When we look at the night sky there is solace in thinking that this is the same sky that has appeared to us throughout our lives, and is the same sky that our parents, grandparents, and great-great-grandparents knew. Although this is true, it is not true that the stars we consider “fixed” are not in motion.
Indeed everything in the Universe moves – and the Earth is no exception. Let’s start with the simple motion that the Earth completes once every 24 hours – its rotation about its axis. The Earth is (roughly) a sphere of radius 3959 miles at the equator. At our latitude of 41.48 degrees north of the equator, Southbury completes a circle of circumference 18636 miles in a day! That’s 776 miles an hour, moving east to west – quite literally “faster than a speeding bullet”. Put another way, every hour and six minutes, we move into the space that Chicago occupied an hour and six minutes ago.
But that’s our slowest motion. Earth completes an orbit about the sun every year. The circumference of the orbit is 584 million miles. That works out to an (average) speed of 66,600 miles per hour! The moon is 224,000 miles from Earth, on average. When the moon is at its last quarter phase (overhead at dawn), it lies roughly in our orbital path. At that time, we pass from where we are to the space the Moon occupies in a mere 3 hours and 22 minutes. Fast enough for you yet?
But that is not the full story. Our sun orbits the center of the Milky Way galaxy. We are 25,000 light years from the center of the Milky Way, and the Milky Way rotates once every 225 million years. That works out to an astounding 492,000 miles per hour. To put that speed in perspective, the average distance to Mars is 140 million miles, and we cross that distance every 12 days at this speed. (Meanwhile, the best human technology can get us to Mars in about 150 days when the planets are favorably aligned to reduce that distance).
As for what direction the sun is heading in, that has been tricky to determine, though we do have an answer. All the stars we see in the night sky are in motion as they make their own orbits about the Milky Way. Using precise measurements of the apparent locations of each star relative to all other stars, astronomers can measure a star’s “proper motion” over a period of years. These motions are so slight as to be completely unnoticed by the casual observer.
Remembering that the stars we see in the night sky are at greatly varying distances from Earth, dozens to hundreds of light years, the proper motions – even if all stars moved at the same speed, which they do not – would also vary dramatically. Nearer stars would appear to move more quickly, distant stars more slowly decade after decade.
The stars also do not move in the same apparent direction – the proper motions are not aligned. This is due to a variety of reasons – the stars all have differing orbits about the galaxy’s center and some are perturbed in their motion by other nearer stars. But key to determining the direction of motion of our own sun is the fact that our sun is gradually overtaking other stars, while leaving others behind us.
Think about driving in a steady snow fall. The snowflakes heading toward your windshield appear to radiate outward from a single point ahead of your car. Their “proper motions” spread in lines away from this point. Similarly, if you look in your rearview mirror (or, more safely, have your passengers look out the back window), the flakes behind you will appear to converge to a single point trailing your car.
When astronomers map out the proper motions of thousands of stars we see in the night sky, they find (after averaging out random motions) that there are two similar points in the sky – one from which the stars on average are moving away from, and one toward which they seem to converge. The first point is located between the constellations Hercules and Lyra (late summer and fall constellations), near the bright star Vega – and that is the direction toward which the sun moves.
If you are not dizzy yet, we can move on to consider how the Milky Way galaxy moves with respect to the rest of the Universe. A major challenge in considering this motion is how to define what it is we are measuring motion against. This is a very subtle question, but leads us to consider one of the premises of Einstein’s theory of relativity – that there is no proper concept of “absolute motion”. When everything in the Universe is in motion, how can we pick a “fixed frame of reference” against which to measure velocity?
There are two possible approaches to an answer. The first is that it doesn’t matter what you pick as being fixed – physics will work out the same way no matter what you pick. That’s not to say that the choice is unimportant – if you were to pick the Earth as fixed (as some would still like us to), then the mathematics necessary to describe everyday observations becomes horrifically complicated! Astronomers usually consider a fixed (but rotating) Earth when thinking about Earth satellites, a fixed Sun when thinking about motions in the solar system, and a fixed galaxy when thinking about the motions of stars.
The other – very subtle and quite advanced – choice brings us face to face with the origin of the Universe – the Big Bang. The Big Bang hypothesis asserts that all of the Universe came into existence in an instant, in a very small region of space (I’m mangling the physics here, but this is as accurate as I can be while introducing the Big Bang in a single paragraph). This Universe-forming event released unfathomable amounts of radiation, and we can still detect that original radiation today, some 14.7 billion years later. The radiation is in the form of microwaves (similar to the microwaves in your microwave oven, but extremely weak in comparison).
Because the radiation was released at (literally) the “dawn of time”, it should be evenly distributed throughout the heavens, and if Earth were motionless we should measure roughly the same amount of energy no matter what direction we look in from Earth. But, very roughly similar to our snowflake analogy used earlier, if we are in motion relative to the “cosmic microwave background”, we should see a point in space where the energy appears higher – in the direction we are moving – and a second location in the opposite direction of lower energy. The amount of energy increase in the direction of motion also tells us the speed at which we are moving.
Astronomers are now capable of mapping the cosmic microwave background with great accuracy using orbiting telescopes. From these observations it has been determined that the Milky Way galaxy moves through the Universe at approximately 1,200,000 miles per hour! This motion carries us in the direction of a point near the center of the constellation Hydra, below our southern horizon at about midnight in early June.
It was while reading the 1909 text that I discussed last week that the author went into a tangent on the motions of Earth, and how far a man travels while attached to this lonely stone through his lifetime. The facts known then limited the distance traveled to dimensions well within our Solar System. But using the motion of the Milky Way through the universe (and neglecting the other motions that occur in different directions), over an 80 year life span, a man will travel an incredible 0.15 light years during his life – travelling through space on our most convenient spaceship of all – our very home.
