Newsletter Archive

The Eclipse Season

May 19, 2021


Welcome to the Spring Eclipse Season!

Any time a lunar eclipse happens, only half of the world gets to see it. And more than a third of lunar eclipses are "penumbral", meaning nothing happens (unless you are paying really, really close attention). Much less than half of the world gets to see any given solar eclipse. So for any given location, dramatic eclipses are fairly rare. However, if you make a list of all kinds of eclipses visible anywhere in the world, you discover that they happen about twice a year ... in pairs. For some reason, whenever there is a lunar eclipse, there is always a solar eclipse two weeks later or two weeks earlier.

In the early 2020s, there will be one pair of solar-lunar eclipses every spring, and another pair every fall. There are two "eclipse seasons" every year, and we are now entering 2021's first eclipse season.

Let me get a few non-eclipse comments out of the way first, and then I'll discuss the first of the two upcoming eclipses. I'll save the second eclipse of the season (a solar eclipse on June 10) for the next issue.

Stars and Planets

The stars and planets haven't changed much since last time. Mars, Mercury, and Venus are still outlined by the Arch of Spring over the sunset. Mars is still pretty dim, and the other two are still very low. Mercury's "greatest elongation" was the 16th, and it will only get lower and dimmer from here on out.

If you have been observing the evening skies for the last several days, you probably enjoyed the lovely crescent moon rising through the sunset scene. Tonight is the night the moon graduates to First Quarter, and it will be high in the south, right above Regulus at the time of sunset. (Regulus, if you recall, is the period at the bottom of the backwards question mark, or the Sickle of Leo.) In another week, the moon will reach the opposite side of the sky from the sun, it will become full ... and it will pass right through Earth's shadow.

Jupiter and Saturn continue to rise ever higher over the sunrise. If you get up before dawn to see them, turn and see if you can catch Scorpius and Sagittarius setting in the southwest. That's where they'll be in the evenings four or five months from now.

For anyone curious about what's been happening with the new novas: As nearly as I can tell they've all fizzled. (Extra Astronomer Points to anyone who spotted the tautology.) I've been unable to find any more information about the two that appeared in Scorpius and Sagittarius, and I'm pretty sure those have vanished into the blackness from which they came. Nobody lacking a powerful telescope ever saw them. The nova in Cassiopeia held steady for awhile, and then flared up briefly in early May and became visible to the naked eye. But it was only barely visible to the naked eye, and it only stayed that way for a few days. As of yesterday, it was fading fast, and sinking back towards obscurity. This Sky & Telescope page has some maps and photos, if you want to try to find it.

The May 26 Lunar Eclipse

The two lunar eclipses of 2021 will be a little unusual. A lunar eclipse is classified as "total" if the moon goes completely into the earth's shadow for awhile, and it is "partial" if the moon goes partly into the shadow but then comes back out again before being completely darkened. When deciding what kind of eclipses to have, the year 2021 seems to have been unable to make up its mind. Next fall's November eclipse is officially a "partial eclipse", but the moon will go so far into the Earth's shadow that it might as well be total. If you measure by the moon's diameter, it will go 97% of the way into shadow. The upcoming eclipse next week is officially "total" ... but only barely. The moon will scarcely leave the inner edge of the earth's shadow before it turns to go back out again. If you measure the gap between the edge of the moon and the edge of the shadow, the gap will never be more than 1% of the moon's diameter, and the moon will only stay in complete shade for 14 minutes. (In a more typical total eclipse, the moon goes deeper into the Earth's shadow and stays inside for over an hour.) Perhaps we could call the two lunar eclipses of 2021 "borderline" eclipses.

So next Wednesday, the 26th, before dawn (in the U.S.), there will be a total lunar eclipse. But the moon might not become as dark or deeply colored as you would like, and it won't stay dark as long as it usually does.

Where and When

Most people in North America will be able to see the first part of the eclipse, but then the moon will set and the eclipse will finish with the moon below the western horizon. Look for the eclipse low in the western sky before dawn, as the moon is about to set. The farther to the northeast you live, the less of the eclipse you will be able to see. In New York City, the moon will set (and the sun will rise) about 10 minutes before the eclipse begins, so New Yorkers won't be able to see any of the event. The farther to the southwest you live, the more you will be able to see. In Los Angeles, the eclipse will end at the same time the moon sets, so with a clear view of the western horizon, you will be able to see all of it. (If you're an Easterner and you're jealous of the Westerners, just wait. There will be another lunar eclipse a year from now in which the situation is reversed.)

If you live in Europe, unfortunately you're out of luck. The eclipse will occur during daytime for you, while the sun is up and the moon is down. If you live in Australia, you're in luck. Australia is probably the best-positioned country for this eclipse. If you live in western Australia, you might miss the initial (and probably uninteresting) penumbral stage, but other than that, you will be able to see the entire thing, in the evening after the sun goes down, as the moon rises in your eastern sky.

To look up visibility and timing for your specific city, I like Time and Date's Event Page. If the eclipse is not visible in your area, you could try looking for a live feed on the internet. The Virtual Telescope Project is planning to host one.

Things to do during the eclipse

As with any lunar eclipse, on May 26th (weather permitting) you will have the special opportunity to see with your own eyes the size and shape of the ground you live on. All you have to do is observe the curved edge of the shadow on the moon, fill in the rest of the circle in your imagination, and you will be able to see that the earth is round, and a few times larger than the moon.

Another thing you might enjoy is to try to rate the brightness and the color of the moon during the eclipse. For this, scientists use the "Danjon Scale", presented below. For the upcoming eclipse, you can ignore all comments about the center of the earth's shadow — since the moon will not go deep into the earth's shadow this time, we will only get to see the colors near the edge of the shadow.

The Danjon Scale
0Very Dark EclipseThe moon is almost invisible. It is hard to find and you can't recognize the face.
1 Dark Eclipse The moon is dark grayish or dark brownish, like an ominous storm cloud. You may be able to recognize the face, but it is not easy.
2 Deep Red Eclipse The moon is deep red or rust-colored. The central shadow is very dark, but the outer edges of the shadow are brighter.
3 Brick Red Eclipse The shadow is red in the middle, with a bright yellow rim.
4 Bright Eclipse The middle of the shadow has a bright copper-red or orange color, and the outer edge has a very bright whitish-blue color, like the color of a summer sky just above the horizon.

If you are a parent or a teacher and you are interested in a more complete presentation of how to observe and think about a lunar eclipse, allow me to advertise my first teacher's manual! It only covers lunar eclipses, but it's pretty extensive and authoritative, if I do say so myself. For a limited time I'll make it available for free at this unpublished link: Thinking About Lunar Eclipses. If you'd like to do me a favor, I'd really appreciate any feedback or publicity you could provide, especially if you're a teacher. If you'd like to do me a huge favor, you could Buy Thinking About Lunar Eclipses on TeachersPayTeachers, and leave me a review. I really need reviews — not only for advertising and "social proof", but also for feedback from people trying to use my stuff in real classrooms. (I'd offer you coupons to buy it at a discount, but durnit if TpT doesn't let me do that. I'll keep it on sale for a few days instead.)

Thanks, and here's hoping for good weather!