Learning the Sky
Like much of this site, please note that this section is currently being written. As it grows, we intend to include some simple animations to illustrate the motion of objects in the sky. Our goal is to give you some simple steps towards finding your way around the heavens. We will also point you to links that go into much more detail and recommend books and programs on the subject. As with everything we do, we invite your comments as we place great value in your feedback.
Learning your way around the sky is in some ways like learning your way around a new city. Often, people look for land marks or sign posts to help them navigate their new surroundings. In the same way, you can use some of the constellations as sign posts to help you navigate the skies. For northern latitudes, we recommend as a start that you familiarize yourself with the location of Polaris (the North Star) and the following three constellations.
Ursa Major (The Big Dipper)
Ursa Minor (The Little Dipper)
Once these are learned, they can be used to point you to other stars and constellations.
One difficulty in learning the sky is that things don't stay in the same place for very long. They are constantly on the move. Actually, it is not really the stars that are moving but us. Because the Earth revolves on its axis, we have the impression that the sky is moving. This is easy to observe. Go out tonight ( unless its cloudy ) and look for the Big Dipper. Make a mental note of where it is. Now go back out a few hours later and look for it again. You will see that it has moved. Fortunately, this motion is not that hard to figure out. For people in the northern hemisphere, the sky seems to revolve around Polaris.
Look at the next few graphics. As you look at these pictures, think of Polaris as being in the center of each picture. These pictures represent the general positions of the three constellation mentioned above in summer, spring, winter and autumn skies.
A note on seeing these constellations:
Depending on how dark your skies are, you may or may not see all of the constellations as depicted above. In general, the Big Dipper is an easy target. Cassiopeia is not as bright and the Little Dipper is even dimmer. If you are having a hard time seeing these, try to walk away from any major sources of light that may be washing out your skies. How well you can see these constellations can also be used as a measure of what astronomers call seeing (more on this later).
The last 2 stars in the dipper of the Big Dipper point right at Polaris and in the general direction of Cassiopeia.
Understanding the motion
Try not to think of the sky as flat. Rather, imagine a small ball inside a very big ball. Now imagine that you are standing on the small ball at its north pole. All around you from your zenith (straight overhead) to your horizon you can see the inside of the big ball. On it are painted the stars and constellations. Now imagine that the ball you are standing on starts to spin slowly on its north /south axis. Remember, you are standing on the top of its northern axis of rotation. You would begin to notice the constellations moving around you. If you looked straight up, you would see the North Star, Polaris. If you watched the rotation of the objects for one full turn of the small ball, you would notice that everything had turned nicely around Polaris. You would also be able to see all of the constellations moving around your horizon. This is very much how you would see the heavens from our north pole.
The following animation illustrates how the Big Dipper would move around Polaris and the Little Dipper.
Now imagine you leave the north pole and travel to London England. From there, if you looked north, you would find that Polaris is no longer right above you but is about half way between your horizon and your zenith. The little ball (Earth) is still spinning exactly the same way that it was when you were at the north pole but you are no longer standing on its axis of rotation. Unless you travel to the southern hemisphere, you will still see Polaris all night long. However, through out the night, you will only be able to see constellations that are close to Polaris. Because you are no longer at the axis of rotation, some stars and constellations will rise from your eastern horizon and set in your western horizon. The further south you go from the north pole, the greater this effect. (Graphic coming soon)
Using constellations as pointers
As we mentioned above, constellations can be used to point to other constellations, stars or other celestial objects. Look at the next picture of the Big Dipper (Ursa Major) and follow the blue lines from it. (While looking at these picture, bear in mind that the orientation of the big dipper is dependant on the seasons as we discussed above. In some seasons, not all of the objects we point to will be visible.)
The last 2 stars in the dipper point right at the north star Polaris. Another line points at Capella and another at Castor. Just below Castor is Pollux. A line drawn from Polaris and just above the first star in the Big Dippers handle leads you to Arcturus, the 4th brightest star in the sky.
We also mention Mizar (shown in red). Mizar is the second star in the Big Dippers handle. It is actually a binary star (or double star). A binary is a double or multiple system of stars gravitationally bound together. One star is in orbit around the other. Mizar is part of a binary system. Even a pair of binoculars (held steady mind you) can easily resolve this pair. The smaller star next to Mizar is called Alcor. Now you know the name of two of the stars in the Big Dipper and how to find some others using this constellation as a pointer.
Imagine you wanted to find the Whirlpool Galaxy M51. Its a great target to test your skills and it will show in even a modest telescope though please do not expect to see spiral structure. Don't try this on a moonlit night as this is a faint object that the Moons light will wash out. What you will see is what many call a fuzzy blob. With really dark skies, and your eyes dark adapted, you might see its companion galaxy NGC 5195. An easy way to find it is by first finding the star (24 Canum Venaticorum) shown on the following diagram. Once you have it found, imagine a triangle as shown by the blue lines and point your telescope there. A Telrad finder really helps here and makes this so much easier.
One more example with the Big Dipper is to use 2 of its stars to point to a beautiful pair of galaxies, M81 & M82. Again, don't expect to see any structure but what you will find are 2 lovely galaxies that fit in one lower power field of view. To find them, draw a line in your minds eye as shown in the diagram below from the star Phecda to the star Dubhe. Now keep that line going till it is double the length between the 2 stars. M81 & M82 are in that vicinity. The line will not take you right to them but you will be very close. Now, gently sweep the area with a low power eyepiece until you find them. You will know when you see 2 fuzzy blobs in the same field of view. If its a really dark night, and you have about 6" aperture, you might happen upon a third galaxy which is close by called NGC 3077. Again, a Telrad finder really helps. Using a Telrad and a few charts we printed from Sky Map, at about 11 years old, my daughter was able to find over a dozen galaxies in one outing (not all in this region of space).
In the next example, we use Cassiopeia to lead you to the Dual cluster in Perseus and the Andromeda Galaxy. Look at how 3 of the stars in Cassiopeia form an arrow that points to another constellation called Andromeda. We have circled "part" of the constellation in red. The middle stars point to the Andromeda Galaxy. On a dark moonless night, you can just see this with you naked eye as a smudge in the sky. It is helpful to use your averted (or peripheral) vision. Your peripheral vision is more sensitive to light than your straight on vision. Also, you should allow your eyes to become dark acclimated. It takes at least 15 minutes before your eyes start detecting more and more light. You have probably noticed this before on dark evenings. As the evening progresses, you can see more and more stars. This is because your eyes have adjusted to the dark. In your binoculars, Andromeda reveals itself as a smudge. Unless you have a very large aperture telescope, do not expect to see the dust lanes shown in time exposed astro-photographs. However, as you look at Andromeda, consider that it is the most distant object one can see with the naked eye. At a distance of 2.2 million light years, it is the closest spiral galaxy to our own Milky Way Galaxy. It is about 1 1/2 times the size of our galaxy and contains some 500 billion stars. It is also called M31 in the Messier catalog (see Messier objects section).
Cassiopeia also leads you to one of the skies most beautiful sights, the dual cluster in Perseus. As with the Andromeda Galaxy, the dual cluster is visible to the naked eye as a smudge. However, when viewed through a pair of binoculars, one begins to see many of the stars in 2 separate clusters. A small telescope with a low power wide field of view offers a truly beautiful image. These 2 clusters are called NGC 884 and NGC 869. In case you are wondering, NGC stands for "New General Catalog".
Using this illustration, you learn the position of a new constellation (Andromeda), a galaxy and a star cluster. See, its not that hard if you approach learning the sky a bit at a time.
These previous examples are meant as suggestions on how to use constellations to point to other objects of interest. The objects we point to are but a sampling of what the sky has to offer. We highly recommend that you purchase a good book on the subject (NightWatch), a star atlas and/or a computer program such as Sky Map or Distant Suns. The next image is a screen shot from Sky Map meant to show you the helpfulness of these great programs. This particular image shows objects to magnitude 6 (see glossary for explanation of magnitude). These programs allow you to adjust the magnitude up or down.
How The Sky Is Measured
The apparent distance between objects in our sky (stars, planets, constellations etc.) is measured in degrees, minutes and seconds. The distance from the horizon to your zenith (straight overhead) is 90°. The distance from the eastern horizon to the western horizon is 180°. The whole sky is 360°.
Unit of measure Symbol Value Degrees o 360° for the whole sky Minutes ' 60 minutes per degree Seconds " 60 seconds per minute
The next time there is a full moon high in the sky, look at it and think ˝°. The moon and sun are both about 1/2° in the sky. Look at the pictures below. This is a simple way to measure objects or the distance from object to object in the night sky.
( I can not claim credit for this system of measurements. I have seen it used often and I hope I am not stepping on any toes using it here. If so, I could redo this with feet but it would be somewhat awkward. The hand model was my daughter.)
Of course, everyone’s hands will differ slightly. Look at the measurements in the Big Dipper below. Use this (print it) and if its clear skies, go out tonight, stretch out your arm toward the Big Dipper and hold your hand up in the positions shown in the previous images and see how your hand matches up to the degrees stated in these pictures. Take a pad with you and make some notes. Now you have a rough but very handy (no pun intended) way to measures distances in the sky.
With a bit of practice, you'll be able to use this system to help you find objects in the sky.
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