The benefits of using H.O. 249 for Selected Stars

There is no greater thrill for the evolving celestial navigator than in obtaining a fix from a star sight. This is the navigational sine qua non and the goal to which navigators aspire. The most difficult aspect of taking star sights — especially if you are older than age 50 — is finding the stars, recording the time, and getting a sextant altitude observation (Hs) in dim light. With or without glasses it can still be a chore. The stars, it might be said, are for young eyes, but with a good red-light flashlight and some practice the physical act of shooting stars can be learned!
The easiest and most fool-proof method requires the use of H.O. 249 Vol. 1, Selected Stars. This volume is published in 10-year editions called epochs — the latest version having come out in 2010. The table is laid out with latitude pages from 89° North to 89° South. The local hour angle (LHA) of Aires for seven stars is listed from 0° to 359°. The seven stars have been chosen for their magnitudes (brightness) and for their ability to give the navigator good cuts — they are positioned about 45° relative to each other. To make life even easier, three stars are chosen as the best stars to shoot for a fix. These stars are marked with a diamond next to their names. For instance at latitude 42° North with an LHA Aires of 315° we are informed that Alpheratz, Altair and Kochab are the best stars to observe. The tables give the computed altitude (Hc) of the stars and the true azimuth (Zn). All one has to do is to take the sight, compare the observed altitude (Ho) to the Hc, and then calculate the intercept. What could be easier?
The wobble of the earth (both polar and equatorial) can be accounted for in the Precession and Nutation tables in the back of the book. The navigator must correct the Hc and the Zn to allow for these “wobbles.” The instructions are easy to follow and should not present any problems.
But, of course, there is a rub. Nothing is for free even in celestial navigation, and in order to use H.O. 249 Vol. 1, the navigator must do some pre-planning. It is well to remember that these tables were originally designed for air navigation, so the navigator on a plane, moving a great deal faster than the 6 or 7 knots we are used to, had to precalculate the LHA and the position of the stars. The same is required for the sailing celestial navigator.
The procedure of pre-planning is straightforward. First, the time of nautical and civil twilight (the time of star sights) has to be calculated from the dead reckoning (DR). These twilight times are given in GMT in the daily pages of the NA. They have to be converted to the GMT of twilight at your DR longitude. You need to take your DR longitude and use the Arc to Time conversion table in the back of the Almanac to find the conversion.
Once you have the time of the twilight in GMT at your DR longitude you go to the daily pages and find the Greenwich Hour Angle (GHA) Aires at the required times. Then one has to either add or subtract the DR longitude, depending on whether one is in west or east longitude. This will yield the LHA of Aires. Once the LHA Aires is found, enter the tables of H.O. 249 under the latitude closest to the DR position. Go down the columns and find the LHA Aires and you will see the seven stars with their Hc and Zn.
The next step is to write those numbers down and mark where the stars will be relative to the ships heading. What I usually do is make a drawing of the boat and extend arrows marked with the Zn and the Hc of the stars that are listed. This way I can preset the sextant at the Hc at twilight and face in the direction of the Zn. Usually the magnification of the optics will make it easier to pick out a pinprick of light that is the star in question — much easier than using no optics at all. Then it is just a matter of bringing the star to the horizon, marking the time and recording the Hs. When the Hs is reduced, it is compared to the Hc in the tables and the difference is plotted. Sound complicated? Let’s do an example and see.
We are at a DR of 40° 30’ N by 70° 15’ W. It is May 28, 2010. Height of eye is 10 feet and there is no index or watch error. Find nautical twilight and calculate stars.
In the Nautical Almanac on May 18 we see that the time of nautical twilight at 40° is at 03:23 GMT in Greenwich. However, since we are not in Greenwich, we convert longitude to time and see that it takes the sun 4 hours, 41 minutes to travel that distance. (See conversion of Arc to Time table at rear of Nautical Almanac). We add that to 03:23 and get the time in GMT for nautical twilight:
3 hours, 23 minutes
+4 hours, 41 minutes
Time of nautical twilight GMT at DR is 8 hours 04 minutes
Now any wise navigator knows that time is approximate. The ship may not be at the DR. If it is further west the time will be later, if it is east it will be earlier. So a good rule of thumb is to be up on deck at least 20 minutes before the time.
We now convert the time into GHA Aires. On the daily pages we go to the GHA Aires column and see that at 08:00 the GHA Aires is 5° 45.6’. The four minute increment adds another degree so the GHA Aires at nautical twilight is 6° 45.6’. We add 360° to that and then subtract the longitude. (In this case just use the degrees. It doesn’t have to be spot on!) thus:
366° 45.6’
– 70°
– LHA Aires 296°
We now enter H.O. 249 and go to the page of 40° North latitude and find the column for LHA Aires. At 296° we see the following: Alpheratz, Enif, Altair, Rasalhague, Alphecca and Kochab. The three best stars to shoot are Alpheratz, Rasalhague and Kochab.
By Ocean Navigator