Summary

• Historically it was believed that the sun, moon and planets all orbited the Earth. This is known as the geocentric model of the solar system.

• The geocentric model has been replaced by the heliocentric model of the solar system, in which the Earth and other planets orbit the sun.

• Our solar system includes the sun, which is orbited by the planets, which are in turn orbited by moons. It also consists of other bodies, such as asteroids and comets.

• The inner solar system consists of the terrestrial planets – Mercury, Venus, Earth and Mars – and the asteroid belt.

• Terrestrial planets are smaller, composed primarily of rock or metal, and lack ring systems.

• The asteroid belt is a donut-shaped region between the orbits of Mars and Jupiter that contains approximately one million asteroids – small rocky object that orbit the sun.

• The outer solar system consists of the giant planets – Jupiter, Saturn, Uranus and Neptune – and the Kuiper belt.

• Giant planets are larger, composed primarily of hydrogen and helium or water, ammonia and methane, and possess ring systems.

• The Kuiper belt is a donut-shaped region outside the orbit of Neptune that consists of millions of icy objects that orbit the sun.

• A planet is a celestial body that:

• Orbits the sun.

• Has a spherical shape, due to the gravity resulting from its large size.

• Has no objects of a similar size near its orbit.

• A moon is a celestial body that orbits around a planet.

• The terrestrial planets have few or no moons.

• The giant planets have many moons.

• Planets move in two ways:

• They move in a path around the sun known as an orbit.

• They rotate on their axis – an imaginary line joining the north and south poles.

• The time taken for a planet to orbit the sun is known as a year.

• A year on Earth is approximately 365 days.

• The further a planet is from the sun, the longer its year, due to having a larger orbit and a slower orbital speed.

• The time taken for a planet to rotate on its axis is known as a day.

• A day on Earth is approximately 24 hours.

• The day length of a planet is largely determined by the rotational speed when the planet was formed.

• All of the planets orbit the sun on an imaginary flat surface called the ecliptic plane.

• Planetary orbits are slightly elliptical, meaning the distance from the sun varies.

• The point where a planet is closest to the sun is called the perihelion.

• The point where a planet is furthest from the sun is called the aphelion.

• Earth’s average distance from the sun is approximately 150 million km.

• This distance is known as an astronomical unit.

(Image: vectorfusionart, Adobe Stock)