“Nature is relentless and unchangeable, and it is indifferent as to whether its hidden reasons and actions are understandable to man or not.” -Galileo Galilei
All of science is rooted in the idea that natural phenomena can be explained naturally, and that if we want to know how anything in the Universe works, all we need to do is ask the Universe the right questions, and the answers will appear.
So what about the question of the night sky, and why it appears to rotate the way it does?
There are two straightforward explanations for this, and from this observation alone, they’re indistinguishable from one another.
- The entire sky — and all the stars in it — spins around the Earth with a period of 24 hours, causing the stars to change position as we observe it from Earth.
- The entire sky is — to the best of our observations — stationary, and appears to spin because the Earth is rotating beneath it.
These two scenarios, although they would both adequately explain this phenomenon, are vastly different from one another.
But the stars appearing to rotate about the celestial pole is not the only observation we have. By making other observations and interpreting them in the context of these two very different models, we can help determine whether one is superior to the other.
So what can we see that can give us a clue to whether it’s the Earth or the sky that’s moving? Although the stars always appear to make this rotational motion throughout the night, the stars that are visible in the night sky — as well as their locations — vary greatly throughout the year.
Given that we’ve got the entire sky to consider, this has to do with the position of the Sun. When the Sun appears during the day in the Summer, the winter constellations are obscured by the sunlight bathing our atmosphere, and when night falls, the summer constellations are visible. Conversely, the summer constellations are obscured by the Sun during days in Winter, while the winter constellations are visible at night.
Again, both models can accomodate this, but they look very different.
If the Earth is truly stationary, then the Sun would have to move to different locations relative to the night sky throughout the year. In addition to its once-a-day orbit around the Earth, it would have to migrate in one additional circle relative to the background stars each year, in order to explain why the visible constellations vary throughout the seasons.
On the other hand, if the Earth is allowed to move, then it can also move around the Sun, explaining why different constellations appear in the night sky at different times of the year.
We also need to explain what the Sun does.
From the point-of-view of us here on Earth, particularly those of us who live away from equatorial latitudes (outside of the tropics), the Sun’s path through the sky varies significantly throughout the year.
The lowest the Sun ever appears — at zenith — above the horizon occurs during the winter solstice, while its highest point occurs during the summer solstice.
In the Earth-is-stationary model, the Sun needs to change its location in the sky significantly throughout the year: in addition to its once-a-day journey around the Earth, it needs to change its location relative to the celestial sphere by a whopping 47 degrees every six months. Why the Sun moves in this path so slowly relative to the celestial sphere but so quickly relative to Earth is not explained in this model.
On the other hand, if the Earth is allowed to move, this would result simply from the Earth moving around the Sun while it rotates on its tilted axis. If the Earth is the moving thing, its rotation and its revolution are allowed to be separate quantities, which could explain the vastly different timescales for days (the period of Earth’s rotation) and years (the period of Earth’s revolution).
Again, both models are still allowed, but the complexity and power of each explanation is different. Let’s throw in just one more object: the Moon.
Much like the Sun, the Moon follows a very similar path throughout the sky: it rises towards the East, sets in the West, and rises and sets once per day. It also appears to migrate relative to the stars, completing an extra circle about once every 29-to-30 days.
The big difference between the Moon and the Sun is noticeable when there’s a Full Moon.
While the Moon never varies by more than 5 degrees from the Sun in terms of its inclination to Earth, there’s a huge seasonal difference between the Full Moon and the Sun. When the Sun reaches its maximum height above the horizon, during the summer solstice, the Full Moon achieves its minimum height above the horizon. And when the Sun is at its minimum height during the winter solstice, the Full Moon reaches its maximum height above the horizon!
If the Earth must remain completely stationary, we must again put the Moon’s orbit in, making an extra circle relative to the celestial sphere every lunar month, and inclined at nearly the same (but not quite) the same amount relative to the celestial sphere as the Sun.
We need this, of course, to explain the observed Lunar and Solar eclipses, which can easily be deduced to be to the interplay of shadows between the Sun, Moon and Earth.
But if you allow the Earth to move, you can not only explain the daily motion of the stars, Moon, and Sun relative to the Earth’s sky by the Earth’s rotation, you can explain the lunar and solar motions relative to the rest of the sky as revolutionary orbits due to the force of gravity.
If you insist that the Earth remain stationary and the celestial sphere rotates, you can make a working model for the Earth, Sun, Moon and stars, but it requires you to put the motions of the Sun (an extra revolution tilted at 23 degrees relative to the celestial sphere per year) and the Moon (an extra revolution tilted at 5 degrees relative to the Sun per lunar month) by hand, with no physical explanation for these motions.
That was exactly what the Ptolemaic Model did, which adequately described these motions without explaining them. This is why we needed the theory of gravity (and why we need scientific theories in general): to explain why the objects in the sky make the apparent motions that they do, and to explain why the path of the Full Moon nearest the Summer Solstice is — to within that 5 degree tolerance — is identical to the path of the Sun at the Winter Solstice.