How long have we been predicting eclipses?

Tomorrow there will be a total lunar eclipse which will be visible in much of North America and Asia. I won’t personally be able to watch it from here in the UK, but if you’ve never seen one you should check it out if you can.

During a lunar eclipse the Earth moves directly between the Moon and the Sun, casting a shadow over the Moon’s surface. That’s different to a Solar eclipse, when the Moon moves between the Earth and the Sun, casting a shadow on the Earth. We understand how eclipses work and we know exactly when they’re going to happen, down to a fraction of a second. These days we take that for granted. After all, we have advanced technology and a fairly solid understanding of the Solar System, so of course we know what’s going to happen! But surely that must be a relatively recent thing?

As it turns out, you don’t need to know that much about the Solar System to be able to predict eclipses (of either the Solar or Lunar varieties). Ancient civilisations – Babylon, Greece, and China – were able to do a reasonably good job even though they thought the Earth was at the centre of everything. That makes sense, because all you really need to know to predict an eclipse is where the Sun and Moon are relative to the Earth. You don’t need to know everything about them. Ancient people observed enough eclipses that they were able to figure out the pattern behind them, even though they didn’t really know why they were happening and they didn’t always know where they’d be visible from.

Back then people were jumpy about the Sun or Moon suddenly disappearing, so astronomers were expected to provide accurate predictions. They weren’t always right, though, and for early astronomers the consequences of getting it wrong could be severe (think: beheading).

Thankfully we’re much better at predicting eclipses now, and people tend to be a bit more relaxed about such things these days. So if you’re lucky enough to be someplace the eclipse will be visible, take a look up at the Moon as it slips into darkness and spare a thought for the ancient astronomers. The disappearing Moon might have been the last thing they would see!

Further reading

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