The Annual Perseids

Greetings, everyone!

Welcome to another edition of my somewhat erratic skywatching newsletter. In this issue, I'd like to review what the planets have been up to since last month's spectacular lineup, and then to discuss an upcoming meteor shower.

On a personal note, I'd also like to give a brief update on what I've been working on lately. I've been trying to support myself in recent months by teaching and tutoring mathematics, and I've decided that it makes more sense for me to put astronomy on the back burner temporarily, and focus instead on creating math content. (I'm currently drafting a book on statistics.) This is a big part of the reason why the newsletter has become less frequent than it used to be. I still have many additions and improvements that I want to make to the astronomy website, but those will have to wait for a while.

The Celestial Wanderers

You may remember the majestic planetary lineup over the sunrise last month, with Mercury, Venus, Mars, Jupiter, and Saturn all lined up in a tilted arc over the sunrise, spanning just over a quarter of the sky, and allowing you to connect the dots in your imagination and to see for yourself the solar system in the sky.

These “wandering stars” have for the most part kept their relative arrangement, but their motion has stretched out the arc and spread them much farther apart in the sky. Saturn is still the farthest from the sun, and in fact it has now crept so far away that it is on the opposite side of the sky, 180° from the sun. In other words, Saturn is “at opposition” this month, which means that it will rise in the east just as the sun sets in the west, it will cross the sky throughout the night, and it will set in the west just as the sun rises in the east. (The exact date of opposition is the 14th, but Saturn moves so slowly that it will be near opposition all month long.)

The rest of the lineup (except Mercury) still fills the 180° arc between the sun and Saturn, and it will follow Saturn across the sky each night like a tail, each planet on its own schedule. Jupiter will rise a couple of hours after Saturn, in the late evening, for most of the month. Mars will rise a couple of hours after Jupiter, and Venus will rise just before the sun, making a low and dim decoration over the sunrise. The lineup will also continue to spread farther apart as the weeks go by — Jupiter will continue to creep away from the sun, reaching opposition in late September, and Venus will continue to sink towards the sunrise, passing the sun in the opposite direction in late October. (It will remain out of sight for a month or two, and then it will reappear as the beautiful Evening Star around Christmas time.)

Mercury has already passed the sun, and is now on the “sunset side” of it, so you may be able to find it in the evenings, very low and dim and a little to the left of the sunsets. However, for Northern Hemisphere viewers it is very close to the horizon, so you will need good timing and a clear view of the horizon. Your chances are much better if you live in the Southern Hemisphere. Because the ecliptic or the “planet highway” stands up at a much straighter angle there, Mercury will stand much higher over the western horizon, even though the total distance from the sun will appear the same. If you wish to hunt for Mercury over the sunsets, your best chance will be later this month. August 27th is the date of “greatest elongation”, and then Mercury will plummet rapidly into the sunset after that.

Hunting for planets is even more rewarding if they have wandered close to some of the eternal beauties on the map of stars. In Taurus there is a lovely blue cluster of stars known as the Pleiades, and a lovely red cluster of stars known as the Hyades, both large and visible to the naked eye (and spectacular in a pair of binoculars). The ecliptic (i.e. the “planet highway”) runs right in between them. Sometimes this pair is known as the “Golden Gate of the Ecliptic”, and Mars will pass right through the Golden Gate late this month. Mars will pass close by the Pleiades on the 19th (just as the moon is also passing through the Gate), and this should make for a lovely sight. Mars will pass through the middle of the “Gate” on the 27th, and it will pass by the Hyades in early September. Go out well before dawn, look towards the east, and see if you can find the two clusters of stars near Mars. (The Pleiades will be the higher blue cluster on the right, and the Hyades will be the red cluster lower and to the left.) Then look down closer to the horizon to see if you can find Venus over the sunrise.

If you've been enjoying the evenings outdoors for the last few days, you may have noticed the crescent moon climbing and waxing over the sunsets. As always, it will continue to march eastward day by day, following the path of the zodiac constellations, and passing each planet in turn. It will reach Saturn (and opposition) at the time of the full moon, on the 12th. On the 15th, a waning gibbous moon will pass Jupiter, and on the 20th, the moon will pass Mars on its way through the Golden Gate. On the 25th, we will be treated to an especially lovely sunrise as the fading crescent moon passes Venus over the sunrise. (As usual, the moon may be behind or ahead of the position I've described on the date that I've given, depending on your longitude. I've described the conjunctions as visible in the Western Hemisphere as best I can.)

Before we leave summer behind, you may want to take advantage of your relatively brief opportunity to catch Scorpius and Sagittarius for the year. Saturn is currently sitting on the eastern horn of Capricorn, and to the right of Capricorn are Sagittarius and Scorpius — a particularly bright and pretty pairing of constellations, although somewhat elusive for viewers in the Northern Hemisphere. I suggest trying to find them at least once before they are gone for the year. If you have dark skies, and you can find the “teapot” of Sagittarius, try looking for the “steam” coming from the spout of the teapot. That's the “Milky Way Core” — perhaps the brightest and loveliest section of the Milky Way. (If that milky ring in the sky is actually a galaxy of stars, seen from the inside, then what do you suppose the extra-bright “Core” in Sagittarius represents?)

The Perseids

If you are an avid stargazer, you may be lucky enough to see, every once in a while, a “star” shooting across the sky instead of standing still. Often these “shooting stars” merely look like dim stars moving very fast, but sometimes they leave glowing colorful trails that last for a moment and then vanish. On rare occasions, they even explode. People used to think that these were phenomena of the atmosphere, like clouds and rainbows (and in a way they really are atmospheric phenomena), so they were named “meteors” and fell under the heading of “meteorology”, the study of the atmosphere.

On a typical night, you may see perhaps one random meteor per hour. If you are a very avid stargazer (or if you pay attention to the news), you may have noticed that sometimes, at the same time every year, we see a faster-than-average flurry of meteors. We call these annual events “meteor showers.”

Some showers are weak and their meteors are almost as rare as the “sporadic” meteors of any ordinary night. Some showers are strong, and you may be lucky enough to see 100 in an hour. But all meteor showers have something rather remarkable in common. Everyday “sporadic” meteors fly in random directions, but the meteors of a shower are organized. If you trace their paths backwards, you will see that they all appear to fly away from the same point in space. Each of the annual showers has its own special place in the heavens from which its meteors appear to “radiate”. We call this point the shower's “radiant”, and we name each shower after the constellation in which its radiant lies. Can you guess why the upcoming meteor shower is called the “Perseids”?

I recently discovered the work of Petr Horalek, a Czech photographer, who devoted 8 years to creating his “meteor shower collection” of composite photos. I have taken the liberty of reproducing a low-resolution thumbnail here (of the Perseid shower), and you can see the rest of his impressive collection on his website.

Can you think of a reason why the meteors would all seem to fly from the same point in space, like railroad rails appearing to spread apart from a common point on the horizon, or like the star streaks that they draw in the movies whenever a spaceship “goes to warp speed”? It's as if we were flying through a cloud in space and watching the debris in the cloud streak past us. If this is so, then we have showers whenever we pass through a cloud in space, the meteors are bits of cloud debris streaking past us as we fly through the cloud, and the radiant is that point in space towards which we are currently moving.

There are several reasons why the Perseids are considered one of the best meteor showers of the year. This August shower tends to have more meteors per hour than most other showers, it tends to have more fireballs than most other showers, and it occurs in summertime (in the Northern Hemisphere), when the nights are much more comfortable than they are for most other showers. Unlike other showers, the “peak” of activity also lasts for several days, so if you miss it one night, you can try the next. The entire shower lasts almost a month, and has actually started already, although rates won't really be dramatic until the peak.

Unfortunately, this year there will be one big flaw: The full moon coincides perfectly with the peak, meaning that all of the dimmer meteors, and probably most of the fireballs or flaming tails, will be drowned out by moonlight, in the same way that sunlight hides the stars during the daytime. The “drowning out” will be made even worse if you live in a city. There should still be plenty of bright meteors that shine through, but on the whole, the viewing probably won't be nearly as good this year as it was last year.

You may prefer to start your viewing now, before the moon becomes full, for this reason. You won't see nearly as many as you would nearer to the date of peak activity, but the moon is not yet full and it is still missing from the pre-dawn sky. So you still have a window of opportunity in which the Perseid shower will sparkle in a dark sky. You could also try waiting until after the full moon, but then the dark moonless skies will be just after sunset rather than just before sunrise. The rates normally improve as the night goes on, and are highest before dawn, so the meteors tend to be much less abundant in the evenings.

If you do hunt for Perseid meteors, you may be tempted to look towards the radiant, which will rise in the late evening in the northeast, and climb higher as the night goes on. However, the oncoming meteors are still far away and outside the atmosphere when they are near the radiant, and they don't become bright until they are some distance away from the radiant (maybe 30-40 degrees?) in your perspective. So just try to take in as much of the sky as you can, focusing your attention on the overhead sky, where the air is clearer, and on the sky farthest from the moon if the moon is up.

If you live in the Southern Hemisphere, you may just want to give up on this one. The Perseids are predominantly a Northern Hemisphere meteor shower, and you may be able to see a few meteors rising up from your northern horizon, but not before midnight. On the other hand, there are a couple of weak but long-lasting and overlapping Southern Hemisphere showers going on this month, so you may want to just try your luck sky-gazing on the next pleasant evening.

Happy Viewing!

John