Earth's tilted axis gives us seasons, and the Moon keeps it steady
Summer and winter exist because Earth spins on an axis leaning about 23.44 degrees from upright. That lean keeps pointing the same way among the stars all year, so each hemisphere takes turns facing the Sun. Without a large Moon, the tilt might have wandered far more wildly.
Astronomers call the angle obliquity: the gap between a body's spin axis and a line standing perpendicular to its orbit. Because the axis stays fixed in direction as Earth circles the Sun, the North Pole leans sunward for half the year and away for the other half, giving the northern hemisphere its summer and then its winter. Venus shows how conventions matter. Measured one way it tilts 3 degrees and spins backwards; by the right-hand rule it tilts 177 degrees, upside down, and spins forwards.
People have measured Earth's lean for a very long time. Pytheas of Marseilles used the shadow of a vertical pointer at midsummer around 350 BCE, the caliph Al-Mamun set Baghdad's astronomers the task around 830 CE, and in 1437 Ulugh Beg arrived at 23.5047 degrees. Medieval scholars mostly believed the angle rocked back and forth over 672 years, but Fracastoro recognised in 1538 that it was shrinking at a fairly steady pace. Today it declines by roughly 46.8 arcseconds each century.
Over long spans the swing is larger. For the past 5 million years the tilt has ranged between about 22 and 24.5 degrees on a cycle averaging 41,040 years, a rhythm that probably helps drive long-term climate change. The Moon damps bigger excursions, though it is slowly drifting away and will do so for less than two billion more years. Mars, lacking such a partner, may have lurched anywhere between 0 and 60 degrees, perhaps explaining why rivers and lakes there came and went.
Other worlds are now being measured too. By 2024 tilts were known for 4 exoplanets, one of which, VHS 1256 b, lies on its side much like Uranus.
Source: Axial tilt
