Therefore, the synodic month takes 2.2 days longer than the sidereal month. The calendar recognizes two types of months&colon; synodic month and sidereal month. The calendar uses lunar months and solar sidereal years. The Vikram Samvat uses lunar months and solar sidereal years. Western astrology takes the tropical approach, whereas Hindu astrology takes the sidereal one. The celebration starts when the sun enters Aries according to the sidereal zodiac system. Ayanamsa systems are used in both Vedic and Western sidereal astrology. There are 27 Nakshatras present in the Sidereal Zodiac. The tropical and the sidereal zodiacs were lined up together about 2,000 years ago. The orbital period is equal to exactly one sidereal day. Geosynchronous satellites revolve around the Earth with a period of one sidereal day. A mean solar day is, therefore, nearly 4 minutes longer than a sidereal day. The latter three periods are slightly different from the sidereal month. Sidereal time is time by the stars. ERA replaces Greenwich Apparent Sidereal Time (GAST).