August events – Get ready for the 2010 Perseid meteors – a special year

• Perseids
• Planets
• Calendar  (summary)

Arrows represent Perseid meteors - which can appear anywhere int he sky and will be of different lengths and brightness, but will appear to radiate from a point in the northeast in the constellation Perseus. Click chart for larger version. (Prepared from Starry Nights Pro screen shot.)

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The Perseids should be really good this year because the Moon won’t interfere when they are near their peak during the morning hours of August 12th and 13th. Here’s a quick guide.

When:

The night of  August 12-13 starting about 90 minutes after sunset, but best after midnight. And if that night is likely to be cloudy, the preceding night of August 11-12 might prove to be just as good, but the best chance looks like the 12-13.

Where:

Any place you have a clear and dark sky – the more horizon visible the better, but in truth you can only look in one area at a time, so a clear, dark sky to the northeast is best. While a Perseid meteor can appear anywhere in the sky, your best chance to see  several will be to scan the sky to the northeast in the general vicinity of the “W” of Cassiopeia.  However,  you don’t have to fixate on one region. Get comfortable, look high in the northeast, and from time to time look around to different sections of the sky to enjoy the sights and stay alert.

What can you expect to see?

Under the best conditions at the peak of the shower, you can expect to see between one and two meteors a minute! But I never seem to achieve those best conditions, so I don’t raise my hopes too high. I’m just sure I’ll see many more meteors than normal, but fewer than I would in a year when the Perseids are at their very best.  To put numbers to it, I’d be delighted if I averaged one every five minutes. For everyone, everywhere, the intensity of the annual Perseid “meteor shower” is in a downswing, but because we’ll have no interference from the Moon, this should be a better than average year.

Special bonus

Go out early – half an hour after sunset – and bid farewell to the planets we’ve been enjoying as they cluster together to put on a farewell performance in the west.

Mars, Saturn, and Venus are so close they may all nearly fit in the same low power binocular field – and using binoculars will make them much easier to pick out in the twilight, though certainly Venus should be enough to guide you to the others. What’s more, a three-day-old crescent Moon (on  the 12th) will round out this picture. It will be between the planets and the horizon.

Meteors and meteor showers are fun if for no other reason than they are a chance to see something happening in the sky. Much of what we look at doesn’t change – or rather changes so slowly we don’t notice the change. Meteors, on the other hand, demand that you be looking in the right place at the right time. Only on the very rare, very bright meteors do we actually have time to alert others and have them turn their heads and see what we see.  And what we see is a space event happening closer to us than any other natural one. What’s more, meteors can have real scientific value.  They are viewed by some as our cheapest “space probe.” They are relatively pristine bits of matter left over from the early days of the solar system and so can tell a story to those who know how to read them.

Meteors – “falling stars ” – can be seen any time. You don’t have to wait for a “shower” like the Perseids; you just have to be lucky. But they are most frequent at certain times in the year when the Earth happens to be plowing through a meteroid-rich area.  We call this occasion a meteor shower. (For your dictionary: A meteroid is a small bit of space rock that becomes a meteor when it collides with our  atmosphere and heats to incandescence as it descends towards Earth. When it gets here – which is rarely as anything except fine, incinerated dust – it is a meteorite. )

The reason for a shower such as the Perseids is that we are passing through the debris trail of a comet. Think about it. The general model for a comet is a “dirty snowball,” and as that dirty snowball nears the Sun it melts, and as it melts it leaves a trail of dirt particles behind it – particles that remain in orbit until something like the Earth sweeps by and captures some of them with its gravity.

The comet itself can vanish entirely – but the result is a river of space dust – a river that is most intense nearest where the comet actually was.  That’s why there are some years – the 1990s in the case of the Perseids – when the meteor shower is more intense than others.  Now we are in a period when we are passing through the trail of the comet that creates the Perseids at a point where that trail is relatively sparse – so there will simply be fewer Perseids than there were  15-20 years ago..

That trail is not encountered all over the sky. It collides with our atmosphere near a particular point in our sky. That point is called the radiant – you might think of it as a hole through which the Perseids fall – and in the case of the Perseids, it appears to be in the constellation Perseus.  But we don’t see all the meteors at this point. We see a meteor only when its collision with our atmosphere is intense enough to make it burn up. The faint meteors we see are made by a speck of dirt about the diameter of a pencil lead. The brightest ones are caused by something about the diameter of the pencil’s eraser.  In either case it will, for all practical purposes, burn up entirely in our atmosphere – 50 to 75 miles up – and nothing significant will remain for anyone to find on Earth. But exactly where it burns up is another thing. That’s why we will see a sudden flare – a falling star – anywhere in the sky.

And that’s awesome! Think about it a moment. If someone struck a match 50 miles away would you see it?  Yet a grain of sand, hurtling into the atmosphere, shows us such a brilliant light we can’t miss it!

When you are watching for Perseids, you don’t have to look near the radiant point, though you will see more there.  A meteor can flare up suddenly anywhere and appear to draw a short (usually 5-10 degrees long) straight line across the dome of the sky. (Bright ones may actually leave a trail, which you can see for a few seconds with the naked eye or longer with binoculars.) If we trace a line backwards along the meteor’s trail we will see it comes from the area near the radiant point.

In the early evening, that Perseid radiant point is low in the northeast. That means nearly half the meteors that are radiating from it are happening below our eastern horizon. That’s why the shower is best in the early morning hours when the radiant is high in our sky. If the radiant is overhead, then we have nearly doubled our chances of seeing a meteor.

There are many meteor showers in the course of a year and some are better than others. The Perseids is one of the most reliable ones and happens to come at a convenient time for northern hemisphere observers when it is comfortable to be out at night, lying on the ground, and looking up.

Personally, I don’t like the word “shower.” It immediately gives the impression that what we are going to see will be more intense than what most of us actually experience. I prefer calling this a meteor “event.” But, we have been calling such events “showers” for years, and too often they are hyped in the press and then people are disappointed when nothing like a shower occurs. So keep your expectations realistic and you won’t be disappointed.

In the final analysis there’s only so much time you can spend lying on your back gazing at the starry sky; though I very much enjoy that time, it’s made much more enjoyable by knowing that at any instant there’s a heightened likelihood that I will see a bright meteor.  That – and the summer Milky Way – make looking for Perseids in a dark and moonless sky always worth the effort for me.

Close encounters of the planetary kind

Jupiter is the real fun for telescope users this month and starts to provide a great naked eye beacon in the eastern sky. It rises a couple of hours after sunset at the start of the month and by the end of the month will become a dominant, planetary beacon in our evening sky. But for early August evenings, the planet show for naked eye and binoculars is mostly in the west – though those who stay up late to see the Perseid meteors will be treated to a brilliant Jupiter as well.

Saturn, Mars, and Venus – with a bit part by Mercury – play out the last act of their current drama on the western stage shortly after sunset, with the climactic event centered on August 7, but fun to watch both before and after that.  This is the sort of thing you need to see in your mind’s eye, bearing some resemblance to waltzing elephants. Well, maybe that’s an unfair image.  Kepler would have watched and I suspect heard the music of the spheres. And we should too.  But the emphasis for me needs to be on size.

We’re watching dots of light dance in the western sky. But what we are really seeing is the precisely choreographed movements of the planets with that mystery of mysteries – gravity – dictating the action. And we’re part of it, of course. What we see depends upon where we stand and we’re certainly not standing still. So here we are on the third rock from the Sun, spinning at the dizzying speed of roughly 800 miles an hour – at mid-northern latitudes – and tilted  at the crazy angle of 23.5 degrees and rushing around  our central star at 66,000 miles an hour, and from this incredibly hurried platform we can watch night after night as the second rock and the fourth rock appear to  approach one another in our twilight sky, while the more stately frozen gas ball – Saturn, with its magnificent rings – looms nearby. And off to one side, nearly lost in the dying glare of our own star, is the first rock, tiny, whizzing Mercury, scooting out from the Sun, but at such a low angle for us it will be a challenege to see, making binoculars extremely helpful if we wish to track it down – not to mention unusually clear skies.

That’s what’s going on. Here’s what it comes down to in more mundane terms.

On August 1, as we look west 30 minutes after sunset from mid-northern latitudes – about 42-degrees –  the actors are here:

Look for Venus first - that will be easy and your guide to the others. It's about 15 degrees - a fist and a half - above the horizon, so you can judge other distances from that. Of course, as time goes on and the sky gets darker, the planets are easier to see - but they also are getting closer to the horizon making them more difficult to see. Click image for larger version. (Prepared from a Starry Nights Pro screen shot.)

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On August 7, 30 minutes after sunset, they should look like this – and that’s about as high as Mercury will get this month:

Venus, Mars, and Saturn should fit easily into the same binocular field of view - they are separated by just under 5 degrees and roughly 15 degrees above the horizon. Mercury will be much more of a challenge, requiring unobstructed horizon and very clear skies. Click image for larger version. (Prepared from Starry Nights Pro screen shot.)

For a printer-friendly version of the preceding chart, download this.

And Venus and Mars hang close, moving at roughly the same speed as seen from our moving platform against the background of stars, until near the end of the month. Then they join the bright blue star, Spica, and this union carries them into the first week of September. This is how they’ll look on August 29th.

The planets are lower now - about one fist above the horizon half an hour after sunset - but Venus still provides a bright guide to finding the others. Venus, Mars and the guide star, Spica, all fit in the same binocular field of view, but Staurn is off by itself, much closer to the horizon and Mercury has headed off towards the morning sky. Click image for larger view.(Prepared from Starry Nights Pro screen shot.)

For a printer-friendly version of the preceding chart, download this.

• August 1 – Check out Venus, Mars, Saturn and Mercury in the west
• August 3 – Last quarter Moon
• August 7 – Mars, Saturn, and Venus closest together
• August 9 – New Moon
• August 11 – Try to catch the thin crescent Moon near Mercury very low in the west. If no luck on this night, don’t despair.   The Moon will still add to the twilight planet show for the next few nights.
• August 11-12 or August 12-13 – These are the nights to look for Perseid meteors. The best opportunity should be after 11pm on the 12th,  but the same start time applies to the 11th. This is really an early morning event.
• August 16 – First quarter Moon
• August 24 – Full Moon
• August  27 – A waning gibbous Moon will rise side by side with Jupiter  about one hour after sunset – Jupiter will be nearly due east, the Moon about one fist north of it.

Look North In August – All hail the Queen! (OK – the “W”)

Click image for larger view. (Derived from Starry Nights Pro screen shot.)

For printer friendly chart, download this.

The easily recognizable “W” of Cassiopeia (kass ee oh pee’ uh), the Queen, is well up in the northeast early on an August evening. Find it and you have a good starting point for tracing the Milky Way on south through Deneb.

When the “W” circles to a point high overhead, it will look like an “M,” of course, but that’s just part of the fun. Some people also see this asterism as forming the chair – or throne – for Cassiopeia. I like it because along with the Big Dipper, it nicely brackets the north celestial pole and provides another rough guide for finding Polaris. As the “W” rises, the Dipper plunges until it may be too close to the horizon for many to see. Both the stars of the Dipper and the stars of the “W” are 28 degrees from Polaris – roughly three fists.  When the Dipper gets on the horizon, the “W”  turns into an “M” directly above Polaris, so just measure three fists down from this “M” and you should be in the right region for finding the North Star.

Normally I do not find constellations or their associated myths too useful. Cassiopeia is an exception. Knowing the myth connected with this constellation will help you remember several important neighbors, and though we’ll meet these in the next two months, I’ll give you a “heads up” now and repeat the story when we meet the others. It goes like this:

Cepheus (King of Ethiopia)  and Cassiopeia (Queen of Ethiopia) have a beautiful daughter, Andromeda. Cassiopeia bragged so much about Andromeda’s beauty, that the sea nymphs got angry and convinced Poseidon to send a sea monster to ravage Ethiopia’s coast. To appease the monster, Cepheus and Cassiopeia  chained the poor child (Andromeda)  to a rock. But don’t worry. Perseus is nearby and comes to the rescue of the beautiful maiden, and they ride off into the sunset on Pegasus, Perseus’ flying horse! These five constellations – Cepheus, Cassiopeia, Andromeda, Perseus, and Pegasus – are all close to one another in the sky and all are visible in the fall, so we will meet them soon.

One of the bright stars of Cassiopeia is also a special aid to finding your way around the heavens, but in a more modern sense. It is part of an asterism known as the “Three Guides.”  These three bright stars are all very close to the Zero Hour Right Ascension circle in the equatorial coordinate system – the system that is roughly the celestial equivalent of latitude and longitude and is commonly used to give a permanent address to stars and other celestial objects. These three bright stars mark a great circle that goes through both celestial poles and the equinoxes and is known by the imminently forgettable name of  “equinoctial colure.”

Click image for larger view. (See note at end of post for source of this drawing.)

We’ll meet the other two stars in this asterism next month, but for now, simply take note of Beta Cassiopeia. It’s marked on our chart and is the bright star at that end of the “W” that is highest in the sky this month. Remember that this star is very near the “0” hour  circle, which you can visualize by drawing an imaginary line from Polaris through Beta Cassiopeia and eventually the south celestial pole. This line will cross the ecliptic at the equinoxes.  Of course, this helps only if you are familiar with the equatorial coordinate system! If that means nothing to you, then don’t clutter your mind with this right now.

The source for the drawing showing the equinoctial colure can be found here.