There's a mildly interesting article on the BBC on today's eclipse, which reminds of another penny which dropped with me a couple of days ago...
Popular myth has it that the sun is in the sky in the day time and the moon is in the sky at night. This is quite untrue of course, there are two main ways of explaining how this myth arose.
A few facts:
* The sun moves through our sky every 24 hours or so, because that's how fast the earth turns on its axis.
* The moon goes round a bit more slowly, about an hour slower than the sun, because the earth's gravity drags it round with it slightly.
* The sun and the moon travel along the same general path through our sky, above the equator, plus or minus a bit. So we know that the sun is always due south at midday, but the moon could be anywhere on its circular path at midday.
* The moon goes through phases, from new moon to full moon and back again every 27 days or so, because it loses an hour or so every day, and after 27 days, it has lost so much ground to the sun that it is back where it started.
* When the moon is opposite the sun, from our point of view, it is a full moon because it reflects a lot of sunlight back to us. When the moon is in the same general direction as the sun, it is between two new moons and all the sunlight shining on the moon is reflected away from us.
Explanation One
If the moon happens to be in the sky at the same time as the sun, it is nowhere near bright enough to be seen if they are in the same general direction. If the sun is setting at the same time as the moon is rising (or vice versa), then the moon is more likely to be visible. The moon is most easily seen at night time. So we are accustomed to only (or mainly) seeing it at night.
That's simple enough.
Explanation Two
This is the nub of the matter, which exaggerates the illusion. The moon is at its most visible when it is a full moon, i.e. where it is 180 degrees in the opposite direction from the sun.
If the moon is in the sky when it is a full moon, by definition, it must be night time - the sun is beneath your feet and the moon is above your head, so to speak. Although the moon is 'full' from the point of view of all observers on earth for a few days, you cannot have a full moon in your personal day time when the sun is above your head, because the moon is beneath your feet i.e. on the other side of the world.
Bonus round
If you know the old trick about using the sun and the time of day to work out compass directions, you can adapt this and use the moon to tell where the sun is, even in the night time, and then work out the compass directions. Right now (for example) it is 6 o'clock in the evening and it is a full moon. Therefore the sun must be 180 degrees in the other direction (even though it has set and I cannot see it). So if I stand facing the moon, due south is to my right.
You can use this as well for other phases of the moon, if it's a quarter moon and the left half is in shadow and the right half is lit up, the sun must be ninety degrees to the right of the moon. So if it happens to be six o'clock, I can stand facing the moon and due south is straight ahead.
Of course, this technique is absolutely useless if we are near a new moon, because the new moon is usually barely visible, being only slightly to the left or right of the sun, and if you can use the sun for navigation you don't need the moon (which is a lot trickier).
If in doubt, take a compass, seems to be the obvious conclusion.
Greek Irony
8 hours ago
