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Jul 5, 2016

According to my well-thumbed and trusty 2016 Nautical Almanac, the declination of the sun was at its northernmost point at 1400 hours on June 20 when it climbed to 23° 26.1’, and it stayed there — as if resting on a rock after a long climb, catching its breath admiring the view — until June 21, when between 0700 and 0800 it roused itself, shook off the cobwebs and moved south 0.1’.

The declination of the sun had finally arrived at the Tropic of Cancer, and the word “tropic” has less to do with the writings of Henry Miller and more with the Greek derivation of the word itself. According to the Online Etymology Dictionary, the word “tropic” derives from the Greek tropikos: “Of or pertaining to a turn or change; of or pertaining to the solstice.” According to Webster “Either of the two parallels of terrestrial latitude (Tropic of Cancer and the Tropic of Capricorn) at a distance of about 23.5 degrees north or south of the equator where the sun is directly overhead when it reaches its most northerly or southerly point in the sky.” The term “solstice” is from the Latin solstitium and means the “sun stop.” The sun stops on the solstice and heads back the other way.

The declination begins to decrease until the equinox in September. Ironic then, that this first day of summer is actually its last day. We navigators know that the sun is already moving southward on its way to the equator. A bittersweet bit of information for those of us in the Northern Hemisphere.

The d correction at the bottom of the daily sun pages in the NA tells the tale. For a few days following the solstice there is no change in the declination and it is 0.0’ until June 29 when it is at 0.2’. The sun is moving south very slowly at actually 0.2’ nm every hour. On the other hand if one looks at the SD correction (semi diameter) it almost seems as if it is a mistake. On June 20 and thereafter, the SD is 15.8’ and remains so until August 25 when it increases by 0.1’. For those of us in summer in the northern hemisphere, this appears to be counterintuitive. It’s hot out, it’s summer — how can the earth be further away from the sun? A larger SD would make more sense. But that’s not the way the system has been set up. Because of the elliptical orbit of the earth as it travels around the sun, we are, in the northern hemisphere, in summer actually slightly farther away from the sun. Go figure. It’s the tilt, folks, not the distance that allows the rays of the sun to strike the northern latitudes more directly, thus making summer perfect or as hot as the blazes. If you were to examine the changes in the SD at the bottom of the daily sun pages, and I hope you do, during the course of the year you will discover that the largest SD is in the winter months of the northern latitudes when it is at 16.3’. Those folks below the line have both the earth closer to the sun and the declination as far south as the Tropic of Capricorn where the sun reaches in declination on or around Dec. 21, rests and begins north again. It is one of the great benefits of studying celestial navigation that we can see and understand the beauty and perfection of this and other celestial movements. The annual change in the sun’s declination is reminiscent of the Timex watch ad of old. “It takes a lickin’ and keeps on tickin’.”

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