The solar eclipse – and what it means

Eyes across the US will be pointing skywards today as the total solar eclipse sweeps across America – the first visible from the continguous continental US since 1979.

The Sun’s corona. Public domain, CC0 license, via

There’s been a buzz about fake ‘eclipse glasses’ on the market, which doesn’t surprise me. Humanity has a very dark side and, of late, it hasn’t been much hidden. Personally I wouldn’t trust any glasses . Looking at the sun is dangerous. If anything goes wrong, at best you’ll be dazzled and at worst you’ll do permanent damage (the eye interior doesn’t have pain sensors, so you won’t know you’ve burned your retina until it’s too late. Just saying.)

Instead, do what I did last time there was an eclipse in New Zealand. Use indirect projection. I punched a hole in a card and rigged a piece of white paper a short distance behind it, allowing the sun to shine through the punched hole. The outcome was that the sun’s image – with the ‘bite’ out of it made by the Moon – was projected on to the paper, and by standing with my back to the sun, I could look at the projected image. NOT the sun itself – just its projected light.

So what is an eclipse? It’s the moment when the Moon comes directly between Earth and Sun, which doesn’t happen very often because the Moon’s orbit is tilted relative to the plane of the Earth’s orbit around the Sun. It’s only when the Moon crosses that plane AND is between Earth and Sun that an eclipse occurs.

Coincidentally, the Moon is also almost the same angular diameter as the Sun, as seen from Earth – so it precisely covers the Sun’s disc. At totality, it’s possible to see a phenomenon known as ‘Baily’s Beads’, in which the edges of sun peek through the serrations around the visible edge, made by lunar mountains. The Sun’s atmosphere – the corona – is also visible.


The weird thing is that this ability of the Moon to precisely cover the Sun happens to be true only just now. Thanks to tidal forces – both induced by the Moon in the Earth and by the Earth in the Moon – the rotation of both bodies is relentlessly being slowed. The Moon’s has already been locked in to one rotation per orbit around the Earth.

Because of the principle of conservation of angular momentum, that energy isn’t lost – it goes instead into slowing the Moon in its orbit. This means that the Moon is gradually moving away from Earth. Hundreds of millions of years ago, it was far closer and both bodies were rotating far more quickly. In future, Earth will be rotating even more slowly – while the Moon remains tidally locked – and the Moon will be further away.

That will end the ability of the Moon to create a total eclipse; its angular diameter (the width it appears to be in the sky when viewed from Earth) will be smaller than that of the sun, and it won’t cover the Sun’s disk. That moment will come in about 600 million years.

Copyright © Matthew Wright 2017


3 thoughts on “The solar eclipse – and what it means

  1. what I learned is that the small deviation in starlight(due to general relativity) is inversely proportional to the distance between the star and our sun’s center.

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