As I stated earlier, the times you take or compute from the Sunrise and Sunset tables are in LMT and must be converted into LST or UTC. The conversion to LST is accomplished using the longitude of the location for which you are computing sunrise and sunset, the standard meridian closest to this longitude, and table 6-7-2, Conversion of Arc to Time.

Step 1. Determine the longitude of the standard meridian closest to the longitude of the location for which you are computing sunrise and sunset. Remember, the standard meridians are every 15 degrees either side of the Prime meridian (0).

Step 2. Compute the difference between the longitude of the closer standard meridian and the locations longitude.

Step 3. Enter the Conversion of Arc to Time table with the longitude difference obtained in step 2, and convert degrees and minutes into time. The conversion factor is 1 degree equals 4 minutes. The last two columns are used to convert minutes of longitude into minutes and seconds of time.

Step 4. Add the time obtained in step 3 to or subtract it from the LMT. Add if the longitude of interest is west of the closer standard meridian, or subtract if it is east of the closer standard meridian.

For example, lets use a sunrise time of 06:50 LMT as computed for NAS Pensacola, Florida, (3021N, 8719W).

Step 1. The standard meridian closest to 8719W is 90W.

Step 2. The difference between 90W and 8719W is 241.

Step 3. Enter the conversion table and convert 241 into time. The 2 equals 8 minutes, and 41 equals 2 minutes 44 seconds (3 minutes rounded off). IF THE SECONDS VALUE IS 30 OR GREATER, YOU ROUND UP. You should find 11 minutes.

Step 4. Subtract 11 minutes from 06:50 LMT (06:50 :11 = 06:39 LST).

To convert Local Mean Time into Universal Time Coordinated, you simply enter the Con-version of Arc to Time table with the lon-gitude of the location for which you are determining sunrise and sunset. For example, if you are computing sunrise or sunset for NAS Pensacola, you must convert 8719W into time. From table 6-7-2, 87 equals 5 hours 48 minutes, and 19 equals 1 minute and 16 seconds, for a total time difference of 5 hours 49 minutes 16 seconds (5:49, rounded off). This time difference is then added to or subtracted from the LMT to get UTC. When the longitude is west of the prime meridian, the time is added. When the longitude is east of the prime meridian, the time is subtracted.

Learning Objective: Define twilight and identify the tables and procedures used in twilight computations.

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