How to Calculate Sunrise/Sunset


The exact times of sunrise and sunset no longer are as critical to the modern mariner as they were in the days before electronic navigation systems like the Global Positioning System (GPS) constellation of navigation satellites made ocean navigation a precision undertaking. One practical remnant of the early rules remains: Rule 20(c) of the Rules for the Prevention of Collision at Sea (the "COLREGS") requires that you display your vessel's navigation lights from sunrise to sunset.

Things You'll Need

  • "Nautical Almanac," current year
  • "The American Practical Navigator"
  • Navigational chart
  • Dividers
  • Look at the pages in your Nautical Almanac for the day you're interested in; there are two pages for each day. The Table of Sunrises and Sunsets will appear on the right page. Sunrise and sunset times will be given for each three days, since variations in non-polar latitudes are immaterial over that span. Find the time indicated for Sunrise/Sunset in Coordinated Universal Time (UTC).

  • Look at the World Time Zones Chart in Chapter 18 of "The American Practical Navigator" (which seamen refer to simply as "Bowditch"). Because the world is round and rotates at a uniform rate, if it is sunrise at a certain time at the Prime Meridian of the World--0.00 degrees--then sunrise will occur at 5 a.m. in each time zone, as the Earth turns to face the sun. If you are located off the coast of California (Time Zone "U" on the World Time Zones Chart) and sunrise occurs at 5:00 o'clock a.m. UTC, then it will occur at 5:00 o'clock a.m. at the center of the center of Time Zone "U." For example, if your projected position is off the coast of southern California, look at the top of the table, and you will see that you are in the "U" time zone. The center of the "U" time zone is located along a line running from the North Pole to the South Pole along longitude 120 degrees West.

  • Plot your current position on your navigational chart if you are underway and making way. Project your position at sunrise/sunset time specified in the Almanac by setting the width of your dividers so that the points are as far apart on the chart's scale as the distance you will travel in one hour. "Walk" the dividers along your course line on the chart, turning them end for end one time for each hour until the projected time for sunrise or sunset.

  • Use the plotted longitude of your position at the time sunrise or sunset is supposed to occur in order to correct the time of sunrise or sunset according to your projected position. If you are east of the center of the time zone, the sunrise will be slightly earlier than at the center of the zone; if you are west of the center of the time zone, it will be slightly later. If your position is 119 degrees 30 minutes 00 seconds West, you are 30 minutes of arc east of the center of the "U" time zone. For navigational purposes, each 15 minutes of longitude the observer's position differs from the zone meridian, the zone time of the phenomena differs by 1 minute, according to "Bowditch." This means that sunrise, if it is to occur at 5:00 a.m., will occur at 4:58 a.m., rather than at exactly 5:00 a.m. If your longitude is 120 degrees 30 minutes West, you are 30 minutes of longitude west of the center of the zone meridian and sunrise will occur at 5:02 a.m., rather than 5:00 a.m.

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