Astronomy and related astronomical terms

A reference to the solar system when it is regarded as having the Earth as its center: The geocentric idea is an obsolete cosmology that put the Earth at the center of the universe.

A theory of the solar system or the universe that places theEarth at the center: The geocentric theory not only placed the Earth in the center of the universe, but also stated that all heavenly bodies and objects circled around the Earth.

A description of the orbit of a satellite that circles the earth above the equator at a speed matching the earth's rotation; thus, appearing to remain stationary, or a satellite in such an orbit: Most communications satellites are in geostationary orbit.

A large star that is highly luminous: When Jamie and Lisa were lying on the meadow watching the stars above on a clear night, they both saw a giant, a terrifically brilliant star, and made a wish.

A spherical collection of stars, the concentration of which increases greatly toward the center: All of the stars in the globular cluster are probably of the same age, and the densely-packed group, as a whole, moves in highly elliptical orbits that take them high above the galactic center.

A globular cluster is a dense groups of thousands to millions of stars.

A spherically symmetric collection of stars, or a globular cluster, have shared a common origin.

A planet that can support life: A Goldilocks planet is neither too hot nor too cold, neither too big nor too small, and neither too near its star nor too far.

A Goldilocks planet is one whose size, temperature and composition are all just right to maintaine life.

The force by which two masses attract each othe: Gravitation is the mutual attraction existing between all objects with mass.

Newton's law of gravitation states that the force of attraction between two bodies varies as the product of the two masses and, inversely, as the square of the distance between them.

Einstein's theory of general relativity views the attraction as being a result of the curving of space and time by a massive object.

"Quantum gravity" considers the force to be the result of the exchange of particles known as gravitons.

The pulling force (force of attraction) between any two masses in the universe: Gravity is the gravitational force at the surface of a planet or other body that pulls mass toward its center.

On Earth, gravity is experienced as a downward force that makes things fall toward the ground.

Common names and translations of Latin terms for the constellations Ursa Major and Ursa Minor in that order: Both the Great Bear and the Little Bear are circumpolar constellations in the northern sky.

The trapping of infrared radiation from the surface of a planet by a dense atmosphere, opaque to heat radiation.

The greenhouse effect occurs on Venus and on Earth.

The interface between the mantle of the earth and the outer core: The Gutenberg discontinuity lies about 1,800 miles (2,900 kilometers) beneath the earth's surface.

The comet that orbits the sun about every 76 years, named for Edmond Halley, who calculated its orbit: Halley's comet is the brightest and most conspicuous of the periodic comets and recorded sightings go back more than 2,000 years.

Halley's comet travels around the sun in the opposite direction to that of the planets and its orbit is inclined at almost 20° to the main plane of the solar system and ranges between the orbits of Venus and Neptune.

Halley's comet is expected to reappear in 2061.

The emission of particles by a black hole, leading to the eventual explosion of the black hole in a burst of gamma radiation.

A theory for the motion of the planets that has the sun at its correct place in the center of the solar system.

A plot of stellar temperatures (or, equivalently, spectral types or colors) and absolute magnitudes (or, equivalently, luminosities).

Most stars are within a diagonal belt, the main sequence, with white dwarfs and giants on either side.