Of Particular Significance

Three Crescents and a Ring

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 05/20/2012

Hi all! I said that posts would be sparse for a couple of weeks, but I didn’t realize quite how sparse! But I’m gradually getting back on-line here.

Right now, what’s occupying my attention for the next 48 hours is all in the sky: three crescents, and a ring. I’ll be brief, but if you want more explanation about the geometry involved, you might want to read my very relevant article about geometry and the beauty of the heavens.

Crescent Venus: If you look just after sunset in the western sky between now and the end of the month, you’ll see Venus shining brilliantly in the twilight, but seeming to plummet, day by day, toward the Sun. This is all in preparation for Venus to pass between the Earth and the Sun, during June 6th’s last-in-a-lifetime “transit.” But if you have good binoculars (and good vision, I’m afraid — if not, you’ll need a small telescope), you’ll discover Venus is not a dot, or a disk, right now — it is a crescent. With a small telescope you’ll see just how thin and large a crescent it is. Venus, in preparation for its transit, is almost as close to Earth as it ever gets — hence it appears large — but since it is closer to the Sun than the Earth, almost all of the part of Venus that faces toward the Earth is facing away from the Sun, and is therefore in shadow. One half of Venus faces toward us; a different half faces the Sun and is therefore lit; and these two halves only overlap along a fingernail crescent.

Crescent Moon: If you were up before sunrise the last couple of days, you’ve seen a thin crescent Moon in the eastern sky, because the “New Moon” (the start of a new cycle of lunar phases) is happening shortly, and before the New Moon there are always crescent Moons in the morning twilight. After the New Moon, there is a crescent Moon in the evening twilight. Look for a fingernail crescent Moon just after sunset in the western sky on Monday and Tuesday nights. The crescent is there for the same reason that it is there for Venus: the Sun lights one half of the Moon; we see one half also; but the visible half and the lit half overlap only on a crescent. The pairing of the crescent Moon and the crescent Venus will make a lovely pair to look at with good binoculars.

Crescent Sun: on Sunday, May 20th (in the United States; it will be Monday the 21st on the other side of the arbitrary International Date Line) the “New Moon” will transit the Sun, just a few days before Venus does the same trick. But there is very different geometry involved — Venus, so far away, will appear to transit the Sun from much of Earth, and will appear so small that it will block only a tiny portion of the Sun’s light —- whereas the Moon is so close that its transit is visible only from a smaller portion of Earth, but there is a small region of the Earth where almost all of the Sun’s light will be blocked. For this reason we call such a transit an “solar eclipse” — but let us remember that it is basically the same thing as a transit. For those who are in the region where the eclipse is visible, you will see a crescent Sun. But note: such crescents are not the same shape as those seen on Venus or the Moon. The geometry is different. Think about it. Hint: there will not be a “half Sun” the way there is a “half Moon”.

A Reminder and Warning: Do not look at the sun if it is only partially blocked, as will be the case everywhere in this eclipse. Make sure your children understand this. There are a very few safe ways to look at an eclipse of the sun; they are all over the internet (but get a second opinion, I have seen errors on some webpages) so I won’t try to summarize them here.

And finally: a Ring.

The Sun minus the Moon: It is a remarkable accident that our Moon and our Sun, at very different distances from Earth and of very different sizes, appear as disks of almost the same size in our skies. This means that when the Moon transits the Sun, it can — in those locations on Earth where the Moon’s center appears to cross the Sun’s center — completely or almost completely hide the Sun. Because the Moon and Sun do not maintain exactly the same relative size in the sky — distances between the Earth, Sun and Moon are not exactly constant, as the orbits of these objects are not exactly circular — it happens that sometimes the Moon will appear a little smaller than the Sun, with the effect that the Moon will not quite cover the Sun even if, from your point of view, it crosses the Sun dead center. This is the case during this eclipse. Rather than a total eclipse of the Sun, where the Sun’s entire disk is covered by the Moon (making the Sun’s outer atmosphere visible), this eclipse will be different (neat but less extraordinary) because a small but still dangerously bright ring of sunlight will remain even when the Moon’s center and Sun’s center are exactly lined up. This ring or “annulus” of sunshine is the source of the name “annular eclipse”.  In short, take the Sun’s disk, subtract the Moon’s slightly smaller-appearing disk, and what remains is a ring of sunshine. Don’t look at it directly at any point during the eclipse! If you do, you’d better have specially designed eclipse glasses or the right kind of welding glass (and don’t take chances with your eyesight, please! remember your eyes can be damaged without you feeling a thing.)  But enjoy it safely, savor the magic of the dance of the Sun and Moon — the former about 400 times larger in radius, the latter about 400 times closer — and consider how lucky this planet is to have these special astronomical treats.

No matter where you live, you can at least enjoy the first two crescents. As for me, with a little luck on the weather, I’ll be looking at all three crescents, and the ring too. Reports to follow.

Share via:


2 Responses

  1. How many planetariums are equipped to give good simulations of transits and solar eclipses?

Leave a Reply


Buy The Book

A decay of a Higgs boson, as reconstructed by the CMS experiment at the LHC


It’s always fun and interesting when a measurement of an important quantity shows a hint of something unexpected. If yesterday’s results from DESI (the Dark

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 04/05/2024

I hope that a number of you will be able to see the total solar eclipse next Monday, April 8th. I have written about my

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 04/01/2024