Understanding changes in times of day

 By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

17 April 2011

Joseph Maina of Nanyuki is wondering whether school books are misleading our children. He read one that says the Earth takes 23 hours and 56 minutes to complete one rotation. “But I have always known that a day is 24 hours and that this is the duration taken by the Earth to rotate. Which is the correct one – is it 23h:56m or 24h?”

I had tackled this problem many years ago [13 September 2003] when I revealed that the duration from noon on one day to noon the following day is 24 hours while that from midnight to midnight is 23h:56m. If you missed that article, the explanation for the discrepancy is that the Earth also revolves (around the sun) as it rotates daily.

Every day, revolves about one degree and this “subtracts” from the rotational motion. Therefore the apparent movement of the sun (rising and setting) completes its cycle one degree later than the Earth’s rotation. This translates to about four minutes.

Consequently, the correct way to measure the duration of earth’s rotation is to time it from midnight to midnight. But what is “midnight”? It is NOT 12 o’clock at night! It is the mid-point between sunset and sunrise.

At that mid-point, one locates a star that is overhead and then times how long it will take to traverse the sky and return to exactly the same location the following day. The result is 23h:56m.

Thing brings me to another question that has been sitting in my inbox for several months. Joseph Nderitu had asked why, despite Nairobi being near the equator, the sun sets earlier when it is at the southern hemisphere than when it is at the north.

First, lets agree on the dates: the sun is at the extreme south in mid December and in the farthest north in mid June. Right now (April) it is on its way north, having crossed the equator mid last month (March).

Are Nderitu’s observations correct? I invite other readers to check and verify… and while at it, please also check the sunrise times. For now, let it be enough to say that these variations arise from the fact that the earth’s axes of rotation and revolution are not parallel to each other. They are at an angle of 23.5 degrees to one another.

School textbooks say that the Earth is tilted, but they don’t explain where it is tilted from…now you know.

This tilt accounts for the south-to-north-to-south movement of the sun through the year – from January to June to December, respectively. It also explains why even for us who live near the equator, the times for sunrise and sunset vary from month to month.