A tilted planet
Earth spins on an axis that is tilted 23.4° from upright compared with its orbit. As Earth travels around the Sun, the axis keeps pointing the same way in space, towards the North Star, Polaris. So for half of the year the Northern Hemisphere leans towards the Sun, and for the other half it leans away. The tilt changes two things that control how much solar energy a place receives: the angle of sunlight and thelength of the day.
Sunlight angle
When the Sun is high, its rays strike the ground steeply and their energy is concentrated on a small area. When the Sun is low, the same beam is spread over a much larger area, and it also passes through more atmosphere. At 50°N the noon Sun is about 63° high in late June but only about 17° high in late December, so a square metre of ground gets far less energy in winter.
Day length
As Earth spins, each place travels around a circle of latitude. Because of the tilt, more than half of that circle is in daylight in summer and less than half in winter. At 50°N the day lasts about 16 hours at the June solstice and about 8 hours at the December solstice. Inside the Arctic and Antarctic Circles (66.6°) there are days when the Sun never sets (midnight sun) or never rises (polar night). At the equator the day is about 12 hours all year. Longer days mean more hours of heating and fewer hours of cooling.
Opposite seasons
When the north leans towards the Sun, the south leans away. So when it is summer in Canada, Europe and China, it is winter in Australia, Argentina and South Africa, and Christmas comes in midsummer in the Southern Hemisphere.
What about distance?
Earth’s orbit is very nearly a circle. Earth is about 147 million km from the Sun in early January and about 152 million km in early July, a difference of only about 3%. That changes the sunlight reaching Earth by about 7% across the year, much less than the effect of the tilt, and it works the wrong way to explain northern seasons: Earth is closest during northern winter. It does make southern summers receive slightly more sunlight than northern summers, but the Southern Hemisphere’s large oceans soften the difference.
Solstices and equinoxes
| Date (approximately) | Event | Sun overhead at noon |
|---|---|---|
| 20 March | March equinox | Equator |
| 21 June | June solstice | Tropic of Cancer, 23.4°N |
| 22 September | September equinox | Equator |
| 21 December | December solstice | Tropic of Capricorn, 23.4°S |
Formulas
The latitude where the Sun is overhead at noon, called the Sun’s declination δ, varies through the year:
δ = arcsin( sin 23.4° × sin λ )where λ is Earth’s position in its orbit measured from the March equinox. For a place at latitude φ:
noon Sun height = 90° − |φ − δ|day length = 2 × arccos( −tan φ × tan δ ) ÷ 15° per hourReal-world connections
- Farming and nature: growing seasons, bird migration and animal breeding follow the seasons.
- Solar panels are angled to face the Sun as squarely as possible, steeper in winter.
- Seasonal lag: the warmest weeks come a month or more after the June solstice, because oceans and land take time to warm.
- Other planets: Mars is tilted about 25° and has seasons like ours; Uranus is tilted about 98°, so each pole has decades of continuous daylight.
Common misconceptions
- “Summer is when Earth is closest to the Sun.” Earth is closest in January.
- “The tilted hemisphere is closer to the Sun, so it is hotter.” The tilt changes the distance by only a few thousand kilometres out of 150 million. It is the angle and day length that matter.
- “Earth’s axis wobbles back and forth every year.” It points the same way all year; Earth’s position around the Sun changes.
- “The Sun is directly overhead at noon everywhere.” It can only be overhead between the two tropics.
What the model simplifies
The noon Sun height and day length use standard formulas for the centre of the Sun, ignoring the bending of light by the atmosphere and the size of the Sun’s disc, which make real days a few minutes longer. The sunlight values are the daily average arriving at the top of the atmosphere; clouds, air and the ground reduce what actually reaches the surface, and they are not air temperatures. Earth’s orbital speed is treated as constant, so the dates of the equinoxes and solstices can be off by a day or two. The orbit view is tilted and not to scale.
Key takeaways
- Seasons are caused by Earth’s 23.4° axial tilt, not by its distance from the Sun.
- The tilt changes the angle of sunlight and the length of the day.
- The Northern and Southern Hemispheres have opposite seasons.
- At the equinoxes, day and night are about equal everywhere; at the solstices, one hemisphere has its longest day and the other its shortest.
- Without a tilt, there would be no seasons.