It has become viral on social media a video showing seven suns at sunset Chengduin the Sichuan province of China, taken on Sunday, August 18 by a resident of the city. It is a seemingly amazing video, which shows several images of the Sun aligned and with different brightness. In the center there is a main image, brighter, surrounded by identical copies of the same image that become increasingly fainter as you move away: there are 4 to the right and 2 to the left. Many are talking about the “mystery of the seven Suns”, but there is nothing mysterious about it: they are simply multiple reflections between the layers of window glass through which the sunset is being observed, probably a double glass with layers that are not perfectly parallel.
As often happens with this type of content, the web and social media have been filled with the most disparate explanations, from a digital effect created with some video editing program at a rare meteorological phenomenonpassing through the inevitable supernatural theories. Initially the phenomenon had been presented as a atmospheric optical effectwith many throwing out terms like Parelion or solar halo. These are, however, completely wrong hypotheses: the parhelion (or “sun dogs”) is a phenomenon that shows two bright spots at a great distance from the Sun (to be precise at an angular distance of 22°), often accompanied by bright arcs. Here instead we have 6 multiple images close together, so it cannot be a parhelion. Also because the parhelion is more frequent when it is very cold, because it is due to ice crystals present in the atmosphere, while in Chengdu in the last few days there were temperatures around 40 °C.
The solar halo should also be immediately excluded, since it does not produce multiple images but a light circle around the Sun (still 22° away). Again, there is no resemblance to what the viral video from Chengdu shows.
Ok, so if it is not an atmospheric optical phenomenon what is it? It is still an optical effect, but created in the glass through which we see the sunset. In social media you can read explanations that bring into play terms of optics such as scatteringwhich however have nothing to do with it because in the video you can’t see any dispersion of light: these are pure and simple multiple reflections.
From the fact that the secondary images are identical to the main one, they are increasingly fainter as one moves away from the main image and become increasingly closer together we can reconstruct a possible scientific explanation. The window glass could be a double glassand the sunlight is reflected back and forth between the two glasses several times. Furthermore, the two glasses would be slightly inclined one with respect to the other, otherwise all the images would be equidistant from each other. From a physical point of view, the angular distance between the images is comparable to the inclination between the two glass plates, and from the video you can see that this distance is comparable to the diameter of the Sun in the sky, which is about 0.5°. The inclination between the two glass plates would therefore be 0.5°, very small but sufficient to create the optical effect.
As you can see at the beginning of the video, the sunlight reaches the window “from the left”, and in fact most of the multiple reflections are on the right of the main image, because they follow the path of the light rays. Here is one geometric pattern which conceptually shows how these multiple images could have been formed:
