Newsletter Archive

The Evening Solar System, and the Leonids

November 14, 2020

Evening Planets

You may remember Mars, Saturn, and Jupiter from last month—they were all lined up in the evening sky. You could face the sunset, imagine the sun below the horizon, and then extrapolate a curve from the sun through all three planets. By doing so, you were visualizing the solar system in the sky. The moon also spent a couple of weeks marching down the highway, although observers in Iowa (like me) were disappointed by lousy weather conditions.

Today, the 14th of November, the moon is new again. It is about to repeat its march across the evening sky, and all three planets are still there, although Mars is not as impressive as it was last month. Starting Monday evening, you may be able to find a thin crescent moon low over the sunset, below Jupiter. Over the next two weeks of evenings, the moon will leave the sunset as a crescent, and end up on the far side of the sky as a full moon, and it will pass Jupiter, Saturn, and Mars along the way. So keep your eyes up as you wander around in the evenings after dinner, and see if you can find the solar system in the sky!

The Moon and the Evening Planets

This picture should give you an idea of what to look for. It presents the view from Iowa, facing south, at around 6PM each evening. The moon will be near Saturn and Jupiter on Wednesday and Thursday of next week (the 18th and 19th), and it will be next to Mars a week after that, on Wednesday the 25th. The planets are a little blurred in the picture, because they move a small amount each day. I'm trying to find a way that I can show you the motion of the moon, without also showing the motion of the planets. But since you can see the motion, you may notice something odd about the path of Mars...It curves.

If you carefully locate planets—those "stars" that trespass within proper constellations‐you can track their progress through the constellations. Normally, Mars, Jupiter, and Saturn march steadily from the west towards the east, just like the moon...except sometimes they go backwards. They stop moving forward through the constellations, turn around, and go backwards for a little while. Then they turn around and go forwards again. Why in the world would they do this? One clue (and the only one I'm going to give you today) is that it always happens around each opposition. During every opposition, planets display "retrograde motion". You may recall that Mars was at opposition last month. If you compared it to the stars around it, you might have noticed that it was also in retrograde motionit was moving backwards compared to the starsat the same time. Well, now Mars is finishing its backwards motion, and it will soon resume its normal eastward progression through the stars. That little comma-shaped path that you can see in the picture is Mars reversing its course, ending "retrograde motion", and resuming normal "prograde motion".

In the picture, and in the sky, you might just be able to catch the Teapot of Sagittarius before it finally disappears into the sunset for the winter. The bright star above is Altair, the tip of the Summer Triangle, which is finishing its summer and fall dominance overhead, and is now also slowly sinking into the sunset.

The Leonids

Also coming up this week is the annual Leonid meteor shower. The Leonids are a fairly slow shower, as meteor showers go. If you are patient and have good observing conditions, you might be able to see a dozen meteors per hour during the peak. One author I read described the Leonids as "modest and reliable". However, the Leonids also have a higher percentage than most showers of "persistent trains" and flaming, colorful fireballs. So if you go out to watch the shower, you probably won't see many, but the ones you do see could be very impressive. The moon, being so close to new, will also not be in the sky for this year's Leonid shower, which will help improve viewing conditions.

(By the way, the Leonids are a pretty sedate meteor shower ... most of the time. On rare occasions, two or three times a century, the Leonids will throw a tantrum and light up the sky with a "meteor storm". In the historic Leonid storm of 1833, there were hundreds of thousands per hour, or thousands per minute! They came down like rain! Regrettably, such a historic mega-storm probably won't happen again in our lifetimes, and there's little chance of any Leonid storm at all for another decade or two.)

If you do want to set up a Leonid-watching station, you will want to encamp overnight on the 16th-17th. That will be the best night, but you may also see a few meteors for a day or two before or after the peak. Face east-ish, and look high into the sky. The best hours will be in the early hours of Tuesday morning, between midnight and dawn. (This assumes you live in North America. If you live elsewhere, you'll have to check your local listings for the best time.) And if you're up before dawn anyway, remember to look for Venus over the sunrise.

If you do happen to see meteors, can you tell whether they actually belong to the Leonids? Or are they just normal, everyday "sporadics"? What's the difference? Follow the meteor trail backwards to see where it came from. If you do this for several meteors during a shower, you will discover something interesting about the meteors of a shower. They are not random, but organized. If you watch for meteors on any normal, non-shower night, you might see a couple per hour, but they won't be organized. They will come from random places and head in random directions, like swarming insects. The meteors of an annual "shower" all come from the same place. They all fly outwards from the same constellation. They "radiate" from the same point in the sky. It's as if there is a cloud of debris out there in space, and the cloud is flying past us (or we are flying through the cloud), and the particles of the cloud are making streaks through space as they pass all around us. And the radiant point shows the direction the cloud is coming from (or the direction we are flying towards).

Each of the various annual meteor showers has its own special constellation that the meteors fly out of. And this gives a good way to name each shower. Why do you suppose we call the mid-November shower the "Leonids"?

The "radiant point" of the Leonids lies in Leo, the Lion, and on Tuesday this radiant will rise in the east after 11PM. Almost all of the meteors you see that morning should trace back to the mane of the Lion. This information might help you to orient yourself...but don't worry too much about keeping your eye on the radiant. The best way to see the streaks is not to watch the point they come from, but to watch them pass you, above or to the side. Also, the sky is always clearest near the "top" or zenith, and haziest near the horizon. So the best place to fix your attention is high in the sky, somewhere near the radiant.

The Radiant of the Leonids, in the Sickle of Leo

This picture shows Leo rising in the east, at roughly 2AM, as seen from Iowa, with the radiant depicted inside the "Sickle of Leo". I'll save a detailed description of Leo the constellation for another time.