Coaching Centre
Day, night and the seasons
Rotation and revolution, why the Earth's spin gives day and night and time zones, how the tilted axis causes the seasons, solstices and equinoxes, day length and the height of the noon Sun in Sydney, the midnight sun, leap years, and why the seasons are not caused by distance from the Sun.
Ask a group of adults why it's hotter in summer and many will say "because the Earth is closer to the Sun". It's a reasonable guess, and it's wrong. The Earth is actually closest to the Sun in early January, and furthest away in early July. If distance caused the seasons, Australia and Europe would have summer at the same time. They don't.
Two motions
The Earth is doing two things at once:
- Rotation: it spins on its axis, an imaginary line through the North and South Poles. One rotation takes 24 hours and gives us one day.
- Revolution: it travels around the Sun in a path called an orbit. One orbit takes about 365¼ days and gives us one year.
Gravity between the Sun and the Earth keeps the Earth in its orbit. The orbit is very slightly oval (an ellipse), but so close to a circle that our distance from the Sun only changes by about 3% through the year.
Day and night
At any moment, the half of the Earth facing the Sun is in daylight and the other half is in its own shadow, which is night. As the Earth rotates from west to east, your part of the planet turns into the sunlight (sunrise, in the east) and later turns away from it (sunset, in the west). The Sun isn't moving across the sky: you are.
Because the Earth turns west to east, places further east see the sunrise first. New Zealand celebrates New Year before Sydney, and the sun rises in Sydney about two hours before it rises in Perth. That's why the world is divided into time zones.
What really causes the seasons
The Earth's axis is tilted at about 23.4° compared with its orbit, and it always points the same way in space (towards the North Star, Polaris). So as the Earth travels around the Sun, first one hemisphere and then the other is tipped towards the Sun.
When your hemisphere is tipped towards the Sun, two things happen:
- The Sun climbs higher in the sky. Its rays hit the ground more directly, so the same beam of sunlight is concentrated on a smaller area and heats it more.
- The days are longer. The ground has more hours to warm up and fewer hours to cool down at night.
That's why the Southern Hemisphere has summer in December, January and February, while the Northern Hemisphere has winter, and the other way around in June.
- Drag the Earth to December, then to June. How many hours of daylight does Sydney get on each day? How high is the noon Sun?
- Compare Darwin, Hobart and Tromsø in Norway. Which place has the biggest difference between summer and winter? Why?
- Set the tilt to 0°. What happens to the seasons?
Solstices and equinoxes
| Date (about) | Name in the Southern Hemisphere | What happens |
|---|---|---|
| 21–22 December | Summer solstice | Longest day of the year in Sydney (over 14 hours of daylight); the noon Sun is at its highest. |
| 20–21 March | Autumn equinox | The Sun is directly over the equator; day and night are about 12 hours each everywhere. |
| 21–22 June | Winter solstice | Shortest day in Sydney (under 10 hours of daylight); the noon Sun is at its lowest. |
| 22–23 September | Spring equinox | Day and night are equal again. |
Near the poles the effect is extreme. Inside the Arctic and Antarctic Circles there are days in summer when the Sun never sets (the midnight sun) and days in winter when it never rises. At the poles themselves there are about six months of daylight followed by six months of night. Near the equator, by contrast, day length hardly changes and there's little difference between summer and winter. Darwin instead has a wet season and a dry season.
Why we have leap years
The Earth takes about 365.24 days to orbit the Sun, not exactly 365. If every calendar year had 365 days, the calendar would drift by about a day every four years, and after a few centuries Christmas would be in winter in Australia. So every four years we add an extra day, 29 February, to make a leap year. (Because 365.24 is slightly less than 365.25, century years such as 1900 and 2100 skip the leap day unless they divide by 400, like 2000.)
Test yourself
Day and night are caused by
One rotation every 24 hours turns each place into and out of sunlight.
Australia has summer in December because
Higher Sun and longer days.
On the equinox
The Sun is directly over the equator.
Which place has the least difference between summer and winter day length?
Darwin is closest to the equator.
We have a leap year every four years because
Four quarter-days add up to one extra day.
Label the Earth's orbit.
What causes it?
The Earth spins on its once every 24 hours. Its axis is tilted at about degrees. The longest day of the year is the summer . When day and night are equal it is an . Every four years we add a day in a year.
Explain why Sydney is warmer in January than in July. Refer to the Earth's tilt, the height of the Sun and day length.
The Earth's axis is tilted at about 23.4° and always points the same way in space. In January the Earth is on the side of its orbit where the Southern Hemisphere is tilted towards the Sun. The noon Sun is high in Sydney's sky (about 79°), so its rays hit the ground almost directly and the energy is concentrated on a small area. The days are also long, over 14 hours, giving more time for the ground and air to heat up and less time to cool at night. In July the Southern Hemisphere is tilted away: the Sun is low (about 33°), its energy is spread over a larger area, and days are under 10 hours, so it is colder.