How to tell that Earth is moving around the sun

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

02 July 2017

Last week, Jusper Mogire engaged me in a Twitter discussion about the motions of the Earth and the Sun. He started off with this tweet: “The sun rises in the East but then we were told the sun doesn't move: that the earth rotates around it. Any idea?”

I pointed out to him that that was not correct: the raising and setting of the Sun daily is not evidence of the movement of the Earth around the sun. This is a common misconception that, unfortunately, is even propagated by some our teachers. As Mogire mentioned later in our discussion, his son had been taught the same in school.

But let’s think about it. The Earth takes about 365.25 days to go around the sun. The duration from sunrise to sunset and back to sunrise again is just one day. How then can anyone argue that a cycle which repeats once every day is evidence of a phenomenon that takes 365 days? Utter nonsense, isn’t it?

To be clear: the daily movement of the sun across the sky is evidence that the Earth is spinning and one spin takes approximately 24 hours (actually, 23h:56m:4s). To get evidence of the movement around the sun, we must observe the stars carefully.

And no; you do not need a telescope! Ancient Egyptians and Greeks knew about this movement many centuries before the invention of the telescope. Here is how to do it: go out tonight at a convenient time, say, 8:00pm, and look at the stars (hopefully, it will be a cloudless night!)

When you look at them, you immediately notice that they are not arranged in any order. It is as if some one scattered a handful of rice on a concrete floor! But look keenly and select a group of about five stars which appear to form pattern that can be easily remembered.

A word of caution: it is tempting to start searching for the patterns of the zodiac (Leo, Virgo Libra etc). Forget about those and just pick your own!

Now, if you go back to the house and come out again at 9pm, you will notice that your group of stars has drifted slightly to the west – just like the sun. This is more evidence of the spinning of the Earth.

Go back and wait for one week. Then return to the same spot outside your house at exactly the same time (8:00pm) and look for your pattern of stars. You will notice that even though it is the same time, the group has drifted westward.

So there are two motions: the fast one observed hourly and a slower one detected weekly. If you continue this weekly observation long enough, your group will drift below the western horizon and no longer be visible at 8:00pm.

At the end of one year, this group of stars will return to exactly the same spot on the sky at 8:00pm. This is the evidence of the Earth’s motion around the sun. It is a simple exercise that can be given to school children