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+--- Thread: Close GHA determination using EoT (/showthread.php?tid=80)
Close GHA determination using EoT - CarlosPindle - 01-13-2017
You can determine GHA, closely, of an integral hour by taking the EoT (Equation of Time) and converting it to an angle and then subtracting or adding it to the GMT integral hour (converted to an angle). Whether you add or subtract the EoT is determined by the time of Mer. Pass listed next to the EoT on each of Sun & Moon pages of The Nautical Almanac.
The bottom of The Nautical Almanac Sun and Moon page has the Equation of Time for each day on the bottom right hand side of said pages. To get a more accurate figure use an EoT that is closest to the present hour of GMT.
At most you'll be off by around 2 minutes of arc. Not bad if you're 1,000 nm from land.
Do you have any other ideas about this?
Carlos
RE: Close GHA determination using EoT - c_davidson - 01-13-2017
Carlos,
For yesterday, January 12, 2017 the EoT for 12h was 08:24.
Is this method correct? If it was GMT 16:00 then the approximate Sun GHA would be-
16-12= 4 hours
4 x 15°= 60°
EoT converted to angle;
8 minutes 24 seconds converted to angle= 2° 06'
60° - 2° 06'= 57° 54' (GHA of the Sun on January 12, 2017 at GMT 16:00)
The correct GHA is 57° 53.1' as listed in The Nautical Almanac.
How does that sound to you?
Clen
RE: Close GHA determination using EoT - craigsl - 01-13-2017
(01-13-2017, 02:32 PM)c_davidson Wrote: Carlos,
For yesterday, January 12, 2017 the EoT for 12h was 08:24.
Is this method correct? If it was GMT 16:00 then the approximate Sun GHA would be-
16-12= 4 hours
4 x 15°= 60°
EoT converted to angle;
8 minutes 24 seconds converted to angle= 2° 06'
60° - 2° 06'= 57° 54' (GHA of the Sun on January 12, 2017 at GMT 16:00)
The correct GHA is 57° 53.1' as listed in The Nautical Almanac.
How does that sound to you?
Clen
That's pretty convenient. The USNO Astronomical Phenomena for the year 2017 page- 18 has a list of the Equation of Time for 00 h GMT for each day of the year. But, it's only for GMT 00 h. Confound it!
It also includes declination of the Sun.
Find it here- Equation of Time & declination
Craig