I first started studying astronomy in my 7th grade in Armenia in an Astronomy Club organized by teacher Armen Oskanyan. One of the first concepts he taught me was the various coordinate systems used in Astronomy, at least 5 of them!
When one thinks of coordinate systems, typically what comes to mind is an x/y square coordinate system.
Image from Wikipedia, made by K. Bolino, https://commons.wikimedia.org/wiki/File:Cartesian-coordinate-system.svg
However, instead of centimeters (or inches or meters) and x-y coordinates, most of the time, we use degrees as units for each axis. It is not so intuitive to conceptualize. However, imagine that as an observer you are sitting at the center of a large sphere, and the stars and planets that you see sparkling on the sky are glued on the edge of that sphere. Yeah, it is similar to the ancient concept of a crystalline celestial sphere with stars glued on it, but now we use the concept of an imaginary sphere.
You can read more here, https://courses.lumenlearning.com/suny-astronomy/chapter/the-sky-above/
Right above you is the Zenith, it is 90 degrees from the horizon. The line of horizon is in itself 0 degrees then. The point beneath your feet (which you cannot see) is the Nadir. Beyond the horizon, the degrees become negative in value. So Nadir is at -90 degrees. So essentially, we are really measuring angles between two points on the envelope of the sphere connected to the observer. (There are several additional reference systems, where the reference point is not the observer, hence at least 5 coordinate systems 😀 ).
When dealing with degrees, we should remember the units are in 60-s :D. There are 60 arcminutes in a single degree, and there are 60 arcseconds in a single arcminute. So 1 degree = 60×60=3600 arcseconds!
Most of the time nowadays in my research papers I use the coordinates of Right Ascension and Declination (R.A. and Decl.). They are tied a bit to our geographical longitude and latitude – isn’t it cool ?!
So, for example, when I give a table of objects that I have observed with the Nordic Optical Telescope, I put into that table tho RA and Decl for each object, as their unique identifiers. When I try to observe in the telescope at night, I also input these coordinates to point the telescope at the right spot on the sky.
More here, https://skyandtelescope.org/astronomy-resources/right-ascension-declination-celestial-coordinates/ .
As you can imagine, a sphere is a 3D object, but often on the paper we put 2D x-y looking rectangular images, such as the grid here in my PhD Thesis. (Here is short explanation of the Figure, https://youtube.com/shorts/s9s_zZNySa4?si=_8Va3uYRWfra3l3G)
When dealing with small angle separation, it can be a reasonable approximation. However, one should always keep in mind the curved or 3D spherical nature of angle arcs on the imaginary celestial sphere between two objects.
I hope you are inspired next time to guess what is the angle separation between two stars that you notice on a clear night sky in your neighborhood. Go Astronomy !
Your astronomer from Helsinki, aka Maria Stone
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