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Hello everyone,

This week, I am on vacation in Nairobi, the capital of Kenya. This city is located around 120 miles south the Equator line, so yesterday I hired a local driver to take me there. The equator line mark is in a town called town called Nanyuki, and the longitude is 37° 4.2’ E.

According to my calculations from the nautical almanac, yesterday, Mach 20^th, the equinox occurred at 3hs 5’ 30” UTC, and the sun GHA was 224° 31.2’, which is somewhere north of Papua Island (135° 28.8’ E). This is also the position of the first point of Aries.

Sitting at the equator line in the morning, the sun was coming towards me right on the equator line (assuming no declination change from the equinox point), so the objective was to calculate the longitude at that position, using the same technique we would use to calculate latitude with the noon sight.

At the equator mark, the sun was going to pass around six hours later the equinox and the declination was going to be around 6 minutes north. For practical purposes, I assumed that declination was zero, so there was not spherical triangle to resolve. In this case, I used my 1941 A-12 sextant, the one I take with me on every trip.

A)       Part 1: I took some sights around one hour before noon (maximum altitude).

Average time: 08h 42’ 38”

Average Hs: 75 ° 55’ (the A12 does measure minute decimals)

Index error: -1’

Dip:0 (bubble sextant)

Ha: 75° 54’

Refraction at 75°: - 0.3 ‘

Bubble Sextant Parallel Correction: 0.1 ‘

Ho: 75° 53.8’ (Zenith Distance: 14° 06.2’)

At the average time of the sight, the sun GHA (from the nautical almanac) was:

GHA Sun: 308° 49.1 ‘

The longitude at the position was going to be just the sum of the sun’s GHA plus de zenith distance:

Longitude (in terms of GHA): 308° 49.1 ‘+ 14° 06.2’ = 322° 55.3’

Longitude (in terms of degrees): 37° 4.7’ E

Longitude (GPS): 37° 4.2 E

B)       Part 2:  I took many sights before and after the local noon (maximum altitude) to determine the time when the sun altitude was 90 degrees.

I took around fifteen sights, but below is a summary. It is important to notice that at difference of the noon latitude technique where the sun seems to be parked at a fixed altitude for a few minutes, at the equator line the sun altitudes change very fast at local noon:

UTC               HS

09 34 40 – 88° 56’

09 35 50 – 89° 12’

09 37 13 – 89° 30’

09 38 10 – 89° 40’

09 39 10 – 90° 00’

09 41 00 – 90° 34’

09 42 10 – 90° 52’

09 43 15 – 91° 15’

09 44 30 – 91° 18’

I assumed that the local noon was at 09 39 10, because the Hs was 90 degrees, the refraction at that altitude was minimal and I discarded the index error (-1 ‘).

By using the nautical almanac, I calculated the GHA of the Sun:

GHA Sun: 322° 57.2’

GHA SUN in terms of longitude: 37° 2.8’ E

GPS Longitude: 37° 4.2’ E

I think that this small difference in longitude ccoul be because of these two factors:

-            At the time of the noon sight, the sun’s declination was around 6’ north. I assumed no spherical triangle when it was a small one.

-            The sextant index error is usually measured at low altitudes, so I am not sure if would remain the same at the other extreme of the arc.

I am incredibly happy with the experience, so I am open to your comments or suggestions.

Thanks!

Martin Caminos

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