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Astronomical Terms and Definitions Glossary

This glossary includes definitions of astronomical terms that are commonly used in the Farmers’ Almanac and on FarmersAlmanac.com.

APHELION: For an object in the solar system, this is the farthest point in its orbit to the Sun  (Perihelion is the object’s closest point from the Sun).

APOGEE: For an object circling Earth, this is the farthest point in its orbit to the Earth (Perigee is the object’s closest point to Earth).

ASTEROID: A small rocky object that orbits the sun.

ASTEROID BELT: The region in the solar system, between the orbits of Mars and Jupiter, where most asteroids exist.

ASTRONOMICAL UNIT: (often abbreviated au) A unit of measurement that is the average distance between Earth and the sun. 1 au is equal to 92,955,807 miles (149,597,871 km).

ASTRONOMY: The field of science that studies objects in space and everything in the universe.

ASTROLOGY: The non-scientific method of charting future events based on the positions of the heavenly bodies.

ATMOSPHERE: The gasses surrounding a planet, star, or satellite.

AURORA: Also called the Northern Lights or Southern Lights. Glowing lights in a planet’s atmosphere, caused by atoms from the sun moving through the planet’s magnetic field.

BLACK HOLE: A region of space where gravity is so strong that nothing can escape it, not even light.

COMET: A body of rock, dust, and ice that orbits the sun in an elongated orbit. When it is near the sun, a comet’s shoots off gasses that form a long streaming tail.

CONJUNCTION: The alignment or close alignment of two or more astronomical objects.

CONSTELLATION: A group of stars that form a recognizable pattern.  Many constellations take the form of animals, mythological people and creatures, and scientific instruments.

COSMOLOGY: The study of the beginning, end, and structure of the whole universe.

CRATER: A circular depression in the surface of a planet, moon, or other object in space, caused by a meteorite impact or by volcanic action.

DEGREES: The unit to measure how far apart objects appear in the sky. You can use your hand to measure degrees. For example your closed fist at arm’s length equals about 10 degrees while the width of your pinkie is about 1-2 degrees. See our illustration below:

DWARF PLANET: An object that goes around the sun and is nearly spherical in shape but has not cleared its orbit like a planet.  Pluto, Eris, Makemake, Haumea, and Ceres are examples of dwarf planets.

ECLIPSE – LUNAR: When the Earth passes between the sun and moon and blocks sunlight from reaching the moon.

ECLIPSE – SOLAR: When the moon passes between Earth and sun and blocks out some if not all of the sun’s light from reaching Earth.

Related: Learn more about upcoming eclipses

ECLIPTIC: The Sun’s apparent path among the constellations and the locations in the sky where the Moon and planets are always near (See also: Zodiac).

EXTRASOLAR PLANET (EXOPLANET): A planet that orbits a star other than the Sun. 

GALAXY: An extremely large grouping of stars, gas, and dust bound together by gravity.

GLOBULAR CLUSTER: A large, densely-packed group of stars that have a round or globe-shaped appearance.

GREATEST ELONGATION: When a planet appears at its greatest angular distance from the Sun. For example you can most easily see Mercury and Venus in the sky when they are at greatest elongation from the Sun.

INFERIOR CONJUNCTION: When a planet or objects comes between the Earth and Sun.  From Earth, this mainly applies to the position of Mercury and Venus.

INTERSTELLAR: The space between the stars.

KUIPER BELT: The region of the solar system beyond the orbit of Neptune where dwarf planets, comets, and other small objects circle the sun.

LIGHT-YEAR: The distance light travels in a year. 5.88 trillion miles (9.46 trillion km).

MAGNITUDE: The measure of an object’s brightness. The brightest stars for example are described as “First Magnitude.” Dimmer stars are second magnitude. The higher the magnitude, the dimmer the object.

MERIDIAN: The imaginary north-south line in the sky. Astronomers use it to indicate when objects are east or west of the meridian.

METEOR: An object that falls from space and heats up in Earth’s atmosphere. A “shooting star.”

METEORITE: An object that falls from space, survives its fiery plunge through the atmosphere and crashes to Earth.

METEOROID: A smaller asteroid.

NEAR-EARTH OBJECT: An asteroid or comet whose orbit brings it close to Earth.

NEBULA: A glowing interstellar cloud of gas and dust.

OCCULTATION: When the Moon or a planet blocks out the light of a star or planet.

OORT CLOUD: A region of the solar system beyond the Kuiper Belt which is the source of long period comets.

OPEN CLUSTER: A group of up to a few thousands stars which were formed from the same cloud of gas and dust.

OPPOSITION: The position of an astronomical object when it is opposite the Sun in the sky. Objects at opposition appear to be largest and brightest.

ORBIT:  The regular path an object in space follows as it revolves around another body.

PERIHELION: For an object in the solar system, this is the closest point in its orbit to the Sun  (Aphelion is the object’s farthest point from the Sun).

PERIGEE: For an object circling Earth, this is the closest point in its orbit to the Earth (Apogee is the object’s farthest point to Earth).

PLANET: A nearly spherical object that circles the sun and has cleared its orbit of similar-sized objects.

PLANETARY NEBULA: The glowing ring of gas that expands from a star similar in size to the sun near as it ages.

SOLAR WIND: A stream of charged particles radiating outward from the sun.

STAR: an object in space that fuses elements to creates its own light and heat.

SUPERNOVA: The sudden and violent explosion at the end of a massive star’s life.

ZODIAC: The ring of twelve constellations in the sky and near the ecliptic that were used to measure the positions of the Sun, Moon and planets including Gemini, Taurus, and Virgo.

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Is there an astronomical term you don’t understand?

Let us know in the comments below! Your questions will be answered.

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Is there a glossary for all celestial abbreviations? For example:
Jan. 1 has Mercury Stat. What does ‘Stat.’ mean?
Jan. 3 has Moon On Eq. What does ‘Eq.’ mean?
Jan. 12 ‘Gr. Elong.’ ?

Thanks!

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