When Am I? Navigation for Time Travelers

1400 was the target we set.
My story in place, I flew through the net.
Dragging my germs with me, back to the plague I flee — 
I’m not supposed to be here!
Fevered, delirious, Lord are you getting this?
I never asked what’s the year? What’s the year?!
(“In a Gown Too Blue”, Three Weird Sisters)

Time travel isn’t perfect. Things can go wrong. And we might only be able to approximate our target to begin with. So, how can you get your bearings where — and when — you’ve jumped?

You might say “ask somebody”, but — even assuming that you’ve jumped to a populated area, and even assuming you share a language — have you thought about how crazy that sounds? How would you react if somebody on the street asked you what year it is?

This question was asked by abyss.7 recently on Worldbuilding Stack Exchange. In this world, the traveler knows the location (and the time machine helpfully accounts for things like planetary drift) but knows that the date will be only approximate. So, abyss asks, how can you find out when you are, for any point in the past, and assuming that you might not be able to just pick up a newspaper or ask somebody? The traveler in this case can carry a backpack; what should he put in it so he can quickly locate himself in time?

This answer by AlexP points out that for recent times you can listen for radio signals, and for more-distant times you need to rely on the sky (a theme that runs through several answers). You can use the movements of planets to place yourself within the last 10,000 years or so; if you’re going farther back, pay attention to stars more than planets. Both planets and stars have cycles of regular motion over decades, centuries, or millennia. An answer by Loren Pechtel makes similar suggestions.

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According to AlexP, the backpack should contain a telescope and a laptop with the right software. David A. Gray suggests, instead, a sextant and a set of star charts, which (along with some math) are good for the last few million years. And they won’t run out of battery, either. Either way, the traveler might need to make observations over a few consecutive nights, so if you have somewhen that you really need to be, try to arrive early so you’ll have time to get there.

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If your destination lies millions of years in the past, Mattias points out that observing the moon can tell you something — it used to be closer. Measuring its diameter — probably over a few nights, because it’s not a perfectly circular orbit — can serve as a yardstick. I don’t know if this is enough to get your arrival down to even a decade, let alone a year, but at the scale of millions of years ago, you’re probably not going to be late for your dinner appointment.

Mattias also suggests looking at atmospheric composition, which has changed over the ages, and Guest suggests measuring the “color” or “heat” of the sun and plotting it on the star’s life cycle. Guest also postulates a “Time-GPS” that works by sending a signal from when you are back to when you came from and measuring its transmission time (presumably against some known measure).

Werrf makes two very different suggestions. First, the answer points out, any successful tool is followed by infrastructure to support that tool — roads, cell-phone towers, ports, and so on. The traveler can, therefore, learn a lot from the infrastructure currently in use. Or… you could hide a clock and transmitter on the moon. Encrypt the transmission for extra safety from those not in the time-travel know. If somebody notices, well, is it that different from transient lunar phenomena? Nobody believes Uncle Ned with his fuzzy pictures of UFOs either; it’s probably safe.

Whatever approach your traveler takes, I suggest leaving a little room in that backpack for some penicillin. You never know; you might need it.