Calculating the altitude of an astronomical body

The altitude is the angle of elevation (height in degrees) of a star or other astronomical object above the observer’s horizon. The altitude of an astronomical object changes throughout the observing session because of the Earth’s rotation. It is not an absolute measurement of position.

Diagram illustraring the altitude of a star

Diagram illustraring the altitude of a star

It’s not vastly useful as a means of measuring sky positions but you might use it as a convenience to shout over to your mate to give them an idea of the position of something interesting in the sky.

The maximum altitude of any astronomical object occurs when the object passes the observer’s southern meridian.

It could also be useful if you can figure out the maximum altitude of an object above the horizon from somewhere eles e.g.  if you are heading abroad for a holiday. That way, if it is faint and only gets 20 degrees above the horizon, it probably wouldn’t be worth looking at as it would suffer from too much extinction.

To work out the approximate maximum altitude of a star is done as follows:

The maximum elevation of the celestial equator for a particular spot is:
90 – LAT.

So for an observer at the latitude of London (51.5 N), the maximum elevation will be 90 – 51.5 = 38.5 over the southern horizon.

Then add (for north declination) or subtract (for south declination) the tabulated declination of the object you want to view.

Say you want to know the maximum altitude that the Andromeda Galaxy M31 will reach. Look up the declination (more or less 41degrees N) so it will be 41 degrees above the celestial equator i.e. 79.5 degrees altitude at most.

I say approximate because odds are you will be higher or lower than the real horizon (90 degrees from the zenith). There is a correction that can be applied using alebraic stuff but unless you need to use it for navigation, the approximate altitude is close enough.


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