Salvador Dali Would Love These Constellations

One of the reasons Arcturus moves faster than some stars across the sky is because its orbit around the center of the Milky Way galaxy is perpendicular to ours. Credit: ESO

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.

This is the current sky as seen in late May when Arcturus is high in the southern sky during the evening hours. Notice the kite-shape of the constellation Bootes above Arcturus and compare to how it will look in the future in the next image. Created with Stellarium

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.

In 27,800 AD, all the constellations are at least a little distorted from the motions of their individual stars. Bootes reminds me of timepieces in  Salvador Dali’s painting The Persistence of Memory. Created with Stellarium

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.

The slow cyclic wobble of Earth’s axis called precession resembles the motion of a top slowing down only the Earth’s not slowing down – it repeats the circle every 25,800 years. The wobble causes the pole star to change and the Sun to moves westward along the zodiac. Credit: Earth and Planetary Magnetism Group ETH-Zurich

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.

Once again, here is the current sky. Compare to the next image to see dramatic change. Created with Stellarium

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.

In the far future, Orion will be humanity’s connection with the distant past. Other constellations, with perhaps the exception of Canis Major, are recognizable but distorted to different degrees. Created with Stellarium

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.

It’s gonna be hard to see Cassiopeia get bent out of shape! Created with Stellarium

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.

** If you’d like to get to know the constellations right now before they’re all weirded out, pick up a copy of my recently published book Night Sky with the Naked Eye at Amazon or Barnes & Noble.

6 Responses

  1. RC

    Bob, I have a couple questions about precession. 13,000 years from now (about half way through the cycle), will the winter solstice be in June? And the summer solstice in December?

    If this is the case, the seasons “move” about a month every 2,000 years. Are the seasons coming earlier than they used to? Or later? 2000 year’s from now, will the winter solstice be in November, or January?

    1. astrobob


      Good question. No, the seasons won’t change months, only the locations of the winter and summer solstices caused by precession. In 13,000 years, the location of the current summer solstice in Taurus will shift to the current location of the winter solstice in Sagittarius. Since the seasons are determined by the tilt of the Earth’s axis, which remains stable for a long, long time, precession does not disturb their timing.

      Right now, the summer solstice – the sun’s high point in its yearly cycle – occurs in Taurus. In the year 7000 AD, the sun will still reach its highest point in the sky on June 21 but will be in the constellation Pisces instead. In 10,000 AD it will be in Aquarius on the solstice and in the year 15,000 AD in Sagittarius. On that distant date, Sagittarius will stand high in the daytime sky on the first day of summer. On the first night of summer, Gemini will ride low across the southern sky for northern hemisphere observers. On the winter solstice in 15,000 AD, Sagittarius will twinkle above the snowdrifts high in the southern sky.

      Does this help to answer your question?

      1. Kelly

        Bob is correct – or to put it another way, our calendar is synched to the “tropical” year (the time it takes for the Earth to return to the same point with regards to the solstices and equinoxes) and not the “sidereal” year (the time it takes for the Earth to return to the same point with regard to the distant stars). The tropical year is slightly less than the sidereal one due to precession (if our calendar were in tune to the sidereal year, instead of skipping leap day in some century years, we’d actually have to add in extra leap days at some point due to the difference in length).

        This means that half a precessional cycle from now, the seasons will begin at (roughly – slightly earlier due to the error in the Gregorian calendar if not rectified before then) the same time on the calendar as they do now, but the constellations will appear at opposite times of the year from now – plus some (like the area around the center of the galaxy) will be higher for Northern Hemisphere observers while others (like Orion and the other stars in the vicinity on that photo Bob posted) will be lower (or not rise at all if far enough north).

          1. Kelly

            I learned about that when someone asked if ~13,000 years from now precession would mean that Christmas would be in the summer or not (for those of us north of the equator). (The answer is no for the reason I gave.)

Comments are closed.