Locating objects in the night sky using RA and DEC

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Look up! What’s in the Night Sky

By Bryan Shumaker

Hello again, night sky aficionados! It appears that most of our observing is confined to “armchair” observing—looking at pictures and reading books since the skies are unwilling to cooperate. That being said, we will visit a few topics that can be very helpful when we can actually get outside and look up at the night sky.

The moon is last quarter on the 24th, and on this day in 1986, the probe Voyager 2 flew past Uranus.

The Mars rover Opportunity landed there on the January 27, 1986.

January 27, 1967 was a sad day in space history—the crew of Apollo 1 perished in a fire during launch pad testing.

Major design changes were made after this to prevent a similar disaster.

The winter constellations are in their full glory, so if you do get the chance to get out under clear skies, I encourage you to do so.

Check out any planetarium program and go outside prepared and try and find the objects you have seen in apps, books, and planetarium programs.

It’s very rewarding to actually be able to find and clearly identify the constellations!

You will notice that star charts have a whole bunch of numbers by each object.

These often are the location in the sky of the object so it can be found again easily.

Just what do they mean and how does one navigate around the sky?

You may also see the terms “RA” and “DEC.”

These stand for Right Ascension and Declination.

Every celestial object has a specific location written out in RA and DEC.

These are points defined in space and have t do with an imaginary grid laid out grid laid out on the night sky.

You can see this grid in most planetarium programs as a toggle on and off feature.

The picture accompanying this article is of the Orion constellation with the RA and DEC grid superimposed on it; the numbers at the edge of the image are part of this grid system.

Last time we started to talk about ways to find objects in space from Earth with a telescope.

Again, all points in space are defined by a RA and DEC grid.

RA stands for Right Ascension and DEC stands for Declination.

RA is simply the celestial equivalent of longitude, so imagine a similar set of lines running from the celestial north pole to the celestial south pole in the sky.

They are typically divided up into hours, minutes, and seconds. Each hour has 60 minutes, each minute 60 seconds, and so on—just like time.

DEC is similar to latitude, but in the sky, and is listed in degrees.

Each degree is comprised of 60 arc minutes.

Each arc minute is made up of—you guessed it—60 arc seconds.

The celestial north pole has a DEC of +90 degrees and the south celestial pole is -90.

The celestial equator (where the line drawn from one pole to the other crosses directly overhead) has a DEC of 0 degrees.

For an idea of scale, the moon is about ½ of a degree wide, or about 32 arc minutes when full.

If you have a telescope mount that can be aligned to the celestial north pole, you can find the object you are looking for by using setting circles.

These are just movable calibrated collars on the mount.

Once you set them for a known object (by looking up the location in an almanac or star chart) and lock them, it’s quite easy to rotate the telescope around to an unknown object if you know its RA and DEC.

The other kind of mount is the ALT/AZ mount (Altitude and Azimuth).

These are easy to use and are similar to a gun mount.

North is 0 degrees, east is 90, and so on.

If you move the scope up, imagine the horizon as 0 degrees and overhead as 90 degrees.

These are great for visual observing, but for serious astrophotography you need a mount that aligns to the celestial pole, often called a GEM, or German equatorial mount.

We will talk a bit more about telescopes in the future.

Enjoy the night sky, and remember if it clears at all, go outside and look up!

Go to www.nomac.net for more information about the night sky.