Astronomical Longitude Based on time difference between event in Greenwich and local occurrence Greenwich sidereal time(GsT) gives time relative to fixed stars GST=1.002737909371+0b+△ cost =24110.54841+8640184.812866T+ ulian Centuries 0.093104T2-6.2×10-673 Universal time UT1: Time given by rotation of Earth. Noon is mean"sun crossing meridian at Greenwich UTC: UT Coordinated. Atomic time but with leap seconds to keep aligned with UT1 UT1-UTC must be measured† 02/18/03 12.540 Lec 04 13 Astronomical Longitude GST =1.0027379093UT1+ J0 GMST { y e Precession 34 J0 = 24110.54841+ 8640184.812866 T Julian Centuries { + 0.093104T 2 - 6.2 ¥10-6 T 3 • Based on time difference between event in Greenwich and local occurrence • Greenwich sidereal time (GST) gives time relative to fixed stars + D 1 2 4 cos 02/18/03 12.540 Lec 04 14 Universal Time • UT1: Time given by rotation of Earth. Noon is “mean” sun crossing meridian at Greenwich • UTC: UT Coordinated. Atomic time but with leap seconds to keep aligned with UT1 • UT1-UTC must be measured 7