Announcements
•Homework set 1 is due today
•Homework set 2: Chapter 2 # 43 & 45 plus
supplemental problems
•Use Exam Formula Sheet as you do the homework to
familiarize yourself with where things are on it.
Terrestrial Coordinates
Longitude is
measured CCW (+) or
CW (-) around from
Greenwich England
Latitude is measured
North or South of the
equator
Both are measured in
degrees, minutes
and seconds
Celestial Coordinates
The angle between the
celestial equator and the
ecliptic is 23.5°
Right Ascension (RA) is
measured CCW from the
Vernal Equinox and is in
hours, minutes and
seconds
Declination (Dec) is
measured above (+) or
below (-) the celestial
equator and is in degrees,
minutes and seconds
See Appendix A6 for more on
celestial coordinates
Finding the CE and NCP at
your latitude
Altitude of NCP above due north horizon along the meridian (the North
Point) is just f, your latitude (+ for north, - for south)
Altitude of the celestial equator above due south horizon along the
meridian (the South Point) is 90°-f
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. However,
one sidereal day is 24 sidereal hours
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
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 + 0.0657098245xN + 1.00273791xUT
where
G = GST at 0 hrs on “zeroth day” of that year
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  Year  2000  N  L
Where N is the day number and L is the
number of leap years since 2000
Examples
Find the current sidereal time (4:45pm) and
the current Julian date
First, find current sidereal time (LST) then
find Julian date
Solution for LST
LST  G  0.0657098245N  1.00273791(CST  6)  ( Long /150 )
Using the formula for LST on the formula sheet, we will
need to know the time (4:45pm), G for 2014, N for
January 28 and the longitude of Clarksville, TN.
In 24 hour decimal format 4:45pm = 16:45 = 16.75
The USNO location for Clarksville, TN is 87°22’ = 87.37°
From the formula sheet G2014 = 6.63886
From the formula sheet NJanuary 28 = 28
Final value for LST
LST  6.63886  0.0657098245  28  1.00273791(16.75  6)  (87.37  15o )
 25.46635587
Subtract 24 since this is greater than 24
 1.46635587  1: 27 : 58.9
You can check the value in Stellarium by determining the Right
Ascension of a star on the meridian at 4:45pm on January 28, 2014
A word about significant figures and time
Solution for Julian Date
JD  2, 451,544.5  365  (Year  2000)  N  L
From the equation for JD on the formula sheet we need N
for January 28 and L for 2014
N = 28
L = 3 (2004, 2008 & 2012 were the three leap years)
Plugging in the numbers
JD  2, 451,544.5  365  (2014  2000)  28  3
 2, 456, 685.5
This can be checked on the USNO Data Services page using their Julian
Date Conversion calculator (Use Sunrise/Sunset – Moonrise/Moonset
Times link on www.apsu.edu/astronomy)