Newsletter Archive

The Size and Shape of the Earth

May 08, 2022


Hello, Eclipse-Watchers!

Today the moon reaches first-quarter phase, which means that it will be high in the southern sky at sunset this evening. (If you live in the Southern Hemisphere, it will be high in the northern sky at sunset, and if you live near the Equator, in will be somewhere near the top of the sky near sunset.) If you have clear skies this evening, you can enjoy the beautiful sunset in the west, then turn a quarter-turn to your left (or right), and notice the D-shaped moon facing the sun and lying a quarter-turn away from the sun in the sky. If you continue to watch the moon each evening this week, it will gradually creep farther from the sunset, waxing as it goes, making larger and larger obtuse angles in the sky. And finally, a week from today, it will become a full moon, rising in the east just as the sun sets in the west.

This sequence of phases occurs every month, but this month something special will happen at the exact moment of the full moon. When the moon reaches "opposition", forming a straight line with the sun and moon on each side and you in the middle, a dark shadow will pass across the moon. You will be able to tell that it is a shadow and not something dark passing between you and the moon and blocking your view of it, because you will still be able to see the moon. It will just appear much dimmer, as things in shadows do. (It will also become colorful, but that's another story.) And given that we only see this shadow when the moon and the sun are exactly on opposite sides of us, it isn't too difficult to figure out what is casting the shadow.

A lunar eclipse gives us the special opportunity to see the size and shape of the Earth with our own eyes, and this is what will happen next Sunday, May 15th.

Who, Where, When

Viewed from the ground, the moon appears to circle around us from east to west, once a day, just like everything else in outer space. It will continue to do this as the lunar eclipse progresses, tracing out an arc across the sky during the 3½ hours of the eclipse event. For the first hour or so of the event, the moon will be partially eclipsed while the shadow overtakes the moon from behind. For the middle hour or so, the moon will be totally eclipsed while the moon lies entirely within the Earth's larger shadow. For the last hour or so, the moon will again be partially eclipsed as the shadow pulls ahead of the moon in their race across the sky.

However, which part of the circle the moon lies in while it is being eclipsed will depend on where you live. The eclipse might occur before midnight, as the moon rises in the east, or it might occur after midnight, as the moon sets in the west, or it might occur during the daytime, while the sun is up and the moon is down, in which case you won't be able to see it.

For most of North America, this will be a "before midnight eclipse", with the moon rising in the east after the sun sets in the west, on the evening of Sunday, May 15th. If you live on the East Coast, the eclipse will begin shortly after sunset, and will trace out a trail across the sky resembling that of this "before midnight eclipse" seen from Athens in 2001:

Eclipse Over Poseidon (copyright 2003, Fred Espenak)

(This composite photo is copyrighted, but I think I can include a thumbnail here. To view the original, and many more beautiful eclipse photos, visit Mr. Espenak's website. Also note that the eclipsed moon is much dimmer than the full moon, even though it doesn't appear so in the photo. To capture both of them in the same composite photograph, a photographer needs to drastically change the exposure settings during the intermediate phase of totality, which makes the colorful eclipsed moon appear much brighter in the photo than it otherwise would have been.)

If you live near the Mississippi River, the upcoming eclipse will begin near the time of sunset and moonrise, so you can still see the entire event, but you will need a clear view of the eastern horizon to see the beginning. If you live in the western United States, the eclipse will begin before the moon rises, so you will miss the beginning of the event, but then the moon will rise and the sun will set and you will be able to watch the rest of the eclipse as the moon rises low over the eastern horizon. The farther to the northwest you live, the less you will be able to see. For those living in Seattle, the moon will rise about a half-hour before the middle point or "maximum eclipse", so you will see a little more than half of the event.

If you live in Central America, your view will be much the same as for the East Coast of America, with the entire eclipse visible in the eastern sky, beginning an hour or two after sunset. If you live in South America, you will see the entire event high in the northern or overhead sky, in the final hours of Sunday night and the first hours of Monday morning.

Those of you in Europe or Africa may be able to catch the beginning of the eclipse, but you are in the opposite situation to Northwesterners in America. Parisians and Londoners will see the eclipse begin as the moon descends in the western sky in the pre-dawn hours of Monday, May 16. You will be able to see about half of the eclipse, but then the moon will set and the sun will rise and you will miss the second half. The farther to the northeast you are, the less you will see, and the farther to the southwest you are, the more you will be able to see.

To look up detailed times for your location, I like the Time and Date website. Note that they also give times for the "penumbral" portions of the eclipse, which are interesting mainly to those who want to challenge their observation skills. Just before and just after the "partial" phases of the eclipse, the moon fades slightly on the side facing the Earth's shadow, and you may find it fun to see if you can spot this fading.

What to Look For

I recommend trying to view the eclipse at least twice, once during a partial phase, and once during totality.

During the partial phase, pay attention to the edge of the shadow. Try to extrapolate it into a full circle in your imagination. This circle is the Earth's shadow, and it reveals the size and shape of the Earth to you directly. The Earth is a sphere, a few times larger than the moon, and you can see this for yourself during a partial lunar eclipse.

(You might argue that the Earth could be a disk, and still cast a round shadow. True, but it couldn't cast a round shadow in different directions. You can observe different lunar eclipses in different parts of the same sky, and the edge of the shadow is always circular. There is only one shape that can cast a round shadow in every direction, and that is a sphere. You could also argue that the Earth's shadow and the Earth itself aren't necessarily the same size. This is also true. You can estimate with your eye that the shadow is roughly three times as large as the moon, but the Earth itself is approximately four times as large as the moon. There is a way to figure this out by using observations of solar eclipses, but I won't go into that here.)

During the total phase, enjoy the colors, and ask yourself why they are there. The clue is in which colors are there. The colors of a totally eclipsed moon vary in a messy way from eclipse to eclipse, but they are always in the red-orange-yellow portion of the color spectrum. Can you think of another colorful natural phenomenon whose colors vary in an irregular way, but are always in the red-orange-yellow area of the color wheel? During a total eclipse, the moon becomes sunset-colored for some reason. And you can see the reason if you imagine what an observer on the moon would see if he looked back towards the Earth during a lunar eclipse. He would see the sun behind the circular horizon of the Earth. Or rather, he would see the dark circle of the Earth surrounded by the colorful glow of the atmosphere. During a total lunar eclipse, the moon is not in direct sunshine, but it is still being shined upon by all of the Earth's sunrises and sunsets at the same time. (On rare occasions, when the atmosphere is exceptionally clear, the eclipsed moon can be a light milky blue color, the color of a summer sky just above the horizon. On the other hand, after the 1991 eruption of Mt. Pinatubo, the next eclipsed moon or two were almost invisible, the color of dark gray or brown storm clouds.)

When is the Next One?

Lunar eclipses tend to occur in "tetrads." We often have four impressive total lunar eclipses in a row, one every six months, followed by a couple of years of dull and unimpressive partial or penumbral eclipses. We are currently in the middle of an "almost tetrad." There was a total lunar eclipse a year ago, in May of 2021. There was also a lunar eclipse six months ago, in November 2021, but that one was only a partial eclipse, and the moon never became totally eclipsed (although it came very close). We will have another total lunar eclipse six months from now, in November 2022, rounding out the group of four. If the previous eclipse had been just a little deeper, it would have been classified as total, and we would be exeriencing a true tetrad.

In any case, if you miss this lunar eclipse, you will have another chance to see one this coming November. But if you miss that one, then you will have to wait until 2025 for the next total lunar eclipse.

That's all for now. Let me know if you have any questions, and happy viewing!

John Krieger