Here's how it works:

1. Leveling: The astrolabe has a plumb line or spirit level to ensure it's perfectly horizontal. This is crucial for accurate measurements.

2. Sight: You then sight the celestial body (sun, moon, or star) through the alidade (the movable ruler on the astrolabe).

3. Reading: The altitude of the celestial body is read from the graduated scale on the astrolabe's rim, where the alidade intersects it.

This information, combined with the time of observation and the user's location, can be used to determine:

* Latitude: The astrolabe was a key instrument in navigation for centuries, allowing sailors to determine their position north or south of the equator.

* Time: By observing the altitude of the sun at different times of day, you could determine the local time.

* Other astronomical data: Astrolabes were also used for various astronomical calculations, such as calculating the time of sunrise and sunset, and predicting lunar phases.

So, leveling the astrolabe is the first step towards making these important astronomical observations.