A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Herman Dekker
Date: 2020 Oct 13, 11:10 -0700
During the last "Office Hours" on Sunday Frank had a nice topic what is informative for everybody.
Can you estimate how long it takes for then Sun goed down?
I have made a smummary of this topic for myself and like to show it here. Comments are welcome.
Estimating when the Sun goes down without electronics or watches.
This approach is usable at Latitudes from 0 to max. 60°
1. Basis is that the earth turns 15° per hour. The Sun moves in the ecliptic 15°/h.
2. In the tropics (~ Lat 0°-15° N or S) it is simple.
For instance in the afternoon the height of the Sun above the horizon is
measured 22,5°. I takes then 22,5/15= 1,5 hour before the Sun goes down.
3. In latitudes from 15° to 60° is its more complicated.
You must find the Suns diurnal path.You measure then the angle from the sun
to the bearing on the horizon where the Sun goes down.
Dived the measured angle by 15, answer is the time in hours to Sunset.
4. How to find the bearing on the horizon where the Sun sets?
5. Round 21 mar and 23 sep suns Declination is zero, Sun rises Due EAST and sets Due WEST.
6. On other dates we must use the Suns Declination to calculate (Roughly because Latitude
also has influence but for simplicity we ignore that).
7. Sunset bearing becomes WEST(270°) ± Suns Declination.
From 21 mar-23sep use +,From 23sep-21 mar use -).
8. Use Magnetic Compass to find bearing on the horizon and measure the angle to the Sun in
degrees by Sextant or Kamal or PaperKamal=Indexcard.
9. Divide the angle by 15 and now you know the remaining time to Sunset.
Info from F.Reed during Office Hours Sunday 11 okt 2020