Axial tilt and the seasons
Earth's spin axis leans about 23.4 degrees, which gives us seasons, solstices, the midnight sun and the polar night.
Earth spins on an axis that is tilted by about 23.44 degrees relative to its path around the Sun. Crucially, the axis keeps pointing the same way in space all year round. So for half the year the Northern Hemisphere leans towards the Sun, and for the other half it leans away.
That axial tilt makes the seasons. When your hemisphere leans sunwards, the Sun climbs higher in the sky, its light strikes the ground more directly and the days are longer: it is summer. The solstices, around 21 June and 21 December, are the turning points when the lean towards or away from the Sun is greatest. Around them the Sun's rising and setting points on the horizon reach their farthest north or south and seem to pause before turning back, which ancient observatories tracked with markers.
Near the poles the effect becomes extreme. Inside the Arctic and Antarctic circles there is at least one day of midnight sun in summer, when the Sun never sets, and at least one day of polar night in winter, when it never rises. Far enough in, the polar night lasts for months.
The most common misconception is that summer comes when Earth is closer to the Sun. In fact Earth is closest in early January, during the northern winter; the tilt, not the distance, is what matters. Over a cycle of about 41,000 years the tilt itself slowly varies between about 22 and 24.5 degrees.
Coming up in future stories
- Peru's 2,300-Year-Old Sun Calendar Made of Towers 4th century BC · Chankillo, Casma Valley, Peru
- The Interned Mathematician Who Timed the Ice Ages 1914-1941 · Budapest and Belgrade → confirmed 10 December 1976
- The Stone Ring That Points at the Solstice Before European settlement · Wurdi Youang, near Melbourne, Victoria, Australia