Precalculating

  
The first and most difficult job in precalculating what celestial bodies will be available and where they will be is to determine the time of twilight. Generally speaking, both the horizon and the brighter navigational stars are visible between civil twilight and nautical twilight. These are both listed in the daily pages by latitude. In the evening, it makes sense to determine civil twilight (which will occur first) and in the morning find nautical twilight (which occurs first then).

When using the table, it’s usually not necessary to interpolate between latitudes. The time extracted is for twilight at 0° longitude, so it needs to be converted to GMT at our longitude.

Take the expected DR longitude at the rough time of twilight and use it to enter the arc-to-time conversion table (the first colored page in the back of the Nautical Almanac). If the longitude is west, then this should be added to the time of twilight (subtract if it’s east). This is the time of twilight at your location in GMT. Use this time to find the GHA Aries and then convert that to LHA Aries. With LHA Aries and DR latitude, enter Vol. 1 and find seven stars listed with their altitude (Hc) and azimuth. Three stars are marked with a black diamond. Those are the three stars that will give you the best cut of LOPs.

The advantage to using HO 249 Vol. 1 is that the stars you precalculated may then be reduced using the relatively quick Vol. 1 sight reduction method. Its drawback is that only seven stars are available through precalculation and not the planets or the moon.

The Rude Star Finder has templates for different latitudes and these will yield a rough altitude and Zn for each of the 57 stars visible. These are good within a few degrees and give the navigator a larger list to select from. To use the Star Finder, determine the LHA Aries at the time of civil twilight. Set the arrow on the template to the LHA Aries printed on the card. All of the stars inside the grid will be in the visible sky. The circular lines are the altitude of the star and the radiating lines indicate its Zn. There is also a red overlay disk with a cutout window that allows you to place the position of the planets and moon on the disk in pencil.