Earth’s axial tilt and precession

In short: Earth’s axis is tilted by about 23.4 degrees relative to its orbit around the sun, which causes the seasons. In addition, the axis itself slowly traces out a wobbling motion over millennia, called precession.

In more detail: Precession arises from the gravitational pull of the sun and moon on the slightly flattened Earth, similar to a wobbling spinning top.

In detail

• Tilt angle: The current tilt of about 23.4 degrees varies slightly over long periods between roughly 22.1 and 24.5 degrees.
• Cause of the seasons: The tilt causes the northern and southern hemispheres to alternately receive more or less direct sunlight (jahreszeiten-entstehung).
• Precession cycle: A full precession motion of Earth’s axis takes about 26,000 years.
• Pole star change: Because of precession, the axis doesn’t always point at the same star; in about 12,000 years, a different star will serve as the pole star.
• Influence on climate: Precession and tilt variations are among the so-called Milankovitch cycles, which help drive long-term climate changes like ice ages.
• Zodiac shift: Precession explains why the astrological zodiac signs no longer exactly align with the astronomical constellations.
• Observation: Precession is so slow that it’s not directly perceptible on a human timescale, but it is clearly measurable.