Millions of Years

3.1. Gradual changes in the tilt of Earth's axis relative to its orbit around the Sun at a cycle of 41,000 years alter the amount of solar radiation that arrives at high latitudes.

Over long time scales, Earth's angle of tilt does not remain constant. In the 1840s French astronomer Urbain Leverrier found that the gravitational attraction of large planets (mainly Jupiter) causes Earth's tilt to vary within a range of 22.2° and 24.5° over a cycle that is 41,000 years long. Every 41,000 years, the tilt goes from a maximum to a minimum and back to a maximum. These tilt cycles are regular in both length and amplitude (fig. 3.1).

These slow variations of about 2.3° in the angle of Earth's tilt alter the height of the Sun in the daytime sky. Although 2.3° sounds like a trivial change, it still matters at high latitudes where the Sun is always very low in the sky. North of the Arctic Circle and south of the Antarctic Circle, the Sun never rises in midwinter (the endless "polar nights"). In midsummer the Sun never sets but instead drifts in a slow circle around the horizon at a very low angle. Even a small shift in the Sun's low elevation makes a measurable difference in the amount of solar radiation it delivers.

You can appreciate the importance of small changes in Sun angle by a familiar example from middle latitudes: gently sloping hillsides facing south receive enough solar radiation to melt a thin layer of snow quickly, while the north-facing slopes of those same hills may remain snow-covered for several days longer. The small difference in the angle of the Sun's rays against the slopes accounts for the different rates of snow melt. Similarly, small changes in Earth's tilt over its 41,000-year orbital cycle can cause significant differences in solar radiation at latitudes poleward of 45°.

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