How do time-travelers calculate the date?

[Daigoro]

Originally Posted by Fred Brackin

The exact distance to the Moon will do the same.

For that matter, the exact average distance over the course of at least a month, but probably longer.

[lwcamp]

Geologists and paleontologists use index fossils to get approximate dates of the strata they are studying if there are not any convenient igneous rocks bracketing the stratum. These are usually non-descript marine shells, but if your time traveler is walking around Laramidia and sees a Tyrannosaurus rex stalking her, she can be sure she is between 68 and 66 million years before present. (She can also combine this with astronomical observations - if she sees both a T. rex and a really really bright light crossing the sky heading in the general direction of the Yucatan, she'll know that it is 66.043 ± 0.011 million years before present and she has to get out of there right now.)

You can also use radiodating, if you have igneous rock of a known origin. So if you are going to take a vacation to the Columbia plateau long before there are any people around, you can use the fact that the layers of flood basalt came in some 15 to 17 million years before present. So sample a piece of basalt from a known layer, use your degree in geology to determine exactly what layer it is and look up when it erupted based on dating methods from modern studies, then get a potassium-argon date using your university lab that you brought with you to tell you when you are to within a hundred thousand years or so. If the basalt is still molten and glowing, you can skip the potassium-argon dating step.

Neither of these methods will get you the exact date, but they can help narrow down your window a bit.

Luke

[whswhs]

Originally Posted by lwcamp

Geologists and paleontologists use index fossils to get approximate dates of the strata they are studying if there are not any convenient igneous rocks bracketing the stratum. These are usually non-descript marine shells, but if your time traveler is walking around Laramidia and sees a Tyrannosaurus rex stalking her, she can be sure she is between 68 and 66 million years before present.

Did T. rex really only last two million years? I had thought of "Cretaceous" and figured that meant, well, the Cretaceous. . . . (I remember noticing a while back how many of the big animals in Jurassic Park were actually from the Cretaceous.)

[ericthered]

Originally Posted by whswhs

Did T. rex really only last two million years? I had thought of "Cretaceous" and figured that meant, well, the Cretaceous. . . . (I remember noticing a while back how many of the big animals in Jurassic Park were actually from the Cretaceous.)

Most prehistoric animals are better known at the Genus level than at the species level, and families tend to be better known still. Allosaurus is a Genus, not a species. Triceratops is a genus, not a species. These creatures could vary as much as Lions and Jaguars. Genus can last a lot longer than Species can, and Family can last a good deal longer still. The fossil record is actually kind of sparse. tyrannosaurus (the genus) has only about 50 or so known fossils, and that's considered to be a lot.

I suspect that if you want back to 71 million years ago, you'd find very few of the species you were expecting, but many of the genera, and our knowledge about families would likely be correct. But consider the differences between various badgers and weasels, or between artic, red, and fennec foxes. You're dealing with that sort of variance beyond what we know about the basic creature type.

[khorboth]

I would imagine that if this is a regular problem then there would be a regular procedure. Maybe even a simple-to-follow flow chart.

Step 1: Check a few easy-to-find astronomical spots to see if they're stars or novas. This will establish an Epoch based on Chart A

Step 2: Depending on your environment, look for local flora & fauna to narrow it down further based on Chart B. (This may have general things like the existence of flowers and land animals as well as specific types of creatures. Some field work will have gone into its creation.)

Step 4: Locate a planet or two and look them up on Chart C.

I hope you remember the middle school science field trip where we did this as a class!

[Ashtagon]

Another approach, good for determining which geological era you are in, is to check atmospheric composition, and compare to known deep time information on atmospheric composition.

[lwcamp]

Originally Posted by whswhs

Did T. rex really only last two million years? I had thought of "Cretaceous" and figured that meant, well, the Cretaceous. . . . (I remember noticing a while back how many of the big animals in Jurassic Park were actually from the Cretaceous.)

Pretty much. I took those dates from Wikipedia, but all my other sources put T. rex's time frame as the very late Maastrichian age (which is the last subdivision of the Cretacious period). The Maastrichian lasted from 66 to 71 mya. It is always possible that someone could find an earlier fossil of T. rex, but so far there's no evidence for that.

There were other tyrannosaurs that lived earlier. Tarbosaurus baatar might actually be in the genus Tyrannosaurus, so if you are noodling around north-east Asia about 70 mya you might still see a Tyrannosaurus, even if not the famous T. rex. But there's also two species of Daspletosaurus (Laramidia, 74 - 77 mya), Albertosaurus sarcophagus (Laramidia, ~ 70 mya), Gorgosaurus libratus (Laramidia, 75-76 mya). Teratophoneus curriei (south Laramida, 77-76 mya), Lythronax argestes (south Laramida, ~80 mya), Lythronax argestes (east Asia, ~73 mya), Bistahieversor sealeyi (south Laramida, ~75 mya) - all of these were very similar to T. rex, albeit a bit smaller. So our hypothetical time traveler should be able to distinguish between all these different but similar species. There were earlier tyrannosaurs, starting from about 160 mya (mid-Jurassic) in Asia (and an even earlier specimen from Europe at about 166 mya), but these tended to be significantly smaller, had longer arms, and had more slender and lightly built heads than their later relatives.

Luke

[lwcamp]

Originally Posted by Ashtagon

Another approach, good for determining which geological era you are in, is to check atmospheric composition, and compare to known deep time information on atmospheric composition.

"Hypoxia ... setting in. Must be ... before ... Cambrian period. <Gasp. Wheeze>"

Alternately,

"Let's check the oxygen levels." <lights match, bursts into flame> "Hmm, must be in the Carboniferous. AIEEE! IT BURNS! PUT IT OUT! PUT IT OUT!"

Luke

[Boomerang]

Time travellers from the future may know how the speed of light (or some other fundamental constant) varies over time, which could allow an approximation of the date independent of location.

[Daigoro]

Originally Posted by Boomerang

Time travellers from the future may know how the speed of light (or some other fundamental constant) varies over time, which could allow an approximation of the date independent of location.

I guess they could take a reading of the Cosmic Background Radiation (CBR).