A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Jorrit Visser
Date: 2018 May 10, 07:51 -0700
My name is Jorrit Visser and I am new to this forum. I got interested in celestial navigation when re-reading "Longitude" by Dava Sobel last year. Last year I bought a Davis Mark 15. To be honest, I never spend time on a ship, so my sighting are mostly on land, but I try to go to the North Sea from time to time to practice sightings with a real horizon instead of an articifial one. After reading Stark's book about the lunar distance I became more interested in the lunar distance and manual sight reduction in general.
About my question:
I am working on a website (celnav.nl) to display almanac data for manual sight reduction and also want to include lunar distances. At a specific time the lunar distance is not relevant for all celestial bodies. For example, currently the lunar distance of Aldebaran does not seem relevant, because the sun is between the moon and Aldebaran (GHA-wise), so Aldebaran and the moon are probably not visible at the same time. Inspired by the tables with lunar distances in the Nautical Almanac of 1767 I tried to define requirements that have to be met in order for a lunar distance to be useful.
The following requirements seem to make sense:
- The angle between the celestial body and the moon is less than 120°. With a sextant this is more or less the maximum angle that can be measured.
- The rate of change of the angle between the celestial body and the moon must be at least 15' of arc per hour of time. If less, accurate time determination using the lunar distance becomes more difficult due to increasing sensitivity to measurement errors. The cutoff value is chosen to be half the right ascension rate of the moon, which is about 30' of arc per hour of time.
- The GHA of the celestial body must differ by more than 15° from the GHA of the sun. Otherwise the celestial body might not be visible. The cutoff value of 15° is quite arbitrary.
- The sun is not in between the celestial body and the moon. This is tested by looking at the GHA. The rationale behind this requirement is that the lunar distance of stars and planets can only be determined if the sun is below the horizon and the moon is above the horizon.
- In case of the lunar distance of the sun, the angle between the sun and the moon must be larger than 40°. Otherwise the moon is not visible. Again the cutoff value of 40° is quite arbitrary.
Any thoughts on this?