Newsletter Archive

When the Sun Looks Like a Doughnut

June 1, 2021


I have good news and bad news.

Next Thursday morning, there will be a solar eclipse. And it will be an "annular" eclipse, meaning some lucky people will get to see a "ring of fire" in the sky. But this eclipse will be visible only in the far northern hemisphere, so you probably won't be one of those people.

However, if you live in the northeastern United States or in eastern Canada, you may at least get to see a spectacular rising crescent sun. If you live in Europe, you may be able to see a partial eclipse in the late morning. And no matter where you live, you can watch it on the internet. NASA is planning to host a live stream of the event.

The Second Eclipse of the Season

If you've been following along, you realize that the eclipse next week will be the second of 2021. Last week, at the time of the full moon, many of us were treated to a lunar eclipse (or at least to part of one). This week, the moon is at third-quarter. If you have been paying attention to the sky as you go about your morning business and the sun is still low in the eastern sky, you will notice the waning quarter moon high in the southern sky. Since last week, the moon has migrated eastwards towards the sun by a quarter-turn. It is not opposite to the sun anymore, but is now a quarter-turn from the sun in the sky. And next week? The moon will travel another quarter-turn, it will reach the sun, and we will have a new moon. But unlike most new moons, instead of missing the sun and passing to one side, this new moon will actually "hit" the sun, passing right in front of it and hiding part of it. Last week, instead of a normal full moon, we had a lunar eclipse. And next week, instead of a normal new moon, we will see a solar eclipse (or at least some of us will).

For some reason, the moon seems to aim really well twice a year. And when it does, it hits the earth's shadow at the time of the full moon, and it hits the sun at the time of the new moon. It gives us a lunar eclipse and a solar eclipse two weeks apart. We are currently in the middle of such a "good aim season". Last week's lunar eclipse and next week's solar eclipse are the two eclipses of the 2021 spring eclipse season.

The Ring of Fire

But why the ring of fire? Why are some solar eclipses "annular"? As you may be aware, by some cosmic coincidence the sun and the moon appear to be almost identical in size in the sky. This means that when the moon covers the sun in the sky, it's a near-perfect fit. But as you may also remember, the moon is sometimes a little closer to us, and sometimes a little farther away, meaning that it appears ever-so-slightly larger or ever-so-slightly smaller in the sky. So what does this mean when the sun and moon come together in the sky? The sun is right in the middle of the moon's range of sizes, which means that sometimes the moon can cover the sun completely (but just barely), and other times it isn't quite big enough (although it almost is).

If there is a solar eclipse at the time of a "supermoon", the moon will block the sun completely, and the world will get cold and dark for a couple of minutes (or at least a tiny part of the world will). We call that a total solar eclipse. If there is a solar eclipse at the time of a "minimoon", the moon can go right across the center of the sun, yet still fail to cover it completely, leaving an annulus of sunshine exposed around the circumference of the moon — a "ring of fire" in the sky. We call this an annular solar eclipse. Whether the moon is a "supermoon" or a "minimoon" determines whether the moon can cover the sun or not, and controls what kind of solar eclipse we will see.

An Annular Solar Eclipse

(For some reason, this ex-teacher is hearing curious children's voices in his head. They are asking: "What happens if there's an eclipse in the middle?" Well, then it's even weirder. Think about the different sides of the earth for a moment. People on the side of the earth facing the sun see it high in the sky. For them it's noon. They are also a little closer to the sun. People on the side facing away are having night, and they are also a little farther away. And people for whom it is sunrise or sunset are in between. During a "perfectly-matched" solar eclipse, the eclipse will start out as a wispy-thin annular eclipse for people who see the eclipse at sunrise, and then it will turn into a just-barely total eclipse for people who see the eclipse high in the sky at noon, because they are just a little closer to the moon, and for them the moon looks a tiny bit larger. Then it will turn back into a wispy-thin annular eclipse for people who see the eclipse at sunset, because they are just a little farther away again. We call this kind of eclipse a hybrid solar eclipse.)

During the lunar eclipse last week, the eclipse coincided almost perfectly with the moon's near point (or perigee), making it a supermoon lunar eclipse. This may have been worth a footnote, but it made virtually no difference to the appearance of the eclipse. The moon doesn't change in size that much, and the earth's shadow is far larger than the moon. You'd never know the difference between a supermoon and a minimoon unless you carefully measured the size of the moon in the sky ... or if the moon passed in front of something nearly the same size. During the solar eclipse next week, when the moon has gone halfway around the cycle, it will then be near the far point (or apogee), and this time it will be a minimoon. But this time the size will make a huge difference. We will have a minimoon solar eclipse, and the difference in size will be obvious. The minimoon will try to cover the sun but fail, leaving a ring of fire instead of a black hole.

Viewing the June 10 Solar Eclipse

In North America, the June eclipse will be an early morning event, visible only to the most northeasterly states and provinces, and only for a few minutes to a couple of hours after sunrise. The "ring of fire" will be visible only to people in a narrow band across northern Ontario and Quebec. For others in the surrounding regions, you may be able to see the moon slide across one side of the sun for a short time after sunrise (i.e. you'll see a partial solar eclipse) but you won't be able to see the ring of fire. In the United States, only New England and the Great Lakes region will get to see any of the event at all. Those in New England will get to see a dramatic sunrise: The sun will be mostly eclipsed when it rises, but then it will return to normal over roughly the next hour. Those very near to the Great Lakes may get to see a weakly eclipsed sun rise, which will very quickly return to normal.

If you live in Europe, you may be able to see a partial eclipse for awhile in late morning or around noon. The farther to the northwest you live, the more of the sun will be covered.

To see maps and to look up visibility and specific timing in your area, again, I like Time and Date. If you like to read magazines, the June Sky & Telescope issue has all the eclipse details, maps, and local timetables. You can of course also watch NASA's live stream.

By the way, here's another weird thing about solar eclipses: Sometimes they seem to want to hide at the poles. Sometimes we have periods of history when the solar eclipses are visible exclusively (and alternately) near the north and south poles. This is what is happening now. The June eclipse will be visible only in the middle and far northern hemisphere. The next solar eclipse in December will flip over to the south pole, and (with very few minor exceptions) will only be visible to the penguins in Antarctica.

The Next Eclipse Season

In case you are wondering, the second eclipse season of 2021 will happen in late fall. We'll have another lunar eclipse on November 19, and another solar eclipse on December 4. The lunar eclipse will be partial, but it will be so close to total that it might as well be total. And it will be visible in most of the United States. The solar eclipse will be total, but it will only be visible near the south pole.

I only have a few more remarks for the remainder of June: Ten days after the solar eclipse, on June 20, will be the summer solstice. On that day, the sun's arc across the sky will be the highest and longest of the year. Through June and July, the constellations in the night sky will finish their transition from their spring positions to their summer arrangements. The Summer Triangle is now climbing higher and higher in the east in the evenings. You can already see bright, beautiful pair Scorpius and Sagittarius in the southern sky if you wait until after midnight, but in another month or two they'll be comfortably visible in the evenings. The Milky Way has been more or less invisible for the last month or several, ringing the horizon and lost in the horizon haze for evening viewers. But now the eastern side of the ring is starting to rise, and in the coming evenings you will see the Milky Way arc higher and higher across the eastern sky. (If you want to look for it, remember that it runs through the Summer Triangle, right along the "neck" of Cygnus the Swan, a.k.a. the shaft of the Northern Cross.)

Here's wishing you clear skies!