Why the sun appears to drift from North to South

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

16 January 2022

Githuku Mungai asked me this straightforward question: “The sun is this month setting in the Ngong side and not Busia side. Is there a good explanation for that?” I replied with a quick “Yes; It’s due to the 23.5-degree tilt….and now a question to you: between which two lines is that angle measured?” He had no answer, only replying that he is now “lost in the forest” and asked me to explain. So, here goes…

The standard classroom globe of the world is always mounted with the earth slanted at about 23.5 degrees. That is; the (imaginary) line joining the North and South poles is not vertical. The question I was asking Githuku is this: using this such a globe as a model, what would a vertical line represent?

The Earth has two motions: rotation about the north-south axis and revolution around the sun. Just as there is an axis of rotation, there is also one for revolution. This second axis is located at the sun and it is perpendicular to the line joining the Earth and the sun.

In the model of the classroom globe of the world, the axis of revolution around the sun is represented by the vertical direction. The angle between the two axes is 23.5 degrees. The next question may be: how do we know this?

This can be good astronomy exercise for school children. Start by planting a vertical pole on the ground. Schools have flagpoles so this is an easy step for them. Then, one sunny morning (or afternoon), draw a line along the shadow of the pole.

Now wait for one month (30 days) and look for the same shadow (morning or afternoon, respectively). You will notice that it is no longer at the position it was at the beginning. Draw a line along the new position of the pole’s shadow.

Repeat this every month trying as much as possible to check the shadow on the same date each time. If there is no sunshine all day, do it the following day. After one year, the shadow will have drifted back to its original position. Science requires a lot of patience!

Measure the angle between the two extreme positions of the shadow. The answer should be about 47 degrees. This is the full swing as the apparent position of the sun moves from one tropic to the next. It is double the tilt angle of the earth’s axis of rotation.