Has it really been so long since there was a post here on the Mule? It would appear so and my only excuse is that I have been busy (isn’t everyone?). Even now, I have not pulled together a post myself but am once again leaning on the contributions of regular author, James Glover.

From pictures of the transit of Mercury you might think that Mercury is really close to the Sun and that is why it is so hot that lead is molten! In actual fact Mercury is about 0.4 Astronomical Units (AUs) from the Sun (Earth is about 1AU) and only receives about a 7 fold increase in sunlight intensity. So it is hot but not that hot. Mercury is about 40 solar diameters from the Sun. If the Sun were a golf ball then Mercury would be about 6 feet away and the Earth about 15 feet away. On Mercury the Sun subtends an arc of 1.4 degrees compared to 0.6 degrees on Earth.

Pictures of the Moon in front of the Earth seem to have the same effect, to me at least, of making it look much closer than it is, whereas in reality the Moon is about 30 Earth diameters away. Roughly the same “size of larger body to distance of smaller one” ratio as Mercury is from the Sun.

This optical effect (modesty prevents me from giving it a name) seems to occur when photographing one astronomical body over another. It can’t be that we are using the relative sizes as a proxy for distance since Mercury/Sun is very small and Moon/Earth is relatively large. Lacking other visual clues, that a terrestrial photograph might provide, my guess is that we use the diameter of the larger body as a proxy for the distance from the smaller one. Mentally substituting  “distance across” for “distance from”. Or maybe it’s just me?

One possible explanation is that there is insufficient information in a 2D photo like this to determine the distance between the objects. But if asked “how far do you think the one in front is from the one behind?” rather than say “I can’t tell”, you choose one of the two pieces of metric information available, or some function of them, such as the average. Perhaps the brain is hardwired to always find an answer, even a wrong one, rather than admit “I don’t know”, “I have no answer” or “I have insufficient information to answer that question, Captain”. That would explain a lot of religion and politics.