Predicted sunset 17:41, observed sunset 17:41

I watched the sun set from my rooftop today and just had to take a picture. The direction is pretty much due south, and the sun is about to set over the mountain called Løvstakken (“the stack of leaves”, so called due to its shape, I guess).

The top picture shows the horizon as seen from my rooftop just before sunset today. The graph is a zoomed view of the computed horizon at suncurves.com.

The top picture shows the horizon as seen from my rooftop just before sunset today. The graph is a zoomed view of the computed horizon at suncurves.com.

The clear weather also enabled me to do some on-the-spot verification of the suncurves.com computations. My colleague Paul and I have made a program that computes the apparent horizon from any location worldwide. This horizon is then combined with data for the sun’s path across the sky. The intersections between the two define the times for sunrise and sunset for any day of the year. Our data is obviously a lot more accurate than the standard sunrise/sunset times that you find elsewhere on the web.

But just how accurate are our calculations? Today our program said that the sun would set at 17:41 (see the table in the picture), and that was spot-on for today. We’ve seen that our times are usually with a 5-minute margin of error. When you accumulate that over a full year, which we do to compute what we call a sun index, the results are very very reliable.

If you’d like, you can find your own sunset and sunrise times for places that you care about. The site is free to use as long as you register. Go on, give it a try in our location finder. Or click here to find a list of our demo pages for sites around the world.

And tell your friends about it if you like it – we think a lot of people out there would think that this is cool.

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