<p>No special name.</p>
<p>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:</p>
<blockquote>
<p>If the center of the Earth is C, then <span class="math-container">$AC/AB = \tan(23.5^\circ)$</span> so <span class="math-container">$AB = 6370/\tan(23.5^\circ)=13000\, \mathrm{km}$</span></p>
</blockquote>
<p>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.</p>
<p>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.</p>

CC BY-SA 4.0
2021-05-28T12:38:56.943Z
@Bookaholic - you have to think in 3D; it's different.
Fattie
CC BY-SA 4.0
2021-05-26T01:57:50.383Z
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?
Bookaholic
CC BY-SA 4.0
2021-05-26T03:56:09.583Z
@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.
nick012000
CC BY-SA 4.0
2021-05-25T19:20:58.197Z
2021-05-25T19:20:58.197Z
James K
<p>I don't have the astronomy vocabulary to ask this question.   You can find a better picture at <a href="https://www.timeanddate.com/astronomy/axial-tilt-obliquity.html" rel="nofollow noreferrer">https://www.timeanddate.com/astronomy/axial-tilt-obliquity.html</a>.</p>
<p>I want to know if the distance between points A &amp; 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.</p>
<p><a href="https://i.sstatic.net/Qhodc.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Qhodc.png" alt="Length of line between A &amp; B" /></a></p>

<rotation><precession>
Does precession affect this measurement?
Bookaholic
# Does precession affect this measurement?

(Asked by: username_0 on 2021-05-25)

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](https://i.sstatic.net/Qhodc.png)](https://i.sstatic.net/Qhodc.png)

**Tags:** rotation, precession

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## 1 answers

### Answer by username_1 on 2021-05-25. Score: 4 | Accepted answer

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.

> **Comments:**
> **username_0**: 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?  
> **username_3**: @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.  
> **username_2**: @Bookaholic - you have to think in 3D; it's different.  

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