diamond geezer

 Wednesday, January 31, 2018

Are you ready for today's super blue blood moon?

The Telegraph describes it as "a once-in-a-lifetime event". The Mirror thinks "we're set for quite the celestial show". The Sun says it'll "appear significantly bigger than a normal full moon". The Independent reckons "the moon is about to do something it hasn't done for 100 years". The Express awaits "a pretty amazing sight, the likes of which hasn't been seen for 150 years." The Guardian promises "a moment not seen in the skies in more than 150 years".

But is an amazingly rare huge bluey-red moon about to appear in the sky? Or is this nothing more than journalists with a shaky understanding of astronomy bashing out a clickbait story because all their rivals have? Spoiler: it's the latter.

Let's start with the supermoon part.

Fifty years ago supermoons didn't exist, or at least the phenomenon did but nobody had given it a name. The man who came up with the definition was Richard Nolle, and he's an astrologer, which should start to suggest why "supermoon" is a mighty shaky concept.

Supermoons are full moons which look bigger than normal. According to Richard, a supermoon is "a full moon which occurs when the moon is within 90% of its closest approach to Earth in a given orbit". 90% is a wholly arbitrary limit, but this generous percentage means supermoons crop up more frequently than the name would suggest. Today's supermoon, for example, follows supermoons on 2nd Dec 2017 and 2nd Jan 2018, and three more will be along early next year. Officially the moon's closest approach to the Earth is called perigee, and its timing varies month by month, so has nothing to do with whether the moon is full or not. In the moon's current orbit perigee was yesterday, and the full moon is today, and that's close enough to count.

To throw some actual numbers at this, the moon's average distance from the centre of the Earth is about 385,000km. This year it gets closest on January 1st (356,566km) and is furthest away on January 15th (406,461km), a gap of about 50,000km. Calculating 90% of this interval requires a supermoon to be within 361,500km of the Earth. The full moon this afternoon occurs when the moon is 359,900km away, which is less than the target number, and that's why it's a supermoon.

The "exciting" thing about a supermoon is supposed to be its size. The moon's at the closest point in its orbit, so appears larger in the sky, and that's supposed to make us go wow. Compared to a full moon taking place when the moon is at its furthest point, the diameter of the lunar disc is indeed 14% larger, and shines about 30% brighter. But compared to an average full moon it's only 7% larger, and 14% brighter, and compared to the last two full moons it's pretty much the same size. Your mobile phone will still take a crappy photo, and if you think you can genuinely perceive the difference with the naked eye you're probably wrong. Indeed a completely separate phenomenon called the "moon illusion" kicks in when the moon is very close to the horizon, so you can always trick your brain into thinking any full moon looks much bigger than usual.

Further reading about the astronomy behind supermoons: here, here, here, here, here and here

Let's move on to the blue moon part.

There are two definitions of a blue moon, one of which has surpassed the other. Originally a blue moon was defined as the fourth full moon in a single season (when normally there'd be only three). But more recently it's come to mean the second full moon in a calendar month, after a journalist misunderstood the concept in 1946 and everybody preferred his simpler explanation. Full moons occur every 29½ days, which is how this month we had a full moon on 2nd January and are getting another on the 31st, the second being the blue moon. But this doubling-up doesn't happen very frequently, because 29½ is very close to 31, hence the expression "once in a blue moon" meaning not very often at all.

The original, seasonal definition of a blue moon comes from a time when our ancestors gave full moons names. For example, the first full moon after the autumn equinox was the Harvest Moon, the second was the Hunter's Moon and the third was the Cold Moon (or some other name, depending on what kind of ancestors you had). But if a fourth full moon squeezed in before the winter solstice then another name was needed, otherwise it'd shift the names for winter moons out of sequence, and so the third in the sequence became a Blue Moon. By this measure there are no Blue Moons this year, the next being on 18th May 2019.

By the modern definition, a blue moon comes around roughly every 2¾ years (technically, seven times every 19 years). The last time we had a blue moon was in July 2015, before that September 2012, and before that March 2010. But the next one isn't very far away at all, and all because February has less than 29½ days. This means the next full moon will be on 2nd March, and then we get another on 31st March, so that'll be another blue moon. Poor old February gets no full moons at all, and the phrase "once in a blue moon" turns out to mean "in two months time".

One further catch. Today's blue moon isn't a blue moon everywhere. The precise time of the full moon is 1.26pm GMT, which is fine in Britain, but in New Zealand it'll be after midnight on Thursday. That means New Zealand doesn't have a blue moon today, but does have a full moon in February, and the same goes for various other Pacific islands. Sometimes this happens on the other side of the dateline, so for example a full moon at 1.02am GMT on 1st June 2007 became a blue moon in the Americas, where it was still May at the time.

Most importantly, a blue moon is never blue, it's merely a quirk of the calendar. And that means going outside specially to observe a blue moon is pointless, because there's literally nothing out of the ordinary to see.

Further reading about the astronomy behind blue moons: here, here and here

And finally the blood moon part.

A 'blood moon' is simply another phrase for a total lunar eclipse - a phenomenon that's properly interesting without the need for hyperbole. What's happening this afternoon is that the Earth will pass directly inbetween the sun and the moon, and so will blot out light on the lunar surface. The moon never gets properly dark, because some light gets refracted through our atmosphere, and when that light is reflected back towards us it's taken on a coppery hue. Under optimal conditions the moon can appear eerily red, but don't expect to see a gory disc hanging in the heavens.

Total eclipses of the moon aren't as stunning, or so rarely seen, as their solar counterparts. Whereas a total solar eclipse can only be seen from a very small sliver of the Earth's surface, a total lunar eclipse is visible from the entire half of the Earth facing the moon, cloud permitting. A total solar eclipse also never lasts more than a few minutes in one location, whereas a total eclipse of the moon can be seen for up to an hour and three-quarters. And a total solar eclipse only ever happens during the day, whereas a total lunar eclipse always happens at night... and here's our problem.

Today's full moon takes place at 1.26pm GMT, when it's daylight in the UK, and the moon is below the horizon. Today's total eclipse starts at 12.52pm and ends at 2.08pm, which means here in the UK we won't be seeing any of it. Japan gets a great view in the evening, and the west coast of America gets a great view before dawn, but by the time the moon rises in Western Europe the eclipse will be long over. US media is rightly excited by the prospect of a total lunar eclipse, but again anyone cut and pasting their words for the benefit of a UK audience needs to add the caveat "but you'll see no blood moon here".

Another total eclipse of the moon will be along in July, and that'll be the longest lunar eclipse of the 21st century, so that's something much more interesting to wait for. It will be visible from the UK, but only after moonrise, so we won't see all of it. Another one's due next January, assuming you can be bothered to wake up before dawn and look, and that'll be a so-called supermoon too. With 85 total lunar eclipses this century, you'll have every opportunity to see a special one eventually, but today is not that day.

Further reading about the astronomy behind lunar eclipses: here, here, here, here and here

In short, today's "cosmic trifecta" is nothing special. Supermoons are not as super as you think they are, blue moons are not blue, and the blood moon isn't visible from anywhere in the UK. Our moon is a pretty amazing thing to see at any time of the year, a full moon especially so. But the idea that today's full moon is a super blue blood moon is, alas, nothing but lunar hype.


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