I don't have the astronomy vocabulary to ask this question. You can find a better picture at https://www.timeanddate.com/astronomy/axial-tilt-obliquity.html.

I want to know if the distance between points A & B on my graphic has a name, and if it would grow shorter or longer as the earth's rotation slows down. Would this distance only reflect the 23° wobble in the rotation of earth or would it also reflect Precession of the Equinoxes.

Length of line between A & B


1 Answer 1


No special name.

The problem that I see here is that the distance will vary over the year. At the moment of solstice, you can easily find the distance by basic trigonometry:

If the center of the Earth is C, then $AC/AB = \tan(23.5^\circ)$ so $AB = 6370/\tan(23.5^\circ)=13000\, \mathrm{km}$

At any other time the distance will be different, and at the equinox the axis of the earth is parallel to the tangent plane of the Earth, so the distance is infinite. So the distance AB is not a fixed value, and varies over the year.

In the longer term the precession of the equinoxes will affect when solstice occurs, but not the angle 23.5. This angle also varies in the long term between 22.1° and 24.5°, over a cycle of about 41,000 years.

  • $\begingroup$ Because the two angles (90° in my graphic) would always add to 180°, why wouldn't your formula be good on any day as long as you adjust for the 22.1° - 24.5° variable? $\endgroup$
    – Bookaholic
    May 26, 2021 at 1:57
  • 1
    $\begingroup$ @Bookaholic Because the line NS rotates as the Earth orbits around the sun, so the point B slides along the plane tangent to the surface of the Earth - and as it travels horizontally, it will also slide downwards vertically. $\endgroup$
    – nick012000
    May 26, 2021 at 3:56
  • $\begingroup$ @Bookaholic - you have to think in 3D; it's different. $\endgroup$
    – Fattie
    May 28, 2021 at 12:38

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