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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.

Dawn patrol: 5 Planets and a crescent moon – this will be cool!

OK, I’m normally up at this hour so it’s a no brainer for me – I’m heading to the Allens Pond Bird Sanctuary parking lotwhere there’s a great view of the eastern horizon, preferably on the morning of May 21. That’s when I expect a wonder-full view of five planets and the waning crescent moon. I plan to be there at 3 am, but it should get especially interesting between 4 am and 4:30 am. Later than that? Well, we’ll be in a race with the dawn – everything will get higher, as dawn approaches, making it easier to see –  BUT – so will the Sun which rises that day at 5:19 am. So the sky will get lighter making things harder to see.  And, of course, there’s always the weather question – but in this case, all the players will be around right into early June in approximately the same positions, except, of course, the moon. So while I will aim for May 21, weather will determine the actual observing date.

All of this is local time and date for 41.5 north latitude and 71 west longitude. But a good guide for elsewhere in the world is to simply find out the time of local sunrise, then start looking 90 minutes  before that. All the action as viewed from the Northern Hemisphere is  between east and southeast and the territory will be nicely defined – and easily found – by bright Jupiter,  brilliant Venus, and the Moon. This makes it an excellent opportunity for folks not familiar witht he outer planets Neptune and Uranus to take alook for them.

The more difficult targets will be Mars and Uranus – Neptune, since it will be so close to Jupiter, should be relatively easy – though binoculars will be needed for Uranus and Neptune and, of course, a small telescope willmake it all more fun. Here’s a chart from Starry Nights software  for what I expect to see, given clear skies of course:

(click for larger chart)

(click for larger chart)

For me the key is to look at this chart – look at the sky – and then keep firmly in mind what is really going on – which is this:

(click to enlarge)

(click to enlarge)

To understand how this display relates to what we actually see in the sky, consider that the Earth is rotating counterclockwise, and all the planets are revolving around the Sun counterclockwise.

That means that as the Sun slips below your horizon on the May 20 Saturn will already be high in your western sky. It will set by 2:30 am. But at around 4 am on May 21 you will encounter Jupiter and Neptune first and they will be highest. (That’s what the first line projecting from Earth represents.) Then as you let your eye move towards the horizon – counterclockwise, towards the Sun – you encounter Uranus, Venus. and last, Mars – as shown by the other lines. Not shown is the Moon which will be in line between Venus and Mars.

This representation is modified slightly from Solar System Live.  While all the planets are roughly on the same plane, if their orbits are represented by a blue line it means they are above the plane of the Earth. If it is represented by a green line – as is the case with the five morning planets on May 21, it means they are below the plane of the Earth.

The most difficult planet to find is likely to be Uranus. If  we have exceptionally clear skies and you have exceptionally good eyes you may be able to see it with your naked eye – but for most people in most locations binoculars will be essential. Here’s a typical 7-degree binocular field showing Uranus and stars to magnitude 8 from Starry Nights software. The star 24 Piscium is the brightest in the field at magnitude 5 – the other named stars in the circle – and Uranus – are magnitude 6. Fortunately, the general position of this field is easy to see by drawing a line between Jupiter and Venus (see the first chart). As you move from Venus up towards Jupiter, count three binoculars fields along this line  – Uranus  should be in the third field. Notice that Uranus is the last “star” in an arc of four reaching upward from the bottom of the field – the forth one being just below the field if you put Uranus at dead center.

(click to enlarge)

(click to enlarge)

Neptune is much easier to find. See this posting for details on it.

This is a good event for naked eye and binocular users. Yes, a small telescope will help. Jupiter and Venus will be fun to see in a telescope. Neptune and Uranus are too small to show anything except a tiny disc and Mars is a long way from us showing a disc only one-eighth that of Jupiter, so no details will be visible there either.

Interesting note: A friend in Austrailia said he would look for this, so I made him a quick chart for how things would look at 6 am in Australia on May 21. Compare that chart (below) with the view we get (first chart on this post) in North America.  Note how high Fomalhaut is for them – not to mention Jupiter and Neptune – and, of course, they look to the northeast as we look to the southeast.

The view from Sydney, Australia on the morning of May 21. (Click for larger image.)

The view from Sydney, Australia on the morning of May 21. (Click for larger image.)

Choosing and using binoculars for astronomy

What binocular should I get to view objects in the night sky? The short answer is almost any binocular will help, but if you’re looking for a one-size fits all answer, get a good 10X50 binocular. Of course, one-size doesn’t fit all, so here’s some background that I hope will help you find the correct answer for you.

Binoculars are an incredible way to extend your reach into astronomy and while I recommend everyone start by observing with the unaided eye, binoculars make a wonderful next step. There are a handful of astronomical objects that are best seen with binoculars and a lot more that become more interesting and beautiful if you point binoculars at them.

Binoculars are essentially a pair of low-power, wide-field telescopes that are easy to carry and easy to use. As with any telescope, their main purpose in astronomy is to gather more light allowing you to see fainter objects. If your naked eye view of the stars ends with stars of magnitude 5, binoculars can extend that view to magnitude 9 or even 10. There are approximately 2,800 stars of magnitude 5 or brighter that can be seen with the naked eye. But use binoculars and that number jumps to roughly a quarter million of magnitude 9 or brighter stars and more than half a million stars at magnitude 10 or brighter. So you see, binoculars make a huge differences!

Binoculars are commonly identified by two numbers representing their power and the diameter of their objective lens. For astronomy, the best all around choice – in my opinion – is 10X50 binoculars. The first number means the binoculars magnify 10 times – which means essentially you instantly cut the distance between you and astronomical object by a factor of 10! The second number refers to the diameter of the objective lens in millimeters.

This last is an indicator of how much light the binoculars gather – and thus how faint the object is that you can see with them. This number is a bit deceptive because what is important is the light gathering area of the lens. Let’s take a look at two simple examples to see how this changes.

To begin with, the eye of an older adult typically opens to about 5 mm.(Yes, the eye of a teenager may open to 7mm or more – but this quickly changes with age in most people.) Let’s assume 5 mm is the norm. This figure means the light gathering area of your eye is roughly 20 square millimeters. A 40 mm binocular lens has a light gathering area of 1256 square millimeters – more than 60 times that of your eye. A 50 mm binocular lens has an area of 1,962 square millimeters – roughly 100 times that of your eye. Notice the big jump between the 40 mm and the 50 mm lens. Area goes up much faster than diameter.

The power number is also important, but only for putting a high end to things. It’s easy to add power to a binocular, but even a person in excellent physical condition can not hold a high-powered binocular steady. In fact, I believe that any binocular above 10X needs to be put on a tripod for a steady view – and that decreases its ease of use tremendously. Frankly, even a 10X binocular can reveal a little more if put on a tripod. I would favor binoculars a bit below 10 power generally, but there’s a catch.

There is no sense bringing more light to your eye than your eye can use. The light from a binocular is focused in a cone known as the exit pupil. Exit pupil – the diameter of this cone as it enters the eye – is easy to determine. Just divide the power into the diameter of the lens. Thus a 10X50 binocular has an exit pupil of 50 divided by 10 which is 5 mm. That 5 mm exit pupil matches the dark-adapted eye of the typical adult and that’s why I recommend this as the limit.

Popular 7X50 binoculars provide an exit pupil of 7 mm. Since this is much larger than the pupil of your dark adapted eye, you actually will see less with these binoculars at night then with a 10X50 pair. The lenses are the same size and they gather the same amount of light – but with the 7X50 binoculars that light is put in such a large cone that nearly half of it misses your eye. Suddenly the 50 mm lens is acting more like a 30 mm lens in terms of the faintest star it shows you!

The last number that is critical is eye relief. This too is in mm and you want something that provides at least 15mm of eye relief. This number simply represents the distance your eye should be from the binocular lens. It becomes especially important for people who wear glasses and only then if you have to wear glasses because of astigmatism. If astigmatism is not the issue, you should simply take your glasses off when using binoculars. Focusing the binoculars will correct your vision. But for convenience sake, some people like to leave their glasses on. With your glasses on you can not get your eyes real close to the binocular eye lens and this may mean that you lose a significant portion of the field of view and light that is gathered. You want binocular designed so that when wearing your glasses you still get the full use of them and this is why you want an eye relief of at least 15 mm.

So does this mean that everyone should purchase 10X50 binoculars for use in astronomy? Absolutely not. Your rule should be this:

Purchase the largest binoculars you can comfortably hold whose exit pupil is lower than 5.

Binoculars get heavy quickly as you turn your head upwards and peer at the sky searching diligently for a star cluster or nebula. And heavy binoculars can just be a pain to carry around all evening.

So, for example, 7X35mm binoculars do not collect nearly as much light as 10X50. But they are still an excellent choice for many people because they are much lighter and because it is easier to hold 7X binoculars steady than 10X ones. Notice that the exit pupil is exactly 5mm – that is 35/7 equals 5. Similarly, 8X40 make an excellent choice.

Don’t get fanatic about these numbers. Any binoculars will help you see more.

Choosing binoculars for astronomy, by the way., doesn’t preclude their use for other activities, such as bird watching. And sometimes their ability to do well in dim light – while critical for astronomy – is also very helpful when looking for a bird or deer in twilight.

Finally, whatever binoculars you choose, do learn how to use them. Most binoculars have a central focusing knob and most people simply hold them up to their eyes and focus them with this knob alone – and this is a mistake.

When you first use a pair of binoculars you should do this:

  1. Aim at a distant object, close your right eye and focus with the central focusing knob using only the left eye and thus the left side of the binocular.
  2. Once focused that way, don’t touch the central focusing knob. Instead, close your left eye and use the diopter adjustment on the right binocular eyepiece (just twist that eyepiece, it turns while the other one doesn’t) to make sure the view through the right side is in perfect focus.
  3. Thereafter you simply use the central focus with both eyes open for any objects you wish to focus on.

It is the second step that is usually ignored – but when you ignore it you are not getting the most out of your binoculars. It’s a bit of a bother, but our two eyes are seldom the same. And binoculars adjusted this way for one observer will not work at their best for another observer. Usually binoculars are a personal item that are used by a single observer. But if someone hands you a pair of binoculars to observe with, you should first adjust them in this two-step process for your vision. Then when you hand them back to their owner, it is polite to remind them that you have changed the diopter adjustment and they will need to change it back.

Of course, they may not know what you are talking about when you say this. I’m amazed at how many people who own binoculars are unaware of this. In that case, you can be helpful and explain it to them.

To sum up:

  • Make sure you’re comfortable with the weight of the binoculars you choose.
  • Get the largest diameter objective lens you can comfortably carry and hold.
  • Don’t exceed 10X magnification for handheld binoculars.
  • Don’t exceed a 5mm exit pupil unless you are young and know your dark adapted eye opens wider than 5mm.
  • If you must wear glasses, make sure the binoculars you choose offer enough eye relief so that wearing glasses does not cut down on your field of view.

A few things to generally avoid:

  • Binoculars with a ruby coatings on their lenses.
  • Zoom binoculars
  • Very wide field binoculars

My personal choice for astronomical observing are 12X36 Canon image stabilized binoculars. If you have been following closely you’ll see this violates the power rule – but only because I’m willing to pay a premium price for a good image stabilization mechanism. before I owned these I had a pair of 15X45 image stabilized binoculars, but I found that while these were excellent astronomy glasses, I frequently did not use them. They were simply too large and bulky for me to want to carry them all the time, or to hold them to my eyes for extended views. So I sold them, sacrificing power and light grasp for a pair that were smaller, but that I use much more often.

And that brings us to the final rule that applies to all astronomical instruments, including telescopes: The instrument that’s right for you is the one you use the most. Don’t get obsessed with the numbers game. Be aware of it, but choose what works for you.

Sliding south to Spica!

Prime time in May finds the Big Dipper high in the north east. Take a slide down its gracefully curved handle, first to  our April guidepost star – Arcturus – and then on to Spica, our May guidepost star, which happens to be  as close to first magnitude as any star in our sky.

If you thought all our guidepost stars were first magnitude, think again. Technically, to be called “first magnitude” a star should be in the range of .5 to 1.5.  But to be a “guidepost” star we use a broader definition. The star must be very bright and help us anchor an important section of sky. So we call Polaris a “guidepost,” yet it is almost exactly magnitude 2. But being almost at the north celestial pole, it certainly is a useful “anchor” for us. And, of course, we call  Sirius a “guidepost” star and it is much brighter than first magnitude at -1.46. In fact, several of the guidepost stars are brighter than first magnitude, including Arcturus which is -.07. .Spica, though is right on target. It’s official magnitude is .96, so it’s as good a representative of a first magnitude star as any. Look at it – then look at Polaris. Spica is about 2.5 times as bright as Polaris – that’s how much difference there is between magnitudes. OK – a difference of one magnitude is actually 2.512. Why? read “Step 6 – How bright is that star?”   for a complete explanation.

A couple notes about the chart below. First, it is adapted from Starry Nights Pro software and when showing this much sky, the horizon is represented as curved upward. This may catch you unawares, for Spica is closer to the horizon than it may look at first glance. Arcturus is about 52 degrees above the eastern horizon while Spica is nearly 20 degrees lower in the southeast.

Second, Alkaid, the last star in the handle of the Dipper, is named because it too is a second magnitude star like Polaris, though Alkaid is a tad brighter than Polaris. Still, you get a good representation of the magnitude system by looking at these three stars – Alkaid, Arcturus, and Spica, since they are magnitude 2, magnitude 0, and magnitude 1 respectively. Once you have located Spica, study the three stars with that in mind – but at the same time, don’t forget that any star lower int he sky will look a bit dimmer than it should simply because you’re looking through a lot more atmosphere than you are when looking at a star that is high.

slide_to_spica

Start your slide on the handle of the Big Dipper, continue down and past Arcturus, and down more to Spica. (Click on image for larger version.)

Step 7: How bright is that star?

magnitude_scale

We’ve been rating stars by their brightness for at least 2,000 years and the operation is really quite simple, though the whole system may seem backwards to you. In any event, it’s worth learning because the different brightness of stars is one of the main ways we can tell one from the other and make some sense out of the night sky.

Stars are ranked in brightness by a numbering system called “magnitude” – the lower the magnitude number, the brighter the star. Generally, the brightest star we see is magnitude 0 and the dimmest we see is magnitude 6. If that feels backwards to you, think of it this way. Stars of magnitude 1 – very bright – are “first class” stars. Dimmer stars are magnitude 2 – “second class” stars, etc.

Simple – yes, but of course there are exceptions and several ifs, ands, and buts that make the whole business of brightness quite fascinating!

First, 10 stars ( if you include our Sun) are brighter than magnitude 1 – so they are magnitude zero and if brighter than that,  we go into a minus system. Magnitude -1 is brighter than 0, which is brighter than one.

The brightest star we see is our Sun. It is magnitude -26.7! The next brightest object is the full Moon, at magnitude -12. Then comes the planet Venus at -4.7 (sometimes) and Jupiter, usually about -2.3 or -2.4.  Other than the Sun, the  brightest star in our sky is Sirius, at magnitude -1.46. We don’t get to an actual  magnitude 1 star until Betelgeuse, which is magnitude .5.  Confused? Don’t be. We call Betelgeuse first magnitude because it falls in the range of .5 to 1.5. No star is exactly a “1” in brightness, though Spica comes very, very close at .96.

Of approximately 6,000 stars bright enough to be seen by the average person from some spot on Earth, only 22 are of first magnitude or brighter. Quite an exclusive club!  These include all the ones we learn as signpost stars and, of course, some of them are visible only from the Southern hemisphere.

While most books will tell you that the faintest star we see with the unaided eye is magnitude 6, this too is only an approximation and doesn’t really represent today’s  world that well.  Generally speaking, in our light-polluted regions, the faintest star we are likely to see is magnitude 4. If you live well away from city lights, you probably see to magnitude 5.   On mountain tops, well above the thicker parts of our atmosphere and free of light pollution, some observers with excellent eyes and skills can see stars as faint as magnitude 8.5 with the unaided eye. In my own area, which is semi-rural, I’m very happy when I can see a magnitude 5 star with my naked eye.

With binoculars or telescopes we see stars that are much fainter, but that’s another story. Here we’re focused on what we can see with no optical aid.

Many familiar star patterns are made up primarily of second magnitude stars, such as Orion’s Belt and most of the stars of the Big Dipper.  The North Star (Polaris) is second magnitude as well as one other star in this asterism. But the other five stars in the Little Dipper  are magnitudes 3, 4 and 5. This makes the Little Dipper a good object to use to get a sense of magnitude.

On a star chart it is common to represent magnitude by making circles of difference size to represent the star and its brightness. That really isn’t the same as brightness, but it does give us an idea of what to expect. So, for example, here’s how the Starry Nights Software represents the stars of the Little Dipper.

Study the four stars that make up the cup of the Little Dipper for agood idea of the magnitude scale. It includes magnitude 2,3,4, and 5 stars.

Study the four stars that make up the cup of the Little Dipper for agood idea of the magnitude scale. It includes magnitude 2,3,4, and 5 stars.

You’ll find that when you get to know stars of different brightness, the magnitude system makes general sense to our eyes. That is, there seems to be about the same gap in  brightness between a magnitude 1 and a magnitude 2 star as there is between a magnitude 2 and magnitude 3 star.

While the magnitude system has been around since the early Greeks, it was given a mathematical foundation about 150 years ago when it was decided that there would be a gap of 2.512 times between each level of brightness. That means a first magnitude star is 2.5 times as bright as a second one – and exactly 100 times as bright as a star at magnitude six – five magnitudes fainter.

At that time it was thought this logarithmic progression matched the way the human eye responded to brightness. It does so, but only approximately.  Still, the system is convenient, stars are routinely listed this way, and the system easy enough to become acquainted with while observing.

One last note – don’t expect to simply walk out of your nicely lit house and see faint stars. At first only the brightest will be visible to you. Your eyes need time to adjust to the dark. After about 15 minutes you’ll see much fainter stars and after about half an hour you can assume that the faintest star you see is the faintest you will see that night.

But also keep in mind that what you can see at any given moment varies in a number of ways, including:

  1. How good your eyes are at detecting faint objects.
  2. How clear your skies really are – astronomers quickly learn that all “clear” nights are not created equal!
  3. Your physical condition at the moment, including how tired you are and whether or not you have been recently drinking alcohol or smoking, both of which can impair night vision.
  4. The height of the star above the horizon – when you look at stars near the horizon, you are looking through a lot more air then when you are looking at stars directly overhead.
  5. Your experience in looking at faint objects   – yes, you can actually learn to see dim objects by looking slightly to one side of them, a technique called “averted vision” that takes practice to do well.

All of this helps make star-gazing more fun. Variety is, indeed, a tasty spice, and no two nights of observing are exactly the same.  Nor is it all as cut and dry as it seems.  An experienced, older observer can actually see more, in some instances, than a younger, inexperienced observer whose eyesight is technically better.

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