Our Solar System
The Solar System contains the Sun and the objects that orbit it, including eight recognised planets, dwarf planets, moons, asteroids and comets. The inner planets are rocky, while the outer planets are much larger and dominated by gases or volatile materials. The Solar System is part of the Milky Way galaxy.
Stars and galaxies
Stars are enormous objects in which nuclear fusion releases energy. Their colour, temperature, size and lifetime vary. Stars are grouped into galaxies, and the observable universe contains vast numbers of galaxies. Looking farther into space also means looking further back in time because light takes time to travel.
Human exploration
Space exploration has expanded our knowledge through satellites, robotic missions, telescopes and crewed flights. Satellites support communications, navigation and Earth observation, while robotic spacecraft can investigate environments that would be difficult or dangerous for people to visit directly.
A useful way to learn astronomy
Start with the objects closest to home: Earth, the Moon and the Solar System. Once those relationships are familiar, concepts such as stars, galaxies and cosmic distances become easier to place in context. Use quiz explanations to identify terms worth exploring further.
Moon phases, eclipses and seasons are different phenomena
The Moon appears to change shape over a month because we see different amounts of its sunlit half as it orbits Earth. The familiar phases are not normally caused by Earth casting a shadow; that happens during a lunar eclipse, when Earth passes between the Sun and the Moon. Earth’s seasons have another cause: the tilt of its rotation axis means each hemisphere receives sunlight at different angles and for different lengths of time during the year. They are not simply caused by Earth moving closer to or farther from the Sun. Keeping these three ideas separate makes questions about the Moon, eclipses and the seasons easier to reason through instead of guessing.
A practical example: why starlight is a look into the past
Light travels at a finite speed. Sunlight takes about eight minutes to reach Earth, so we see the Sun as it was roughly eight minutes earlier. Light from much more distant stars can take years or centuries to arrive. Astronomers therefore observe distant objects as they appeared when their light began its journey, which makes telescopes useful for studying the history of the universe. Try the same reasoning with a hypothetical star 20 light-years away: the light you receive today began travelling about 20 years ago. A light-year is a measure of distance—the distance light travels in one year—not a length of time. That distinction helps explain why astronomers use light-years for large distances while also describing how far into the past a telescope can see.
Further reading
These links provide further reading from established organisations, including subject explanations, source material and reference databases. They are not citations for every sentence in this guide.