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Announcements

Announcements. After lecture we will migrate to the APSU Farm Homework: Chapter 2 # 2, 3, 4, 5, 7 & 8. Time. Keeping time by the Sun isn’t very accurate. The Equation of Time. Since solar time varies by location, we use standard time.

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Announcements

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  1. Announcements • After lecture we will migrate to the APSU Farm • Homework: Chapter 2 # 2, 3, 4, 5, 7 & 8

  2. Time

  3. Keeping time by the Sun isn’t very accurate

  4. The Equation of Time

  5. Since solar time varies by location, we use standard time Each zone is 15° wide with allowance for political boundaries

  6. The Earth’s rotation rate isn’t constant Terrestrial Dynamical Time corrects for the changes in Earth’s rotation rate

  7. Sidereal Time Sidereal time is the time with respect to the background stars. One sidereal day is the true rotational period of the Earth. Uncorrected, it is 23 hours 56 minutes 4.091 seconds

  8. Calculating Sidereal TimeStep 1 First: convert standard time to universal time For Central Standard Time UT = CST + 6 hours For Central Daylight Time UT = CDT + 5 hours If result is greater than 24 hrs, subtract 24 and add 1 to the date.

  9. Calculating Sidereal TimeStep 2 Determine the “solar interval” and convert to sidereal interval. The solar interval is simply the time since 0 hrs UT Solar to sidereal interval conversion: multiply by 1.00273791

  10. Calculating Sidereal TimeStep 3 Calculate the Greenwich Sidereal Time (GST) Look up the sidereal time at 0 hrs Greenwich for the date and add the sidereal interval to it. If you don’t own a current Astronomical Almanac, use the following formula to find GST GST = G + 0.0657098245N + 1.00273791UT where G = GST at 0 hrs on “zeroth day” of that year (see Appendix 3 of textbook) N = number of days since the beginning of the year

  11. Calculating Sidereal TimeStep 4 Correct for local longitude Divide local longitude by 15° and add (if east of Greenwich) or subtract (if west of Greenwich) to GST to get Local Sidereal Time (LST) LST = GST ± (Longitude/15°)

  12. Julian Date Useful for calculating time interval between two dates. Julian dates start at noon UT The Julian Date (JD) is the number of days since January 1, 4713 BCE JD = 2,451,544.5 + 365 x (Year – 2000) + N + L Where N is the day number and L is the number of leap years since 2000

  13. The Julian Calendar Instituted around 46 B.C. 1 year = 365.25 days Years evenly divisible by 4 have 366 days. All other years have 365 The Tropical Year, sidereal year plus the effects of the Precession of the Equinox, is 365.242190 days so the Julian calendar “drifts”

  14. The Gregorian Calendar First instituted in 1582 1 year = 365.2425 days Years evenly divisible by 4 have 366 days except century years. Only century years evenly divisible by 400 have 366 days. All other years (including century years not evenly divisible by 400) have 365 days Modified Gregorian Calendar (Never officially adopted) 1 year = 365.24225 Years evenly divisible by 4000 are not leap years. Otherwise the same as the Gregorian Calendar

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