The Twists of Time (or, the consequences of stellar velocities for stargazing)

As a kid, I read a lot of juvenile nonfiction on astronomy and celestial objects, which definitely contributed to my (totally original) career goal of being an astronaut. Among others, I know I read a lot of Seymour Simon’s planet books (whichever ones were around 20+ years ago), which were pretty cool. But the book that sticks in my memory as the first was the one that made the greatest impact, and it is this one:

Image courtesy of GoodReads

I don’t know what changes have been made since the 1989 edition, but the book I remember covered some very useful basics about the Big Dipper, including how to find Polaris (the North Star), the different stories and names for the Dipper in various cultures, and its history as a navigational aid for both sailors and the Underground Railroad.

All of this is great, but the part that absolutely blew my little-kid mind was when the book dived into the far past and the distant future. What can I say, I’m a sucker for sprawling, big-picture knowledge about the universe. This particular knowledge was so vastly cool because it linked the things I intellectually knew about the constant, unimaginably swift movements of “outer space” to what I thought of as the unchanging patterns I could see with my own eyes in the sky.

Long story short, the constellations are not unchanging. Your great-great-etc.-etc.-grandchildren, if they’re still on Earth, will see different shapes and even a different North Star. Why?

The Terrestrial Top

Let’s start with the North Star. The culprit here is precession, which means that the Earth’s axis of rotation is slowly changing orientation like a wobbling top.

The north celestial pole (the imaginary point where the north end of Earth’s rotational axis meets what we can think of as the dome of the night sky) currently points very close to the star Polaris, which is considered the North Star. However, Polaris didn’t always align so well with the North Pole—in fact, up until about 500 AD, it wasn’t even the closest star to the pole!

For much of recorded history, the North Star has been far less accurate than the one we currently enjoy. This is a bit of bad luck for our ancestors, since a more exact North Star would have come in handy a lot more for navigation in 500 BC or so. Instead, they had Kochab, another star in Polaris’s constellation Ursa Minor, or (going back a few more thousand years) Thuban, a rather dim star in Draco that is often not even visible in today’s skies.

The change in the stars we see close to the North Pole is actually a cycle that lasts about 26,000 years. During this time, several stars take turns vying for the honor of being closest to the pole, with Vega (in Lyra) and Deneb (in Cygnus) being strong contenders for bright and therefore useful pole stars. However, neither of them ever comes nearly as close to the actual pole as Polaris. Here’s what the whole counterclockwise 26,000-year circle looks like, with some of the potentially most useful stars highlighted:

Basically, we have the best one right now.

The Deforming Dipper

Let’s go back to the Big Dipper. As I said earlier, the constellations as seen from Earth will change over time, albeit more slowly than the North Star’s precession, since each star has its own velocity and many of them are traveling in totally different directions. The Dipper is just one example, and not even as dramatic as it could be. Five of the seven major stars actually do appear to travel together from our perspective, as part of the larger Ursa Major Moving Group. The two on the ends of the handle and bowl, however, move in a different direction, so that over time the Dipper will take on a significantly different appearance. Take a look at how it’ll be in 50 thousand years:

This is happening with all the stars visible in our sky, of course, not just the Big Dipper. Some of them are in groups that will maintain a similar structure from our perspective, and some will move in different directions. Whatever the particulars are, any constellations you know will be changed visibly, if not beyond recognition.

As I learned about the changes in the North Star and the Big Dipper, I remember feeling not quite fear or fascination, but a pleasantly shivery sensation of lonely coldness. Imagine stepping out of a time machine and seeing stars that are close to what you know, but just different enough to be unmistakably alien. (Actually, I did have a very similar feeling reading H.G. Wells’ The Time Machine, when the protagonist abandons trying to talk to people in the future and simply goes further and further forward in time, traveling alone as he witnesses the end of life on Earth and presumably the death of the Sun.)

The idea that my distant ancestors saw, and my distant descendants would see, these same stars change on a timescale so far beyond my comprehension, was both beautiful and creepy to me twenty years ago. It still is, and if human nature is any indication, I suspect the stargazers of 52017 will feel the same way.

 

[There are some really cool resources out there for looking at the positions of the stars in our sky over time! My favorite is Stellarium, which is open-source and has all kinds of nifty options including atmospheric effects, equatorial and azimuthal grids, and constellation art from different cultures.

Also, for further reading, Wikipedia has a pretty extensive article on axial precession and the change in the pole stars. It talks about the southern hemisphere as well, which I totally neglected here.

Cool visuals of constellations over time, going further forward than my Big Dipper example. And another one!]