Dear Tom,
On a recent Hawaiian trip the paper not only listed the actual time of sunrise and
sunset but also times for civil, nautical and astronomical twilight? Can you explain?
Joe Kirasich western Springs
Dear Joe,
The time of sunrise and sunset is defined as the moment when the top of the sun's disk
is on the horizon. Civil, nautical and astronomical twilight are defined as when the sun
is six, 12 and 18 degrees below the horizon, respectively, as it is rising or setting. The
times as determined by U.S. Naval Observatory are used to define the gradual changes
in visible light that occur between total darkness and sunrise or sunset. Astronomer
Dan Joyce tells us that there are small time variations during the year, but there is
generally a period of almost 90 minutes for the transition from total darkness to
sunrise or from sunset to total darkness.
