Last updated February 8, 2018 at 10:41 am
Blue moons are not quite as rare as the old idiom would have it, but blue, super and blood all at the same time? That’s quite a different story, as Alan Duffy explains.
The Moon Illusion makes our nearest neighbour loom larger. Credit: iStock
At the end of January our closest celestial neighbour will sport a bizarre name. As the second Full Moon in a month, it becomes known as a Blue Moon (hence the expression of a rare event as once in a Blue Moon, but really that’s once every two and a half years so not that rare!)
But it is also a perigee syzygy, aka a Supermoon, as the Full Moon occurs during a relatively close approach (perigee) which can be 50,000 kilometres closer than the furthest point (apogee) making it both appear larger (by about 14%) and brighter (by some 30%).
The Moon grows larger
For most people it’s almost impossible to notice that the physical extent has grown larger however this is where a trick of the eye comes in known as the Moon Illusion.
Make sure to view the Moon during moonrise, at the same time as sunset, so its close to the horizon and nearby structures. It will appear much larger than later on in the night when it’s higher in the sky. This is entirely a trick of the eye as you can test by giving the Moon a thumbs up (with outstretched arm) and note how much of your thumb it takes to cover it (usually the thumbnail) then repeat later in the night when it will look much smaller but note that it’s the same area of thumb required.
Finally, it’s a Blood Moon because it is a lunar eclipse, when the Moon travels into the Earth’s shadow.
A shadow creeps across the face of the bright full Moon, but thanks to our atmosphere some sunlight is bent (or refracted) towards the Moon, illuminating it in a blood red colour.
The Blue Moon is for everyone, the eclipse is not
A nicer way to think of this is that all of the sunsets and sunrises of Earth are shining on to the Moon that is now reflecting that light back.
While the entire world can enjoy the Blue Moon, only the side of Earth facing the Moon during the exact geometric alignment of Sun–Earth–Moon can enjoy the reddening.
Related: To the stars and beyond – reasons to look up in 2018
Essentially East Russia, China, East-Asia, Japan Indonesia and Australia as well as Hawaii and Alaska across the Atlantic can expect to see the full eclipse.
For Australia it will be a late one, with the eclipse occuring around midnight on the night of the 31 January/1 February. To see when you should look up, you can check your location here.
Perigee syzygy – it will never catch on
Finally, for all those who dislike the term supermoon (first coined by an astrologer) it’s worth keeping in mind that perigee syzygy is not quite as catchy or appealing to the public and anything that inspires them to look up and enjoy a celestial sight is worth considering.
It would be a shame to see such an inspiring term go to waste just because it has the unfortunate heritage of being coined by an astrologer.
Astronomy has a proud history of owning “unscientific” terms, such as the black hole, itself a term not of inspiration and scientific rigour but rather a derogatory term (in fairness coined by a rather famous scientist).
Whether the name appeals or not, the sight surely will.
Related content:
Alan Duffy’s Astronomy Glossary
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