August 2013 – last good look at Saturn, and a Moon-free Perseids shower

The Big Dipper's handle can guide you first to bright Arcturus, then to yellowish Saturn and blue Spica - both will be about the same brightness. Venus is much birghter, but best seen about half an hour after sunset when it is about 10 degrees above the western horizon. By an hour after sunset it ishalf that or less and even if you have an unobstructed horizon, may be lost in mist and twilight.

The Big Dipper’s handle can guide you first to bright Arcturus, then to yellowish Saturn and blue Spica – both will be about the same brightness. Venus is much brighter, but best seen about half an hour after sunset when it is about 10 degrees above the western horizon. By an hour after sunset it is half that or less and even if you have an unobstructed horizon, may be lost in mist and twilight. CLick for larger image. (Prepared from Starry Nights Pro screen shot.)

For a printer friendly version of the above chart, click here.

If you have a small telescope, August 2013 will give you your last good look at Saturn for the year and if you live on the right side of the globe – not where I live – the Perseids  meteor shower should be spectacular this year with no interference from a waning Moon.  Venus, meanwhile, continues to reign low in the western sky just after sunset.

The sky north of east early on the morning of August 12, prime time to watch for Perseids meteors. (Created froma Starry Nights Pro screenshot.

The sky north of east early on the morning of August 12, prime time to watch for Perseids meteors. (Created from a Starry Nights Pro screenshot.)

For a  printer friendly version of the above chart click here.

The Perseids should reach their peak on August 12 at about 19:00 UTC. To find what time that is for your region, go here.  For about half the world that’s good news, for the other half it’s bad because you really want to see this shower in the early morning hours and you will get the best show if the shower’s peak falls during those hours for your time zone.

Locally, on the East Coast of the United States, I’m going to watch the weather and if either the morning of August 11 or the morning of August 12 is forecast to be clear, I plan to start observing about 2 am. But I am not expecting a big Perseids show – just a nice summer night with a much better chance than usual of seeing a bright meteor.

Meanwhile, I’m bracing myself to hear a lot of promotional blather about the Perseids locally from TV weather folks and others who should know better, but the truth is in North America the timing of this year’s shower could hardly be worse.  The shower is best for a couple hours either side of its peak and its peak is forecast to come at 19 hours GMT on August 12 – for Eastern Daylight Time that translates to 3 pm – broad daylight.  What’s worse, even if the peak was in the early evening hours, the Perseid’s radiant point doesn’t get high in the sky until the early morning. That’s why the best time to see Perseid meteors – regardless of the peak time – is still  between midnight and  a couple hours before dawn.

So can we in America hope to see any Perseids at all? Yes, of course we can.  Just don’t expect a “shower.” In fact, I have to say that i always wince a little at the times and rates of meteors frequently given in news reports. Hey, just the word “shower” implies a lot more than most people usually see, especially from their typically light-polluted back yards.  When someone reports that the Perseids will peak at better than 100 meteors an hour, they usually fail to mention that three conditions have to be met for you to see that peak.

1. You need the Perseids radiant point to be nearly directly overhead – for EDT that occurs in a twilight sky, but is reasonably high from midnight on. The meteors may appear in any part of the sky, but they will appear to radiate from that point, so the higher it is, the better chance we have of seeing a meteor.

2. You need very dark skies – skies that will allow you to see magnitude 6.5 stars, if you are going to experience those real high rates. I have never experienced such dark skies, but they certainly exist. However,  with my reasonably dark skies I am very happy when I can detect a star of magnitude 5.

3. And, of course, you need the shower’s peak to coincide with the radiant point being very high in your sky.

One more caution – anything can happen. This is a forecast and usually reliable. But there could be a burst of meteors at a different time. You may get lucky.

And if all these  condition aren’t met for your location? Well, it’s reasonable to expect to see a Perseid meteor about every 10-15 minutes – of course you  may get two or three in a row hardly separated at all, then not see another one for  an hour. But be patient and you will get results – just not the meteor spectacular that some reports will imply. Last year they were coming in at a rate of 15-20 an hour four hours either side of the peak.

And yes, a Perseid can show up days either side of the peak.  How will you know it’s a Perseid? Draw a mental line extending the path of the meteor back towards the Perseid’s radiant point. If your line points back to that area of the sky – see map above – then you saw a Perseid. But there are always strays around – random meteors that have no connection to the shower – and at this time of year we have a couple weaker showers that may produce a few meteors going in other directions.

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 meteoroid-rich area.  We call this occasion a meteor shower. (For your dictionary: A meteoroid 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! Consider this: 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! Why? Well, for one thing it is hitting our atmosphere at something in the order of 133,000 miles an hour – that makes a “speeding bullet” look like the proverbial turtle!

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.

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.

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