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A component of the coordinate system used to designate positions on the celestial sphere. Declination is analogous to latitude on the Earth, and ranges from +90 degrees to -90 degrees

The mass of an object divided by its volume

The apparent daily motion of the objects of the solar system against the background star

Our home planet and the third from the Sun. The only planet with abundant water in liquid form and a vast biodiversity

A value between zero and one which represents the shape of an ellipse or an orbit. A low eccentricity is near zero and is a perfect circle, while a high eccentricity is near one and very oval

The event in which one celestial body passes in front of another, blocking the light from the more distant object. An eclipse can refer to the Moon passing in front of the Sun

The imaginary line on which the Sun travels through our sky

A flattened circle. Johannes Kepler first postulated that the orbits of celestial bodies are elliptical and not circles

A system of millions or billions of stars and interstellar gases and material, held together by gravity

Greenwich Mean Time

The true model of the solar system in which the Sun is the centre around which the planets orbit

The angle between the orbital plane of an object and the equatorial plane of the parent object

A small particle which has a magnetic charge

The German astronomer and mathematician who formulated the three laws of planetary motion, helping prove the heliocentric model of the solar system

The common term given to radiation. Although it generally refers to the visible portion of radiation, radiation of other wavelengths is also referred to as light

The distance traveled by light in one year, equal to approximately 9,460,536,000,000 kilometers. A parsec is another common unit of distance in astronomy, and is equal to 3.26 light years

The spacecraft which traveled to Venus in the 1990's and accurately imaged the surface using radar

The region surrounding an object which has a changing electric field. Most stars and planets have magnetic fields which will affect charged particles coming in contact with them

The apparent brightness of an object in our sky. It is a logarithmic scale with negative numbers being the brightest

A fictitious sun conceived to move eastward along the celestial equator at a rate that provides a uniform measure of time equal to the average apparent time. It is used as a reference for reckoning mean time, zone time, etc. Also called ASTRONOMICAL MEAN SUN

A north-south reference line, particularly a great circle through the geographical poles of the earth. The term usually refers to the upper branch, the half, from pole to pole, which passes through a given place; the other half being called the lower branch

The prime meridian passes through longitude 0°. Sometimes designated TRUE MERIDIAN to distinguish it from magnetic meridian, compass meridian, or grid meridian, the north-south lines relative to magnetic, compass, or grid direction, respectively. The meridian through any particular place or observer, serving as the reference for local time, is called local meridian, in contrast with the Greenwich meridian, the reference for Greenwich Time. A celestial meridian is a great circle of the celestial sphere, through the celestial poles and the zenith. Also called CIRCLE OF LATITUDE. See also ANTE MERIDIAN, POST MERIDIAN

The phenomenon occurring when a solid particle from space enters the earth’s atmosphere and is heated to incandescence by friction of the air

An annual display of several to hundreds of meteors visible every hour. A meteor shower occurs because the Earth passes through the debris left by a comet

The galaxy of which the sun and its family of planets are a part. It appears as an irregular band of misty light across the sky

The Earth's satellite. When visible it is the brightest object in the night sky

The elliptical path of an object that is gravitationally bound to another object

The point in its orbit where the Moon is at its closest to the Earth. Opposite of apogee

The point in its orbit where a planet is at its closest to the Sun. Opposite of aphelion

Small deviances in the location of a star, visible through large telescopes

An object which does not produce its own energy and orbits around a star

The motions of the plates which make up the crust of the Earth

A component of the coordinate system used to designate positions on the celestial sphere. Right ascension is analogous to longitude on the Earth, and is designated as units of hours, minutes and seconds, the sky being broken up into 24 hours

Bending of light rays as they pass through the interface between two transparent media

The term given to the moons in orbit around the planets, or any man-made object in orbit around the Earth

Any system of planets and other objects in orbit around a star. Our solar system includes the nine planets along with the numerous asteroids and comets in orbit around the Sun

When an astronaut leaves an orbiting spacecraft and floats free in space while in a special spacesuit