Precession

In this lecture, we're going to talk about another important concept in astronomy called precession, and this is going to connect us to the idea that our friend Polaris, what we now call the North Star, is not always going to be the North Star.

What exactly is precession? Well, it turns out that anything that spins, so rotates around and has a tilt to it, so if you spin around and you have a tilt to it, then you're going to go through something called precession.

Precession is sort of a wobbling during your orbit. Think of it like a top. When you spin a top around, the top doesn't stay straight up and down and spin around. And if it's got a tilt like this, it doesn't just exactly like this and spin around on its axis. Instead, as it's spinning around on its axis, the top itself is slowly making a big circle like this. That circle, that wobble, is what we call precession. Anything that has a tilt to it and spins around will go through precession.

Well, think for a second about the Earth. We learned about the seasons. We learned that the Earth has a tilt to its rotation axis, about twenty-three and a half degrees. We also know the Earth rotates once every 24 hours. So the Earth rotates, has a tilt, the Earth also goes through this wobble or precession.

What does that mean? Well, in our picture here, right now, directly over the Earth's North Pole is the star Polaris. But in a few thousand years, the Earth's rotation axis will be such that the North Pole is not right here over Polaris anymore, but over here somewhere here in space. A few thousand years after that, somewhere over here in space. A few thousand years after that, somewhere over here in space. Since the stars up here are not moving, they're just fixed out there in space, as the Earth's rotation axis moves, it means that Polaris will not always be directly over the North Pole. Polaris will not always be our North Star.

The Earth completes a wobble like this around in the sky once every 26,000 years. So we can look and say, for instance, that right now Polaris is basically the North Star. But six thousand years from now, in 8000 AD, the North Star, if there was one, would be somewhere over here. Well, it turns out there will be no North Star in six thousand years from now, 8000 AD. When you keep going, there will be times when there are sometimes pretty good North Stars. Like about 3000 BC, there was a star called Thuban, and that was, you know, directly over the Earth's North Pole. It was a better North Star even than Polaris.

Interesting thing about Thuban, 3000 BC, this is about the time of the ancient Egyptians building pyramids and things like that. We look at the records of the Egyptians, and for them, this star Thuban was super, super important. Why was it so important? Well, for them, five thousand years ago, 3000 BC, it was their North Star.

Twenty-six thousand years from now, after the Earth's rotation axis has made one complete loop around here, then Polaris will be the North Star again. But every 26,000 years or so, Polaris will be the North Star, but sometimes there will not be a North Star at all.

What about a South Star? Right now, there is no South Star. There is no star directly over the South Pole. We can see during it, during the 26,000 years, there will be some time, particularly in the future here, six thousand years in the future, 9000 years, 7000 years in the future, sorry, where there will be stars pretty much directly over the South Celestial Pole. So there will be times when there is a South Star, but not even a North Star.

One last thing about precession. This 26,000-year cycle also changes the days that the Sun seems to be in each of the 13 constellations that it goes through during the course of the year. This is the reason then why the astrological dates for the zodiac don't match up anymore with the days the Sun is actually in the constellations. We said they were off by about a month. Why are they off by about a month? Well, the, the date, the astrological dates for these zodiac signs were set about 2,000 years ago. In 2,000 years, there's been quite a bit of precession, enough that now the dates are off by about a month. If we wait another two thousand years or so, the astrological dates will be about two months off from when the Sun is actually in the constellations. If we wait another ten thousand years from now, they'll be six months between the astrological dates and the dates the Sun is actually in the constellations. If there was no precession, then the astrological dates would match up quite well with the days the Sun is actually in each constellation.