The beginning of a sidereal day (at midnight, sidereal time) at any location on Earth is defined by the passage of the:

Question

Quizplus · Accepted Answer

The beginning of a sidereal day is marked by the passage of the vernal equinox through the upper meridian at a given location. This is because a sidereal day is measured by the Earth's rotation relative to the fixed stars, specifically using the vernal equinox as a reference point.

The Beginning of a Sidereal Day (At Midnight, Sidereal Time)