Sunday, December 03, 2006

Surprised by the sun

I was surprised to notice recently that the sun here does not always set in the same place. Being very close to the equator, our days are pretty much the same length, but the sunset is definitely moving. This was disguised because it is the rainy season and sunset is usually behind cloud, but I had expected that it would always be virtually due west. The technical explanation is provided by my dad, who used to teach navigation.

“Being on the equator greatly simplifies the calculation of the bearing of the sun at sunrise and set. Cyangugu is at 2* 29' S of the equator but that will not affect the situation to any extent.
The amplitude or bearing at sunrise is the number of degrees from 090--- N from the Spring to the Autumnal equinoxes and S of 090 for the rest of the year. The same applies to 270 at sunset. At the equinoxes it rises at 090 and sets at 270.
The maximum angle away from the E---W line occurs at the solstices and is 23.5* The sun therefore rises at 066.5 and sets at 293.5 on 21st June and rises at 113.5 and sets at 246.5 on 21st December.
These values for the amplitude of the sun at the equator are the same as the sun's declination on the respective dates. Minute by minute value for declination is given in the nautical almanac for each year. You could get an approximate enough value by drawing a cosine curve or even a straight line graph of angle against time”.
Reading this led me to another surprise – the sun is not always directly overhead at mid-day, as I had imagined it would be. This will only be true at the equinoxes.
If all this technical writing detail seems difficult, there is also a very simple parable here. I don’t understand half of what goes on in Rwanda, even simple things like the rising and setting of the sun!

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