1. Announcements
Dark Sky night Saturday night is looking iffy. Check 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.

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

3. Time

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

5. The Equation of Time

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

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

8. 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 seconds

9. 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.

10. 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

11. 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 N UT
where
G = GST at 0 hrs on “zeroth day” of that year (see handout)
N = number of days since the beginning of the year

12. 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°)

13. 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, x (Year – 2000) + N + L
Where N is the day number and L is the number of leap years since 2000

14. 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 days so the Julian calendar “drifts”

15. The Gregorian Calendar
First instituted in 1582
1 year = 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 =
Years evenly divisible by 4000 are not leap years. Otherwise the same as the Gregorian Calendar