There’s a scene in episode three of Marvel’s Moon Knight with a unique kind of time travel. Oscar Isaac’s character Marc Spector stands underneath a clear night sky in Cairo, Egypt, where the band of the Milky Way twinkles in purple and blue. With help from the god Khonshu, they rewind the night sky to see how it looked 2,000 years in the past. Once the magic begins, stars zip past in a clockwise direction until eventually stopping at the ancient sky they’ve been searching for.
In the Disney+ show, it’s the thing of magic — but the European Space Agency has an ambitious mission that can help scientists fast-forward and rewind the sky, at least on computers.
This scene got a lot of things right, minus the Egyptian deities. The night sky does change year after year, as one particular space observatory can prove. The European Space Agency’s (ESA) Gaia mission is using stellar motions and highly-precise optics to create a three-dimensional map of the Milky Way to show how stars travel over time.
How does Gaia work?
In 2013 ESA sent the Gaia spacecraft to Lagrange Point 2 (L2), a gravitationally stable spot in space located behind the Earth with respect to the sun, almost 1 million miles away from our planet. Gaia has been sharing this location with NASA’s James Webb Space Telescope since January 24.
Gaia is far enough away to get a clear view of the Milky Way without the Earth interfering, but it is close enough that it communicates back to scientists while accompanying Earth on its yearly trip around the sun.
As part of the mission, Gaia scientists want to learn how the sky looked in the past, and how it will look in the future. Collisions from ancient galaxies and the Milky Way’s eventual merger with the Andromeda Galaxy are events that happen on billion-year timescales. But Gaia’s special finesse can see incremental changes over much shorter periods of time, leading to a more detailed understanding of how stars move. With a few years data gathering position and movement, that can help reconstruct the past and future movements of our celestial neighbors.
The first piece to solve this puzzle is accounting for funny tricks of perception.
Gaia uses parallax, that interesting phenomenon where you close one eye, stare at an object, then close it and open the other eye, and the object appears to have moved.
One way to envision Gaia’s technique is by pretending that the Sun is your nose and Earth’s orbit stretches between your eyes. If you only have your right eye open, you’ll see the objects in your room — the Milky Way surroundings — as they appear in summer. Close that one and open the left eye, and this will be what they look like half a year later from Earth’s new spot in space. Gaia studies the sky in periods of 180 days, roughly six Earth months. So Gaia’s watching the sky when it’s on one side of the Sun, then again when it’s on the other side.
Each period, stars in the Milky Way will appear in an ever-so-slightly different place with respect to incredibly-distant bright beacons like quasars that don’t move quite so much. “Quasars are very distant objects located outside our Milky Way and they have a proper motion that is zero, because Gaia is not able to measure it. So, if we see proper motions, it means that this is due to the acceleration of the Sun,” Antonella Vallenari, Gaia contributing scientist, tells Inverse.
Stars that are closer to Earth will appear to move more than those farther away, revealing their distances. Moon Knight’s rewinding sky sometimes showed a similar effect, when the big bright stars shifted more than the backdrop stars as the sky spinning slowed down.
This is an apparent shift, meaning it isn’t real. But by knowing how this observational trickery works, scientists can account for this effect and nail down where an object actually is located in space. Not much can be gleaned from a year of observations. But Gaia is a long-haul mission, and ESA plans to track the positions of 1 percent of stars in the Milky Way for Gaia’s 12-year mission, monitoring about one billion galactic stars.
Over time, as scientists account for parallax and another funky force caused by Earth’s velocity through space called aberration, they notice that the stars are ever so slightly moving through space.
What was the night sky like in the time of dinosaurs?
Timo Prusti, Gaia Project Scientist, said that if we rewound the clock a few thousand years, the stars in the Milky Way would appear to move in linear fashion. But if we zoomed out of the Milky Way and tracked the sun’s motion over millions of years, we’d see that the sun and its planets would be trying to escape the galaxy but then get pulled back in, then down in the other direction, bobbing up and down as they travel around the Milky Way’s center to create a curved line.
Everything in our corner of the Milky Way would be traveling alongside us at roughly the same speed, Prusti said. But the sky would appear much different, and that’s again thanks to parallax. Stars that are closer to us have high proper motion. Each year they appear to move much farther along the canvas of the night sky than stars much farther away.
If you went far enough back in time, you would have a whole different night sky. The dinosaurs of the mid-Cretaceous geologic period would have been on the other side of the Milky Way disk because the sun takes roughly 220 million years to travel once around the galactic core.
“I think the sky of the dinosaurs was totally different,” Prusti tells Inverse. “Of course, more or less everything in the neighborhood moves roughly at the same speed. However, it’s only roughly. It means that in this million-year timescale, the sky would look totally different.”
At larger timescales you may also notice some stars moving against the flow of the others. These are the remnants of the crash from Gaia-Enceladus-Sausage, the ancient galaxy that crashed into a proto-Milky Way 10 billion years ago.
Zodiac blues — But, back to Moon Knight. Even just 2,000 years of night sky drift is enough to disrupt where constellations are found. Zodiac signs correlate to certain months because the ecliptic, the Sun’s apparent path through the sky, give rise and fall to constellations, which are all unique from the vantage point of the Solar System. Fast forward thousands of years after this association was first noticed, and the ecliptic no longer runs through the same set of stars. Think you’re a Virgo? That’s a lie, you are something else.
