A glossary of words used in this book:

• Arachnoid: a scientific term for something shaped like a spider.

• Asteroid: a large rocky object that orbits a star, but is too small to be a planet.

• Astronomer: a person who studies stars and planets.

• Astronaut: a person who travels beyond the atmosphere of the Earth.

• Atmosphere: a layer of gases around a planet.

• Basalt lava: molten basalt, a kind of rock.

• Carbon dioxide: a gas that animals breathe out and plants take in.

• Cetaur: an icy planetoid that orbits the Sun between Jupiter and Neptune.

• Channel: a groove in the surface of something.

• Comet: a small icy object orbiting a star.

• Conjunction: when two objects orbiting the same body come closest together.

• Continent: a huge landmass on a planet, usually made of tectonic plates that have locked together.

• Core: the center of a planet or star.

• Corona: a region of very hot gas that surrounds the photosphere of a star.

• Crater: a dent in a planet's surface made by a meteorite falling on it.

• Crust: the outermost layer of a planet's surface.

• Eclipse: the shadow made when one object comes between another object and the Sun.

• Energy: what you use to do work.

• Environment: the conditions on a planet.

• Equator: an imaginary line around a planet, perpendicular to the axis of rotation.

• Erosion: the slow wearing away of a surface, usually from wind, water, and temperature changes.

• Galaxy: a huge mix of gas, dust, stars, planets and other objects that are held together by their own gravity.

• Gas giant: one of the four outer planets made out of giant balls of gas.

• Gravity: the force that pulls on anything with mass (see the About gravity, mass, and weight section).

• Hemisphere: one half of a planet's surface.

• Lagrange point: the places where the gravity from two orbiting objects balance each other.

• Lava: molten rock above a planet's surface.

• Latin: the language of the Roman Empire that was later used by scientists to name things.

• Mantle: a layer of molten rock below a planet's crust.

• Maria: a large sea of magma that has cooled into solid rock.

• Mass: the amount of matter that something is made of (see the About gravity, mass, and weight section).

• Meteor: a small or medium-size rock from space that has not entered a planet's atmosphere yet.

• Methane: a gas that makes up most of the gas giants.

• Near Earth asteroid: an asteroid that has an orbit that brings it very close to the earth.

• Newton: a unit of measurement the describes how hard gravity is pulling you down (see the About gravity, mass, and weight section).

• Orbit: the path that an object takes around a larger object.

• Orbit System: a planet and its moons rotating around a star.

• Organic compounds: compounds (collections of atoms) containing carbon.

• Phase: how a planet or moon looks to us at some part of its orbit, when it is lit by the Sun.

• Planet: the celestial body that has a greater mass than all other objects of the same orbit system together and that describes a well-defined, special orbit around a star.

• Planetary nebula: a great cloud of gas that was blown off by an old star.

• Photosphere: the layer of a star that releases light and other energy into space.

• Prominence: an eruption of hot gas at the surface of the Sun.

• Radar: radio waves used to find distances to and make maps of things.

• Regolith: loose soil on the Moon created by rocks hitting the surface at very high speed.

• Retrograde motion: a rotation that is the opposite way from the rotation of most of the Solar System.

• Retrograde orbit: an orbit that is the opposite way from the orbit of most of the planets and moons in the Solar System.

• Rotate: to turn around on an axis.

• Satellite: an object in a stable orbit around a much larger object.

• Scarp: a type of cliff.

• Solar day: the time for a planet or moon to rotate so that the Sun is again overhead.

• Siderial day: the time for a planet or moon to rotate so that a distant star overhead is again overhead.

• Silicate: an object composed mostly of the element silicon, which makes rocks.

• Solar wind: a very hot gas that is being blown away from the Sun at a high speed.

• Spectrum: the colored band of light made when white light passes through a prism.

• Star: a huge ball of gas that is so heavy that it causes nuclear reactions inside itself. This produces heat and light.

• Sulfuric acid: a strong type of acid that is used in car batteries, and contains the element sulphur.

• Supergiant: a star near the end of its life that puffs out into a huge body many times larger than a normal star.

• Surface area: the area on the outside of something.

• Tectonic Plate: a solid part of the crust that very slowly moves across the surface of a planet

• Telescope: a system of lenses or mirrors that are used to see distant objects.

• Terrestrial planets: the four planets closest to the Sun.

• Tide: the rise in the surface caused by gravity from another object, such as the Moon or Sun.

• Tidal lock: when tides have slowed rotation so that a moon or planet is always facing the same side toward the planet or star.

• Trojan asteroid: an asteroid in the same orbit as a planet or moon that always stays the same distance ahead or behind.

• Volcanic: something that relates to volcanoes.

• Volume: the size of a three-dimensional object.

• White dwarf: a star that has run out of fuel to burn and is slowly cooling off.