This webpage contains updates to the observing program described in my book “Gauging the Solar System”, available at

2016 was a great year for following the superior planets.  I measured complete or partial plots of retrograde motion for Mars, Jupiter, and Saturn.  For Mars especially the measurements cover a greater fraction of the orbit than I have previously recorded, and that shows up the limitations of my analysis programs.  I am still working on getting as much information as possible from the graphs.  I also completed a half plot for Uranus.  It was a challenge because from my home the whole region of the sky that it was moving through looked completely blank to the naked eye.  It usually took a few tries to get a photograph with Uranus near the center.

On another front, I have modified my attitude about sextants!  In my book I explained that I have been using a Davis Mark 3 sextant for almost thirty years.  Because of its limitations I could measure latitudes only with a 2′ uncertainty, and this limited my calculation of the radius of the Earth to plus or minus 6%.  Actually, I am still happy with that as a result in itself.  My problem is that in the later parts of the book I made quite a lot of use of this value, and that 6% looked less and less acceptable.  I have just upgraded to a Davis Mark 25 sextant and that looks much more powerful.  I need to spend some time using it before I try to take final values, but I am hoping to cut down the uncertainties by a factor of ten.  Davis are at