ACARA v9 CONTENT DESCRIPTION “describe the movement of Earth and other planets relative to the sun and model how Earth’s tilt, rotation on its axis and revolution around the sun relate to cyclic observable phenomena, including variable day and night length”
Builds on knowing that the sun rises and sets and that a day lasts 24 hours. Here we explain those everyday patterns with three movements of Earth: it spins on a tilted axis, it travels around the sun, and the fixed tilt makes the day length change through the year.
Earth spins, and that makes day and night
Earth turns, or rotates, on its axis once every 24 hours. The sun only lights the half of Earth that faces it. The lit half has day, the far half has night. As Earth spins, your place is carried from night into sunrise, up to noon, on to sunset and back into night. The sun looks like it crosses the sky, but it is really the spinning Earth that brings the sun into view and then carries it away.
Spinning Earth makes day and night
Step a town around the globe. The half facing the sun has day; the half turned away has night.
Earth spins, or rotates, once on its axis every 24 hours. The sun only lights the half of Earth that faces it, so that half has day while the other half has night. As your town is carried around by the spin it passes from night, through sunrise, into the bright noon, then to sunset and back into night. The sun is not really moving across the sky; it is the spinning Earth that brings the sun into view and takes it away again.
Earth travels around the sun
At the same time as it spins, Earth moves around the sun along a path called its orbit. One full lap takes one year, about 365 days. The orbit is very nearly a circle with the sun near the centre. Because Earth is always moving along this path, its position in the orbit slowly changes from month to month through the year.
Earth travels around the sun
Move Earth one quarter at a time. One complete lap of the sun is one year.
As well as spinning, Earth travels right around the sun along a path called its orbit. One full lap takes one year, about 365 days. The orbit is very nearly a circle, with the sun near the centre. Stepping Earth a quarter of the way at a time shows the whole journey: Earth is always moving, so where it sits in its orbit slowly changes through the year.
The tilted axis changes the day length
Earth's axis is tilted by about 23 degrees and keeps that same lean as it orbits. So at one time of year the northern part of Earth leans toward the sun, and half a year later it leans away. When your part of Earth tilts toward the sun, more of each daily spin is spent in daylight, so the day is longer. If Earth had no tilt at all, every place would get the same 12 hours of day all year round.
A tilted axis changes the day length
Tilt the axis toward the sun. The more the north leans sunward, the longer its day.
Earth's axis is the imaginary line it spins around, and that axis is tilted by about 23 degrees. The tilt does not change, but as Earth orbits, the northern end leans toward the sun at one time of year and away at another. When your part of Earth tilts toward the sun, it spends more of each spin in daylight, so the day is longer. With no tilt at all, every place would get the same 12 hours of day all year.
Earth is one of several planets
Earth is the third planet out from the sun, and it is not alone. Mercury, Venus, Mars, Jupiter and the others all move around the same sun, each along its own orbit. Planets closer to the sun have smaller, faster orbits, while those further out take much longer to go around. Every planet moves in the same basic way Earth does: spinning as it travels around the sun.
The planets all circle the sun
Pick a planet. Earth is one of several planets, each on its own orbit around the sun.
Earth is not alone. Several other planets move around the same sun, each one travelling along its own orbit. The planets closer to the sun, like Mercury and Venus, have smaller, faster orbits; the planets further out, like Mars and Jupiter, take much longer to go around. Earth is the third planet from the sun. All of them move in the same way Earth does: spinning and orbiting, held by the sun.
Putting it together: the cycle of long and short days
The three movements work together. Rotation gives the daily cycle of day and night. Revolution carries Earth around the sun across the year. The fixed tilt means your part of Earth leans toward the sun for half the year and away for the other half. Leaning toward the sun gives long summer days; leaning away gives short winter days. Because Earth repeats this orbit every year, the change in day length repeats too. That is what we mean by a cyclic observable pattern.
Why summer days are longer than winter days
Switch between summer and winter. The same tilt, leaning toward or away, sets the day length.
Now put the three movements together. Rotation gives the daily round of day and night. Revolution carries Earth around the sun through the year. The tilt stays fixed in space, so for half the year your part of Earth leans toward the sun and for the other half it leans away. When it leans toward the sun you spend more of each spin in daylight, so summer days are long; when it leans away the days are short. That steady, repeating change is the cyclic pattern of the seasons.
Why this matters
The same few ideas explain so much of what we see in the sky: sunrise and sunset, the changing length of the day, the seasons, and even why different planets have years of very different lengths. Modelling Earth as a tilted, spinning ball that orbits the sun lets you predict these patterns instead of just noticing them, and it sets you up for later science about the Moon, eclipses and the rest of the solar system.
Quick self-check
1. What causes day and night on Earth?
2. How long does Earth take to travel once around the sun?
3. Why is the day longer in summer than in winter?