In the demonstration, 360∘ of rotation (one full rotation) represents a sidereal day. You can actually measure the length of the sidereal day by measuring the time from when __________ crosses your meridian on one day (or night) until it crosses the meridian on the next day (or night). Mastering astronomy

You can actually measure the length of the sidereal day by measuring the time from when a specific star (other than the Sun) crosses your meridian on one day (or night) until it crosses your meridian on the next day (or night). A sidereal day is approximately 23 hours, 56 minutes, and 4.091 seconds long.

A sidereal day is measured by the Earth's rate of rotation measured relative to fixed stars, rather than the Sun. Since the Earth is not only rotating on its axis but also orbiting around the Sun, after 24 hours, it has moved a bit in its orbit, so from the perspective of the Sun, it has to rotate a little more than one full 360-degree rotation to bring the Sun back to the same position in the sky. This is why a solar day (the time it takes for the Sun to return to the same position in the sky) is approximately four minutes longer than a sidereal day.

The concept of the meridian is important in astronomy. The meridian is an imaginary line that runs from the north geographic pole, through the point directly overhead (the zenith), to the south geographic pole. When a star or any celestial object crosses the meridian (this event is called a meridian transit), it is at its highest point in the sky at your local position and thus the best time to observe it.

In the demonstration, when measuring the length of the sidereal day using the stars, you can check both the star Sirius and the star Vega. A sidereal day is defined as the time it takes for the Earth to complete one full rotation relative to the background stars. Therefore, it doesn't matter which fixed star you use as long as it's visible from your location. You can measure the time it takes for either Sirius or Vega to cross your local meridian (an imaginary line that runs from due north to due south) on one day and then measure the time it takes for the same star to cross the meridian again on the next day. This period is the length of one sidereal day.