Astronomical clock installed at Hampton Court Palace in Greater London, England around the year 1540. Besides the day, month, and lunar phase, it’s supposed to show where the Sun is in the zodiac. What it actually shows is where the Sun was in the zodiac 2500 years ago. Image by Mike Cattell via Wikimedia Commons.
One of the themes of this blog has been that our units of time are based on the way things appear, not the way they really are. A day is based on when the Sun appears highest in the sky, not how long the Earth actually takes to rotate once. The month is based on when the Moon appears to complete a cycle of phases, not how long it actually takes to orbit the Earth. The last couple of posts have been on the year, which I defined as one cycle of the seasons.
I never mentioned anything about the Earth orbiting the Sun once a year. Does it?
Just like there’s a sidereal day and a sidereal month, there’s a sidereal year which is one orbit of the Earth around the Sun. In theory, you could measuring how long that is by seeing which star the Sun is lined up with at some time and waiting for that to happen again. In practice, that’s not possible because we can’t see the stars that are up during the day, let alone those that are directly behind the Sun. What you can do is see which stars rise just before dawn. One sidereal year later, the Sun will return to the same position relative to those stars. Unfortunately, this will actually happen during the day (because a sidereal year is around 365.24 days) and so you won’t get to see it. You might have to track the position of the Sun relative to the stars for several years before you can accurately measure the length of the sidereal year.
The point is that it’s not surprising ancient peoples didn’t use the sidereal year. Defining a year as one cycle of the seasons makes much more sense. That is called a tropical year.
But do we actually need two different definitions of the year. Aren’t they exactly the same length? The time it takes the Earth to orbit the Sun should be the same as one cycle of the seasons, right?
The animation below (which was also in the last post) certainly makes it look that way. When the Earth is on the right side, the Northern Hemisphere is tilted towards the Sun and the Southern Hemisphere is tilted away from it. So it’s the summer solstice in the Northern Hemisphere and the winter solstice in the Southern Hemisphere. After one orbit, we get back to the same alignment.
There’s just one problem. The animation’s wrong. Or you could say what it depicts is a convenient lie.
The direction of Earth’s tilt changes. This is called precession. It’s the least obvious motion of the Earth because it happens so slowly. It takes around 26,000 years to complete one precession cycle.
If the animation had been accurate, the direction of the tilt will slowly rotate away from you. Initially, you wouldn’t have noticed. Within one orbit or even a hundred orbits, the tilt won’t change much. But if you read this entire post and then scroll back up, you should see that the tilt is pointed in a different direction. You should but you won’t. Because it’s wrong. If you find an animation that shows the Earth orbiting the Sun and also precessing, let me know.
A sidereal year is around 20 minutes longer than a tropical year.
Focus on the Earth when it’s on the right side in the animation. Now imagine that the tilt rotates away from you slightly. This means the Earth wouldn’t quite have to complete one full orbit before the Northern Hemisphere is tilted towards the Sun again. In other words, we don’t have to wait until the Earth is all the way to the right. Just before then, when it’s slightly on the near side, the Northern Hemisphere will be tilted towards the Sun. One tropical year has been completed. When the Earth makes it all the way to the right side (after one sidereal year), the tilt is not pointing directly towards the left but mostly towards the left and ever so slightly away from you.
Thirteen thousand years from now, the tilt will point towards the right in the animation. At that point, it will be summer in the Northern Hemisphere when the Earth is on the left side. The summer solstice will still take place in late June but the constellations we see then will be the ones on the left side, not those on the right side (because the left side of the Earth is the nighttime side).
Even if you observe the heavens carefully all your life, you won’t notice precession happening. It doesn’t produce any changes that are visible to the naked eye within a human lifetime.
However, the changes add up over time. Currently, there is a bright star, aptly named Polaris, that is almost perfectly aligned with the North Pole. But this has only been true for a few centuries. In fact, it was only named Polaris during the European Renaissance. When Stonehenge and the Pyramids of Giza were being built, a star called Thuban was our North Star. In the coming millennia, other bright stars will serve as our North Star at certain times and there will long periods without a North Star. Polaris will once again be our North Star 26,000 years from now.
The figures below show the stars that are aligned with the North and South Poles at different times. The numbers shown are calendar years (+2000 is 2000 CE, -2000 is 2000 BCE, etc.). Since the entire cycle takes 26,000 years, -2000 is the same as +24000.
Polaris is at the end of the Little Dipper’s handle, just to the left of +2000 on the figure at the left. Currently there is no bright star that is aligned with the South Pole (+2000 on the figure at the right).
The path of the North (left) and South (right) Celestial Poles among the stars due to precession. By Tao’olunga via Wikimedia Commons.
Imagine how perplexing precession would have been for an ancient astronomer. You find records from a thousand years ago indicating that a constellation is visible just before dawn on the spring equinox but it isn’t. Or the most famous monument in the land was built to line up with certain stars on certain nights but it doesn’t.
It took an extraordinary astronomer who also had access to centuries of astronomical records to realize that the seasons are indeed drifting relative to the stars. That person is believed to have been the Greek astronomer Hipparchus, a who lived in the 2nd century BCE.
Over the next two millennia, precession was alternately accepted and rejected by astronomers in different parts of the world. Many who accepted it argued incorrectly that it doesn’t go in one direction but back and forth every few hundred or few thousand years. This went on until the 17th century when Isaac Newton provided a physical explanation for precession. Even Newton didn’t have the last word on the matter because his equations didn’t work and had to be refined by later scientists.
A great barrier to accepting precession was the deep attachment many astronomers had to zodiac astrology. This is the idea that there is an enduring connection between you and the constellation the Sun was in when you were born.
During the course of the year, the Sun wanders through a dozen constellations. Some are larger than others and so the Sun takes longer to cross them. But for convenience, the path of the Sun was divided into twelve equal parts with one constellation in each.
The Babylonians first did this around 2500 years ago. Since then, the zodiac has drifted backwards by one full constellation due to precession. So if your zodiac sign is Cancer, the Sun was actually in Gemini when you were born and so on.
In ancient times, studying the stars was often not an end in itself but a means to explain and predict life events. The idea that the stars which are supposed to guide our lives are themselves not fixed must have made an ancient astronomer extremely agitated.
Even more than you were when you found out the Earth doesn’t go around the Sun once a year.