Stars appear to rotate around a point in the upper left corner of this time lapse image taken from a campsite in Arizona’s White Mountains.
You probably know that astronomers in Ancient Greece thought that the stars spun around a fixed, immovable Earth, but that we now know that the Earth moves – rotating around an axis while orbiting around the Sun. But how did we, humanity, figure this out? After all, we had to understand the motions of the Earth, Moon, and planets before we could launch satellites and take lovely images of the Earth like the one below or in the time lapse footage from the International Space Station.
The western hemisphere of the Blue Marble, created in 2002.
Credits: Image Credit: NASA’s Earth Observatory
To understand the debate between the ancient and modern views, you have to first recognize that the motions of the stars and the Sun when observed with your eyes alone (without a telescope) cannot be used to distinguish between the two views. The motions of the stars and of the Sun that we see with our eyes are exactly what we would see if ancient astronomers had been right.If this comes as a surprise, think about this. The ancient astronomers used their observations of the stars and the Sun to build their model. Of course, they built a model that matched what they saw.
The simplest example of this is to consider the way that the stars appear to move (their apparent motions) during a single night. Watch the video below, which I picked for its simplicity. You can see that the stars appear to circle about a single, fixed point (the center of the final star tracks).
It looks as if the stars are all painted on a giant ball that spins around the Earth. Ancient astronomers assumed that this was the truth; they called the giant ball the Celestial Sphere. In contrast, we now know that the sky only appears to spin because the earth is rotating around an axis, but it takes more than a night of observing the stars to figure this out.
Ancient astronomers also knew that the stars’ motions would look identical if the Earth were rotating or if the sky (the Celestial Sphere) was rotating. They argued, that since it doesn’t feel like the Earth is moving, it isn’t. This is intuitively logical. If the surface of the Earth is spinning at speeds up to 1,000 mph while hurling around the Sun at about 67,000 mph, why don’t we feel this motion?
One anwer to this question is that we don’t feel motion; we feel accelerations, or changes in motion. While we may be moving very quickly, the acceleration that we experience as the Earth rotates is very small.
The difference between motion (or velocity) and acceleration – and the difference in how we experience the two – is at the heart of physics. Galileo Galilee convincingly argued that constant motion cannot be felt, Isaac Newton codified this observation in his first law of motion, and relativity takes it further – starting with the postulate that there is no difference between being at rest and constant, unchanging motion (no change in speed or direction).
For some beautiful time lapse footage of stars, skip to about 1 minute in this video taken in Death Valley, taken by the Sunchasers team.
