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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.

Prime Time observing for October 2009

Seeing a bow, a demon,  and a few hundred billion stars  – meanwhile, Jupiter slams it into forward!

Please note: All charts with this post are for observers in mid-nothern latitudes centered on 40° N. If you are 10 or more degrees south or north of that – or if you’re not sure of your latitude – please go here to make your own custom star charts.

On tap this month is a new asterism,  the bow; a variable, Algol, the “demon star;” a neighboring galaxy you can see with the naked eye or binoculars; and yes, Jupiter, which appears to abruptly change directions as it moves against the background stars.

To begin our monthly exploration of the night sky, you can take a slide down Andromeda’s Couch to Mirfak and the Bow of Perseus in the northeast, assuming you learned these last month. If these are new to you, simply start by looking for the rising low in the northeast.

As usual, go out 45 minutes after sunset and watch the stars emerge. It may take another 15 minutes for  to see the bow  clearly, but what you are looking for is three stars in a vertical arc, with the middle one – Mirfak –  the brightest. How big an arc are we talking about? Just make a fist and hold it vertically at arm’s length, and your fist should just cover these three stars. How high? The bottom one should be about a first above the horizon.  Here’s a chart modified from Starry Nights Pro software..

Click chart for much larger view. If you observed last month you know the Great Square and Andromeda's Couch and can slide down the "Couch" to Mirfak, the brightest star in the bow of Hercules. If this is your first month of learning the sky, simply look to the northeast and find the bow.

Click chart for much larger view. If you observed last month you know the Great Square and Andromeda's Couch and can slide down the "Couch" to Mirfak, the brightest star in the bow of Perseus. If this is your first month of learning the sky, simply look to the northeast and find the bow.

Now if you want to be a stickler about mythology, Perseus doesn’t carry a bow – he wields a sword instead, which he is holding in his right hand high over his head, while in the left hand he holds the severed head of Medusa. Here’s how the 1822 “Urania’s Mirror” depicted it.


Perseus - click for alarger version.

Oh boy – and if you can see all that in these stars, then you have a very vivid imagination. I never would have learned the night sky if I had to try to trace out these complex constellations as imagined by ancient cultures and depicted in star guides up until fairly recently.  And for the purposes of helping you find your way around the night sky I think remembering the Bow of Perseus is easier. Mirfak, is just a tad dim to serve as one of our guidepost stars, but it does come in handy when identifying the “Demon Star,” whose proper name is Algol.

Getting sharp about brightness

As you start to learn the stars, it may surprise you how precise you can be about their brightness.  At first you may have difficulty just telling a first magnitude star from a second, but if you get to know Algol, the “Demon Star,” I bet you’ll find that you can quickly become quite sophisticated in assessing brightness and shaving your estimates down to a tenth of a magnitude.

Imagine a star that regularly varies in brightness every few days – that’s what Algol does. Exactly every 2 days, 20 hours and 49 minutes it begins a 10 hour period where its brightness dims more than a full magnitude. If you look during the right two hours, you’ll catch it at or near its dimmest – and most of the rest of the time you’ll catch it at peak brightness. And it’s quite easy to judge. But first let’s find it. Here’s the chart we’ll use.


Notice how Algol makes a very nice triangle with two companions, and all three stars are close to the same brightness – Almach, the bottom star in Andromeda’s Couch; Mirfak, the central star in the Bow of Perseus; and Algol. That brings us to our first challenge: Go out any clear night and study these three stars and decide which is the brightest. Two are equal in brightness, but one is a tad brighter than the other two. Which is it? Algol? Mirfak? Almach? (The answer is at the end of this text so you can ignore that answer until you actually have an opportunity to test yourself.)

However . . .

Because Algol is a variable, sometimes when you look at it, Algol will actually be significantly dimmer than either Mirfak or Almach. In fact, there’s a reasonable chance it will be dimmer than either of Mirfak’s two fainter companions that make up the Bow of Perseus. If when you test yourself, this is the case, congratulations! Make note of the date and time.

Algol is a special kind of variable star known as an eclipsing binary. That is, what looks like one star to us is really two stars, and when we see Algol’s light start to dim it means its companion is passing between Algol and us causing an eclipse. Since the stars are locked in orbit around one another this happens with clockwork regularity.


The above diagram came from this astronomy class web site which includes amore detailed scientific explanation.

Since either star of the pair can cause an eclipse, there is a much fainter, secondary eclipse of Algol – really too faint to be noticed by most observers. Why is one eclipse fainter – because one star is blue, Class B – and much hotter/brighter than the other star which is “K” class. (Remember – OBAFGKM.)  It is when the cooler star is in front that we see the dramatic change in light.

It’s fun to catch Algol in mid eclipse, but I suggest you not read about when to do this right now. Instead, do the little challenge first. Then when you’re ready, go to the final item in this, which explains how and when to catch Algol in eclipse and in the process, tells you the brightness of its companions.

OK – second project – Jupiter changes direction!

I described this in an earlier post an am quotingit in its entirety here.

On October 1, 2009 a nearly full moon joins Jupiter, Uranus, and Neptune in the southeast as shown here about an hour after sunset as seen from latitude 42 degrees north and longitude 71 degrees west. Chart from StrayyN oghts Pro software. Click for larger image.  .
On October 1, 2009 a nearly full moon joins Jupiter, Uranus, and Neptune in the southeast as shown here about an hour after sunset. (Jupiter is made large to indicate its relative brightness – ut it will look like a very bright star – not a small moon!) This is how the sky appears from latitude 42 degrees north and longitude 71 degrees west. Chart from Starry Nights Pro software. Click for larger image.

The idea here is simple – connect what we can see in the sky this month with what’s actually going on. We’ll do this by watching Jupiter, the easiest object to find right now since it is the brightest “star” fairly high in the southeast shortly after sunset.

With just a few quick checks with binoculars we should be able to track the movement of Jupiter in relation to a bright, nearby star. You should start this project on or before October 1, 2009 if at all possible and plan to observe two or more nights between your start time and October 13. Then observe again in about a week and again near the end of the month.Your first couple of checks should show Jupiter in “retrograde” moving westward among the background stars. Your next two checks should show Juputer has resumed it’s normal eastward movement.

Use the following chart as both your guide and your log. That is, click on it to get a version you can print, take out under the stars, and record your observations on with a pencil.

Click for larger version, suitable for printing.
Click for larger version, suitable for printing.

So why does Jupiter appear to first go one way, then the other? Afterall, it isn’t really doing that, is it? Like the other planets – and us – it’s simply continuing a steady, eastward journey around the Sun. But so are we – and we are moving much faster because we’re much closer to the Sun. So what you are seeing is partly the movement of Jupiter – but also the apparent change in its position caused by our rapidly changing position.

I made the following animation from Solar System Live charts. It shows how Jupiter’s position changes slowly in relation to Earth and the other planets, particularly Neptune. The animation starts with September 1, 2009  and moves a month at a time for six months. The arrow shows our changing view of Jupiter with relation to Neptune, a much more distant – and even more slowly moving, planet. Notice that in late December Jupiter makes another close approach to Neptune – the third this year – which will make especially easy at that time to find this distant and faint planet. Right now you can use the chart above to track it down – it would be just visible in binoculars on a moonless night.


So let’s review the movements we’re dealing with here.

1. The daily rotation of the Earth causes Jupiter to appear to rise inthe east and move westward as the night progresses.

2. The revolution of the Eartha round the sun at a much higher speed than Jupiter makes it so that for some time the huge planet appears to be moving westward in relation to background stars and the more distant planet Jupiter. That apparent westward motion comes to a stop October 13, 2009.

3. Jupiter’s own motion is more apparent after October 13, as it appears to move eastward against the background stars. This general motion will carry it about 30 degrees eastward – very close to where Uranus can be found now – in about a year. It takes Jupiter almost 12 of our years to make a complete circuit of the sky.

The idea here is simple – connect what we can see in the sky this month with what’s actually going on. We’ll do this by watching Jupiter, the easiest object to find right now since it is the brightest “star” fairly high in the southeast shortly after sunset.

See a few hundred billion stars at one glance!

Yes, you can do it if you have good dark skies, you have allowed your eyes to dark adapt, and you are looking at the right place.  Once again, Andromeda’s Couch is our guide, and what we are looking for this time is the Great Andromeda Galaxy aka M31.

This is our “neighbor” in space if you can wrap your mind around the idea that something “just” 2.5 million light years away is a “neighbor.” ( As you try to do that remind yourself that a single light year is about 6 trillion miles – of course, good luck if you can imagine a trillion of anything!)

But seriously, you can see this with  your naked eye – and even in normal, light-polluted skies, you can see it with binoculars. In fact, this is one object where the binocular view can be almost as rewarding as the view through a telescope. Here’s a wide field chart for mid-month and about 90 minutes after sunset. At that point the galaxy should be roughly half way up your eastern sky.


Click image for larger version.

Starting with the preceding chart – and moving to the chart below:

  1. Locate the Great Square
  2. Locate Andromeda’s Couch off the northeast corner of the Square.
  3. Go down to the middle star in the couch, then count up two stars and bingo!
  4. You can also find the general vicinity by using the western end of the “W” of Cassiopeia as if it were a huge arrow head pointing right at the Andromeda Galaxy.
Click image for larger chart.

Click image for larger chart.

Well, “bingo” if you have been doing this with binoculars. With the naked eye it’s more an “oh yeah – I see it – I think!” But what do you expect? Think about it. The light from the near side of this object started its journey about 150,000 years before the light from the more distant side did! And think of where the human race was 2.5 million years ago when these photons began their journey – or for that matter, where all these stars really are today! Nothing is really standing still -everything is in motion.

You might also want to think about the folks who are on a planet orbiting one of those stars in the Andromeda Galaxy and looking off in our direction. What will they see? A very faint patch – probably fainter than what we see when we look at the Andromeda Galaxy, but in binoculars and telescopes roughly similar in size and shape.  Both Andromeda and the Milky Way Galaxy we inhabit are huge conglomerations of stars. We’re about 100,000 light years in diameter – Andromeda is about 150,000 light years in diameter. The Milky Way contains perhaps 100 billion stars – the Andromeda Galaxy maybe 300 billion.  (Don’t quibble over the numbers – even the best estimates are just estimates. )

And yes, in a few billion years we will probably “collide” with the Andromeda Galaxy, for we are hurtling towards one another. Such galaxy collisions are not that unusual  and probably aren’t as violent as the word “collision” makes them sound – but they do, in slow motion, bring about radical changes.

But all that is for the professional astronomers to concern themselves with – for us, there’s the simple beauty and awe of knowing that with our naked eye – or modest binoculars – we can let the ancient photons from hundreds of billions of stars ping our brains after a journey of millions of years.

And now the truth about Algol and companions

Have you done the Algol test yet? Looked at Algol, Mirfak, and Almach and tried to decide which is brightest? If so, you can check your answer by continuing to read. If not, I suggest you first do that exercise, then come back to this.

Chances are that when you look at Algol, it will be at its brightest – but how can you tell? Well, as we mentioned, you can compare it to Mirfak – but there’s an even closer match with another nearby bright star – Almach.  That’s the third star in Andromeda’s Couch  – the one neareast Algol.

Mirfak is the brightest of the three at magnitude 1.8.

Almach is magnitude 2.1 – the exact brightness of Algol when Algol is at its brightest – which is most of the time. OK – for the hair splitters, Almach is a tad dimmer, but the difference is far too little to be able to tell with your eye.

Here’s a chart showing the magnitude of the stars near Algol that you can use to compare it to and see if it is going through an eclipse.  People who look at variable stars use charts like this, but with one important exception – the numbers are given like they were whole numbers so you will not confuse a decimal point with another star. Thus, a star like Mirfak, of magnitude 1.8, would have the number “18” next to it. I broke a convention here because there are just a few bright stars on the chart, so I didn’t worry about the possible confusion of a decimal point being another star.


So If Algol and Almach are the same, no eclipse is going on at the moment.  If Algol appears dimmer than Almach, then an eclipse is in progress. If it’s as dim or dimmer than either of the companions of Mirfak in the Bow, then you can be pretty sure you’ve caught Algol at or near  its darkest. In two hours – or less – it will start to brighten and will return to full brightness fairly quickly.

Catching Almach at its dimmest is fun, but not as easy as it may seem. Why? Because although  an eclipse happens every few days, it may happen during the daylight hours, or in the early morning, or some other time when it’s inconvenient. And, of course, you need clear skies.  So when I want to observe an Algol eclipse, I go to a handy predicting tool on the Web that you can find here.

I then note the dates and times and pick out only those dates when the times are convenient to me – that is, happening during my early evening observing sessions. Then, given the  iffiness of the weather, I usually find that there are only one or two times a month when I’ll get a good look at an eclipse of Algol.

If I do this for October I find that out of 11 Algol minima, just three hit at the right time for me. Those dates and times are:

  • 10/01/2009  9:09 pm EDT
  • 10/21/2009  10:50 pm EDT
  • 10/24/2009  07:39 pm EDT

Of course the dates and time may be different for you, depending on where you live, and none of us can escape the whims of the weather! So here’s hoping for clear skies for you so you can find a winking demon, follow the actions of Jupiter, and capture in your own eye the photos from a few hundred billion stars in the Andromeda Galaxy!


3 Responses

  1. […] wrote about Algol the “Demon Star” in the posting for October, but it’ s still well placed for viewing January and if you look at the right time, […]

  2. […] wrote about Algol the “Demon Star” in the posting for October, but it’ s still well placed for viewing in February, and if you look at the right time, […]

  3. […] April 14 – The most convenient minima of Algol, the “Demon Star” occurs this month on April 14th at 9:40 pm. Should be fun to check on it about an hour after sunset, then again around 9:40 pm. The only problem – it’s getting very low in the northwest. At 9:40 it will be just 14 degrees above the horizon and will set in a couple of hours – hardly ideal conditions, so I expect this will be the last time I’ll check on it until next fall. For more details on Algol and its eclipses, see the post here. […]

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