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The first point of Aries

Apr 5, 2016

Ah, spring! The peepers peep. The woodpeckers peck. The daffodils dazzle and the redwings are back in town, while on the celestial sphere — the region that we celestial navigators think about —­ the sun has moved from south of the celestial equator to the north on a day referred to as the vernal equinox.

This year, the day of the vernal equinox was March 20, when between 0400 hours GMT and 0500 GMT, the declination of the sun changed from 0° 0.5’ S to 0° 0.5’ N. The little d correction for the date is 1.0’, which of course — and we know this to be true because, after all, we are students of celestial navigation — means that the northerly component of the apparent sun is moving at a rate of 1 nm per hour. How cool is that? Also on this date, and at the exact moment that the sun crosses the equator and of great interest to those of us whose heads are in the stars, it is the moment we celebrate as the first point of Aries. The symbol for this is the ram’s horns. It is true there are no parties, no gala celebrations, but nevertheless for celestial navigators the position of the first point of Aries cannot be overstated. It is after all the spot in the celestial sphere that measures the distance westward to the 57 navigational stars that we use as celestial navigators. The angular measurement of this distance we know as sidereal hour angle.

Now most of us already know from our readings — and after all we do read about this, right? — that back around 150 B.C., the great Ptolemy made the call that the sun was actually in the constellation of Aries on the first day of spring; but it is, alas, no more due to the earth’s precessionary wobble. Yet it is this intersection, when the path of the sun — called the ecliptic — crosses the celestial equator, that marks the alpha point in the heavens from which the stars are measured.

No longer in the constellation of Aries, but still referred to by that name, the first point of Aries now lies somewhere between Pisces and Aquarius on the ecliptic — just another of those confusing facts that make the study of celestial navigation so interesting. According to information gleaned from sleepless nights staring at astronomy websites — this is from earthsky.org — the vernal equinox "drifts westward (along the ecliptic) through the constellations of the zodiac at about 1 degree (2 sun diameters) in 72 years, or to put it another way 30 degrees in 2160 years. Hence, the March equinox point passed out of the constellation Aries and into the constellation Pisces in 68 B.C." So, there you have it. Defining the first point of Aries is one way celestial navigators celebrate spring.

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