The moon is Earth's only natural satellite. It travels around the Earth in about one month (27 days 8 hours). It is about 240,000 miles (384,000 km) from Earth. The temperature on the Moon ranges from -320°F to 248°F (-160°C to 120°C). The moon has no atmosphere.

New Moon = The new moon is the phase of the moon when the moon is not visible from Earth, because the side of the moon that is facing us is not being lit by the sun.

Nuclear Fusion = Nuclear fusion is an atomic reaction in which many nuclei (the centers of atoms) combine together to make a larger one (which is a different element). The result of this process is the release of a lot of energy (the resultant nucleus is smaller in mass than the sum of the ones that made it; the difference in mass is converted into energy by the equation E=mc2). Suns are powered by nuclear fusion, mostly converting hydrogen into helium. In this process, 4 hydrogen nuclei combine to form a single helium nucleus; about 0.3% of the original mass is converted into energy.

Nucleus = The nucleus is the frozen center of a comet's head. The nucleus contains most of the comet's mass.

Observatory = An observatory is a place set up with a device (or devices) for observing astronomical or meteorological phenomenon, like stars, planets, nebula, etc. Observatories often have powerful telescopes (visual, radio, or other types).

Oort Cloud = The Oort Cloud is a cloud of rocks and dust that may surround our solar system. This cloud may be where long-period comets originate. The Oort Cloud was named for Jan H. Oort, who proposed its existence in 1950. It has been hypothesized that the Oort Cloud is responsible for the periodic mass extinctions on Earth.

Orbit = An orbit is a closed path that an object takes as it revolves around another body. Orbits are generally elliptical, but may be perturbed by the presence of yet other bodies and may even form unusual figures.

Outer Planets = The outer planets are those planets that orbit far from the Sun. They are: Jupiter, Saturn, Uranus, Neptune, and Pluto. They are mostly huge, mostly gaseous, ringed, and have many moons ( the exception is Pluto which is small, rocky, and has one moon, Charon).

Partial Eclipse = A partial eclipse happens when the moon partially blocks the sun or the Earth's shadow partially blocks our view of the moon.

Penumbra = The penumbra is the area of partial shadow (compare to umbra).

Period of Revolution = period of revolution for the Earth is one year.

Period of Rotation = The period of rotation of an orbiting body is the time it takes for it to make one rotation around its axis. The period of rotation for the Earth is one day.

Phases of the Moon = 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.

Quarter Moon = A quarter moon looks like half a circle. This moon has completed one quarter of an orbit around the Earth from its full or new position, and one quarter of the moon's surface is visible from Earth. It is also called a half moon.