There will be a very special astronomical event this Saturday which, weather permitting, you’ll not want to miss.
As the full moon rises, around 8:25pm (in the UK) above the south-eastern horizon on the evening of the 5 May, it will appear spectacularly large – larger, in fact, than it has for about 100 years, when this set of circumstances last occurred.
Popularly dubbed the ‘Mega Moon’ this effect is caused by a combination of factors, not the least of which is that the Moon will be at the ‘perigee’ of it’s elliptical orbit around the Earth, meaning it is at it’s closest to the earth, some 27,000km closer than it’s mean orbital distance and some 50,000km closer than at it’s ‘apogee’, which is when it is furthest from the Earth.
The fact that it is rising full will also have a significant impact on our perception of it, simply because we are far more familiar with it hanging in the middle of the sky, and not emerging from the horizon where there are points of reference which our brains use to assist with perception of depth, but in fact completely confuse our primitive and ingrained instincts.
The ‘Moon illusion’ as this is known, seems to stem from the fact that we perceive the sky as a flat dome, rather than the high hemisphere it actually is. Familiar daytime objects, such as clouds and birds appear beneath our flattened celestial roof, but they appear smaller and foreshortened when closer to the horizon (compared to their counterparts flying at a similar height nearby). Mist and haze in the atmosphere, more noticeable in objects closer to the horizon merely adds to this perception.
In essence, our poor brains are befuddled by all these effects and so totally misinterpret the distance of objects in the sky, which is particularly apparent with the full rising Moon, which in turn leads us to perceive it as larger than we expect.
So, in the UK at 8pm on Saturday, we shall be praying to the weather Gods to provide a clear view of the south eastern horizon, from the vantage point of our local hill – it should be a sight to behold.