Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all
Known as the ‘North Star,’ Polaris won’t stay that way forever.
Planet Earth spins a full 360°, about its axis, every 24 hours.
The Earth in orbit around the Sun, with its rotational axis shown. Earth’s axial tilt does not change appreciably in direction or magnitude over the course of a day or even a year, which is why the pole star (or proximity of stars to the pole) doesn’t change even as the hours and seasons change. (WIKIMEDIA COMMONS USER TAUʻOLUNGA)
From either hemisphere, the night sky always rotates about our celestial poles.
This 2016 time-lapse photo of the southern skies shows the stars rotating around the south celestial pole, but with no bright stars that could be considered a “pole star” with the current configuration of Earth’s rotational axis. Thousands of years in the future, however, this will be a different story. (A. DURO/ESO)
The southern hemisphere has no bright “pole star,” but the northern hemisphere has Polaris.
A timelapse series of photos stitched together shows the stars of the northern hemisphere rotating over multiple hours around the celestial north pole. Polaris, the bright star closest to the pole, only rotates slightly, while the stars of the Big Dipper (at right) rotate significantly, as they are farther from the pole. (Alan Dyer /VW PICS/Universal Images Group via Getty Images)
Terminating the Little Dipper’s handle, Polaris lies within 1° of Earth’s true north pole.
With a clear view of the night sky from the northern hemisphere, one can see the asterism of the Little Dipper between the “W” of Cassiopeia and the towering Big Dipper, with the tip of the Little Dipper’s handle corresponding to the star Polaris, which is presently located within 1 degree of the true celestial north pole. (E. SIEGEL / STELLARIUM)
But this won’t always be true.
At present, the Earth spins on its axis, which is tilted at 23.44 degrees away from the plane of Earth’s orbit. However, gravitational effects largely arising from the Sun, Moon, and the other planets will cause long-term changes in the direction of the tilt (due to precession) and the magnitude of the tilt (obliquity) over multi-thousand year timescales. (NATIONAL ASTRONOMICAL OBSERVATORY ROZHEN)
Gravitational tugs lead to precession, causing our tilt’s direction to migrate.
Earth’s rotational axis will precess over time due to two combined effects: axial precession (shown here) and apsidal precession, as its elliptical orbit also precesses. As a result, Earth’s axis will sweep out a large circle in the direction it points over ~26,000 year timescales, changing the direction of our celestial poles. (NASA/JPL-CALTECH)
Additionally, our axis “wobbles” over time, varying in inclination.
Over time periods of ~41,000 years, Earth’s axial tilt will vary from 22.1 degrees to 24.5 degrees and back. Right now, our tilt of 23.5 degrees is slowly decreasing from its maximum, which was reached just under 11,000 years ago, to its minimum, which it will achieve a little less than 10,000 years from now. (NASA / JPL)
Over ~26,000 year timescales, our axis completes a 360° rotation.
Today, in the year 2020, Polaris lies extremely close to the exact north celestial Pole. The red circle traces out the direction that Earth’s axis will point along over time, indicating which star will best serve as a pole star in both the far future and the distant past. Vega, the brightest star in this vicinity, will serve as our pole star in approximately 12,000 years. (WIKIMEDIA COMMONS USER TAUʻOLUNGA)
Bright Polaris will pass within 0.45° of “true north” after 80 more years.
The familiar asterism known as the Summer Triangle has three bright members: Deneb, Altair, and Vega, the last of which is the brightest of the three and the second-brightest star in the entire northern celestial hemisphere. In approximately 12,000 years, Vega will be within 5 degrees of the north celestial pole, making it the brightest pole star of all. (A. FUJII)
~12,000 years later, Vega becomes Earth’s brightest pole star: within 5° of true north.
Located along the imaginary line connecting the Little Dipper to the Big Dipper, Thuban, a naked eye star belonging to the constellation of Draco, was the best pole star in terms of proximity that the northern hemisphere has to offer. It was the best pole star from 3942–1793 BCE, achieving its closest approach to the pole in 2830 BCE: where it was less than 1/6th of a degree from the true celestial pole. (TRISHA SHETTY / HTTPS://ALCHETRON.COM/)
5000 years ago, Thuban was the closest pole star of all: within 0.2° of celestial “north.”
Without a bright pole star in the southern hemisphere, one needs alternative methods to find the south celestial pole. Either connecting an imaginary line from the “pointer” stars (alpha and beta centauri, yellow) with one from the southern cross (orange) will do it, or to drawn an equilateral triangle with the two Magellanic Clouds. In the future, however, the pole will change, eventually acquiring a series of good pole stars. (FRASER GUNN OF NEW ZEALAND; ANNOTATIONS BY E. SIEGEL)
Meanwhile, the southern pole has been “starless” for millennia.
The southern celestial pole, at present (in the year 2020), has no bright stars near it at all, and hasn’t had any for thousands of years. Beginning in another few thousand years, however, a series of excellent pole stars will appear, as the pole passes through the asterism of the “false cross” during the years 8000–9000. (WIKIMEDIA COMMONS USER TAUʻOLUNGA)
Beginning in the year ~5000, a series of excellent “south pole” stars will appear.
Beginning in another few thousand years, Earth’s axis will precess sufficiently so that the southern celestial pole passes through many of the stars in the constellation of Carina, which is quite dense with visible stars. Many of the stars will, at one point or another, serve as excellent pole stars, in contrast to today’s southern skies. (IAU AND SKY & TELESCOPE MAGAZINE (ROGER SINNOTT & RICK FIENBERG))
The southern sky is an unfamiliar sight for most northern hemisphere observers, and includes the asterism known as the Southern Cross. It also includes two other crosses: the Diamond Cross and the False Cross. While the Southern Cross presently points to the south celestial pole along its long axis, the true celestial pole will pass through the False Cross, visible at the top of the image, in a few thousand years. Each of the stars on the short axis on the False Cross will serve as pole stars about 1000 years apart. (VW Pics/Universal Images Group via Getty Images)
In the year ~7000, both poles will simultaneously possess pole stars.
In the year ~7000, both the northern and southern celestial hemispheres will have good pole stars: a rarity for both hemispheres at once. Although neither star will be as bright as Polaris is today, this simultaneous alignment will be fortuitous and only occurs with extreme rarity. (WIKIMEDIA COMMONS USER TAUʻOLUNGA; ANNOTATIONS E. SIEGEL)
Enjoy Polaris while we have it; after 2102, today’s “North Star” will consistently worsen.
This long-exposure photograph shows star trails from the northern hemisphere above a peanut field, with reliable Polaris illuminating the celestial north pole. While Polaris will reach its peak as a pole star in the next 80–82 years, it will steadily decline thereafter, giving way to other stars as the centuries and millennia continue to pass. (Edwin Remsberg / VWPics/Universal Images Group via Getty Images)
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.
