Principles of Celestial Navigation

Basics » Measuring Angles and Altitude What we actually measure with a sextant is the angle between the horizon and a star; this is called the star’s altitude. It is simply how high in the sky a star appears from the horizon. In other words, altitude is angular height. In fact, navigational "shorthand" for altitude is "H". But note that this isn't height as we normally think about it in terms of feet or meters, it is height in terms of an angle from the horizon.

This is an important point, so let's spend a few moments on this. If we were to ask, "how high in the sky is that star?", how would you answer? You may think in terms of feet, or meters, or miles. But these really don't make sense for something in the sky that is unimaginably far away. Instead, to measure the height of something in the sky we use angles. To illustrate, the distance between our horizon and our zenith is about 90 degrees. So an object about halfway between our horizon and our zenith would have an angular height of about 45 degrees. We say its altitude is 45 degrees. Roughly speaking, if you hold out your arm, the width of your little finger is about 1 degree and your closed fist is about 10 degrees. So remember that altitude is height, but it is measured by an angle. It is abbreviated H. We will use "H", meaning altitude, later in this module. There is a very simple relationship between zenith distance and altitude. Zenith distance = 90 degrees minus altitude. For example, if a star has an altitude of 89 degrees, its zenith distance is 90 - 89, or 1 degree. That is, it is very close to being directly overhead. A star near the horizon might have an altitude of 5 degrees; its zenith distance would be 85 degrees; a very big angle between the zenith and the star.

Up to this point we have been talking about the zenith distance of objects. But because what we actually measure is altitude, the navigational publications tend to use altitude, not zenith distance. This does not cause a problem, and actually keeps us from having to convert between the two.