Tools Astrocalculator Sidereal Time module The Sidereal Time calculator module Sidereal Time This module converts between Universal Time and Local Sidereal Time. First, select whether you will use Universal Time or Sidereal Time as an input value in the Input Selection section. You must also specify a geographic longitude, and a date for the calculation, in addition to either the Universal Time or the Sidereal Time value. When you press the Compute button, the corresponding value for the other Time will be displayed.