The Sun, Earth and Moon System

The Sun, the Earth and the Moon form a system that is always moving in a regular, repeating way. Because their positions change in a predictable pattern, the events they cause on Earth are also predictable. The NSW syllabus puts it like this: predictable and observable phenomena on the Earth are caused by the relative positions of the Sun, the Earth and the Moon (SC4-OTU-01). "Relative positions" just means where each one sits compared with the others.

In this lesson we focus on three of these events:

• Day and night, caused by the Earth spinning on its axis.
• The year, caused by the Earth travelling all the way around the Sun.
• The seasons, caused by the tilt of the Earth's axis.

The Moon also causes events such as tides and its own changing shapes, but we cover those in later lessons. What makes all of these events so useful is that they are predictable. Because the motions repeat in regular cycles, scientists can work out exactly when the Sun will rise next Tuesday, or when summer will begin, years in advance. The Bureau of Meteorology and astronomers rely on this every day.

The Earth spins, or rotates, around an imaginary line through its centre called its axis. One full turn takes about 24 hours, which is why we have a 24-hour day. The Sun shines on only one side of the Earth at a time. The half facing the Sun has daytime. The half facing away is in shadow and has night. As the Earth keeps turning, your part of the world is carried from the dark side into the sunlit side, so the Sun seems to rise.

This is the key idea: the Sun is not really moving across our sky. The Sun only appears to move from east to west because we are riding on a rotating Earth that turns toward the east. It is a bit like sitting on a spinning roundabout: the playground seems to sweep past you, but it is really you that is turning. The Earth turns toward the east, so the Sun seems to come up in the east and go down in the west, day after day.

Because the rotation is so steady, day and night are completely predictable. We know the Sun will rise tomorrow at a time we can print in advance, anywhere in Australia.

As well as spinning, the Earth travels in a giant loop around the Sun. This path is called an orbit. One full trip around the Sun takes about 365.25 days, and we call this a year. Notice the extra quarter of a day. Those quarters add up, and every four years we add one extra day to the calendar (the 29th of February). That year is called a leap year. This is another sign of how predictable the system is: the timing is so regular that we can plan our whole calendar around it.

It is important not to mix up the two motions:

• Rotation is the Earth spinning on its own axis. It causes day and night and takes 24 hours.
• Orbit is the Earth travelling all the way around the Sun. It causes the year and takes about 365.25 days.

So in one year, the Earth spins about 365 times while also making one full lap of the Sun. Both motions are happening at once, and both repeat in steady, predictable cycles.

Here is the part most people get wrong. The seasons are not caused by the Earth being closer to or further from the Sun. The real cause is that the Earth's axis is tilted by about 23.5 degrees. The axis keeps pointing the same way all year, so as the Earth orbits the Sun, different parts of the planet get tilted toward the Sun at different times of the year.

When your part of the Earth is tilted toward the Sun, sunlight hits the ground more directly, the days are longer, and the energy is concentrated. That is summer. When your part is tilted away from the Sun, sunlight hits at a low, slanted angle, the days are shorter, and the energy is spread thinly over the ground. That is winter. It is the directness of the sunlight that matters, not the distance.

Busting the distance myth: if distance caused the seasons, the whole planet would have summer and winter at the same time. But it does not. When Australia has summer, the United Kingdom has winter, even though the whole Earth is the same distance from the Sun on any given day. That only makes sense if the cause is the tilt.

Australian framing: when the Southern Hemisphere is tilted toward the Sun, it is summer in Australia, around December, while the Northern Hemisphere is tilted away and has winter. Six months later it flips: the Southern Hemisphere tilts away and Australia has winter around June, while the Northern Hemisphere has summer. This is why a Sydney Christmas is hot and a London Christmas is cold.

The reason these events are so useful is that they are predictable. The Earth's rotation and orbit are extremely steady, and the tilt of the axis stays the same. Because the motions repeat in regular cycles, we can predict them far ahead of time.

• We know the Sun will rise and set tomorrow, and at roughly what time, because the Earth keeps rotating at the same rate.
• We know summer in Australia comes around December and winter around June, every year, because the tilt and orbit repeat.
• Australians who lived here for tens of thousands of years also tracked these regular patterns, using the sky to know when seasons changed. Aboriginal and Torres Strait Islander Peoples developed detailed seasonal calendars from this kind of careful observation.

This is exactly the idea in the syllabus dot point: predictable and observable phenomena on the Earth are caused by the relative positions of the Sun, the Earth and the Moon. Because the positions change in a regular way, the events they cause are regular too.