365daysDate: June 11, 2009

Title: Round, Round, Round We Go


Podcaster: Larry Sessions


Description: We are taught that the Earth circles the Sun once a year, but is this really true? If it is not true, where do we really end up each time the Earth completes an orbit? Motion in space is relative, and as Einstein discovered, you don’t always end up where you think you are. So where is the Earth today, compared to where it was last year on this date? The answer may surprise you!

Bio: Larry Sessions is a former director and staff astronomer at Denver’s Gates and Fort Worth’s Noble planetariums, and now is an instructor for Metropolitan State College and the Community College of Aurora, Colorado. He also is the webmaster and editor for the Southwestern Association of Planetariums, as well as his own website, North American Skies, and a contributor to both and A NASA/JPL “Solar System Amabassador,” he has every copy of the Royal Astronomical Society’s annual handbook since 1971.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Cryo, amateur astronomer from Austria, and is dedicated to every astronomer, from the the early days of humankind to the present, and to those who will come. Thank you for your endless efforts on expanding our view of the universe.


Einstein was wrong. And if I am allowed to take it a bit out of context, Thomas Wolfe got it right. You can’t go home again. I’ll tell you why in a moment, but first let me give you a little background. I am Larry Sessions, former planetarium director and now an astronomy instructor in Denver. I also write a little, produce the website North American Skies, and serve as the webmaster for the SouthWestern Association of Planetariums, or SWAP.

If you are listening to this podcast on the originally scheduled date, it’s my birthday. It’s exciting to think that if I go out tonight and look at the stars and constellations, they will appear in virtually the same positions at the same time of night as they did on the night I was born. The Moon and planets won’t match because they have their own independent motion, but the stars and constellations will appear virtually the same as they have been for thousands of years. You can use this idea to commemorate other, more important events, as well, such as the birth of your child, the end of World War II, the first flight into space, or anything important to you.

But in truth, the Earth is not in exactly the same place in its orbit at the same time each year. We are used to thinking that the Earth spins like a top, and in fact it does, and like a top, the spinning Earth wobbles slowly on its axis. The result is to slowly change the dates of the beginnings of the seasons. This is called the “precession of the equinoxes.” The difference is small, amounting to something like one day in 70 years. Given the Earth’s orbital speed, that amounts to about 36,000 kilometers or 23,000 miles difference each year. Relative to the distant stars, this is too small to notice in a human lifetime, but astronomers have known about it for more than 2,000 years, and Einstein must have known, too. So this was not his error.

However, in the mid 1920’s a Swedish astronomer named Bertil Lindblad discovered something curious. He detected small motions in the stars near the Sun, which led him to determine that the entire Milky Way Galaxy was turning, carrying along the Sun, the Earth and other stars with it. Einstein certainly did not know this originally, but this was not his error, either.

It does say something about the Earth’s motion, however. Seeing the same stars on my birthday — or your birthday or whatever you wish to consider the anniversary of — gives the impression that we are in the same spot relative to the cosmos. The stars and constellations proceed statefully across the sky each evening, drifting slowly through 360 degrees until we come full circle on the same date each year. Or so we thought.

With Lindlbad’s discovery, and confirmation by Dutch astronomer Jan Oort, things changed. It became obvious that in just one year, the Sun — and the Earth along with it — moves a large distance, coincidentally toward the direction of the constellation Hercules. The standard estimation for this velocity is about 800,000 kilometers per hour. This yields a linear distance of about 7 billion kilometers (a bit more than 4 billion miles) in a year. Of course we are not traveling in a straight line, but it is such a small segment of a circle’s circumference that it might as well be straight! It takes our Sun more than 200 million years to go around the galaxy just once. And recent estimations show that given the revised size of the Milky Way, our motion through it may need to be revised upward as well.

It is sobering to think that my youthful musings of returning to the same place in space each year on my birthday are wrong. Instead of being at the same position in space that our planet occupied on the day I was born, we are more than four billion miles farther away each year! Four billion miles is roughly the distance between the Sun and Pluto.

Even if he didn’t know about it, this motion of the Galaxy was not Einstein’s error either. But, in 1915, when he published his General Theory of Relativity, Einstein thought that the Universe as a whole was static and unchanging. While stars and planets within a galaxy might move, the galaxies themselves remained in one position, and this never changed, or so he thought. Einstein made this assumption, because … well, everybody else believed it and there was no evidence to the contrary. Still, he knew that his own theory dictated that the Universe could not be static — it would either contract under gravity or expand under the influence of some other force. To keep it static, as he believed it was, he introduced a term referred to as the “Cosmological Constant.” This was a kind of repulsive force that exactly balanced the inward pull of gravity to keep the Universe from collapsing. At best, Einstein’s arrangement was an unstable equilibrium, which he quickly discarded when Edwin Hubble discovered that in fact the Universe was not static after all, but expanding. Einstein later referred to the Cosmological Constant as his “greatest blunder,” but in fact current evidence suggests that it may exist after all as a variation on what is now called “Dark Energy.”

So in reality, Einstein’s mistake was not the Cosmological Constant, but in assuming that the Universe was static in the first place.

The Universe and everything in it is moving. A few close galaxies, such as Andromeda, are actually moving toward us (or we toward them, depending on how you look at it!). Compared to where we were just a year ago, today we are about 5 billion miles closer to Andromeda. And then there are the farther galaxies. In fact, the farther they are, the faster they are moving away from us (or us from them). This is Hubble’s Law. The farthest single object ever observed appears to be more than 13 billion light years away and traveling away from us at nearly the speed of light. Thus, relative to that primitive galaxy at the edge of the observable Universe, we move nearly a light year farther away, for every time we orbit the Sun! Truth is, it is almost meaningless to talk about our motion relative to objects so far away. Still, the bottom line is that the Universe does not stay still and carries us along with it. We never come back to where we started.

In a way it is unsettling to think that Einstein was wrong, even though he later realized it, and that we are in fact billions of miles away from our “birthplace” in space each year. On the other hand, it makes life interesting.

So go out on your birthday and look at the stars. But take a good look, because you will never be in that exact position again!

These and related topics are discussed also in the February 5 podcast by James A. Brown. You also can listen to cosmologist Michael Turner discuss the Cosmological Constant and Dark Energy in the podcast for May 12. You can follow my Twitter page, Thanks for listening.

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365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at or email us at Until tomorrow…goodbye.