Monday, December 22, 2008

How to Select the Best Stars or Planets for a Fix

Star selection problems are one of the more difficult and time consuming of the problems found on USCG license exams. In star selection problems, you have four choices of combinations of three stars, planets, and or the moon in each problem. You must determine which of the four combinations would result in the most reliable fix for the date, time, and DR position. In making a choice the following three things must be considered, listed from highest priority to least priority:

First Priority - The azimuth differences between the bodies should be sufficient to give a reliable fix. The ideal azimuth spread would be for the bearings of the bodies to differ by 120 degrees.

Second Priority - The bodies should be at altitudes between 15 degrees and 70 degrees. Unusual refraction can introduce large errors in low altitude sights, and accurate sights at very high altitudes are difficult to obtain.

Third Priority - The magnitude of the star. Obviously, first magnitude stars are easier to see and to shoot while the horizon is still clearly defined.

In analyzing the four choices the following should be avoided.
1. Two of three bodies are very close together in bearing.
2. All three bodies azimuths fall within the same 180 degrees of bearing.
3. Two of the bodies are reciprocal in bearing or nearly so.

To make a list of stars and planets available for observation at morning or evening twilight for a fix, you would setup the starfinder with the LHA (Aries) and have the planets plotted. Then use the following guide lines for selection of bodies:
1. Altitude 10° to 65°
2. 1st magnitude
3. Bearing 120° apart
4. Always try to get Polaris, it gives you a latitude line of position if you are in the northern hemisphere, and any body who has a bearing of 000° - 180° will do the same thing.
5. To check your course, select bodies with bearings perpendicular to your course.
6. To check your speed, select bodies that are parallel to your course.

In working these Coast Guard exam problems, some groups can be eliminated if one of the bodies in the group is below the horizon for that observer's DR position and time of star time.

Here is a Coast Guard exam problem.
On 23 July 1981, your 1700 zone time DR position is Lat. 27° 29.0' N, Long. 129° 26.0' W. You are on course 079° T at a speed of 20 knots. Considering their magnitude, azimuth, and altitude, which group includes the three bodies best suited for a fix at star time?

A. Arcturus, Jupiter, Denebola
B. Spica, Sabik, Vega
C. Antares, Polaris, Altair
D. Jupiter, Saturn, Polaris

Step 1: Compute the ZT of evening twilight or star time (approx. 1857), use this DR position for your form.

Step 2: Compute the LHA (Aries) and the RA and Declination of the planets.

Step 3: Plot the planets on your starfinder, and then place your blue template over the star base on your LHA (Aries). This will be the stars and planets available at star time.

Step 4: Find the best answer. I use a diagram to help me locate the answers.