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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.

Look north in July 2014 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. (Developed 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 reasonably well 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 2014 the last two weeks should work pretty well for the evening hours – as will the first day or two 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 Southeast in July 2014 – Colorful Stars and Planets, Great Asterisms – even a Great Constellation!

We’re going to cheat a little this month and look quite a bit south of east, rather than due east. The reason is we have some wonderful stars getting as high as they get if we look that way – AND we have two bright planet and colorful planets, Saturn and Mars which make for some interesting comparisons with nearby stars. We also have a couple of really cool asterisms and even a great constellation.

I’m not a big fan of constellations. Most don’t look anything like their names imply; some are quite obscure; and many simply can’t be seen in typical suburban skies these evening because of light pollution. Scorpius is an exception. It looks like the Scorpion of its name – a truly beautiful constellation with its graceful, curving tail. What’s more, many of its brighter stars actually do hang out together – they are not just an accident of our line of sight.

The Scorpion as Bayer saw him in his 1603 illustrated star atlas, Uranometria. Click for a much larger image. (Used by permission from the Linda Hall Library of Science, Engineering & Technology.)

It dominates our southeastern sky in July, just as the Summer Triangle – a terrific asterism, dominates our eastern sky this month. And we have two fascinating new “guide” stars – the intriguingly close and rapidly spinning Altair – and the incredibly huge and red Antares that is right at the heart of the Scorpion!

Antares begs comparison with Mars – both being red. But Mars is also very, very close to a bright blue star, Spica. Mars will be just a bit brighter than either of these comparison stars. Saturn –  between Mars and Antares and also very bright, has a yellowish hue.

But the real treat at this time of year remain these southern stars. They never get real high and from mid-nothern latitudes we only get a couple hours on a summer night when they are really well in view above the southern horizon. To top it all off the Milky Way runs from Deneb in the  Summer Triangle to the tail of Scorpius,but you have to wait a couple hours after sunset before this comes out.

Let’s take a look at the chart, then examine Scorpius along with its quaint little companion, a very real looking teapot complete with “steam” coming out of its spout! Wow! Summer nights may be short, but they sure offer some nice visual treats!

Oh - about that "teapot." We won't discuss it, but you can clearly see it tagging behind the scorpion. If you have real clear skies, the Milky Way is beautiful in this area and looks like steam rising from the teapot. More on this next month. Meanwhile, click image for a larger version. (Developed from a Starry Nights Pro screen shot. )

Click to enlarge! This chart covers a bigger section of sky then we usually show. Vega, for example,  will be six fists up. Oh – about that “teapot.” We won’t discuss it, but you can clearly see it tagging behind the scorpion. If you have real clear skies, the Milky Way is beautiful in this area and looks like steam rising from the teapot. More on this next month.  (Developed from a Starry Nights Pro screen shot. )

First up is the Summer Triangle – it’s an asterism that you can’t miss, and it will grace our evening skies right up into early winter. If you’ve been following for a few months, you’ve already met its lead star, brilliant Vega. And last month we were introduced to Deneb on the other corner. In fact, we saw that we could make a quite impressive Northern Triangle out of Deneb, Vega, and Polaris. But far better known than that asterism is the Summer Triangle shown above of Vega, Deneb, and Altair.

Altair is hard to miss. It is the brightest star low in the east early on a July evening, but it is also distinctive because it has two reasonably bright companions, close on either side, that form a straight line with it. This is appropriate because it’s not hard to see Altair and those two companions as representing an eagle in flight, and that’s good because they are the major stars in a constellation known as Aquilla, the Eagle.

Altair is white, much like Deneb and Vega, and is even closer to us than Vega. Vega is 25 light years away, Altair just 16. That’s in contrast to Deneb, which you may recall is an astounding 1,425 light years (at least)  from us – astounding because even at that distance it is almost as bright as its much closer companions and some experts believe it is much more distant.

Altair also distinguishes itself by spinning incredibly fast. It takes our Sun almost a month to complete a rotation on its axis. Altair, almost twice as large as our Sun, spins once on its axis in just 10 hours. Why, I don’t know, but it’s one more reminder of how these stars, which all look pretty much the same to us because they’re so far away, all have their special traits that distinguish them as individuals.

The most obvious special trait for Antares, our other new guide star this month, is its redness – and it’s one of only four guide stars that is quite close to the ecliptic – the path of the planets. That means that reddish Mars comes close, sometimes, to reddish Antares, and that’s appropriate because the name “Antares” actually means “like Mars.” However, science tells us something else about Antares. It is huge. I mean BIG.

Get out your calculator and do a little simple math. (OK, I’ll do the math, but really – this is simple, and I think you would appreciate the numbers much more if you did the calculations yourself.) One possible source of confusion:  To visualize a sphere I use its diameter. To actually calculate things I need the radius – since a radius is half of a diameter  you’ll find me jumping back and forth between these two terms – don’t let it confuse you.)

So try this. Start with something manageable, like the Earth. It’s about 8,000 miles in diameter and that’s a number that’s fairly easy to imagine. Let’s reduce Earth to a ball 2 inches in diameter. It would have a radius, then, of one inch.

Now let’s make a scale model Sun to go with our Earth. That’s easy. The radius of the Sun is 109 times the radius of the Earth. That means the Sun will have a radius of 109 inches – roughly 9 feet. So now we have a one-inch Earth and a 9-foot Sun. So our scale model has two balls – one two inches in diameter to represent the Earth and one 18 feet in diameter to represent the Sun.

That certainly should tell you that the Sun is a lot bigger than Earth, but my problem is, these linear measures don’t give us a really good sense of the size difference. We need to visualize spheres in terms of volume. We can get a rough approximation of the  volume  of a sphere by simply cubing the radius and multiplying it by 4. If we do this for our scale model Earth we have (1 x 1 x 1) x 4 – or four cubic inches. Now to calculate the volume of our scale model Sun – in cubic inches – we multiply 109 x 109 x 109, then multiply that by 4. Wow! Well, if you tried it on your calculator I hope you said “Wow!” You should get 5,180,116. That means you can fit well over one million Earths in our Sun! That to me is a lot more impressive than the linear measure where we find the diameter of the Sun is about 109 times the diameter of Earth.

Now let’s do a similar exercise with Antares. Antares has a radius more than 800 times the Sun. Do the math. Our scale model Sun has a radius of  9 feet – our scale model Antares will have a radius in feet of 9 x 800. Man, that’s big. About 7,200 feet!  (Just remind yourself that a mile is 5,280-feet.)  So now we have three models – a 2-inch diameter Earth, an 18-foot diameter Sun, and a 14,400-foot diameter Antares – that last is approaching three miles!

Don’t bother to calculate the volume. Unless you use scientific notation, your calculator probably won’t handle it. But you get the idea. That little dot of red light we call Antares is B-I-G. And don’t forget – on this same scale the huge planet you are standing on is just 2-inches in diameter.

Here’s a graphic representation courtesy of Sakurambo:

Notice the artist didn’t even attempt to represent the Earth on this scale!

Think of it this way. If Antares were our star, both the Earth and Mars would be orbiting inside it!

That’s huge – even bigger than Deneb – which we noted last month was a “supergiant” – the same class that Antares belongs in. But Deneb would only reach about halfway to Earth – Antares would go past both Earth and Mars. Deneb, however, is a very young, very bright, very hot star, which is why it shines so brightly from such a great distance. Antares is much closer – about 600 light years vs at least 1,425 for Deneb. But Antares is old – a star in its dying stages, and is large and bright because it is so bloated. It really is quite cool as stars go – that’s why it appears red to us. But it has such a huge surface area that even from a distance of 600 light years it appears bright to us – a bit brighter in our sky than Deneb, actually.

So let’s briefly consider these four guide stars together – Vega is our “standard” star – white, about the size of the Sun, and quite close at 25 light years. Altair has some unusual features, but is still rather normal as stars go. Deneb is distinguished by being large and hot; Antares by being even larger, but relatively cool.

Vital stats for Altair (AL-tair), also known as Alpha Aquilae:

• Brilliance: Magnitude .77; its luminosity is the equal of 11 Suns.
• Distance:16.8 light years
• Spectral Types: A, main sequence
• Position: 19h:50m:47s, +08°:52′:06″

Vital stats for Antares (an-TAIR-ease), also known as Alpha Scorpii:

• Brilliance: Magnitude 1.09; its luminosity is the equal of 65,000 Suns.
• Distance: 600 light years
• Spectral Types: M, supergiant
• Position: 16h:29m:24s, -26°:25′:55″

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