Celestial Ephemerides Options for Sun and Polaris
I thought I would play around with what is available online now to get ephemeris data. Jerry Wahl’s Cadastral site is still up and there are some old ephemerides still available (link below), so I could use it to check against the settings for various sites currently available that would be needed to get the right data for azimuth determination. I looked at the US Naval Observatory site (links below) and the Jet Propulsions Laboratory Horizons site (link below). I was very pleased with what I was able to do ultimately, but it does require some effort to make sure the settings are correct and a bit of conversion to get the Greenwich Hour Angle that we are accustomed to using. I picked a date of January 1, 2014 for the three sites and compared results (comparison results are at the end).
I looked at Jerry Wahl’s site and was able to find tables from 2014 at:
Sun and Polaris Ephemeris for Surveyors – January 2014 (cadastral.com)
Cadastral Ephemeris
On Jerry’s site (Cadastral) he had this for January 1, 2014:
Sun
Declination: -23°01’17.2″ GHA0: 179°10’23.0″ Eq of Time: -3m18.46s Semi Diameter 16’15.9″
I then went the US Naval Observatory Astronomical Applications Department site:
USNO Ephemeris
On the USNO site I went to Geocentric Positions of Major Solar System Objects and Bright Stars.
Geocentric Positions of Major Solar System Objects and Bright Stars (navy.mil)
For Position Type, I selected Apparent Geocentric Right Ascension and Declination
For Celestial Object of Interest, I selected Sun
For Date, I selected 01/01/2014
For Time, I selected 12:00:00.000 AM
The results were:
Declination: -23°01’17.21″ Right Ascension: 18h45m35.521sec Eq of Time: -3m18.5s
To get Greenwich Hour Angle (GHA) from Right Ascension (RA), you must also have the Sidereal Time at Greenwich (GST). The formula is
GHA=GST – RA
The Sidereal Time is available on the USNO site:
For Date, I selected 01/01/2014
For Time, I selected 12:00:00.000 AM
Tabular Interval, 1.00 Days
Iterations 1
Location 0 Latitude and 0 Longitude
The results:
Greenwich Sidereal Time Mean: 6h42m16.4224s
Greenwich Sidereal Time Apparent: 6h42m17.0580s
I used the Greenwich Sidereal Time Apparent in the formula and got:
GHA = GST – RA
GHA = 6h42m17.0580s – 18h45m35.521s
GHA = -12h03’18.463s
This converts from HMS to DMS by converting the HMS to H.hhh, then dividing by 24 (hours per day) then multiplying by 360 (degrees per full revolution) then from D.ddd to DMS which gives:
-180°49’36.945″.
To make this positive add 360° which gives: 179°10’23.055″
GHA = 179°10’23.055″
The Diameter of the Sun is available from the USNO site under Topocentric Configuration of Major Solar System Bodies:
Topocentric Configuration of Major Solar System Bodies (navy.mil)
This tool also has the Right Ascension and the Declination, but the values are rounded off.
The Diameter of the Sun given from the USNO site on 1/01/2014 is 32’31.7″ which divided by 2 to get the semi-diameter gives:
Semi-Diameter: 0°16’15.85
So the GHA (derived from Right Ascension and Sidereal Time), Declination, and Semi-Diameter (derived from the diameter) from the USNO match very well (within rounding errors) of the Cadastral data when all of the settings are put right (mostly using 12:00 AM for time and using 0 lat, 0 long for position, and Geocentric for position).
JPL HORIZONS Ephemeris
JPL Horizons is another option:
For this you set up what you want and can get all of the data at one time. Like the USNO site you cannot get GHA, but you can get Sidereal Time and Right Ascension which can get you to the Greenwich Hour Angle.
I used these settings:
Ephemeris Type: Observer Table
Target Body: Sun [Sol]
Observer Location: 0°E, 0°N, 0km
Time Specification: Start=2014-01-01 UT, Stop=2014-01-02, Step=1 (days)
Table Settings: custom
The Table Setting items I selected are:
2. Apparent RA & DEC
7. Local apparent sidereal time
13. Target angular diameter
Reference Time: ICRF
Date/time format: calendar format
Calendar type: mixed
Time digits: HH:MM:SS.fff
Angle format: sexagesimal format (hours/degrees minutes seconds)
Refraction Model: no refraction (airless)
The results from Horizons are:
RA: 18h45m35.48s
DEC: -23°01’13.7″
Sidereal Time: 6h42’16.957s
Angular Diameter 1951.695″
The RA and Sidereal Time give a GHA of
GHA: 179°10’22.155″
The Semi-diameter converts to
Semi-Diameter: 0°16’15.85″
Comparisons
Cadastral
GHA: 179°10’23.0″ Decl: -23°01’17.2″ SD: 0°16’15.9″
USNO
GHA: 179°10’23.055″ Decl: -23°01’17.21″ SD: 0°16’15.85″
JPL
GHA: 179°10’22.155″ Decl: -23°01’13.7″ SD: 0°16’15.85″
- This discussion was modified 5 months, 1 week ago by shawn-billings.
- This discussion was modified 5 months, 1 week ago by shawn-billings.
- This discussion was modified 5 months, 1 week ago by shawn-billings.
cadastral.com
Sun and Polaris Ephemeris for Surveyors - January 2014
Sun and Polaris Ephemeris for Surveyors - January 2014
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