Alright, so an eclipse is caused by the earth’s passage between the moon and the Sun, yes? And the various phases of the moon are caused by, well, the same thing, only it takes a hell of a lot longer. I mean, if you see a crescent moon out, it’s because the earth is casting a shadow over 90% of the side of the moon we can normally see. So why does one take weeks (phases) while the other takes minutes (eclipses [eclipsi?])?
And if the earth is usually between the sun and the moon to some degree, why is the moon only very rarely between the earth and the sun? Why no “phases of the sun?”
I’m sure that there is an elementary answer to this that I’m overlooking (and I’m sure that it would be easier to explain with the help of diagrams), but any help would be greatly appreciated.
A solar eclipse is caused by the moon which comes between the earth and sun, obscuring the sun from our view.
A lunar eclipse is caused by the earth coming between the sun and the moon. The earth casts a shadow on the moon.
The phases of the moon are not caused by shadows of the earth. The phases occur because of the relative positions of the earth, moon and sun. One side of the moon faces the sun and is illuminated; the other side faces away from the sun and is not illuminated. If you stand next to the sun, you’ll see the whole sunlit side of the moon, so it looks like a bright disk. If you stand 90 degrees away from the sun, you see half the sunlit side, and half the dark side. So you see a disk with half of it illuminated - i.e. a half moon. If you were almost but not quite opposite of the sun, you will only see a thin strip of the sunlit side - that’s the crescent moon.
The sun is not illuminated by something else, it shines by itself. There is no “dark side of the sun,” so there are no phases of the sun.
First, get yourself a lightbulb, a baseball, a golfball and a duck…
Actually, the phases of the moon are not caused by the Earth’s shadow. Only half the moon is lit by the sun at any one time, just like the Earth. When the moon is full, we see the whole side that is lit by the Sun. When you see a moon half-illuminated you are looking at it from the side, so you see half of what the Sun is illuminating and the other half where it is night on the moon. During a new moon, we are looking only at the night-time side of the moon. The side we cannot see is fully illuminated by the Sun.
Alright, so an eclipse is caused by the earth’s passage between the moon and the Sun, yes? And the various phases of the moon are caused by, well, the same thing, only it takes a hell of a lot longer.
Actually, the phases of the moon are due to the moon’s revolution around the earth (with one revolution taking about one month). Rather than me trying to describe it, check out this page which has nice depiction.
You actually had a teacher say it was the Earth’s shadow causing the phases? (sound of AWB crying)
There was an article in Reader’s Digest about all the flagrant errors in textbooks. Among them:
[ul]
[li]a dozen incorrect definitions for basic math concepts such as perimeter[/li][li]A teacher’s edition comment about a word problem dealing with increasing a population figure by 239%. It said to simply multiply the population by 2.39. (Wrong. You have to add that to the initial population.)[/li][li]Richard Nixon resigned rather than face impeachment by the Seante (it was the House).[/li][li]fish have scales so they won’t leak.[/li][li]Carter was the first Democratic president since Truman. (What about Kennedy and Johnson?)[/li][li]Truman ended the Korean War by dropping the atomic bomb.[/li][li]Napoleon won at Waterloo.[/li][li]Copernicus believed that the planets revolved around Earth. (He though [correctly] that they went around the Sun.)[/li][li]Congress ratified the 19th Amendment in 1920. (States ratify amendments.)[/li][li]Columbus set sail for India in 1942. (1492)[/li][/ul]
And the reason that we don’t get an eclipse every month (or one of each type), is that the orbit of the Moon isn’t quite in the same plane as the orbit of the Sun, so usually, it goes above or below the Sun. The Earth’s shadow at the distance of the Moon is much bigger than the Moon, so it’s a lot easier to hit the shadow, so we have lunar eclipses more often than solar.
Not much more to add here, but I need to at least add my 2 cents in each astronomy thread…
Total solar eclipses are possible because at this time, coincidentally, the moon and sun have the same apparent angular size in the sky. As the moon’s orbit continues to distance from the Earth over the ages, total solar eclipses won’t be as complete.
Yes, but the earth is much larger than the moon, so it is easier to hit with the moon’s shadow. I believe the incidence of solar eclipses (full, annular, and partial) and lunar eclipses are about the same–twice a year.
The new moon is very close to the sun, and it is virtually impossible to see because of the brightness of the sun. Otherwise, you would indeed see the small crescent at the top or the bottom.
The reason you don’t see the dark one on a full moon is, it is dark and the full moon is very bright. The moon is only slightly out of round, not enough to notice.
Lib, there is a very small crescent around the edge of a new moon, and a very thin shadow around the edge of a full moon. But you can’t see either one. With a new moon, the moon is so near the sun that you can’t see the thin crescent - it’s not bright enough to show through the atmosphere that’s being lit up by the sun.
And when the moon is full, the bright full moon obscures the thin dark edge.
When the moon comes between the sun and earth, you get a solar eclipse.
When the earth comes between the sun and moon, you get a lunar eclipse.
When the sun comes between the moon and earth, you get a really bad sunburn.
VarlosZ, where do you live? If you still live near where you went to school, I would see if that teacher still teaches at the school you went to. Then I would try to get her fired.
I’m serious about this. I taught third grade for a year, and this was one of their science chapters. They didn’t have a difficult time understanding this. If that teacher couldn’t figure out the little pictures in the books, then that person doesn’t belong in a classroom.
You know, I’m a Junior in college, and the whole phases/eclipses etc. issue has never seriously come up in any of my schooling. While skirting over it in early grade school (2nd Grade? 3rd?), I remember the teacher mentioning the thing about the phases. It just stuck with me, for some reason. It’s one of those things that wasn’t even really in the lesson plan, she was just answering a question (IIRC). I think her ignorance on this matter is pardonable (as is, I hope, mine).
Now, I’m going to hijack my own thread. The moon’s “day” (rotational period, whatever) is one lunar year long; that’s why we only see the one side of the moon from earth. This happens to all satelites eventually, the tighter the satelite’s revolution, the faster it happens (Mercury is well on its way, e.g.). This phenomon has a name, but I forget it. Anyone know what causes this?
Tidal friction, or tidal braking, are as good as any.
Before we organize the lynch mob for that teacher, I want to second VarlosZ admirable thought that this is a pardonable offense. A comment off the top of ones head may have been a brain fart, or it may have been a result of a deep-seated understanding caused some previous teacher, or it may have been the result of a misinterpretation of those comments. (Lib, the skin on the back of my neck is beginning to crawl. Has anyone seen Bob Eubanks around here? Let’s not do that.) On the other hand, with a textbook, we have physical evidence, and a textbook should be held to a high standard.
When I was a kid, I remember being confused about lunar eclipses for a while. Seeing a normal moonrise (reddened by the atmosphere), I thought it was eclipsed! Nobody had told me that it was, I just got the idea somehow.
I finally figured out it really wasn’t an eclipse, but for quite a while I didn’t fully understand why there wasn’t a lunar eclipse every month.