I have a printed star map. I need to break it down to get alt and azimuth.

I need to figure out the exact location. I just have no idea how to do it.

  • 3
    $\begingroup$ You should probably attach a scan of the map to your question. It might not be possible to work out an exact location without a date and time that the star map is supposed to depict, and exact azimuthal coordinates on the depicted stars. $\endgroup$
    – notovny
    Oct 29 '21 at 15:30

Your star map shows the right ascension and declination of the stars. The right and left edges of the map are the 0 (and 360) degrees right ascension (or sidereal hour angle). The Earth rotates, so all the stars have an apparent motion east to west. They're always in the same north-south position, for your purposes. So to begin, you need to know exactly what time it is, and where the right ascension 0 degree line is at that time. As I'm writing (1200 UTC) the line is over the 140 degree east longitude meridian - near Japan. (I looked it up.) I'm at 75 degrees west longitude so a star directly overhead (which I can't see because it's daytime) would be at 145 degrees right ascension. I'm at 39 degrees north latitude, so it would be at 39 degrees north declination.

That's the short explanation for how you could find the spot on the Earth that the star is directly above (its geographical position (GP)). After that, all you have to do is solve a spherical triangle with vertices at the North Pole, you and the star's GP. The angle at the pole is the difference between the longitudes. The two sides are 90 degrees minus the latitudes of you and the GP. You can solve for the angle at you, which is the azimuth of the star, and the length of the third side, which is 90 degrees minus its altitude.


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