As I said on the home page, I have upgraded my sextant to a Davis Mark 25. After some time to get used to the new sextant, I have had four successful runs using the false horizon method in my backyard. With the higher magnification of the telescope in the Mark 25 sextant, my simple bowl-of-water false horizon was very sensitive to breezes and to passing traffic, but I persevered. The average of the four runs is a latitude at my home of 39° 53′ 21″. This is a good result, but based on the spread of the four runs I have to put the uncertainty at ±20″.

To complete an improved estimate of the diameter of the Earth, I now need a fine Fall day for me to drive down to Cape May and re-measure the latitude there.

An extra use for these new values is to try to refine my value for the length of the Tropical year. I have been making sporadic observations of the Sun’s altitude since 1988, and the value quoted in my book, of 365.25 ± 0.01 days, was based on measurements made up to 2011. The new values, for September 2017, don’t increase the base line very much, but I might as well try! The most successful method of analysis I have found seems to be to plot all the measurements with the dates reduced to a single period using the modulo function in Excel, with an instruction “=MOD(Julian date, guessed year)”. With the new data, I judge that a guess of 365.24 gives as good a fit as 365.25, and may even be a smidgen better. I am putting my best value as half way between these, but keeping the uncertainty at 0.01. So 365.245 ± 0.01 days is my current best estimate.