Using Sight Reduction Tables to Precalculate

Even if you don’t have a celestial computer you can precalulate pretty closely what the sextant altitude (Hs) will be for a LAN shot.

We can use the sight reduction tables — in this case HO 249 — as a hard-copy celestial computer. Here is what we know about the meridian altitude shot: We know that at the time of meridian passage that the assumed longitude will be the same as the GHA of the sun. We know that by definition the LAN shot can be defined as the time when the LHA = 0° and that the bearing of the sun is either due north or due south (90° or 180°).

So let’s say we are at sea and we want to precalculate the Hs of the sun at the time of meridian passage. Since we have been keeping track of our DR and presumably, have taken sun sights already during the course of our trip, we have an idea of what our assumed latitude is, as well as the declination of the sun for the day of the shot.

By using this information we can enter in the sight reduction tables on the page of latitude and look up the column under the declination until we find the LHA that is 0°. All we then have to do is see what the Hc is at the moment when the LHA is 0°. This will not be spot-on accurate but it will give the navigator a chance to pre-set the sextant in preparation for the shot. Let’s look at an example:

We are at a DR of 27° N by 65° 15’ W. The day is October 26 and the year is 2007. We want to precalculate the Hs for LAN. According to the Nautical Almanac the declination of the sun on October 26 is S 12°. We don’t have to have the exact declination because this exercise is just a method of obtaining a ballpark figure. So given that we have the declination and we have latitude of 27°, we enter HO 249 and look for the tables that read declination contrary to declination. In this case that puts us on page 164. Go to the bottom of the page and look to the left-hand side: the last entry for LHA is 0°. Notice that the azimuth is 180°, as it should be at the time of LAN. The Hc is 63° 00’. Now this isn’t what the sextant will read at the moment of your sight but you now know that th e Hs of the sun will be in that vicinity so you can take the sight of the sun with less wasted time, since you can preset the sextant to 63° and then wait until it reaches its maximum height.

The thinking behind this procedure has more to do with increasing your understanding of the relationship of the numbers in the sight reduction tables to the reality. The sight reduction tables can be intimidating because of all the numbers but if you can begin to visualize relationships between the numbers and the actual you will be more comfortable with the tables.