Are the sun and the moon the same size when overlapping? (ala total eclipse..)

Same relative size, of course, from an earth’s eye view.

Very close, the moon appears only slightly smaller, which is why you can see a thin ring of the sun on a perfect eclipse.

Yes, they’re about the same.

And there’s no physical reason for it to be that way, either. It’s just a lucky coincidence.

They’re both usually listed at about 0.5° or 30 arcseconds, which is close. I think they’re both between 31 and 32 arcsec, and of course they vary as their distance from Earth varies.

Ah! I found a great page with pretty diagrams!

NASA’s da bomb, yo.

So how close are they in percentage terms?

95%?

99.9%?

Depending on when the eclipse occurs, the moon sometimes appears very slightly larger than the sun thus covering it completely, or very slightly smaller when you get the “diamond ring” effect, or Bailey’s beads.

Because both the moon and the earth’s orbits are eliptical, the sun ranges from about 944 to 976 arcseconds, and the moon from 882 to 1006 arcseconds. So, as Q.E.D. mentions, it depends on the timing to calculate the exact relative size.

Arcminutes. I meant arcminutes, not arcseconds. Sorry.

Also, I believe that friedo’s numbers are radius, while mine are diameter. That explains the discrepancy.

Ok, according to that NASA link, the moon can be anywhere from 7% bigger to 10% smaller.

Question asked, answer given.

I love the Straight Dope.

Here’s a follow up that I don’t believe anyone’s ever figured out- We know that sometimes we get annualar eclipses when the moon happens to pass in front of the sun while furthest away from earth. We also know that the moon is slowly receding from earth as a consequence of the conservation of angular momentum. Therefore, at some point in the future, there will be one last total eclipse, and henceforth the moon will always be too far away from earth to completely block the sun. Has the date of that last total eclipse ever been determined?

This question has come up before and I posted a table giving the ranges of apparent semidiameters of both the Sun and Moon.

You know, I don’t know. But if the moon is receding at 5.6 cm/yr with a current angular size of 0.0087 radians, then the total eclipse path widths will be shrinking by about 0.049 cm/yr. If the widths are now around 100 km, it will take them 200 million years to dwindle to zero. This is just a first-order approximation, though.

If the 200 My figure is correct, then there’s no way to know when the last total eclipse will be. That’s a geologic timescale, so among other things, the elevations on Earth will be different then, and an eclipse which is total on a mountaintop may be annular at sea level.

Annular eclipse, where the moon doesn’t quite cover the sun.
Total Eclipse, where the moon is larger than, and covers the sun.
Eclipse sequence, from partial through total.

Another way of looking at the last total eclipse- We know that at furthest apogee, which is about 407,000 km, the moon is too far away to cause a total eclipse. At closest perigee, at about 356,000 km it of course is close enough. So if the moon recedes 51,000 km we will have no more total eclipses. If the 5.6 cm/year figure is accurate, then in 911 million years the perigee will be at the current apogee. So I guess I believe the 200+/- million year figure, since the distance of the moon to cause an annular eclipse must be somewhat less than 407,000 km.

Or one better… at what point will the moon go behind the sun?

There was an SF novel (Vernor Vinge’s Marooned in Realtime?) which was set in a future ~20 million years in the future. One of the differences was the fact that there were no longer solar eclipses

Several nitpicks. Both the diamond ring and Baily’s beads usually occur when the Moon appears larger than the Sun, not smaller. This is because both are associated with the beginning or end of a total eclipse, while if the Moon is smaller than the Sun you just get a rather dull annular one. The sort of borderline case is when the two are almost exactly the same size. Then the Moon can obscure the entire disk of the Sun, except for a handfull of tiny patches shining through lunar valleys, producing a very short eclipse, but with Baily’s beads (there’s a famous photo of this from the eclipse of 20th May 1966).
The more usual circumstance is that such beads only appear at the start or end of totality, as the edges of the two disks sort of coincide. What you then get at the end of the totality is that enough of part of the solar disk becomes unobscured enough to dazzle. That’s the centre of the diamond ring, with the rest of the ring formed by the edge of the Sun extending beyond the Moon’s disk. My understanding is that the perfect eyewitness vision is to see a few fleeting beads in the final moments of totality, followed by the much more intense diamond ring marking the end of it.

Yes- that’s the one;
an important book (to me) in that it was the first time I ever read about the concept of the Singularity…

but putting that aside,

Marooned in Realtime was set 50 my in the future, and the Moon had moved far enough away for eclipses to no longer be possible; this doesn’t agree with Achernar’s estimate very well, but as dicussed in this thread long ago the retreat of the Moon is partly dependent on the flow of the liquid oceans on Earth, so any calculation would have a large margin of error over long periods of time.


Sci-fi worldbuilding at
http://www.orionsarm.com/main.html

It won’t.

The moon is moving farther away by taking energy away from earth’s spin. Eventually the earth will be tidally locked to the moon, and the moon will stop receding. The sun is likely to die long before this happens though, since the speed of recession will slow as the distance increases and the tidal forces weaken.