Total Failclipse 20/03/15

View over St Andrews' West Sands beach Image: HenryLeggPhotography
View over St Andrews’ West Sands beach
Image: Henry Legg Photography

Not until 2026 will a similar reason for sacrificing a morning lie-in crop up, as it did this morning when the UK and some parts of Europe witnessed a celestial phenomena of coincidental Sun-Moon-Earth alignment, otherwise known as a Solar Eclipse. The lucky souls over in Faroe Islands were well-placed to see the total solar blackout; here in the UK, most of us could only witness around a 90% dimming – still enough to cause confusion amongst birds and humans alike! The partial eclipse I witnessed here was, unfortunately, not quite as spectacular as the rather more impressive stratus obscuring the entirety of the South-Eastern sky, thus only a noticeable temperature drop and a sudden graying of the sky made it known that yes, indeed the Earth momentarily lay entirely in the Moon’s shadow circa 9:30am on 20th March 2015.

Not since 1999 when I was a kid, and not until 2026 when I might even be able to take my own kids out and see this phenomena in a place where the sky doesn’t necessarily always resemble a state of partial eclipse as it does here in the UK…

I could talk you through the science of why these beautiful coincidences happen,  but New Scientist or Astronomy Now or your physics teacher can do a more thorough job than I could. Instead, I’d like to bring to your attention this: in the theorised infinite future of our universe, only a finite number of such occurrences are due to happen. Why? Well, most of us are aware that the Moon’s gravity exerts an attractive force on he Earth’s oceans, causing a high tide on the nearside of Earth, as well as the far side. This is because not only the waters are pulled in, but also the non-solid core inside the Earth is affected, edging the planet a little closer and “pulling it away” from the oceans on the far side, shall we say. But anyway you probably know this, so allow me to explore the grittier parts of the story: these tides slow down our planet’s spin by creating greater than normal friction underneath the ‘bulges’ as it rotates, increasing the length of the day by about 1.6×10-3 seconds per century. Hey trivia! But this means that its angular momentum decreases overtime. Conservation laws make it a rule that the total angular momentum in the Earth-Moon system must remain the same (unless an external object such as a giant extraterrestrial cookie perturbs the peace), so in turn the Moon’s angular momentum increases in order to balance the act. Additionally, the sea bulges exert a minimal (but nonetheless effective) attractive force on the Moon, increasing its speed and therefore also its orbit, by about 3cm each year.

And here’s a highly scientific, informative diagram to explain it all. Drawn to scale, of course:

Drawn to scale. Obviously.
Drawn to scale. Obviously.

This, by the way, has all been studied by the big guys in astronomy over many decades; but it’s also something I pulled out from BBC’s Stargazing Live. Ah, Dara O’Brien you huggable madman! [As an aside: I have full appreciation of that show – it was the final trigger of inspiration to become a physicist a good few years ago! As much as Cox’s wonderous smirk annoys me now, I fully thank him for the inspiration and recognise that he is doing a fantastic job with making science look downright cool in the public eye!]

But I still haven’t mentioned why there will be less total eclipses. That is due to the beautiful coincidence that the Sun’s diameter is approximately 400.484604 times larger than the Moon’s, and it is roughly 389.169363 times further away. Mathematical intuition yells at me to not do this but…. let’s make that roughly 400 and 400, respectively. Kill me now. Anyway. These conditions are almost geometrically perfect for the Moon’s diameter to be able to fully obscure the Sun’s core, apart from the corona we could see during this year’s eclipse. But as the moon moves further away, this approximate 1:1 ratio distorts, making only partial eclipses possible in the very distant future as the Moon won’t be able to cover up the entirety of the Sun from Earth’s perspective. Sad times, man. Sad times.

Eclipse meme
source: BBC News. Great British weather.


One thought on “Total Failclipse 20/03/15

  1. Even with the 400-coincidence, apogee and perigee mean the moon’s distance from the earth varies quite wildly, resulting in annular, hybrid and total solar eclipses. And there’s still all the stuff about lunar nodes and Saros cycles!

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