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Astronomy L0l Lecture 券l Astronomy 101 Lecture #2 Today: Time in Astronomy Charts and Catalogs Reading: Ch. 2, 3 Tomorrow, and tomorrow, and tomorrow, Creeps in this petty pace from day to day. ± William Shakespeare What then is time? If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know. ± Saint Augustine Astronomy 101 Lecture #2 I. The Length of Day Sidereal Time = RA of stars crossing the meridian or HA of Vernal Equinox. For any star: ST = RA+HA Sidereal Day = time between two successive meridian crossings of VE . Astronomy 101 Lecture #2 Solar Time = HA of the Sun +/- 12h Solar Day = time between two successive meridian crossings of the Sun. Difference between solar and sidereal days. Astronomy 101 Lecture #2 Solar Time is non-uniform due to: 1) ellipticity of the Earth©s orbit 2) tilt of the Earth©s polar axis with respect to the ecliptic. Mean Solar Time (MST) = HA of hypothetical mean Sun +/- 12h. Mean Solar Day = time between two successive meridian crossings of the hypothetical mean Sun. 1 solar day = 86,400 solar seconds. Astronomy 101 Lecture #2 Universal Time (UT) = MST at Greenwich observatory in England. Also known as Greenwich Mean Time (GMT). Difference between PST and UT is 8 hours in winter and 7 hours in summer (because of daylight savings time change). Astronomy 101 Lecture #2 How long a minute is, depends II. Precise Timekeeping on which side of the bathroom door you're on. -Zall's Second Law Before 1958: 1 s = 1/86400 solar day t / t≃2×10−10 1958 ± 1867: 1 s = 1 ephemeris second = = 1/31556925.9747 tropical year After 1967:Sun 1 s = 1 atomic (SI) second = 133 = 9192631770 cycles of Cs hyperfine transition. t / t≃10−14 Astronomy 101 Lecture #2 Atomic Time (TAI, Temps Atomique International) = based on SI second. TAI clock agreed with UT clock in 1958. 1 SI second = 1 solar second in 1820! Now the offset between UT and TAI clocks is About 1.2 s per year. Universal Coordinated Time (UTC) = based on SI second, but is adjusted on June 20 and/or Dec 31 by a adding a leap second to match UT to within 0.9 s. Astronomers typically time-stamp their data using UTC! Astronomy 101 Lecture #2 Effects of General Relativity 1. Moving clocks run slower relative to a clock at rest. 2. Clocks in a gravitational potential well run slowly relative to a clock in away from sources of gravity. Barycentric Dynamical Time (TDB) = time kept by a clock on a surface of a hypothetical Earth in a circular orbit around the Sun with the same period as the real Earth. Eliminated annual fluctuations with amplitude of about 1.7ms relative to TDT. Barycentric Coordinate Time (TCB) = time kept by a clock infinitely far from the Solar System barycenter and at rest relative to it. Runs faster than TDB by 49s per 100 years. Astronomy 101 Lecture #2 Heliocentric Time = time recorded by an observer on a hypothetical Earth located at the Solar System barycenter. Runs at the same rate as BDT, but eliminates time of travel variations caused by the varying distance between the Earth and the target due to Earth©s motion around the Sun. Earth Sun Star Earth Astronomy 101 Lecture #2 II. Calendar Julian Date (JD) = number of days since 12h noon on Jan 1, 4713 BC. At 0h UT on any day after Jan 1, 2000: JD = 2,451,544.5 + 365 (Y ± 2000) + N + L Y = year of interest N = day number L = number of leap days between Jan 1, 2001 and the day of interest. Epoch = J2000 was set at 12h Jan 1, 2000 (TDB). JD(TDB) = 2,451,545.0(TDB) +(365[E±2000]). Astronomy 101 Lecture #2 II. Calendar Sidereal Year = time it takes the Earth to orbit the Sun relative to distant stars. Sidereal year = 365.256363 days Tropical Year = time between two successive vernal equinox crossings of the Sun. Tropical year = 365.242190 days .
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