| WORD |
MEANING |
| Galactic
Centre |
The
nucleus, or centre, of a galaxy, usually densely populated by stars. The
galactic centre of the Milky Way is about 28,000 light years from Earth. |
| Galatea |
Moon of Neptune, fourth
closest to the planet. Galatea is 175 kilometres (109 miles) in diameter
and orbits Neptune at an average distance of 61,953 km (38,496 miles). It
was discovered by Voyager 2 in 1989. |
| Galaxy |
A large group of stars, bound together by gravity. Quite
often, the galaxy will form a spiral or circular shape. |
| Galilean Satellites / Galilean Moons |
Jupiter's four largest moons, discovered by Galileo in 1610.
These are (in order of increasing distance from Jupiter) Io, Europa,
Ganymede and Calisto. Ganymede is the largest moon in the Solar System. |
| Galle, Johann Gottfried |
German astronomer who made the first observation of Neptune,
assisted by Heinrich Louis d'Arret. This observation was based on
predictions of the planet's position by French mathematician Le Verrier. |
| Gamma
Rays |
A form of energy such as
light, but made from shorter and more powerful waves. |
| Ganymede |
Moon of Jupiter, largest moon in the Solar System.
Ganymede's surface is more heavily cratered on one side than the other. It
is about 5,262 km in diameter (a greater diameter than both Mercury and
Pluto) and takes 7 days and 3 hours to complete an orbit of Jupiter. |
| Gas
Giant |
A
planet with a small, possibly rocky core, surrounded by a deep atmosphere.
This atmosphere is made up mainly of hydrogen and helium gases. There are
four Gas Giants in your Solar System. These are Jupiter, Saturn, Uranus
and Neptune. The majority of planets discovered around other stars are
also Gas Giants. |
| Geostationary
/ Geosynchronous Orbit |
An
orbit of a satellite around Earth which is in a stationary position above
a set point on Earth, taking the same length of time to orbit Earth as it
takes for Earth to spin on its axis (24 hours). Communications satellites
(for example, those used to broadcast satellite television) and weather
satellites are most commonly placed into geostationary orbits. |
| Gravity |
A force which attracts objects
of mass to objects of a greater mass. This force is what created the stars
and planets, and keeps people stuck to Earth. The more massive an object,
the stronger its gravitational force. The Sun's gravity is able to keep
planets many millions of miles away from it in orbit around it. |
| Gravity
Assist |
A method used to propel a
spacecraft to a planet or moon using the gravity of another planet or
moon. This is done by the object entering into the orbit of a planet,
therefore now travelling around the Sun at the same speed as the planet.
It then gets slung out of this orbit and uses the velocity it has built up
to travel faster to its next destination. The first mission to use this
was Mariner 10 which used the gravity of Venus to take it to Mercury. The
most famous mission to use gravity assist was the Voyager which used the
gravity of the four Gas Giants to propel the craft to each planet and now
on their ways out of the Solar System. |
| Greip |
moon of saturn |
