Celestial navigation

The ancients believed that the stars were hung on a giant piece of black cloth, or were stuck on the inside of a giant crystal sphere and that was how they stayed up in the sky.

We now know this not to be true but the concept helps us to measure the positions of stars (and other objects) in the night sky so that we can reliably locate them.

To locate objects in the sky we need two coordinates: How far around; and how far up should we look. There are a couple of coordinate systems that are used. The first is Alt. / AZ. or Altitude and Azimuth.

The Azimuth is just another name for a compass heading and Altitude is how far above the horizon the star (or object) is.

This system is nice and simple, by the values have to be calculated for your specific position on Earth and the time of day.

The second system is RA / Dec or Right Ascension and Declination. With this system an imaginary line has been drawn onto the inside that crystal sphere that runs all the way around it. This is the line of Right Ascension and it is split up into 24 hours (with minutes and seconds). Declination is then a measure of how far above (or below) that line the star (or object) is. The benefit of this system is that the coordinates relate to the stars and not the Earth, so an RA DEC coordinate is the same for me here as it would be in Sydney, or Paris!

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