On 20th March there is a deep partial solar eclipse over the  UK which is total over the Faroe Islands and Svalbard. I set about trying to determine the time of maximum eclipse, and percentage eclipsed at a given location.

I used the pyephem module which is a Python implementation of the numerical routines that are behind the excellent Xephem. The module will allow a user to calculate the positions of astronomical objects (eg Sun, Moon, etc) from a given location at a given time. Using the positions of these objects it’s possible to determine the separation of the centres of these objects.

To work out the percentage eclipse at that time, I used the formula on the Wolfram Mathworld page on “Lunes” (author: Weisstein, Eric W). A “Lune” is the “plane figure bounded by two circular arcs of unequal radii, i.e., a crescent.” which exactly describes the visible surface of the sun during an eclipse.

With the area of the lune calculated, it’s trivial to work out the percent eclipse at that time – and by looping over time, it’s possible to get a list of lists to search for a maximum.

The code can be downloaded using this link: eclipse_percent.py

The result I get (using the location of the Science Oxford setup on 20th March – the SBS in Oxford, UK) is as follows:

Max Eclipse occurs at: 2015-03-20  09:30:11 GMT
Max percentage eclipse: 85.86
First contact: 08:24:21 GMT
Last contact: 10:40:04 GMT

All we need now is some decent weather…

The featured image here is an H-Alpha filtered frame taken by me on 3rd Oct 2005 from Denia, Spain, during the early phases of an annular eclipse.

Eclipse Calculations using Python by Graeme Coates is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.