How can I tell I've travelled 20 million years into the future?

Story starts out about 100 years from now.

While in orbit around Earth preparing for an interstellar mission, our spacecraft encounters some hand-wavey spacetime anomaly such that we're still in orbit around Earth, but approximately 20 million years into the future.

How would we be able to tell how far into the future we have traveled?

For the purposes of the story, H. sapiens has long since gone extinct on Earth (within a million years of the present day), and it's not obvious if they've left any descendants, or if they traveled to the stars. No other obviously intelligent or technologically advanced species has risen since then. The state of things is roughly the same as it was 20 million years before the present, way before the first humans showed up.

Some things that I've thought about:

• Stellar motion (constellations won't look anything like they did before, and we should have a fairly accurate catalog for navigation purposes);
• Continental drift (after 20 million years some motion should be evident, although since this meant to be an interstellar mission, we probably wouldn't have detailed maps of the Earth's surface to compare against);
• Earth-Moon distance (Moon is receding at a rate of a few cm a year, should be measurable after 20 million years);

Since this ship was designed for interstellar travel, assume we have plenty of $\Delta{V}$ to tool around the solar system and landers to explore the surface.

What man-made structures (if any) would survive for 20 million years on Earth? If we find what used to be Yucca Mountain (or a similar site), could we somehow find and use the radioactive material that was stored there to estimate how long since it was buried?

What shape would the Apollo landing sites be in (or any other man-made structures on the Moon) after 20 million years?

What other things should I look at that would provide a somewhat reliable estimate?

Edit

To add some clarity and address some questions:

1. For the purposes of the story, the estimate only has to be accurate to within a couple of million years - the exact number of years doesn't really matter. And they have to be able to determine they've moved into the future, rather than the past.

2. The effect of the hand-wavey anomaly puts the ship in orbit around the Earth where the Earth will be 20-ish million years from now (magically preserving angular momentum), meaning they've travelled through space as well as time; however, from their perspective, they never really moved (although for some cheap initial drama, I may have it deposit them in a higher or lower orbit, or in a different inclination). And, for the purposes of the story, how they got there really doesn't matter.

3. This was supposed to be an interstellar exploration mission using a hypothetical reaction drive with an insanely and likely impossibly high $I_{SP}$ allowing the ship to reach substantial fractions of $c$, such that the trip to the original target would have taken on the order of a decade within the ship's reference frame (centuries within the Earth's reference frame - the crew know what they're signing up for). Thus, they have plenty of $\Delta{V}$ to tool around the solar system (if they choose to do so), although high $I_{SP}$ usually correlates to low thrust, so it may take a while to get anywhere.

4. While there are planetary scientists on board, the purpose of the trip is to analyze bodies in another star system, not Earth. They will have some Earth reference data, but not highly detailed maps of the surface.

posted about 7 years ago

John Bode‭

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