It seems I can’t go too long without hearing about everyone’s favourite celestial event: the Supermoon. It’s a somewhat rare event, typically occurring once every 14 months or so, but given the amount of attention it seems to get on various news and social media sites you’d be forgiven for thinking it’s more akin to a total solar eclipse. Considering what actually happens during this event, I.E. the coincidental alignment of the moon being full whilst also being at it’s closest point on its orbit around Earth, you wouldn’t expect there to be much interest in it but everyone seems to be wowed at just how huge the moon appears when this happens.
Problem is though that it’s not that much bigger at all.
As you can see the supermoon on the right hand side isn’t really that much bigger than the more regular moon on the left. Indeed if I could somehow show you both of them in the sky at the same time you probably wouldn’t be able to tell the difference between them. Still come tomorrow morning I’m sure there’ll be dozens of pictures showing the moon in all of its super glory, towering over buildings and dwarfing every other source of light in the night sky. So the question is then where do all these pictures come from and why is the moon so gosh darn huge in them.
The answer is somewhat complicated, and we don’t have an exact answer for it yet, but it comes down to something we’ve dubbed the Moon Illusion. Essentially any stellar object in our sky, be it the moon, sun or even stars, will increase in apparent size the closer it is to the horizon. All those amazing pictures you’ll see of supermoons around the globe tomorrow will likely be taken when the moon is rising when this effect is at its most pronounced. This makes the moon appear much, much bigger than it would normally but over the course of it’s full rotation it’ll begin shrinking back down to a more reasonable size. This is why you don’t see any pictures of it up in the middle of the sky, it just doesn’t look as massive as it would otherwise.
Photographers are also somewhat guilty of exacerbating the moon’s super-ness during this time by using telephoto lenses that compress the visual space significantly, often putting other objects near or in front of it to make it appear much bigger than it would with your eyes. Don’t get me wrong it makes for a stunning picture but it also feeds into the idea that the moon appears that huge at all points in the sky. The reality is unfortunately nothing like that.
I know I’m probably being a killjoy for some people mentioning this but honestly things like this fall into the same realms as those claiming we’d have 2 moons when Mars was at its closest approach to Earth. Sure it’ll be bigger than it would be otherwise but the effect is usually beyond our ability to perceive and the photos just give a false impression of what a supermoon actually is. To be fair though the term supermoon isn’t a scientific term at all (it comes from astrology) so I guess I shouldn’t be too surprised about the smoke and mirrors that surrounds it.