Nov 24: Angular Separation
Amateur astronomers sometimes have the need to know the angular separation between two objects in the sky. In my case, I have been planning a session of astro-imaging using my DSLR camera on a tripod. (I am being optimistic about the weather for our next MAS observing session). It saves precious time in the field if you already have an idea of the target 'field of view' and which camera lens (and zoom setting) will cover that for the particular group of objects to be imaged.
There are two methods for obtaining the angular separation: (a) estimating; and (b) calculating.
Estimating is quite simple if you can remember the rules but it is not of much help with my image planning. If you are out in the field without a computer and need to estimate the angular distance between two objects, hold your fist at full arm-length. The angular separation of your knuckles is about ten degrees. This image gives a few other options but I can't remember them all:
A more accurate option is to calculate the angular distance. I found a good software application to do this going by the very original name of "Angle Separation". This programme is designed for the geographic coordinates Longitude and Latitude but it works equally well with the sky because the celestial sphere coordinates Right Ascension and Declination work on the same principle as geo-coordinates.
Let's say we wish to measure the angular separation between Beta Centauri and Beta Crucis. In the following picture, these are the uppermost of the two bright "pointer" stars to the left of the pane; and the bottom left of the four brightest Crux stars just above the pane, on the left. The Celesial co-ordinates of these two nearby stars are:
Beta Centauri: RA: 14hr 03m 49.4s; DEC: 60d 22m 22s S
Beta Crucis: RA: 12hr 47m 43.2s; DEC: 59d 41m 19s S
Enter the RA coordinates as Longitude and the Declination coordinates as Latitude:
That's a lot of decimal places but the angular separation between the two stars is effectively 9.5 degrees. Now you can go outside and confirm that by holding your fist out!
The free Angular Separation sofware can be downloaded from:
http://www.softpedia.com/progDownload/Angular-Separation-Download-211326.html
It requires Java.
