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    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.

Look north in July 2013 and take the measure of your skies and eyes!

Light pollution is a big issue these days. How does it impact you? Summer is a good time to check by looking north about two hours after sunset and seeing what stars you can see in and near the Little Dipper. Why summer? Because this is when the Little Dipper should be highest in your sky – standing upwards from Polaris, the North Star. Here’s what you should see on a typical July evening when you look north from mid-northern latitudes.

In summer the faint stars of the Little Dipper are high above the North Star. Click image for larger view. (Derived from Starry Nights Pro screen shot.)

For a printer-friendly version of this chart, click here.

The Big Dipper is diving downward in the northwest but is still very high, and its handy “pointers” should get you quickly to the North Star, Polaris. Roughly opposite the Big Dipper you should see the “W” of Cassiopeia starting to make its way upward in the northeast. And unless you suffer from really terrible light pollution, you should see the two “Guardians of the Pole” – the second and third magnitude stars that mark the end of the Little Dipper. The brighter of these two is just a tad dimmer than Polaris, but since it’s higher in the sky right now and thus shining through less air to get to you, it will probably look just the same as the North Star in brightness.

To do this test you first have to wait until it is genuinely dark, and in summer that’s a bit longer than in winter. Twilight actually is divided into three steps. We have civil twilight which goes from sunset until when the Sun is six degrees below the horizon. Nautical twilight is the next period, which continues until the Sun is 12 degrees below the horizon. Then you have Astronomical Twilight until the Sun is 18 degrees below the horizon. At that point it is as dark as it will get and will remain that dark until we run the sequence in reverse as the eastern horizon nears the Sun. As a rough rule of thumb, you can consider each twilight period to last half an hour – but the exact length depends on where you are on Earth and the time of year. If you want to get precise, go to the U.S. Naval Observatory site, fill in the form you’ll find there, and you can get a table that will give you the start and end of these twilight times – or for that matter when the Moon rises, or the Sun sets. It’s very handy. (Note: the preceding link takes you to a page for US cities and towns – but there’s a second page here where you can put in the latitude and longitude for any location in the world, including in the US. )

The second thing you need to do is make sure your eyes are dark adapted. They are casually dark adapted after you have been out for 15 minutes and have not looked at any white lights. But it can take from half an hour to an hour of protecting your eyes from any white light for them to become fully dark adapted. That doesn’t mean you have to sit around in the dark doing nothing waiting for this to happen. In the last hour or so before full darkness there are plenty of things to see – just avoid bright lights. That also means moonlight. You’re going to want to do this when the Moon is not in the sky, for it will make it difficult to see faint objects anywhere near it. In July of 2013 the first two weeks should work pretty well – as will the last four or five days of the month.  Other evenings, the Moon will dominate the early evening sky.   (A good Moon-phase calendar can be found here, though for this purpose I find the table from the Naval Observatory for local Moon rise is also handy!)

So here’s the test:

How many stars can you see in the Little Dipper?

Remember that in the magnitude system the higher the number, the fainter the star.

The Little Dipper consists of seven stars. Three are easy – Polaris and the two “Guardians” marked “21” and “30” on the chart below. If, once you are dark adapted, you can see only one of the “Guardians,” then your skies are limited to magnitude 2 stars and brighter – very poor. If you see both, but no other stars in the Little Dipper, then your limit is magnitude 3.

On our chart below, the magnitude of each star is listed as a whole number so as not to put decimal points on the chart because they might then be confused with faint stars! So when you see a star listed as “21” that means “magnitude 2.1.”

For a printer-friendly version of this chart, click here.

Even in good, dark skies the other four stars in the Little Dipper may not be that easy to see – and the faintest ones may require averted vision – that is, don’t look exactly where the star should be. Instead, look a little to one side or the other, and the star may pop into view. That’s because the center of your eyes are not as sensitive to faint light as the outer regions of your eyes.

Here’s another little trick that may help you locate these faint stars – use binoculars. With typical, hand-held binoculars you may be able to fit all four stars of the Little Dipper’s “cup” into the same field of view. If not, get the “Guardians” in your field of view, then move just a little to where the other two stars of the “cup” should be. This does not count, of course, for the light pollution test. For that test we’re trying to determine the faintest star you can see with the naked eye. But looking first at the stars with binoculars helps assure you that they really are there! You also can trace out the handle this way, though you will have to move your binoculars to do so.

If you can locate all the stars in the Little Dipper with your naked eye, you have very dark skies – congratulations. To see how good they are – and continue to test your eyesight and dark adaption – look for the stars marked “55” and “60” on our chart.

The star marked “60” is traditionally thought of as the faintest you can see with your naked eye. That’s a magnitude 6 star. In really pristine skies, such as those over Mauna Kea in Hawaii, experienced observers with excellent eyes can detect stars down to magnitude 8 with the naked eye. Personally, I’m happy when I can see all the stars in the Little Dipper and especially happy if I can get that “55” star – I’ve never seen the “60” one with my naked eye. But relative to the heavily light-polluted eastern seaboard of the US, I have dark skies.

This is not simply a good guide to light pollution in your area. It also is a handy guide to tell you just how good  – how “transparent” – the skies are on any given night – and to show you how well you have dark adapted at any given moment. So whenever I go out to observe I frequently glance at the Little Dipper to test both my developing night vision and the clarity of the skies. (It never fails to amaze me how much and how quickly my night vision changes. )

To the casual observer all clear nights are equal. But the experienced star gazer knows they are not, and the stars in and about the Little Dipper are a good guide, especially in the summer months when they are so high in the sky.

Look North in October 2011 – and find a really bright star!

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

For a printer-friendly version of this chart, click here.

When I say “really bright,” I mean Capella – and I say it because so many folks assume, quite understandably, that the North Star will be the brightest in that section of the sky. It isn’t.

Capella is one of our brightest stars, and you may catch it this month just peeping up over the northeastern horizon about an hour and a half after sunset. And it is brilliant!  When you see Capella,  compare it with the North Star  and you will get a good idea of what the word magnitude means. There are a solid two magnitudes difference between the two – or if you want to get technical, Capella is magnitude 0 and Polaris is magnitude 2.

Yes, the magnitude system goes backwards – the lower the number, the brighter the star. Each magnitude difference represents a change of about 2.5 times in brightness. So a zero magnitude star is 2.5 times as bright as a first magnitude star – and 2.5 x 2.5, or 6.25 times as bright as Polaris, a second magnitude star. And while we’re on this subject, the faintest star in the Little Dipper – it’s over in one corner of the cup – is just about magnitude five. That means Capella is 100 times as bright as that star.

And while we’re on the subject of magnitude, the Little Dipper does give us a great range. Polaris, as we said, is magnitude 2. The other bright star in the Little Dipper is magnitude 2 also – it’s at the end of the cup away from Polaris. Sharing that end is a magnitude 3 star, and these two are known as the “guardians of the pole” as they make a circle around it every 24 hours. Most of the other stars in the Little Dipper are magnitude 4, with the one star in the corner of the cup being magnitude 5, as noted – well, actually 4.9. There’s a chart with these magnitudes in this post. Many people who live in light-polluted areas can see only three stars in the Little Dipper – Polaris and the two Guardians of the Pole.

How faint a star can you see? Depends upon your eye sight, the light pollution in your area, the transparency of the skies on any given night, how high the star is in the sky, and, of course, your vision and dark adaption. But as a general rule of thumb magnitude 6 has been accepted as the typical naked eye limit. However, if you live in a typically light polluted suburb you may see only to 4, or worse yet, magnitude 3. And observers on mountain tops with really clear skies reliably report seeing stars as faint as magnitude 8. All of this is with the naked eye. Add ordinary binoculars and you will certainly add three to five magnitudes to the faintest star you can see. In fact, you take a huge jump in the number of stars seen just by using binoculars with 50mm objective lenses. That’s why many amateur astronomers with fine telescopes also are likely to have a pair of 10X50 binoculars that they keep handy. I always did, but more recently I stumbled upon some very inexpensive Celestron 15X70 binoculars that I love for quick peaks, though you can’t hold them steady for serious observing.

Capella will be a “look east” guide star next month, but you can get to know it this month as you look north. And if you live in mid-northern latitudes, it will become a most familiar site. In fact, where I live – about latitude 42°N – it is in the night sky at some time every night of the year. This is because it is so far north and as it circles Polaris only dips below the horizon for a few hours at a time.

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