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Moon near former pole star on January 9

Tonight – January 9, 2017 – the waxing gibbous moon passes in the vicinity of Aldebaran, an ex-pole star, a famous zodiac star, and the brightest star in the constellation Taurus the Bull. In fact, if you live in Asia, you can actually watch the moon occult – cover over – Aldebaran for a portion of the night tonight. Aldebaran will disappear behind the moon’s dark side and reappear on its illuminated side. So the disappearance will probably be easier to spot than the reappearance.

Click here if you’re in Asia and want to watch the occultation

The rest of us will see Aldebaran near the moon on this date. It’s a wonderful time to learn to identify this star, even though you might have to squint a bit to see it in the moon’s glare.

Aldebaran is a bright reddish star, a good star to come to know. Did you know that Aldebaran is also 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).

View larger. | This illustration shows the view 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.

So watch for Aldebaran near the moon tonight, and think back to 450,000 years ago, when 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

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Bottom line: People in Asia will see Aldebaran pass behind the moon on January 9, 2017. The rest of us will see Aldebaran in the moon’s glare. Plus … the story of Aldebaran when it was part of a double pole star.

Bruce McClure

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