Navigatio Part IV

 I made an ark from the light of the stars
And I hewed two mesas of bright silver metal
That sang in the sun like a sail in the wind
And I weave my way back to you --- Paul Kantner


Making use of the Nautical Almanac is in large part a matter of locating the stars. After all, all those fancy coordinates we confirmed when we used our sextant mean nothing if we just base them on any random point of light. So how do we find stars in the heavens?

The first thing to realize is that the "fixed" stars identified by Ptolemy are only fixed relative to one another; that is, the constellations don't change over time (actually, they do, but very slowly, so that an observer, in the course of her one brief human lifespan, won't see much if any movement except in the case of a few very extraordinary stars. In this, as in so much else, Ptolemy was flat wrong). 

Still, for our purposes the fixed stars are fixed and the constellations are always the same.

We don't see the same constellations all the time. Over the course of the year, the constellations revolve around Polaris. Some, higher up in the sky and closer to Polaris, are always visible (in the northern hemisphere). There are others that are always visible in the southern hemisphere. But there are many that rise and set along the celestial equator or against the horizon of the earth, and there are others that are higher and lower in the sky at various times of the year. 

Ptolemy's explanation was that the dome of the sky revolved above his flat Earth, the same way the planets moved in orbits and epicycles around the Earth. Remember, he believed not that stars were gigantic balls of flaming gas (as amazing as that fact is) but were pinpricks the fabric of the night sky which allowed little bits of the radiance of the realm of the gods to reach Earth.

Whatever Ptolemy believed, he understood that there is a lot of movement out there (why he assumed the Earth was immobile in the midst of all this movement is one of the great unanswered questions of all time). So it wouldn't do to just look up in the sky, wish upon a star, and then use that star to navigate everywhere, because that so-called "fixed" star isn't always in the same place. 

Instead, navigators had to invent star charts and astrolabes in order to find certain stars at certain times and in certain places, because not only did stars revolve and rise and set, but the further men sailed away from home the more likely they were to see unfamiliar stars. Canopus, for instance, wasn't visible in Greece, but it was visible in Egypt, only a few hundred miles away to the south.

Human ingenuity being what it is, navigators came up with the idea of fixing the celestial year at a particular moment in time, and calculating all star movements from that moment. The moment they picked was the Vernal (Spring) Equinox, when Earth's day and night are precisely twelve hours long each. They named that moment the First Point of Aries, because (in ancient days) the Sun "entered" the Zodiacal constellation of Aries at that moment (it also explains why astrological calculations of signs and houses begin with Aries, the first sign of the Zodiac). 

The First Point of Aries is the Prime Meridian of the sky, dividing the celestial sphere into 24 equal segments reaching from celestial pole to celestial pole, the same way the Greenwich Meridian divides the Earth into 24 equal segments running from terrestrial pole to terrestrial pole. On Earth we call any north-south point "Latitude" (Lat). In the sky, we call any north-south point "Right Ascension" (RA). 

And just as on Earth, navigators divided the sky into 24 equal segments running east to west with the celestial equator as the dividing line. On the ground we call those east-west lines Longitude (Long); in the sky, we call them Declination (Dec).

Every star in the sky is given a set of coordinates, RA and Dec, based on its location in the sky relative to the First Point of Aries. On the map, the First Point of Aries is represented by the centerpoint of the two hemispheres of the map where it all converges. The blue lines are RA. The green lines are Dec. The red wavy line, the Ecliptic, is the path of the Sun through the sky.

So, the first day of Spring is the center of the map, and the first day of Autumn is the outer edge of the map, and every star superimposed on the map has a set of coordinates. Imagine that the intersection of the terrestrial equator and the Prime Meridian was the Zero Point on Earth (as it is), and that every place on Earth had a set of coordinates relative to that Zero Point (as they do); it then becomes possible to find any point on the Earth. The same rule applies in the sky. We can find Peking on Earth without having to see Peking, just as we can find Canopus in the sky without having to see Canopus. 

The two coordinate systems correspond (essentially, they are one system, and if we still used the First Point of Aries as our New Year as we did in ancient times, celestial and terrestrial coordinates would exactly correspond). Today, if we draw a line from a point on the surface of the Earth (Lat and Long) to a point in the night sky (RA and Dec) we know the equivalent relative positions of those two points, day or night. Having that knowledge will tell us where we are. We use the brightest stars in the sky as our markers. And that, shorn of its math, is Celestial Navigation.