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Using the Rude Star Finder

Sep 25, 2015

Everything is easy once you practice. This is certainly true of what seems to be the complicated subject of precalculating available stars for a morning or evening twilight star sight. Actually, the procedure can be broken down into a number of small bite-sized steps that are simple to follow. It takes very little time to precalculate what stars might be available at a given dead reckoning latitude and longitude while out at sea. Precalculation of these stars, if one is preparing for a passage, can be done at home on the kitchen table. It’s not difficult that way to get some practice, and when you get aboard you’ll know exactly what you’re doing. There are various methods of calculating five or six or seven or eight stars that might be available at a given position at sea. The method I’ll discuss this time is the use of the analog Rude Star Finder 2102-D. I’ll say more about this later, but let’s start with the procedure.

Let’s say I want to do a morning star sight. As we know, what we need to do is calculate for nautical twilight. This does not have to be done for the exact second, as twilight is an extended period of time — pinpoint accuracy as to when it begins is not critical. Nautical twilight, by the way, places the center of the sun at 12° below horizon, giving the early-morning riser time to have a cup of coffee and a smoke before the sky gets so light that the stars are no longer visible. 

For instance, let’s say we’re at a DR of 42° 25’ N, 70° 15’ W, and we are looking for the time of nautical twilight for a morning star sight. The day in question is Oct. 10, 2015.

We go to the daily sun pages in the Nautical Almanac for Oct. 10 and look up nautical twilight at sunrise. At latitude 40° N, local time, it occurs at 0507. We then go to the Arc to Time conversion table and enter our longitude we find that at 70° 15’ it has taken the sun 4 hours 41 minutes to move to our longitude. We then add these two times of 5 hours 7 minutes plus 4 hours 41 minutes and we get a time of 9 hours 48 minutes GMT for the time of nautical twilight.


Next we go to the opposite page in the NA for Oct. 10, which is the stars and planet page. We look up GHA of Aries for the time of 9 hours 48 minutes. The GHA for 9 hours is 153° 39.1’. We then go to the 48 minutes page in the increments and correction table and see that the GHA of Aries has changed 12° 2’. Thus we have the GHA of Aries for the time as 153° 39.1’ plus 12° 2’, making 165° 41.1’ the GHA of Aries for the time of nautical twilight.

What we are trying to solve for is not the GHA of Aries but the local hour angle (LHA) of Aries and we do this by using the method of subtracting our assumed longitude from the GHA if we are in west longitude. If we are in east longitude, we add these numbers. So, 165° 41’ minus 70° 41’ equals 95°, which is the LHA of Aries for our position at twilight.

Okay, take a breath.

Now we’re done with the NA and we turn to the Rude Star Finder to help us see what available stars, their altitudes and their azimuths are for the given LHA.

The Rude Star Finder is a collection of clear two-sided plastic discs that go from 5 degrees to 85 degrees at 10-degree increments. One side of each disc is marked “N,” for north, and the other is south. These discs sit on a white plastic two-sided centerpiece, also labeled North and South, that has stars positioned in place on it and an outside scale running its circumference that represents the local angle of Aries.

In the northern latitudes, we use the sides that are labeled North. In this case, we find the North 45 degrees plastic disc and place it on the star disc. It is held in place with a plastic pin. Now, to find the available stars for our local area, rotate the disc on the white plastic disc until the hour corresponding to the LHA on the inner white scale — in this case, 95 degrees. Now we can find the true bearing and altitude of the local stars. In this instance, Sirius is available to shoot at about an altitude of 28° bearing about 173° True. Regulus is about 32° bearing 106°. We want good cuts, so we look for at least three stars that are more than 45 degrees apart. Remember: it is better to shoot three stars four times than 10 stars once. At twilight, in order to find the stars, you look over the compass, factor in the variation to get true course, set your sextant angle at approximately the right altitude, look out toward the horizon and — whammo! You’ll see the star. It’s that simple. And yet, like many other things, it’s only simple if you practice. 

Now there are other methods of finding available stars, most notably using Volume I, HO249. This volume is good for 5 years, costs about $26, and the latest epoch began in 2015. In our next newsletter, I’ll explain how to use these tables. 

Meanwhile, I am off to sea on a trip to Bermuda and I will let you know, if I get back, how it all worked out.

Edit Module

Oct 6, 2015 06:37 pm
 Posted by  gary217

I have a couple of comments on the authors technique.

First he gives an example of finding the time of morning nautical twilight, then computing LHA Aires for setting HO 2102D to locate which stars will be available to shoot when the horizon becomes visible. Well, you don't need to go through all that trouble, it's dark outside, just go out and look at the sky and see the stars, they will be there when the horizon becomes visible. And if it isn't the first night at sea then you should remember which stars you shot yesterday morning, at sailboat speeds they will be there this mornign too at altitudes close enough to be found in the index mirror. Using the HO 2102D only makes sense for evening stars since precomputing the altitude and azimuth will allow you to find the star and take the observation when the stars would not have been visible yet to the naked eye so you can take the sight earlier in the evening while the horizon is as clear as possible.

Second, his method for figuring out the LHA of Aries is unneccisarily complicated. He says find the time of twilight from the Nautical Almanac table, adjust that time for the DR longitude to find the GMT of twilight at your longitude, use that GMT to re-enter the NA and take out GHA Aries, and then subtract out your longitude to determine LHA Aries. See what he has done here, he first adds the longitude and then, at the end subtracts out the longitude, these two operation cancel out. All you have to do is determine the time of twilight at the Greenwich meridian which is what is tabulated in the twilight table, this is GMT is the local time at Greenwich. Then look at the GHA Aries for that time and that is also the LHA Aries at Greenwich. That will be the same LHA Aries at the point of twilight at your longitude. And the precision he uses is kinda silly since he starts out with the tabulated time of twilight, tabulated to the whole minute of time and may be off by two minutes of time.

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