Motion in the Universe

The hierarchy from Earth to Universe

Everything in space sits inside a tidy nested structure. Working from biggest to smallest:

• The Universe is everything that exists. All the matter, energy, space and time, taken together. The Universe contains roughly 100 billion galaxies
• A galaxy is a vast collection of billions of stars held together by their mutual gravity. Our galaxy is the Milky Way, and it contains around 200 billion stars
• A star is a huge ball of hot gas (mostly hydrogen) powered by nuclear fusion in its core. The Sun is one such star, fairly average in size and brightness
• A planetary system is a star together with everything that orbits it. Our planetary system is the Solar System
• A planet is a roughly spherical body that orbits a star, big enough for its own gravity to pull it into a sphere, but not big enough for fusion. The Earth is the third of the eight planets in the Solar System
• A moon is a smaller body that orbits a planet. Earth has one moon; Jupiter has 95 known moons

Other objects in the Solar System

• Asteroids are small rocky bodies, mostly orbiting the Sun in a band between Mars and Jupiter called the asteroid belt
• Comets are small icy bodies that orbit the Sun on highly stretched paths, swinging in from the outer Solar System
• Artificial satellites are human-built objects placed in orbit around the Earth or another planet, for communications, weather, GPS, science, and spying

Where everything orbits what

• Smaller bodies always orbit larger bodies. The bigger gravitational pull belongs to the larger object, so the smaller one is the one that swings around