How far can you see with that?

I was asked the other day, "How far can you see with your telescope?" To answer that question I need to deal with it on two levels. One, distance in miles (or kilometers); and two, in time.

For the first we need to realize that when we look up into the night sky we are seeing objects that are, in some cases, millions to hundreds of millions of miles away. Often the vastness of space is hard to comprehend when dealing with numbers of that kind.

That is why astronomers use distance measured by the speed of light, 186,000 miles per second. When dealing on that level the moon, which on average is 250,000 miles away, is one and one-third seconds away. The sun is 93 million miles or eight and one-third minutes away.

All of the stars we see in the sky are part of our own galaxy, the Milky Way and they can range from hundreds to thousands of light years away. The closest star to our sun, Proxima Centuri, is about four light years away. The galaxy itself is some 90,000 light years across and some 3,000 light years thick in the middle.

Take just the stars of the Big Dipper for instance. They range from 78 to 124 light years away. So when we look up into the sky we are seeing not just in distance but into time.

So, to answer the question of how far can I see it is more than just a matter of distance, it is a matter of time.

Using just the eyes alone, how far, in time, can the average person see? That one is easy. The great Andromeda Galaxy is the farthest the average person can see without any visual aid, that is, no binoculars or telescopes.

The galaxy is located in the Andromeda constellation which can be found by first looking almost directly overhead and finding the Great Square of Pegasus.

Next find the star Alpheraz which is the easternmost, or lowest star to the horizon (since at this time of year the "square" is a "diamond").

Next follow the two lines of stars extending east toward the constellation Perseus. Count two stars out from Alpheraz on those lines, estimate the distance between the two lines and go that distance up, or left, to locate a faint, fuzzy-looking blob. That blob is the Andromeda Galaxy.

When you find it you are seeing as far as you can see using just the eyes alone, some 2.9 million light years or a little more than 18 million trillion miles.

Kind of staggers the mind doesn't it.

SKY WATCH:

New Moon on Dec. 8. I hope you are watching the planet Mars on these early evenings. It is well above the eastern horizon after 8 p.m. and well worth looking at. It was a pretty sight last week when it was very close to the moon. If you missed it the moon will be very close again on Dec. 23, when you might be able to see it, Mars, and the star cluster M35 in the same binocular field of view. If you are out Mars watching, compare its color to the red giant star Betelgeuse which is the right shoulder star of Orion, the Hunter just to the right of Mars. Now is a good time to start looking for a dark sky place to observe the Geminid meteor shower next week. More about that later.

Next time:

More astronomical blathering.