Why Earth Has Seasons

Earth spins around an axis that is tilted by about 23.5 degrees compared with the plane of its orbit. As Earth moves around the Sun, the axis keeps pointing in nearly the same direction in space. This means each hemisphere leans towards the Sun for part of the year and away from it for another part.

When the Northern Hemisphere leans towards the Sun, it has summer while the Southern Hemisphere has winter. About six months later, the situation reverses. The tilt, not a sudden change in the Sun, creates the yearly pattern.

Earth's distance from the Sun changes slightly during its orbit, but that is not the main cause of seasons. If distance caused summer, both hemispheres would warm at the same time. Opposite seasons are the clear clue that the direction of the tilt matters far more than the small change in distance.

Direct sunlight spreads its energy over a smaller area, so it warms the surface more strongly. Slanted sunlight spreads the same energy over a larger area and travels through more atmosphere, so the heating is weaker.

In summer, the Sun appears higher in the sky and its rays strike the ground more directly. In winter, the Sun stays lower and the rays arrive at a shallower angle. This difference is especially strong far from the equator, which is why high latitudes experience larger seasonal changes.

The equator receives fairly direct sunlight throughout the year, so day length and temperature change less there. Near the poles, the tilt creates extreme differences, including very long summer days and very long winter nights. The same tilted planet therefore produces very different seasonal experiences depending on latitude.

A hemisphere tilted towards the Sun also receives more hours of daylight. Longer days give the land and water more time to absorb solar energy. In winter, shorter days and longer nights reduce the daily heating.

The seasons do not change instantly at the solstices because oceans and land take time to warm and cool. This delay is called seasonal lag. Keep four clues together:

• •Earth's axis is tilted.
• •The tilt keeps a steady direction.
• •Sunlight becomes more direct in summer.
• •Summer days are longer.

In the Northern Hemisphere the longest day is around 21 June, yet July and August are usually hotter, and the same lag appears at the other end, with the coldest weeks arriving in January and February after the December solstice. The reason is that the ground and especially the oceans store heat. As long as the Sun delivers more energy each day than the surface loses at night, the temperature keeps climbing, and that surplus continues for several weeks after the days start shortening.

Places surrounded by water lag more than continental interiors, which is why coastal summers peak later. The equinoxes are also not the halfway point most people expect: because the oceans are still warm in September and still cold in March, autumn days are typically warmer than spring days with the same amount of sunlight. Seasonal lag is the single most useful idea for explaining why the calendar and the thermometer disagree.

A neat check on the tilt explanation is that Earth is actually closest to the Sun in early January, during the northern winter. Other planets make the point even more vividly. Jupiter's axis is tilted by only about three degrees, so it has almost no seasons at all. Mars is tilted by roughly 25 degrees, similar to Earth, and it does have seasons, with polar ice caps that grow in winter and shrink in summer as carbon dioxide frost forms and vaporises.

Uranus is the extreme case. Its axis is tipped over by about 98 degrees, so it rolls around the Sun almost on its side. Because one orbit takes about 84 Earth years, each pole spends roughly four decades in continuous sunlight followed by about four decades of darkness. Mars also shows that distance can add a smaller effect on top of tilt: its orbit is more stretched than Earth's, so southern summers, which happen near its closest approach to the Sun, are shorter and warmer than northern ones.

You can estimate how high the Sun climbs at midday with one simple rule. At the equinoxes, the noon Sun's height above the horizon is roughly 90 degrees minus your latitude. At the June solstice, a place in the Northern Hemisphere adds the tilt of about 23.5 degrees, and at the December solstice it subtracts it. For London, near 51.5 degrees north, that gives a noon Sun about 62 degrees above the horizon in late June but only about 15 degrees in late December.

The sunlight landing on each square metre of flat ground depends roughly on the sine of that angle. The sine of 62 degrees is about 0.88 and the sine of 15 degrees about 0.26, so at midday London's ground receives over three times as much sunlight per square metre in June as in December, before even counting day length. The tilt also draws lines on the globe. The tropics, at about 23.5 degrees north and south, are the furthest places from the equator where the noon Sun can stand directly overhead, and the polar circles, at about 66.5 degrees, mark where at least one day a year has no sunset.

The 23.5 degree tilt is a snapshot. Over a cycle of about 41,000 years, Earth's tilt rocks between roughly 22.1 and 24.5 degrees. A larger tilt makes seasons more extreme, with hotter summers and colder winters at high latitudes, and at present the tilt is slowly decreasing. The axis also wobbles like a spinning top, a motion called precession, tracing a full circle in about 26,000 years. Today the northern winter falls near Earth's closest approach to the Sun, but in something over 10,000 years northern summer will fall there instead, making northern seasons a little sharper.

In the early twentieth century the Serbian scientist Milutin Milankovitch calculated how these slow changes, together with variations in the shape of Earth's orbit, alter the sunlight reaching high northern latitudes in summer. He argued that cool northern summers, which let winter snow survive and build up into ice sheets, could explain the timing of ice ages. Strong support came in 1976, when researchers analysing chemical records in deep-sea sediment cores found climate cycles matching the predicted rhythms. The same tilt that gives you summer and winter also helps pace the great ice ages.

Earth's seasons come from axial tilt, changing sunlight angles and changing day length. Picture the two hemispheres leaning towards and away from the Sun, and the whole seasonal cycle becomes easier to explain.