Measuring the moon’s motions without a telescope

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

28 February 2021

At school, children are taught that the evidence that the moon moves around the earth is in the changes in its phases from crescent to full moon and back to crescent again. But this is not true!

Last week, I decided to demonstrate the correct evidence of the moon’s motion to a standard 8 pupil. At 9:12pm on one evening, we stood outside at a specific point at the car park in our estate.

We then looked for the moon in the sky and noted its position in relation to the rooftops of buildings in the estate. The following day at the same time (9:15pm), we went back to the same spot and looked for the moon.

We observed that it was not where we had seen it the night before. It had moved by a discernibly large distance westward. The pupil was amazed! She had never thought that one could see the movement so clearly.

We stood there for a few minutes discussing what we had observed and then realised that the moon was drifting eastward. Another discovery was made: when observed on a minute-by-minute basis, the moon moves eastward; but when observed day-by-day, it moves westward. Two motions in one!

At this point I thought it would be interesting to make some quick measurements. I had a newspaper in my hands. I gave it to the pupil and asked her hold with her arms stretched out straight.

Then, with one eye closed, make sure that the point her left hand was holding coincided with the previous day’s moon position while the right hand was at its current location. This formed a triangle: two arms held straight out and the edge of the newspaper

We went back to the house and took measurements. The length of her stretched out arms was 60cm. The distance between the two points on the newspaper was 10cm. Next, I asked her to make a scale drawing of this triangle.

She drew it and measured the angle between the arms. It turned out to be about 10 degrees. This was our estimate of the angular displacement of the moon in one day.

By our measurement, it would take the moon 36 days to make a complete 360-degree journey around the earth. The correct number is 27.3 days.

I think its pretty good for such rough measurements even though it is off by more than a week. Why did we get it so wrong?