Newsletter Archive

Summer Evening Stargazing

June 18, 2021


Did anyone have a chance to see either of the two recent eclipses? I spent one lovely pre-dawn hour in May watching the lunar eclipse. Or at least I watched the beginning of the eclipse. Where I live, the sun rose and the moon set just before the full eclipse began. But I did manage to take a nice picture of a partially-eclipsed setting moon inside the "Belt of Venus":

An eclipsed moon, setting in the west, inside the Belt of Venus
An Eclipsed Moon Inside the Belt of Venus

The next time you are enjoying the beauty of a sunrise or sunset, try turning around and facing in the opposite direction for a moment. If the sun is a little bit below the horizon, then when you turn around you might notice an arc of pinkish sunrise-colored air over the horizon opposite to the sun, with a wedge of shady atmosphere below it. If it is evening, this wedge will rise up from the eastern horizon as the sun descends below the western horizon. If it is morning, this arch will descend down the western sky as the sun approaches the eastern horizon from below, and it will join the western horizon at the moment the sun rises. Venus often appears opposite to this arch, over the sunrise or sunset, as if the arch was trying to wrap itself around the horizon to reach Venus on the other side. This arch is poetically called "Venus's Girdle" or the "Belt of Venus". (A more technical name is the "anti-twilight arch".) Can you figure out what causes it?

When I took the photo above, I was facing west, with the sun just about to rise behind me in the east. The sun was casting the earth's shadow out in front of me. What you see in the picture is the shadow of the earth striking the distant moon, and shading the earth's own atmosphere, at the same time. The rays of pink and orange sunlight around the edge of the shadow are the same rays that turn the moon colorful during total eclipses.

The solar eclipse on June 10th was not visible where I live, and I have to confess that I made no effort to watch a live feed on the internet. (Part of me wishes I had. The other part thinks that if you are going to watch an internet feed, you might as well just watch a recording after the event. Both parts of me would welcome additional opinions on the matter.) This solar eclipse marked the end of the 2021 spring eclipse season, but there will be a second eclipse season later this year, beginning with another lunar eclipse in November.

Summer Evening Stargazing

The next big event of the summer will be an impressive meteor shower in August. In between now and then we have some pretty stars and planets to study in the summer evening skies.

If you go out stargazing after dark in the near future, try to find the "Arcturus Arc" marking out the western sky. Assuming you are one of my Northern Hemisphere readers, start by finding the Big Dipper in the northwest, then follow the arc of the Dipper's handle towards the left, or towards the south. It will lead you to an especially bright orangish star. That's Arcturus, the brightest star in the constellation Boötes. If you extend the curve through Arcturus and keep going, you will come to another bright star in the southwest. That's Spica, the brightest star in Virgo. Starting from the Big Dipper, you "arc to Arcturus, then speed on to Spica". (Officially, "Spica" is supposed to rhyme with mica, and I've also heard it said that you should "spike to Spica". But somehow I just don't like that expression as well, even though it makes the rhyme better. So I still speed on to Spica.) Both Arcturus and Spica are hard to recognize by their constellations, because all the other stars in Boötes and Virgo are pretty dim. But if you can find the Big Dipper, you can recognize Arcturus and Spica easily, by following the arc of the handle. If you keep going after Spica, you can hook around a little and you will end up at a small kite-shaped quadrilateral of stars low in the southwest. That's Corvus, the Crow. (I guess after you "speed on to Spica" you can "continue to Corvus"?) On June evenings, Leo slides down the western sky following the sun, and the "Arcturus Arc" makes a grand arch across the western sky, over Leo's behind.

(For Southern Hemisphere residents, the Arcturus Arc will be rotated into the northern sky, burying the Big Dipper, and lifting the second half of the arch high into the northern sky. In your evenings, the Big Dipper will probably lie below your northeastern horizon, with Arcturus and Spica above the northeastern horizon, and Corvus high in the north. Perhaps you should start with Corvus, flying high across the northern sky, and then follow the arc in reverse.)

What about the planets? You may remember that Venus gradually sank into the sunrise last fall, and vanished completely for a few months over the winter. Then it reappeared over the sunset … but very faint and very low. Finally Venus is starting to shine bright enough and climb high enough over the sunset to blaze as the beautiful "Evening Star" again, although it is still pretty low and you still need an uncluttered horizon to see it. The Evening Star is especially beautiful when the crescent moon joins the scene, which it did over the past week. If you've been watching the sunsets, maybe you observed that since eclipsing the sun, the moon has been growing and climbing ever higher over the sunset, and it now stands at first-quarter, a quarter-turn from the sun. It will circle around and decorate the sunset scene again a month from now, and then again a month after that, and Venus will be higher and brighter every time. We have some gorgeous sunsets ahead of us.

You may also remember that Mars dominated the evening skies last October when it was at opposition. It was in the "full moon position" opposite to the sun, where it is especially bright, and up all night long. After leaving opposition, Mars has been creeping gradually closer and closer towards the sun. It passed "quadrature" in February, meaning it was in the "first-quarter moon" position, a quarter-turn away from the sun, and high in the south at sunset. Quadrature is the halfway point between being opposite to the sun, and being next to it. Mars will reach the sun in September, almost a year after it was at opposition, and at that point it will finally vanish into the sunset.

Notice that Venus is gradually creeping out of the sunset, and Mars is gradually creeping towards it. What's going to happen? These two wanderers are traveling in opposite directions along the "planet highway" (i.e. the zodiac), and they will eventually pass in the night. Technically we say that there will be a conjunction between Mars and Venus, and this will happen on July 13th. In the coming weeks, you can watch these two travelers approach each other over the sunsets, and then pass each other in the middle of July. (You might also notice that the 13th of July is a little less than a month from now, meaning that the moon will have completed a loop around the racetrack, and it will be back over the sunsets as well. From the 11th to the 13th of July, a lovely crescent moon will join the two planets over the sunset.) So keep your eyes on the sunsets: Leo is tilted forwards, sliding into the sunset, and both planets and sometimes the moon will be in front of him.

By the way, there's an especially bright "fuzzy spot" in the sky, about halfway between Leo and Gemini. It is the most interesting thing in the constellation Cancer, which is otherwise virtually invisible. (I think of Cancer as "the dark spot between Leo and Gemini".) It is possible to see the fuzzy "Beehive Cluster" without a telescope, but you do need fairly dark, clear skies. Mars and Venus will both pass through the Beehive Cluster in late June and early July, and this could make for a lovely sight in binoculars. It will be quite low over the horizon after sunset, and I think that the haze of the horizon and the glow of the sunset will probably make it impossible to see by eye. But if you own a pair of binoculars, you may want to just explore the sky around Mars next week, between June 22-25, and explore the area around Venus just before Independence Day, on July 2-4. If you can catch the planets inside the Beehive Cluster, it could make for a lovely binocular sight. If you want to search, I suggest starting with Leo. He'll be tilted and sliding down the sky in the west after sunset. Mars will be the dim red dot, low over the horizon and in front of Leo. If you have a clear horizon, Venus will be hard to mistake — it will be the brightest "star" in the sky, hovering (very low) over the western horizon about an hour after sunset.

The June Sunset Scene
(Northern Hemisphere, late June, about an hour after sunset)

After enjoying the beauty of the sunset, if you turn around and face east, you will see the rising of the Summer Triangle — three of the brightest stars in the sky (Deneb, Altair, and especially bright Vega) in a roughly south-pointing triangle. Low in the southeast, you can see the "Capital 'T'", the front half of Scorpius starting to climb out of the horizon, with bright red Antares in the middle. If you wait until after midnight, the sky will foreshadow its evening appearance in a few months. The beautiful pair of Scorpius and Sagittarius will be crawling across the southern sky, and the Summer Triangle will have risen to dominate the overhead sky.

Welcome to Summer!

One quick final note: Tomorrow, the 19th of June, will be the day of the 2021 summer solstice. On that day, the sun will be at its most northerly station for the year. For those of us in the northern hemisphere, on the June solstice the sun's arc across the sky will be the highest and longest of the year, giving us more minutes of more intense sunshine than any other day. (If you live in the Southern Hemisphere, the sun's arc will be the lowest and shortest of the year, tomorrow will be the shortest day of the year, and you will probably call it the winter solstice. If you live near the equator, tomorrow will be a day like any other.) The solstice is the day of the exact mathematical extreme, but to be fair, the sun moves very slowly near the solstices, and all days within a week or two of the solstice will be pretty much identical, sunshine-wise.

Incidentally, have you ever noticed that the warmest hours of the day are typically not around noon, when the sun is highest and the sunshine is the most intense, but a few hours later? Apparently the world takes a little while to heat up and cool down, and the warmth of the world lags a little bit behind the pulses of sunshine. The same thing applies to the seasons. Have you ever noticed that the hottest weeks of the year are typically not around the summer solstice, when we receive the most hours of the highest sunshine, but a month or two later? Similarly, the coldest weeks of the year are typically some time after the winter solstice. This is why, when we pick the official start and end dates for the seasons on the calendar, we use the solstices and equinoxes to mark the beginnings of their respective seasons, instead of the middle. We don't use the summer solstice to mark the middle of summer, but the start of summer. Sometimes, to clearly distinguish what the weather is doing from what the sun is doing, scientists call these invervals between the solstices and equnioxes the "meteorological seasons". Tomorrow, the 19th of June, is the first day of (meteorological) summer.

Cheers!

John