How would an astronaut conclude he's on Earth, but 600 million years in the future?
Say we have a group of space travelers who have been frozen inside a sleeper ship for an enormous amount of time that left when Earth was recognizable. When they thaw out and reach their destination, they land on a planet which has only one super-continent and possesses a desert ecosystem which evolution has rendered almost completely alien, compared to present day flora and fauna.
Since the solar system has orbited the galactic core multiple times, the stars are in completely different positions and the amount of time that has passed has noticeably increased the physical size of the Sun. Also, during the time that passed, an extra-solar gas giant passed close by Earth in which the Moon was knocked from orbit to become a planet in an independent orbit, as well as sending Mercury crashing into its parent star.
Civilization it turns out, collapsed shortly after the astronauts left for the stars due to severe and abrupt climate change and any artificial traces of humanity has naturally eroded away into nothingness while the surviving species reverted back to something reminiscent of a house-cat sized purgatorius.
Disregarding exactly how the ship and crew managed to survive time itself, how would our unfortunate star sailors realize that the planet they are on is not alien at all, but instead they have traveled in one unfathomably huge circle?
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1 answer
The night sky will still tell you, your planet is orbiting our Sun.
Just ignore the stars of the Milky Way. The Milky Way is just one galaxy of many. It is moving, but in 600MY it will have moved about 11 times its width.
But it is only 100,000LY across; other galaxies are a million (M100 is), even 1.5 million LY across (Hercules A). They are moving, but Hercules A (or M100) will be so close to their original positions, from Earth's POV, that they can't be anything but those galaxies; no matter how their stars may have moved about in that time. Nothing else that big could have appeared that quickly.
Given just a few dozen large distant "lighthouse" galaxies in the universe, we can compute our position by their relative positions and angles with each other; the lines between them will form a kind of 3D mesh that is unique to each position in space. Their rate of movements is already known, we can compute how this map will look at any future point in time: In one million year steps would suffice. Using that, they can discover they are 600M years in their future, and using that, determine their location is our Sun, in our orbit within the Milky Way, and that the planet they are (due to its size, mass [measured by strength of surface gravity] and orbit) has to be Earth.
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