Wooden Boat Plan

Having taken our sights, graphed them to estimate Hs, and applied corrections to determine Ho, we are now ready to calculate the latitude of my position when I took the sights.

Since Ho is the observed height of the sun above the horizon, you might be wondering if Ho is the latitude value. No, its not that simple, but almost.

Here is a diagram that explains the simple relationship between Ho and Latitude.


Lets take this diagram step by step.

First, the inside circle represents the Earth. The outside circle represents the Celestial Sphere. Forget Copernicus and all that Earth rotating around the Sun nonsense. Copernicus was clearly wrong. The Earth is at the center of the universe, and the Sun, Moon, stars, and planets all rotate around the earth, on the surface of the Celestial Sphere.

At the top of the Earth, you can see me with my sextant, taking my sights.

There are three lines drawn through the center of the earth, and projected out onto the celestial sphere:

1. The Zenith line starts at the center of the earth, goes through me, and straight up to the Celestial Sphere. My Zenith is the point on the celestial sphere directly above my head
2. The Horizon is at a right angle to the Zenith line
3. The Equator is, well, the Equator

Study these three lines until they make sense to you, before continuing. Theyre pretty simple, but if you gloss over these three lines without really taking them in, the rest of this explanation will make no sense to you.

Got it? Okay, my Latitude is the angle between my Zenith line, and the equator. This is the angle we are trying to find.

Note that the angle of Latitude is equal to the sum two other angles, labled Dec. and Z.D.


Z.D. is the Zenith Distance of the Sun -- the angle between the Sun and my Zenith. Since the Zenith and Horizon lines are at a right angle, its pretty obvious that you can calculate the zenith distance by subtracting Ho from 90°.


Get it? Again, stop and study the diagram until this makes sense. If you just skip over this part, you wont really understand whats going on. Its easy, but you must stop a second to understand it.

Dec. is the Declination of the Sun -- the height of the Sun above the Equator at solar noon on 12 Jul 2010.

Note the difference between Ho and Declination: Ho is the height of the Sun above the horizon, and Dec is the height of the Sun above the equator.

How do we know what the Suns declination was at noon on 12 Jul 2010? We look it up in the Nautical Almanac.




Actually, I circled the wrong entry. Its an easy mistake to make, so its helpful that I made it (again!).

We dont want the declination of the Sun when it was solar noon in Greenwich (12:00 GMT), we want it when it was solar noon at my location, roughly 17:00 GMT.

You can see that the Suns declination at 17:00 GMT on 12 July 2010 was 21° 54.7. The N indicates that the Sun was North of the Equator. (In January, for example, it would be South of the Equator.)

So, we are almost there.

We used Ho to find the Suns Zenith Distance. We looked up the Suns declination in the Nautical Almanac. If we just add those two angles together, we should get an angle that is equal to my Latitude:



So thats was my Latitude, according to my sextant sights. If  youve followed along, and studied the diagram above just a bit, it should make sense to you. And note that the only math weve done is some simple addition and subtraction. No rocket science, and hopefully, no mystery.

Questions?

Next time we tackle the slightly more difficult problem of Longitude.

>>> Next Episode: Time of Meridian Passage

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