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.
Saturday 10 December 2011
"Skywatchers enjoy lunar eclipse"
Subscribe to:
Post Comments (Atom)
19 comments:
There's alway the old Norse method for sailing to Iceland. Sail west along a line of constant latitude that lies to the south of Iceland. When you meet the whales, turn to starboard and follow them; they'll take you there.
I've wondered, but when it is not a full moon but only a partial moon, then isn't it the earth's shadow that gets in the way and makes part or all of the moon not visible, even in the night sky and thusly isn't that the same as a partial to full lunar eclipse on a recurring basis month after month, do you know?
D, finding Iceland is easier now, you just walk up and down the high street until you find it.
Anon, nope, the shadow of the earth only falls on the moon every year or three, that's what's called an 'eclipse' like today. By a bizarre coincidence I took a photo of the last one and you can see that a chunk is missing from the moon in the bottom right hand corner.
Sadly the process fails when the prat next door leaves a high powered unshaded outside light shining all night every night because he is scared of having a dark back garden, 'cos you can't see a bloody thing in the sky.
How can everything you state possibly be true when
(a) the earth is flat, and
(b) the moon is actually just a mouse treat made of cheese, a new one of which is casually tossed across the universe by a truly gigantic being from another galaxy at regular intervals (quite close to one of our days) each day for his equally gigantic, but to us, invisible and capable of travelling faster than light speed pet mouse to chase, with the mouse eventually catching it and eating it every day ...
Anon - that is patent nonsense. Cheese is yellow. The sun is yellow. Therefore the sun is made of cheese. Everyone knows that.
Very hot cheese that's looking for some toast to lie on.
The moon is white so it is quite evidently made of white bread and the hot cheese sun is trying to catch it and toast it. As it gradually succeeds throughout the month, it overdoes it and burns the toast black.
So the Toast King (who lives on the moon - proof is on YouTube if you don't believe me) has to rebuild it by baking a new white bread moon at the start of every month. As soon as he's finished, Hot Cheese King (who is French and lives on the sun) is after his bread again.
It's a never-ending cycle that can only be broken by throwing a loaf of bread into the sun.
When that day comes, the world will end because we're only here to keep Hot Cheese King away from Toast King.
When you have told this to your children at bedtime, you should then tell them that NASA has a probe on the way to the sun and it's sponsored by Warburton's.
Best to leave home at that point.
W42, that's annoying, but can't you go in the front garden instead? I find that it helps a lot if you hold up a large object between your eyes and the bright light and then look up.
LI, your version does seem rather more plausible than Anon's theory.
Yay. I love astronomy.
The sun the moon and the earth all lie on one orbital plane. The sun goes around the earth once every 27 days; the earth goes around the sun once every 365 days. The earth spins on an axis once a day (definitionally), and that axis is tilted with respect to the orbital plane.
Why then, is there not a lunar eclipse once every 27 days? Every 27 days, the three bodies must line up with the moon further from the sun than the earth. How is a full moon possible, to be 180 degrees from the sun, it would have to be behind the earth?
Logic tells us this: contradictions do not exist, check your premises.
The premise that is wrong is this: the moon does not orbit in the same plane as the earth and the sun. Not only is the earth's rotation tilted, the moon's orbit is too.
Phew...
The moon's orbital plane is fixed with respect to the earth (ish); but not with respect to the sun. An eclipse occurs when that plane has rotated so that the moon's vertical displacement (from the earth-sun plane) is zero, and at the same time is behind the earth.
Bonus info:
The moon originally spun on its own axis at some independent speed. There was a different "day" on the moon. The moon is not perfectly spherical (or of uniform density) though, so at times a sticky out bit was closer to the earth than it would otherwise have been. Earth's gravity pulls that sticky-out bit a little bit more than the rest of the moon, and it slowed its rotation. Wait a few billions years, repeating once every rotation and the moon's rotation synchronises with its orbit and the earth only ever sees one side of it.
The moon is doing the same to the earth, over time the earth's rotation will slow and the moon will appear stationary in our sky; meaning one side of the earth will never see it.
Yes indeedy, LI's summation of the true position does seem eminently more plausible than the one I gave - without though dealing with the flat earth angle and how that is imho skated over in the "standard version" as originally set out by MW ...
Any hoo, having had one explanation given to one as a child completely rubbished (I must have been really dumb to have believed that one !) I do hope no one is going to dispute another piece of information I was given when but a wee sprout ; that being that thunder and lightning are what we mortals experience when the Gods in the Sky are playing their version of Marbles, which utilises really vastly huge cast iron spheres - the lightning being the sparks given off when two of these collide, and the thunder, the sound of two of these colliding ...
OP: "The sun the moon and the earth all lie on one orbital plane"
Very approximately, yes, in actual fact, usually no, or else there'd be a lunar eclipse every month, as you point out later on.
Anon, that marbles story was clearly nonsense. Surely every child know that thunder is when big heavy clouds bump into each other, they also give off sparks, it would appear. Although that explanation skirts dangerously close to reality - lighting is in fact caused by friction/static electricity in falling/rising water droplets, i.e. clouds (I think).
You are all, of course, talking utter tosh - especially Onan Proboscis (is that name right, Ed? No idea. Ed.); I mean "orbital planes"!? Don't you know planes cannot fly in space?
LI, it is the MOON that is made of cheese - I know that for sure. Have you not seen Wallace and Gromit? Who do you think was behind the camera?
Yep. C'es't mo'i. You'res truely.
Gromit and I were planning our next expedition to the sun, but he may have tried to get ahead of me, because I hear that he has been toasted.
RSP
With Wallace navigating, they could have landed in the open-cast cheese mine at Cheddar Gorge.
Anyway, by a remarkable coincidence, the moon was still visible this morning so I took a photo. The Toast King must have been round the back.
RSP, you obviously weren't watching the film closely enough, the Moon is actually made of plasticene.
LI, it's not a coincidence at all. We are at the full moon stage, so the moon is 180 deg in the opposite direction to the sun, ergo, the moon is setting at the same time as the sun is rising (and vice versa) give or take a bit. At other phases of the moon, it is visible at the same time as the sun.
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.
Erm, is that right?
I agree with the first sentence. But surely if you stand facing the full moon, and the sun is 180 degrees in the other direction, then the sun is behind you, not to the right of you?
I should have said that I agree with the first two sentences.
FD, I hope so.
Moon in front of me (east)
Sun behind me (west)
Therefore, south is to the right of me.
I agree now - since your moon is a rising moon, and your sun is a setting sun.
But if your full moon was setting in the west, and your sun was rising in the east, south would be to your left.
And if at midnight you could see the full moon, then if you stood facing it the south would be behind you (though I imagine that you can only see the full moon at midnight in high northern latitudes).
So a more complete rule should perhaps be that if you face the full moon, then south is to the right of you at 6 pm, and behind you at midnight, and to the left of you at 6 am.
Or something vaguely along those lines...
FD, that's why I had to also add the fact that I wrote that post at 6 o'clock in the evening.
If you are facing the full moon at midnight, then clearly the moon is due south, so due south is straight ahead of you (not behind you), that's easy.
And if the full moon is just setting at 6 in the morning (in the west, by definition), then due south is to your left hand side, so we're agreed on that at least.
Yes, you're right. Since the moon has changed places with the noon sun in the south.
But I have a little diagram in front of me which show the sun and the earth and the moon in a line (in that order). The high northern latitudes of the tilted earth are in shadow. i.e. it is night there, and the sun never rises. Someone standing in these high latitudes would see the moon low in the north at noon (not midnight, as I first suggested).
Enough. I think I prefer Leg-iron's far simpler cosmology.
Post a Comment