The sky — astronomy

Why We Have Seasons

Seasons exist because Earth's axis is tilted by about 23.4° relative to its orbital plane. As we orbit the Sun, each hemisphere alternately receives more direct or more slanted sunlight and longer or shorter days. Distance from the Sun plays only a minor role: Earth is closest to it in early January, when it is winter in the north.

Every year we move through warm and cold months, long days and short ones. It is tempting to think summer happens when Earth is closer to the Sun. Science gives a different and more interesting answer: seasons are caused by the tilt of Earth's axis.

The common distance myth

Earth's orbit around the Sun is nearly circular, but not perfectly. It is a slight ellipse. In early January Earth is at perihelion (its closest point to the Sun), about 147 million kilometres away. In early July it reaches aphelion (its farthest point), about 152 million kilometres away. The difference is roughly 3%.

If distance drove the seasons, the Northern Hemisphere would have summer in January. In reality it is winter there. That fact alone shows the cause lies elsewhere. Distance has a small effect on the total energy received, but it cannot explain why summer and winter alternate.

Another clue: if distance were the cause, the whole planet would have summer at the same time. Yet Australia and Europe have opposite seasons.

The tilt of the axis

Earth spins around an imaginary axis tilted by about 23.4° from the perpendicular to its orbital plane (the path of the Sun across our sky is called the ecliptic). Throughout the year the axis points in nearly the same direction in space, toward the region of the North Star. Over very long periods that direction slowly shifts, a topic covered in the article on precession.

Two effects follow:

• Height of the Sun. When the Northern Hemisphere leans toward the Sun, sunlight arrives at a steeper angle. The same light is concentrated over a smaller area, and the ground warms more. When it leans away, the light arrives at a shallower angle, spreads over a larger area and is weaker.
• Length of the day. In summer the Sun travels along a higher arc and stays above the horizon longer. There is more time for heating and less for cooling at night. In winter the opposite holds.

At the two solstices (around 21 June and 21 December) one hemisphere is tilted as far toward the Sun as it gets, and the other as far away. You can read more in the article on the solstice. At the two equinoxes (around 20 March and 22–23 September) the axis is tilted neither toward nor away from the Sun, and day and night are nearly equal. See the equinox article.

Tilt also explains why near the equator day length stays about 12 hours all year and seasons are mild. Near the poles the differences are greatest: there are periods of polar day, when the Sun does not set, and polar night, when it does not rise. The latitudes of about 23.4° north and south, the tropics, mark the limits where the Sun can stand directly overhead at noon.

Opposite seasons in the two hemispheres

When the Northern Hemisphere is tilted toward the Sun, the Southern Hemisphere is tilted away. So the seasons are reversed:

• in June, July and August it is summer in the north and winter in the south;
• in December, January and February it is winter in the north and summer in the south.

Christmas in Australia, New Zealand or South Africa falls in the middle of summer. Even though perihelion occurs in early January, it does not change this pattern. The cause of the southern summer is still the tilt, and the large oceans of the Southern Hemisphere further moderate any small difference from distance.

Tilt is not unique to Earth. Mars is tilted by about 25° and has seasons too. Uranus is tilted so far that it effectively rolls around the Sun on its side, giving it extreme seasons. Mercury has a very small tilt and essentially no seasons of this kind.

Seasons and the night sky

Seasons also show in the night sky. As Earth travels around the Sun, at night we face a different part of the celestial sphere, so the constellations we see change through the year.

In the Northern Hemisphere, Orion rules winter evenings, while summer evenings feature the Summer Triangle, formed by bright stars from three different constellations. See also our guide on how to watch the sky.

Your latitude also decides which parts of the sky you can reach. Observers in the Southern Hemisphere see constellations hidden from mid-northern latitudes, such as the Southern Cross. Winter nights are longer, which gives more dark hours for stargazing, even if it is colder.

The Sun itself shifts too: in summer it rises and sets farther north along the horizon, and in winter farther south. Mark a point on your horizon over a few months and you will see it move.

In short

• Seasons come from Earth's axial tilt of about 23.4°, not from distance to the Sun.
• Earth is closest to the Sun in early January, when it is winter in the north.
• The two hemispheres have opposite seasons.
• Tilt changes the Sun's height and the length of the day, and with them the amount of heat received.
• The night sky also changes through the year, another sign of our orbit around the Sun.

For a wider view of our planet, read the article on Earth.

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