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  • Rapt in Awe

    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

August 2012 – Mars, Saturn, Spica in the west – and a spectacular Perseids shower

Should you watch the planet show in the West in the evening? Or the meteor shower in the morning? Why not both? And if you’re getting up early to see the Perseids at their best, be sure not to miss brilliant Venus and Jupiter – well, how could you?

Over in the west we have Mars showing you how fast a planet can appear to move as it runs between  Saturn and Spica – over a period of three weeks making a colorful red, blue and yellow display.  And all over the sky for several days this month you are likely to pick up a brilliant, Perseid meteor – but particularly on the morning of the 12th of August with the 13th a good back-up – and this year’s show should be especially good because the Moon will not put in an appearance until early morning and will not be bright enough to ruin the show.

Let’s start with the west – week-by-week the changing scene will look like this low in the southwest about an hour after sunset:

The circle is 7 degrees – roughly what you can expect from a low power binocular view.

 

A fun night because a young, crescent Moon joins our trio.

You will need a clear and unobstructed  western horizon for this one because these three objects are roughly 10 degrees – one fist – above the horizon. (Early in the month they’ll be a bit higher – late in the month they get quite low.  Now what I love about this event is it demonstrates three things –

  • Color in the sky – Saturn is yellow, Mars, Red, and Spica blue. But these colors are subtle. You’ll see them better if you use binoculars and you might want to review this post on star colors to better know what to expect.
  • Perception and the effect of motion – from night to night Saturn will hardly appear to change positions relative to the background stars at all,  and Spica won’t  change – but Mars will whip right along and this will be amply clear as you check it’s position against the other two.  The reason is simple – Mars is much closer to us at about 158 million miles; Saturn is about 939 million miles and, of course, Spica is so far away we measure its distance in light years – 263.
  • The time dimension is on display as well – Mars is roughly 14 light minutes from us, Saturn is  well over a light hour, and Spica  263 light years. So what appears to be a two dimensional scene is revealed to be three dimensions as you observe the rapid motion of Mars and picture the solar system – and when you put your mind to it, you understand that the instantaneous nature of the scene is an illusion – that you are really looking into the fourth dimension and what happens simultaneously from your perspective is really happening at much different absolute times.

To help grasp the situation, take a look at this Orrery view of the Solar system for August 7. Keep in mind two things. The east-to-west motion we see as the night goes on is due to the spinning of the Earth.   The night-to-night westward – downward – drift of all  is caused by the motion of the Earth in its orbit around the Sun.

Orrery view for August 7, 2012, (Click image for larger version.)

The yellow arrow in the image above shows our view of Mars and Saturn in the evening sky. As the Earth rotates counter clockwise on any given evening, first Jupiter, then Venus come into view in our morning sky – red arrow.  If you then picture the Earth moving ahead in its orbit it’s not hard to understand why Saturn and  Mars will eventually be lost from view, while Jupiter will appear earlier each evening. Venus is a bit more complex. It too will get lost in the glare of the Sun, but since it is moving faster than us the change will appear to take place fairly slowly. Maybe I’m just slow, but it has taken me years to move from these abstract representations of what we see in the sky and how the planets are moving, to get to the point where I can look up and have a genuine, intuitive sense of what’s going on. Very satisfying and worth the effort, but even if you don’t do that, it’s a wonderful show! (The Orrery view is obtained from Solar System live web site. Go there and play with the dates to see the changing motions of the planets.)

Perseids in the morning

OK – so much for the evening sky. The morning sky is really spectacular because we’re looking at a section of sky that contains a lot of our brightest stars and  two terrific star clusters, plus the two brightest planets. What backdrop for a brilliant meteor shower!

The red oval represents the area opf the sky from which the Perseid meteors appear to radiate – however, they can go in any direction from here and might appear any where in the sky. (Click image for larger version. Prepared from Starry Night Pro screen shot.) Here’s a quick guide.

Perseids – a quick guide

When:

The night of  August 11-11 starting about 90 minutes after sunset, but best after midnight. And if that night is likely to be cloudy, the next night of August 12-13 might prove to be just as good, but the best chance looks like the 11-12. (There’s no doubt you should see meteors either night – but there is doubt as to exactly when the shower will peak.)

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. My most memorable Perseid skimmed the horizon to the north.

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 down swing, but because we’ll have little interference from the Moon, this should be a better than average year.

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!

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.

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