October 2017

A little October fun!

Each month I will describe sights of interest in the night skies of South Carolina. These sights will be broken down into four sections; what you can see with the naked eye, with binoculars, with a small refracting telescope and with a Dobsonian reflector. The best time to view the night sky is at and around the times when the Moon is not visible, what is known as a New Moon; which will occur this month on October 19th. For October, your best viewing nights will be from October 10th to the 24th. The Star chart below is set for Florence, SC on October 15th at 9 pm. Note: A table of contents for earlier columns is located in the October 2016 column.

When to Take Out Your Telescopes:

The night sky has been unfriendly to astronomers for most of this year. In addition to the general problem of light pollution, there are two major natural situations that prevent amateur astronomers from enjoying the night sky. The first situation is cloud cover. This factor has ruined many ScienceSouth observing nights, and also caused us to miss the solar eclipse last August at Moncks Corner, SC. The second situation is the Moon. The light of the Moon one week before and after the full Moon prevents all but planetary viewing. You cannot adjust for clouds, just wait for a clear night. The Moon however moves with clockwork precision, so you simply plan your viewing the week before and after the new Moon.

Therefore, you should simply choose a viewing time without cloud cover and near the new Moon. This brings into play the real villain for amateur astronomy; heat.If the Earth’s surface is hot, as in summer, two heat factors ruin your nighttime viewing. First is excessive moisture in the atmosphere; we call this high humidity. How does this affect astronomy? Basically, atmospheric water doesn’t transmit light as well as clear air. A second factor is thermoclines. An atmospheric thermocline is the area where hot and cold air meet. I have been in deserts at midday, and the distant objects such as mountains are quite blurry. The reason is that the light is passing through many different variable densities of air, resulting in refraction, or bending of light.  Notice the wavy lines above a hot car hood or a hot highway distorting your view; see below.

These wavy lines are known as the Schlieren Effect. When you look through your telescope trying to find a distant galaxy, you will not actually see the Schlieren Effect, you just won’t see the dim object.  So as the hot wet air mixes with the cooler upper air mass, we end up with distortion, and of course, the hotter the surface air, the more distortion. This heat distortion is also affected by the amount of wind turbulence in the atmosphere. The end result is that usually the summer night skies are least favorable for astronomy.

To summarize: An apparently clear/cloudless night does not mean it is a good night for astronomy. There are three ways that I determine if I should bring out my telescopes on a clear Moonless night.

First I go on the computer and check an astronomer’s weather site called the “Clear Sky Chart”;  http://www.cleardarksky.com/c/FMUObSCkey.html?1.

This site reports the viewing conditions for over 5000 locations in North America. The link above is the Francis Marion University location. At first glance, this chart is quite confusing; see below.

I suggest you study the website to understand all of its functions. Focus primarily on the first three lines, cloud cover, transparency and seeing. When the squares on all three lines are dark blue, this is the best seeing that they can report. The first line is obvious; heavy cloud cover, white squares, no viewing, blue squares; no clouds. When the first and second lines are both dark blue, it will probably be a good viewing night. A dark blue third line just adds to your chance of a perfect night. If you note the image above, there were no good viewing nights for this time frame. So put the link to this site on your desktop or smart phone to quickly see the weather conditions for astronomers.

The second method I use is a naked eye view of the sky. If at any time of the year I can clearly see all seven stars in the Little Dipper, it should be a clear viewing night. From September through January, if I can see the Andromeda Galaxy (M31) in Andromeda, and/or the Circlet asterism in Pisces, it should be a good viewing night.

From February through May, if I can see the Beehive Cluster (M44) in Cancer, it will be a good viewing night. All through the summer, if the Milky Way is clearly visible, it should be a good viewing night.

If I can see the Lagoon Nebula (M8) in Sagittarius with the naked eye, you are in a great dark sky location. Finally, if you can see many stars inside the bowl of the Big Dipper, you are in one of the darkest skies on Earth.

The third approach, if the sky looks good with the naked eye, is to bring out your binoculars. I know various dim objects that I can see through simple binoculars. This time of year I will look for the Dumbbell Nebula (M27) in Cygnus; shown below. If I can easily see it, it is a good night for stargazing.

So let’s hope for cooler clearer drier skies as we head toward the winter months.

Naked Eye Sights: The Northern Cross asterism in Cygnus. The Great Square asterism in Pegasus.

Binocular Sights (7 to 10 power):  Andromeda Galaxy. The Double Cluster in Perseus. The Coathanger asterism in Vulpecula.

Telescope Sights (60-100mm):  Andromeda Galaxy. The Double Cluster in Perseus. The double star Albireo in Cygnus.

Dobsonian telescope (6 -8 inch):  Andromeda Galaxy. The Double Cluster in Perseus. The Ring Nebula (M57) in Lyra.

See you next month!

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