Unintended Extra Travels…

Just for once, I’d love to see a time travel story where the protagonists know their probe has worked because they see a starfield when it drops backward to 9,000 BC (or whenever).

This is a minor point that often annoys me with time travel stories: if you actually had a time machine or viewer, you’d better hope it’s a space machine as well. Why? Because the Earth (and the Sun) move.

And they’re actually quite fast.

The average orbital speed of the Earth is 29.8 kilometres per second.

If I recall correctly, when measured with respect to the centre of mass of the galaxy, the Sun’s orbital speed is something like 200 kilometres per second. The Earth, of course, moves with the Sun as well as being in its own orbit. Also, the Earth’s atmosphere is generally treated as being about 100 km deep. (This idea is actually complete rubbish – there’s a detectable exosphere out to a thousand or so kilometres – but in fairness, something like 75% of the mass of the atmosphere is found in the bottom ~10 kilometres, so…)

Or, to put it another way, if your time-machine goes back more than 0.5 seconds and isn’t also a space machine, then you’d better pack an oxygen tank or two, as you’re going to need it. And perhaps a parachute might be a good idea too!

A handwave for this issue is somewhat-possible through a conservation-of-momentum argument (the time viewer/machine is on the Earth, and so moves with it at both “ends”), but that runs into some very subtle niggles. (The Earth’s orbital speed isn’t constant across its orbit, and over long periods of time the shape of the orbit also evolves too – the eccentricity has a long-term cycle driven by play-offs between Jupiter, Venus and the Moon. And if you want to get crazily-technical there’s also the barycentric correction to consider as well.)

((I was reminded of this niggle by a recent run-in with a time-viewer story that turned out to be one of the “peeping on Jesus” subspecies. To be fair to the story, it did actually make a note of the velocity issue, at least with respect to the Earth’s orbit. It said nothing about the Sun, though. And over the course of 2,000 or so years, the Sun will have moved about 0.3 parsecs or so, which means what you’d actually have is a good view of a starfield.))


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