If one of us was to fly above the plane where the Sun and the planets are, what would we find in the center of our solar system? Well, this question led to a historical debate and took a while for astronomers to answer it. The debate was between what we know as the geocentric model and the heliocentric model. According to the geocentric model, the earth is the center and according to the heliocentric model, the sun is the center of the solar system. As we discussed in class, Nicolaus Copernicus, a renaissance-era mathematician and astronomer formulated the heliocentric model. Until then, everyone believed that the universe was geocentric model. But, the essential question is, although initially the heliocentric model was developed, why was the geocentric model of the universe adopted?
But before delving into that, allow me to explain another concept which goes hand in hand with the models above. When we perceive the sky from Earth, the planets seem to move from east to west. Sporadically, the planets will look as if they are moving in a reverse direction–this phenomenon is known as retrograde motion. Ancient astronomers had trouble clearing up this motion. Ptolemy’s proposed explanation of this movement was epicycles: “In this case, the planet moved on a little circle, the center of which rotated on the circumference of the large circle centered on the Earth,” according to Rice University’s Galileo Project website. So here comes the heliocentric model which solves this problem. But how? Well, the heliocentric model assumes that planets farther away from the sun orbit slower than the closer ones. This idea was reiterated in our lecture tutorial on “Observing Retrograde Motion” where we learned that the in retrograde motion the planet would move across east to west of the sky in a single night – rising in the east and setting in the west.
With the context given above, allow me to get back to our topic. As mentioned earlier, Copernicus proposed the heliocentric model (earth-center model), but there was a slight problem with his conjecture. He theorized that the planets moved in perfect circular trajectory around the sun and due to that, the model was questioned as it was not accurately predicting the position of the planets. So, what did we conclude? Well, accepting heliocentrism was a gradual process and by the 18th-19th century, it was clear that the sun was not the center of the universe, but only a star among many others. In fact, we learned that the sun is not even the center of the solar system but rather it is the concentration of the elliptical orbits crossed by the planets.
Okay, this all makes sense. But the discussion doesn’t end there. Another important matter that needs attention is, it is well recognized that the earth circles the sun, rather than the other way around as had been deemed previously. However, if the earth and the other planets go around the sun, is the sun motionless in space? Or is it going around something as well sequentially? Now, this was my favorite part of my research. The answer is. . . yes! It is going around something and it is not slow in its movement. In the piece, “Myth Debunked: The earth orbits the sun, but does the sun orbit anything?” it is detailed, “the sun orbits the center of our galaxy, the Milky Way, pulling us and all the other bodies in the solar system along with it at a breakneck speed of about 230km every second. Even at this speed, however, the sun takes around 225 million years to make one complete turn round the center of the galaxy. The Milky Way is enormous, around 100,000 light years across. That means that light, travelling at 300,000km every second, would take 100,000 years to cross from one side of our galaxy to the other. And what is at the center of the galaxy? It is now thought that at the center of every large galaxy is a massive behemoth, hiding in the light of the billions of stars that make up the galactic bulge; a super-massive black hole. Sagittarius A*, the super-massive black hole at the center of the Milky Way is estimated at a mass of over four million times that of the sun”.
I. . . just. . . love. . . this! Wow! Who would of thought! Not me, for sure.
It’s clear that the discussion of the heliocentric model and the geocentric model begin in the 17th century but it certainly did not end there. It has not only answered questions but also birthed new questions and perspectives. Honestly speaking, I hated the struggle to find some cool, recent discoveries for this conceptual objective. The struggle was reallllllll! However, what I found mesmerized me completely and made the struggle worthwhile. Phew!