Announcements Dark Sky night Saturday night is looking iffy. Check www.apsu.edu/astronomy before heading out to see if it is cancelled. Homework: Chapter 2 # 2, 3, 4, 5, 7 & 8 Don’t forget the Lunar Observations lab. Telescopes can also be checked out over the weekend (not this one). Chapter 1 homework is due now. I will accept it up to tomorrow afternoon.

Where: APSU Farm When: ~4:30 tomorrow!

Time

Keeping time by the Sun isn’t very accurate

The Equation of Time

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

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

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

Calculating Sidereal Time Step 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.

Calculating Sidereal Time Step 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

Calculating Sidereal Time Step 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 handout) N = number of days since the beginning of the year

Calculating Sidereal Time Step 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°)

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

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”

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