Enjoying EarthSky? Subscribe.

130,081 subscribers and counting ...

Moon and Aldebaran, former pole star, on January 27

As always, the moon moves eastward through the constellations of the Zodiac. The green line depicts the ecliptic - the sun's annual path in front of the "fixed" backdrop stars.

Tonight is Jan 27, 2015

Moon Phase Courtesy U.S. Naval Observatory

Tonight’s waxing gibbous moon – January 27, 2015 – is moving toward the star Aldebaran, brightest star in the constellation Taurus the Bull, and the moon will be even closer to this star on January 28. Did you know that Aldebaran is a former pole star? It’s true, and it’s a fascinating story.

Many people know that Polaris is the present-day North Star, but few know that Aldebaran reigned as the North Star some 450,000 years ago.

What’s more, Aldebaran appeared several times brighter in the sky then than it does now. Plus – 450,000 years ago – Aldebaran shone very close to the very bright star Capella on the sky’s dome. In that distant past, these two brilliant stars served as a double pole star in the astronomical year -447,890 (447,891 BCE).

EarthSky astronomy kits are perfect for beginners. Order today from the EarthSky store

This illustration shows the view from Seen from present-day Arizona in 447,000 B.C., when Aldebaran and Capella served as double pole stars.  Illustration via Carina Software and Instruments

This illustration shows the view from Seen from present-day Arizona in 447,000 B.C., when Aldebaran and Capella served as double pole stars. Illustration via Carina Software and Instruments

At this point, we should probably insert a note about astronomical dating. In ancient times, there was no zero year, so the year AD 1 followed the year 1 BC. However, present-day astronomical calculating is made simpler by equating the astronomical year 0 with the year 1 BC. Thus, the astronomical year -1 corresponds to 2 BC and the astronomical year -2 corresponds to 3 BC. And so on . . .

But back to Aldebaran and Capella as dual pole stars. The identity of the pole star shifts over time, due to the 26,000-year cycle of precession. To read more about that, click into this article about Thuban, another former pole star.

Still, how can it be, you might wonder, that the stars Aldebaran and Capella were once so near each other on the sky’s dome? They’re not especially close together now. Aren’t the stars essentially fixed relative to one another? The answer is that, yes, on the scale of human lifespans, the stars are essentially fixed. But the stars are actually moving through space, in orbit around the center of the galaxy. In our solar system, galaxy and universe … everything is always moving. So the sky looked different hundreds of thousands of years ago than it does today.

Here’s another kind of motion you might pay attention to on the night of January 27, 2015. It’s the spin of the Earth you’re standing on, which causes the moon and Aldebaran to shift westward throughout the night. And yet another motion … the moon is also going eastward relative to the fixed stars (which we now know aren’t really fixed), because of the moon’s orbit around Earth.

In other words, tonight and for the next several days, if you look carefully over a period of hours (or from one night to the next), you can watch the moon traveling eastward in front of the constellation Taurus.

Aldebaran is a noted star of the Zodiac – the band of stars in front of which the sun, moon and planets make their rounds.

Live by the moon with your 2015 EarthSky lunar calendar!

As always, the moon moves eastward through the constellations of the Zodiac. The green line depicts the ecliptic - the sun's annual path in front of the

As always, the moon moves eastward through the constellations of the Zodiac. The green line depicts the ecliptic – the sun’s annual path in front of the “fixed” backdrop stars.

But Aldebaran wasn’t always a star of the Zodiac. Some 450,000 years ago, Aldebaran and Capella teamed up together to serve as Earth’s double north pole star!*

*Source: Page 363 of Mathematical Astronomy Morsels V by Jean Meeus

Bottom line: Many people know that Polaris is the present-day pole star, but few know that Aldebaran – the star near the moon on the night of January 27, 2015 – reigned as the pole star some 450,000 years ago. What’s more, at that time, Aldebaran had the company of a fellow bright pole star, Capella.