When the Sun draws in the Sky

This picture of the week shows the Sun’s path on the sky as seen from ESO’s Paranal Observatory in Chile. The picture was taken with a pinhole camera with an exposure of 8 months, from 17 April to 11 December 2018. The tall structure in the foreground is the “seeing monitor”, which measures the blur/twinkling of the stars due to the atmospheric turbulence. In the background we can see several domes; the ones to the centre-left are part of the Very Large Telescope, and the rightmost one is the VLT Survey Telescope.

But what are those bright coloured lines behind them? They are created by the Sun as it moves in the sky throughout the day, leaving an imprint on the sheet of photographic paper placed inside the pinhole camera.

The camera was facing west, so here we see hundreds of sunsets. But as the Earth moves around the Sun, each day the Sun sets at a different point along the horizon. The shortest arc all the way to the right of the image corresponds to the winter solstice (June 21 in the Southern Hemisphere), when the Sun reaches its lowest elevation. From that point onwards, the Sun moves southward day after day, to the left in this image. The arcs become longer and higher, and the days longer and hotter. Then, on the summer solstice (December 21), the Sun reaches its highest point, which is not visible in this picture, and draws the longest arc to the left of this image.

The dark lines are due to clouds blocking the Sun. Luckily for astronomers, Paranal Observatory boasts over 300 clear days per year, hence the very few dark bands in this image.