Neil's News

Solstice Calculations

21 December 2002

[Earth] According to the BBC, today's sunrise/sunset times for Inverness are 09:00 and 15:30 respectively. From this one can tell that noon/midnight in Inverness is shifted about 15 minutes away from the centre of the GMT time-zone. Therefore one can calculate that our longitude is 360°/24/60*15=3.8°. The accepted value of longditude for Inverness from the Ordinance Survey is 4.0°W.

Those sunrise/sunset times specify 6.5 hours of daylight, or 27% of the day. Consider the formula SQRT(1-(1-(90°-x-i)/(90°-i))2)/2=p where x is the latitude, i is Earth's inclination (23.5°) and p is the percentage of daylight (yes I know it can be optimised, but I've already got one headache from developing it). If you solve for p=0.27, x becomes 56°. The accepted value of latitude for Inverness from the Ordinance Survey is 57°N.

Both calculations assume that Earth is a perfect sphere. No direct measurement of any of this is possible, since we haven't actually seen the sun since about mid-August.

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