BBC Sky at Night Magazine
The Sky Guide challenge
Can you solve the mystery of the clair-obscur effect known as Plato’s Hook?
This month’s challenge is to observe an intriguing clair-obscur effect known as Plato’s Hook. It was famously recorded by Patrick Moore and HP Wilkins on the night of 3 April 1952, although there’s some controversy surrounding the precise timing of the observation. The hook refers to the shape of the shadow cast by one of the peaks around Plato’s rim, the so-called ‘gamma peak’. At certain times
when morning light has flooded most of
the crater’s interior, as the eastern rim shadow is retreating back towards the rim, the gamma peak casts its pointed
shadow across Plato’s floor.
What Moore and Wilkins reported while using the 33-inch refractor in Meudon, Paris, was a curiosity with this shadow. It looks curved where you’d normally expect it to look straight. As such it gives the appearance of a hook rather than the more triangular, sharp tooth shape you might expect. This begs the question what is going on?
One theory cast into the pot suggested the shadow was falling onto a low-lying hill complex and as it falls down the hill, so it takes on the curving shape of the surface. This seems fairly plausible except that some interesting simulations made by an Italian team using computer and Plasticine models suggest a) the gamma peak never exhibits curvature as shown in Moore and Wilkin’s drawings and b) the shadow cannot appear where the hook is reported in drawings.
The Italian explanation seems more plausible than the gamma peak shadow falling onto rolling hills. Their suggestion is that the curved shadow is formed where the main rim shadow is interrupted in the southeast by a complex set of hills within Plato’s rim terraces. Looking back at our own archives, we have one shot which is illuminated this way and, well, it’s not exactly clear.
The only way to verify what’s happening is to observe it yourself. Indeed, modern high-resolution imaging should be able to put this topic to rest completely. However, in order to do this the terminator needs to be in the correct place, the seeing reasonably good and the weather clear. Plato’s Hook should be visible during the day on 1 May, around 14:30 BST (13:30 UT), or better placed on 1 June around 00:49 BST (23:49 UT on 31 May). However, with the disagreements stated an extended observation time would be appropriate to see how things develop.
We don’t know what you’ll see or what you’ll record and that’s fascinating. Astronomy isn’t always about being told what you’ll see. This is a real observation that could potentially solve a mystery which has endured for many decades.