A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Tony Oz
Date: 2018 Jul 17, 04:16 -0700
So far I know of two formulas to obtain the GHAAries from the date&time:
- the higher-precision, T = 367 · yyyy - int(1.75 · [yyyy + int((m + 9) / 12)]) + int(275 · m / 9) + d + UT / 24 - 730531,5; GHA°Aries= 0,9856474° · T + 15° · UT + 100,46062°; where yyyy - calendar year, m - calendar month, d - a day within a month, UT - universal time; courtesy Henning Umland, "A Short Guide to Celestial Navigation"
- the lower-precision, GHA°Aries = (DD + UT / 24) · (360° / 365) + UT · 15° + 0°15' - (0°15' for each year since the last leap year); where DD - number of days from Sep 21st (positive if past, negative if before); courtesy David Burch, "Emergency Navigation"
The lower-precision is easier to remember, but it requires to calculate the DD from a calendar date. Not clear (to me yet) where a big negative value "overlaps" into a big positive - mid-March?
Are there any more formulas or easy ways to obtain GHAAries from a given date&time?
Thank you in advance.