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Glossary of Astronomical Terms

Aphelion: The point in an planet’s orbit when it is farthest from the Sun.

Apogee: The point in the Moon’s or planet’s orbit when it is farthest from the Earth.

Aurora: The glow in the Earth’s ionosphere caused by the interaction between the Earth’s magnetic field and charged particles from the Sun. It is know as Northern Lights or Aurora Borealis in the Northern Hemisphere and the Aurora Australis in the Southern Hemisphere.

Comet: An icy object in independent orbit about the Sun that is smaller than a planet, usually having a highly elliptical orbit extending out beyond Jupiter.

Conjunction: When two bodies appear close together in the sky usually within 15 degrees or less.

Constellation: An grouping of stars which form a pattern.

Earth shine: Light reflected from the Earth’s atmosphere onto the dark part of the New Moon.

Eclipse: When the Moon passes directly between the Sun and Earth.

Ecliptic: The visible path of the Sun and planets as seen against the stars. The plane of the Earth’s equator is inclined at 23.5 degrees to its orbit and the ecliptic is inclined to the celestial equator by the same angle. The ecliptic intersects the celestial equator at the two equinoxes.

Equinox: This is the time when the Sun crosses the celestial equator. There are two equinoxes: Vernal (Spring), around March 21st and Autumnal (Autumn) around September 23rd. On these dates, day and night are equal.

Full Moon: The Moon when it lies directly opposite of the Sun. The Moon is full two weeks after New Moon. The full moon rises at sunset and sets at sunrise.

Galaxy: Vast star systems containing thousands of billions of stars, dust and gas, held together by gravity. There are three main classes of galaxies: elliptical, spiral and barred, named after their appearance. The Milky Way is a spiral galaxy.

Inferior Conjunction: When Mercury or Venus are directly between the Sun and Earth.

Inferior Planets: The planets (Mercury and Venus) that orbit between the Earth and the Sun. Also known as the inner planets.

Meteor: Also known as a shooting star or falling star, it is a bright streak of light in the sky caused by a meteoroid as it burns up in the Earth’s atmosphere. A meteor that survives its trip through the Earth’s atmosphere and lands on the ground is called a meteorite.

Meteor Shower: An increased number of meteors appearing to radiate from a single area within a constellation at certain times of the year when the Earth crosses the debris trail of comets.

Milky Way: The name of our own spiral galaxy and the band of light from the combined glow of stars and galaxies that lie along the galaxy’s equatorial plane.

Minor Planets: Another term for asteroids.

Moon: A naturally occurring satellite or relatively large body orbiting a planet.

Naked Eye (Unaided Eye): A term used to describe observing without the aid of optical instruments.

New Moon: The Moon when it lies in the same direction as the Sun and the beginning of a cycle of lunar phases. The New Moon rises and sets with the Sun.

Opposition: A position of an outer planet when it appears opposite the Sun (inner planets cannot come into opposition).

Orbit: The path of a celestial body around another due to the influence of gravity.

Perigee: The point in the Moon’s or planet’s orbit when it is closest to Earth.

Perihelion: The point in an planet’s orbit when it is closest to the Sun.

Retrograde: The movement of a planet when it appears to “travel” backwards.

Solar Cycle: The 11-year variation in sunspot activity. More sunspots are seen at the solar maxima with a quiet Sun occurring during the minima.

Solstice: This is the time when the Sun reaches its most northerly or southerly point. It marks the beginning of Summer and Winter in the Northern Hemisphere and the opposite in the Southern Hemisphere. Summer solstice occurs on June 21st and winter solstice on December 22nd.

Star Cluster: A loose association of stars within the the Milky Way. Examples are the Pleiades (Seven Sisters) Hyades clusters.

Sunspot: A cooler region of the Sun’s photosphere which appears as a dark spot on the Sun’s disc. It is caused by concentrations of magnetic flux occurring in groups or clusters. The number of sunspots varies according to the Sun’s 11 year cycle.

Superior Conjunction: This is when Mercury or Venus are behind the Sun.

Superior Planets: These are the planets beyond the Earth's orbit (Mars, Jupiter, Saturn, Uranus, Neptune and Pluto). Also known as the outer planets.

Transit: The visible journey of Mercury or Venus across the Sun’s disc or of a planet’s moon across the disc of that planet.

Twilight: The time preceding sunrise and following sunset when the sky is partially illuminated. Civil twilight occurs when the central point of the Sun’s disk is between 90°50' and 96°, nautical twilight from 96° to 102°, and astronomical twilight from 102° to 108°.

Waning Crescent: The phase of the Moon between third quarter and new moon. Waning means declining or fading.

Waning Gibbous: The phase of the Moon between New Moon and Last Quarter.

Waxing Crescent: The phase of the Moon between New Moon and First Quarter. Waxing means increasing.

Waxing Gibbous: The phase of the Moon between First Quarter and Full. Gibbous refers to when the Moon is more than half lit but less than fully lit.

Zodiac: The twelve constellations (Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces) that formed an ancient calendar of the Sun’s progress in the sky during one Earth-year. Also, it is the visible path followed by the Sun, Moon and most planets, lying within 10 degrees of the celestial equator.

Our Solar System

Our solar system is located on the edge of a spiral arm called Orion’s Arm, and is one-half to two-thirds of the way (28,000 light-years) from the center of our Milky Way galaxy.

The Sun, Sol

The Inner Planets
    The Moon

The Asteroid Belt
  Dwarf planet, Ceres
  Asteroid, Ida
  Asteroid, Vesta
  Asteroid, Eros

The Outer Planets

The Kuiper Belt
  Dwarf planet, Pluto
  Dwarf planet, Eris

The Oort Cloud
  Dwarf planet, Sedna
  Comet Halley
  Comet Hale-Bopp

  Glossary of Terms
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