Anticrepuscular rays

Two days ago, just after sunset, we had this view from the balcony:

Anticrepuscular rays over Shinagawa hotel area

The pink beams seem to be emanating from the hotels and corporate headquarters around Shinagawa Station. The sun was not setting behind these buildings, as you might think, but rather in the opposite direction in the sky. What was their origin? Although a great theory, they are not related to any imminent re-emergence of the Japanese Empire, as suggested by Anders.

Since I did not immediately understand this phenomenon, I posted the photo on Google+1 and asked if anybody did. Jakob had a hunch that they were caused by shades cast by high-rise buildings or mountains behind me, but I did not really buy into this. How foolish of me, because he was basically right!

Christina, an astronomer, knew the phenomenon and its name well: Anticrepuscular rays. They are closely related to the rays that you often see emanating from the sun. In this case, the pink light from the setting sun was being obstructed by scattered mountains or clouds, resulting in these rays. Usually you would see the rays in the sky around the sun, but here they continued to the opposite corner of the sky. Since the rays are essentially parallel2 they appear to converge at a point at infinity.

Rather naively, I expected the rays to continue spreading away from the sun, even with my back to the sun. A bit embarrassing, as the infinity convergence is quite obvious once you know it. But at least I got a nice conversation out of it.

Meguro, Ebisu, Shibuya and Shinjuku after sunset

I did look towards the sun at the time when I took this photo, but there were no clear crepuscular rays to be seen there. The photo above is roughly in the direction of the sunset, but taken 15 minutes later. It does not show anything relevant to this discussion, I just think it is a pretty sunset pic (click for larger version). The high-rise buildings in the centre and centre-left are in Meguro, Ebisu and Shibuya, while the ones in the background to the right are in Shinjuku.

  1. Google+ is pretty awesome. Please join and circle me there and share the awesomeness! 

  2. Remember, the sun is really, REALLY far away. From its viewpoint the mountains or clouds are therefore more or less in the same point.