Weather: Earthly misconceptions
Friday 10 October 1997
Question one: How many times does the earth rotate on its axis in a year?
Question two: When does the moon come out?
Question three: Why is it cold in
If you answered 365 (or 3651/4); at night; and because we're further away from the sun, then you are in good company. For I have been posing these questions to many of my acquaintances - all supposedly intelligent and well-educated - and those were the answers most commonly received. They are all wrong.
Let's take them in reverse order. The earth's orbit round the sun is an ellipse which is so close to being circular that our distance from the sun at different times of year has little perceptible effect. What does make all the difference is the angle at which the sun's rays strike us. The earth's own axis of rotation is inclined at 23.4 to its plane of rotation about the sun, which produces the effect of letting the sun's rays strike us directly in summer but obliquely in winter. The same amount of solar heating is spread over a wider area in winter than in summer. And, of course, while we in Britain are tilted away from the sun, Australia is tilted towards it, which is why Christmas in Sydney is so pleasant.
That's done the sun. Now for the moon, which rotates about the earth roughly once a month, always keeping its dark side towards us. But whoever wrote the lyric "you've got the sun in the morning and the moon at night" has a good deal to answer for. Just because in general we can see the moon only at night, does not mean it's not there. As you will see from the figures below, it's getting up just after lunch and going to bed at about midnight these days. Sometimes it's not there at all at night, yet few of us realise it was there all day, only the sky was too bright to see it.
The opening question, however, was the one that caused the most problems. The earth goes round the sun once a year, it turns on its axis once a day, so that's a total of 365-and-a-bit rotations. Wrong! The correct answer is 366-and-a-bit, and here's why. A day, we know, is the time from one sunrise to the next. A day, we also believe, is the time the earth takes to complete one revolution about its axis. But these two definitions are not the same. In fact, the earth completes a revolution about its axis once every 23 hours and 56 minutes; the other four minutes are spent catching up with the sun.
Try this experiment: face an object a few feet away from you. Then start turning on the spot in a circle. After one complete rotation, you are back facing it again. But after one rotation of the earth, it has also moved on a little in its rotation about the sun. When you shift yourself a little, in the same direction as that of your rotation, keeping the same distance from the fixed object, you realise that you have to turn a little bit extra to face it again. All those little bits add up to another complete rotation in a year: 365 days, 366 turns.
Going into it a little more deeply, we can understand why sundials are so often wrong. Our 24-hour day is only the mean time between one sunrise and the next. Some days (defined by our position relative to the sun) are in fact longer than others. But to keep clock-makers and train drivers happy, we all adopt Mean Solar Time (by which all days are the same length), while our sundials register Apparent Solar Time which leaves them some 15 minutes adrift in February and November.
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