Newsletter Archive

The End of an “Almost Tetrad”

Oct 27, 2022


Hi Everyone!

Two days ago (Tuesday) the moon became “new,” and began a fresh cycle of phases. This means that if you go outdoors tonight to enjoy the beauty of the sunset, you may discover an extra decoration hanging in the sky just over the sun — the beautiful thin sliver of a waxing crescent moon. If you go outdoors each evening after that, you will notice the moon growing each day, and climbing a little higher and a little farther from the sun in the sky. In about two weeks, the moon will become full as it reaches the far side of the sky, 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 to the full moon. When the moon reaches the exact point of “opposition,” a dark shadow will pass across the moon. You will be able to tell that it is a shadow and not a large solid object blocking your view, because you will still be able to see the face of moon. It will just appear much dimmer, as things in shadows do. (The moon will also become colorful, for some curious reason.) 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 on November 8th. (More precisely, it will happen before dawn on the 8th for the Americas, and after sunset on the 8th for Asia.)

The Schedule of Events

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 westward race across the sky.

However, depending on where you live, this sequence could occur anywhere along the circular “racetrack” around you. It might occur before midnight as the moon ascends in the east, or it might occur after midnight as the moon descends 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. Where will the eclipse fall on your local “sky-circle”? That depends on your position around the world, or more precisely your longitude.

For viewers in North America, this will be an “after midnight eclipse”, occurring in the early hours before dawn, as the moon descends in the western sky and the sun prepares to come up in the east. Residents of the West Coast will be able to see the entire event, running from roughly 1 am to 5 am (not counting the invisible penumbral phases). Residents of the East Coast will be able to see the first half of the event, but then the moon will set and the sun will rise, and the second half of the event will occur in daylight with the moon below the western horizon. (To give some specifics: In New York City, the partial phase will begin at about 4 am, the total eclipse will begin at approximately 5:15 am, and the deepest point or “maximum eclipse” will occur at about 6 am with the moon close to the horizon. Then the moon will set at about 6:40 am.)

Viewers in Central America will see the first half of the event, on a similar schedule to New York. Viewers in South America won't see much of the eclipse, although they may be able to see the initial hour or so. At the eastern extremity of Cape Branco, the eclipse will begin just after moonset and the entire event will occur below the horizon. Viewers to the west may catch a glimpse of the initial stages just before the moon sets, and the farther west you are, the more of the event you will be able to see.

Most of Asia and Australia will miss the beginning of the eclipse, but will be able to see the end. Japan and New Zealand will see the whole thing, as a “before midnight eclipse” in the eastern skies. China and Australia will see the moon rise while it is already eclipsed and will be able to see roughly the second half of the event, and India will see only the final minutes. For residents of Europe and Africa this will be a “daytime lunar eclipse”, meaning the moon will be below the horizon the entire time, although residents of Scandinavia living north of the Arctic Circle may catch a glimpse of the final stages just after the moon rises.

To look up detailed times for your location, I recommend the Time and Date website. Note that they also give times for the “penumbral” portions of the eclipse, which might be interesting if you want to challenge your observation skills. Just before and just after the “partial” phases of the eclipse, the moon is very near the Earth's shadow but not touching it (i.e. the moon is outside the Earth's “umbra” but inside the “penumbra”). At these times, the moon fades slightly on the side facing the Earth's shadow (or umbra), as if someone turned down a dimmer switch slightly on that side, and you may find it fun to see if you can spot this fading.

What to Look For

Assuming you aren't going to have an all-night “eclipse party,” I recommend going outdoors to view a total lunar 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 it turns out that the Earth itself is approximately four times as large as the moon, comparing diameter to diameter.)

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 rocky 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.)

If you watch this eclipse with your kids, make sure to point out these things to them. Use your imagination to find where the sun went after it set, or where it is coming from before it rises, and point to it with one arm. Point to the moon with the other arm. Your arms should form a straight line (or close to it). The moon is up in the sky, and the sun is opposite to it, below the horizon somewhere. This is the main clue that the dark thing is the shadow of the Earth, cast by the sun onto the moon. If you are watching the partial phase before or after the total phase, be sure to notice the edge of the shadow, use your imagination to fill in the rest of the circle, and notice the size and shape of that round shadow.

After the Tetrad: Eclipses in 2023 and 2024

The November 2022 lunar eclipse marks the close of the second “eclipse season” of 2022, and also the finale of an “almost tetrad”.

If we pay attention to lunar eclipses for many years, we will often see four impressive lunar eclipses in a row, one every six months, followed by two more years in which we see only feeble partial or penumbral lunar eclipses. Whenever four total lunar eclipses occur in a row, astronomers call that a “tetrad.” Six months ago, in May of 2022, and a year before that, in May of 2021, we saw a total lunar eclipse. One year ago, in November 2021, we saw a partial eclipse, in which the moon never became totally eclipsed. It came very close, however. If that eclipse had been just a little deeper, it would have been classified as total, and we would be experiencing a true tetrad. So I am choosing to call the past two years an “almost tetrad.”

After the November 2022 eclipse, we will have to wait until 2025 for the next total lunar eclipse (at which point we will start another “almost tetrad”).

However, nature provides its own compensation. Whenever we are in the middle of a lunar “tetrad” (or “almost tetrad”), the solar eclipses all seem to happen near the poles, for some reason. This is part of the reason I have said very little in this newsletter about solar eclipses for the past couple of years. But when the impressive lunar eclipses are in abatement, the solar eclipses turn up the fireworks, so to speak. In 2023 and 2024 we will have only penumbral or feeble partial lunar eclipses, but we will have four total (or annular) solar eclipses visible somewhere in the inhabited world, including two that will cross North America. I'm especially looking forward to the annular solar eclipse that will cross the southwest United States a year from now, in October 2023.

For more details regarding eclipses in 2023 and eclipses in 2024, you can click on the respective links, which will take you to informative lists on the Time and Date website.

The Leonids

Before I sign off, I want to make a few comments about the Leonids meteor shower, which will be active through most of November, and will peak on the night of the 17th-18th. The performance is expected to be modest this year, perhaps a dozen meteors per hour, except that at least one astronomer says that there might be a brief but very impressive flurry on the following day, the morning of the 19th. According to the Sky & Telescope website: “…more than one meteor dynamicist predicts that this year’s Leonid display will be enhanced by a pulse of particles that were ejected by the comet in 1733 — nearly three centuries ago. The predicted arrival is 6:00 to 6:30 UT on November 19th, which is excellent timing for the Americas.” One scientist is predicting that rates over 200 meteors per hour are a possibility.

If I have done my conversions correctly, the given time corresponds to 2:00-2:30 am on the 19th for the East Coast of the United States, and 11:00-11:30 pm on the 18th for the West Coast. This is nearly the optimum time of night for viewing the Leonids anyway. Meteor viewing is often worst in the evenings and improves in the hours leading up to dawn, except that on this day the moon (10 days after the eclipse) will be just after third quarter. This means that it will rise a bit after midnight, and will continue to climb as the early morning hours go on, and the brightness of the moon will tend to drown out the dimmer meteors. So the optimum time for this year's Leonids will be the middle of the night.

For more information, you can try Time and Date, or the American Meteor Society.

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

John Krieger