The Duration of Twilight graphs (the lower two graphs of table 6-7-4) are very similar to the Semiduration of Sunlight graphs in that they consist of a date scale, a latitude scale, and skewed semiduration lines. Two areas on each of these graphs also are without semiduration lines. On the left above the 0-hour semiduration line on the upper Twilight graph and on the right above the 0-hour semiduration line on the lower graph are the areas of "No Twilight or Sunlight." On the right above the 8-hour semiduration line on the upper Twilight graph and on the left above the 8-hour semiduration line on the lower graph are the "Continuous Twilight or Sunlight" areas.

The Duration of Twilight graphs are used to determine the beginning and ending of civil twilight. When a latitude and date inter-sect within the span of semiduration lines, you must determine the semiduration period. The semiduration time is one-half of the total twilight time. On the upper Twilight graph of table 6-7-4, Duration of Twilight January to July, note that each semiduration line is labeled and that there are two 1-hour time lines and two 2-hour time lines. Also, note that just above the 3-hour time line on the right side of the graph, some of the lines run together at various points. If you align these points, the line that is formed will correspond to the 0-hour duration line of the Semiduration of Sunlight graph. Along this line, sunrise may or may not occur. To the right of this line, the time represents one-half of the total duration of twilight when the Sun remains below the horizon. The sky lightens but the Sun does not rise above the horizon. To the left of this line, the time represents the length of each twilight period (morning and evening) for each day having a sunrise.

For further explanation on how to use these graphs, lets do an example: Date, 26 October; latitude, 77N.

Step 1. Locate 26 October along the date scale on the lower graph and extend a line vertically to the top of the graph.

Step 2. Locate 77 and extend a line hor-izontally until it intersects the line drawn in step 1.

Step 3. Where the two lines intersect, read the duration of twilight. In this example, the dura-tion of twilight on 26 October is approximately 2 hours and 15 minutes.

Note: The attainable accuracy for twilight time using these graphs is no greater than 15 minutes.

Step 4. Subtract 2 hours and 15 minutes from the time of sunrise to obtain the time of first light, and add 2 hours and 15 minutes to the time of sunset to obtain the time of last light. REMEM-BER:  

THE TIMES ARE LMT AND MUST BE CONVERTED TO LST OR UTC.

To further your knowledge of sunrise, sunset, and twilight, I recommend that you refer to the latest addition of Air Almanac. You may also want to complete the Practical Training Publica-tion Preparation of Astronomical and Tidal Data, NAVEDTRA 40390-A.

4. Thule, Greenland (7729N, 6912W) on 21 October

Have your immediate supervisor or leading chief answer any questions you may have, and then have one of them check your computations.

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