Yesterday we dipped into the deep past, hungering to know more about the first generation of suns that formed after the Big Bang. Today we look into the future at what the sky will look like thousands of years from now, when the cumulative motion of the stars will render many of the familiar constellations unrecognizable.
We touched on Arcturus as being one of the few stars that has moved enough since the days of ancient Greece that a sharp-eyed observer from that era would look up and notice that it’s not exactly in the same place in 2017 as it was in 100 BC. There are a couple of reasons for this. First, Arcturus is close as stars go, only 37 light years from Earth. Second, it’s really trucking, moving around the center of the galaxy at 76 miles a second (122 km/s). Third, Arcturus doesn’t share in the motion of the general stream of stars in the flattened disk of the Milky Way like the sun does. Instead it cuts perpendicularly to the sun’s orbit, making its motion even more obvious over a fairly period of time.
Even the slowest-moving stars must yield to time. We can follow not only where they’ve been in our sky but also where they’re going by firing up a planetarium-style software program like Stellarium and setting the clock to a time in the distant future. Or the past. But today, let’s look forward in the spirit of daylight saving time. Yes, it’s back. Don’t forget to advance your clock an hour ahead this Saturday night before going to bed.
How far into the future shall we have our time machine take us? I’m going to pick the year 27,800 AD. Earth’s axis experiences a periodic wobble like a top slowly losing speed. Called precession, it takes about 25,800 years to complete one cycle. The north end of that axis points like a finger at Polaris, the North Star. As the Earth very slowly wobbles, the axis describes a circle in the northern sky. Right now, it’s aimed at Polaris but between about 3900 – 1900 B.C. it pointed at Thuban in the constellation Draco the Dragon. In 14,000 AD, Vega will be the polestar.
The tippy-top effect is caused by the twist-pull of the combined gravity of the sun and the moon on the Earth’s equatorial bulge. Our planet’s not a perfect sphere but instead is slightly wider around the equator than around the poles. The moon and sun’s attraction on the bulge is enough to cause the axis to precess or gyrate. The wobble repeats every 25,800 years, so Polaris will return as polestar much where we see it now around 27,800 AD.
The individual motions of the stars really add up over the millennia and stretch and compress the current constellations in strange and bizarre ways. Thanks to the movement of Arcturus, the constellation Bootes really gets stretched! Sirius wanders away from Canis Major, and the Big Dipper’s handle gets squeezed.
Interestingly, Orion’s shape changes little over so much time, which could be because those stars are revolving in similar orbits as the sun around the center of the galaxy, so we keep pace with each other.
Whenever you feel like peering into the future a planetarium program makes a perfect time machine. Download and have some fun! And if you’d like to see great animations of the constellations as they appeared tens of thousands of years in the past and as they will in the future, make sure you stop by Tony Dunn’s Proper Motion of the Constellations. There you can watch it happen right before your eyes.