365 Days of Climate Awareness 18 - Milankovitch Cycles 3: Rotational Variations

 

365 Days of Climate Awareness

18 - Milankovitch  Cycles 3: Rotational Variations

The earth rotates on its axis, with slowly decreasing velocity. Earth's days were significantly shorter a few billion years ago, but the gradual effect of the moon's gravity has slowed the earth's spin rate. Eventually, so long as the solar system exists, earth's face will become locked to the moon, as the moon's face is locked to the earth (always showing the same face, with the same craters, to us).

There are two major types of regular variation in the earth's axis: the angle with respect to the ecliptic, and the orientation with respect to the (so to speak) fixed stars.

The orbit of the earth forms a plane through the sun known as the ecliptic. The earth currently rotates on an axis which is 23.5° away from perpendicular to that plane 63.5° away from the plane itself). However, that angle is not constant, but oscillates between 22.1° and 24.5°. (The angle is now decreasing with respect to perpendicular, and will reach its minimum inclination in about 11.8K years.) This decrease in inclination will lead to a smaller difference between summer and winter, and likely an overall cooling effect.

    

Rotational Milankovitch variations.

The other change is the gradual rotation of the axis itself on which the earth spins. It backs around in a circle with respect to the surrounding stars, clockwise when viewed from above the north pole, with a period of roughly 25.8K years. (Anyone who has spun a top and watched the orientation of the handle rotate slowly as the top spun rapidly can picture this motion perfectly.) After a few thousand years the earth's axis will no longer point at Polaris, and there will be no north star. The rotation of the axis itself changes where in earth's orbit the seasons occur, which changes not only which constellations are visible at night but also when during the year the earth is at perigee--closest to the sun, absorbing more solar energy--and apogee, furthest from the sun and absorbing less. (Currently perigee occurs on January 3, apogee on July 4.)

Tomorrow: Milankovitch cycles 4: climatic significance.

Be well!