Why must astrolabes be level in science?

Answer 1

When navigating by the stars, or measuring an angle, there has to be a reference point. Oftentimes, we assume that a measurement is referenced to a particular point. (For example, a compass gives degrees in reference to magnetic north, not true north.) However, the use of a reference point is very important.

In celestial navigation, the height, or angle, of certain stars and constellations is measured in reference to the horizon. Basically, you take the angle your astrolabe/sextant/stadimeter, whatever, and measure the angle from the horizon to the star. This forms a rough triangle with vertices at your location, the point of the Earth beneath the star, and the star itself.

Obviously this is not terribly accurate, so you need to figure in a calculation taking into account the curvature of the Earth, and the expected position of the star. By using the correction and Nautical Almanac, you can create an ellipse that your ship must lie on if it is on the Earth's surface.

Take measurements from two or more stars and find where they intersect--that's roughly (+-50mi or so) your position.

The importance of shooting the horizon (i.e. being level) is apparent in the definition of Nautical Twilight--the level of light where you can use both the horizon and major stars as reference points.