Here's why:

* Altitude: This refers to the angle between a celestial object and the horizon. It's a crucial measurement used for navigation and timekeeping.

* Leveling: The astrolabe's accuracy depends on its alignment with the horizon. If it's tilted, the measured altitude will be incorrect.

* Reading: The astrolabe uses a system of graduated scales and sights to determine the altitude. Leveling ensures these scales and sights are aligned properly.

Therefore, ensuring the astrolabe is level is essential for obtaining accurate readings of the altitude of celestial objects.