Early people noticed that the stars seemed to follow circular motions across the sky at night. This led them to believe that the earth was at the center of everything. The sun and moon also appeared to go around the earth. In this section, we will follow the development of how we came to understand our place in the universe.
We will begin with this geocentric model, which was replaced with a heliocentric model, with the sun at the center of the universe. Eventually, we became to understand that we are not the center of the universe, but rather, we inhabit a planet that orbits a star, which orbits the center of a galaxy. We are one, among many planets in the galaxy and in the universe.
In order to understand the motions of the planets in our solar system, we will study the underlying physics, including Kepler's laws and Newton's laws.
Ancient stone configurations like Stonehenge are probably evidence of early peoples' recognition of astronomical cycles. The lengthening and shortening of the daylight portion of a day means that twice a year, the sun will "rise" at the same position on the horizon for a given location on the surface of the Earth. Stone markers would make it possible to recognize these days, necessary for an early attempt to create a calendar.
Other examples of possible early observatories around the world include these 2300 year old structures at the Chankillo archeological site in Peru.
The Beijing Ancient Observatory was built during the Ming dynasty, around 1440 AD. It contains the longest complete succession observation period of astronomical records among ancient observatories.
This time lapse video from the Very Large Telescope array shows the apparent motion of the stars, as they seem to move across the night sky. The telescope array is located high in the Andes mountains in Chile, far from city lights.
This video was produced from the same footage as the video above, but edited to hold the sky fixed and attribute the motion to the rotation of the Earth, as it would look to an observer outside of Earth's reference frame.
This curious "star" seems to wander around in the night sky against the background of the stars. These images, taken at the same time each night over a span of three years, show the background stars remaining still, compared to each other, with a rogue "wanderer." The ancient Greeks noticed a few of these strangely behaving stars, and gave them the name "planet," which was their word for "wanderer.
In this sequence of photographs, taken over the span of three years, Saturn can be seen to move forward, then backward, then forward again in what we call "retrograde motion."
A Greek astronomer named Ptolemy, around 140 AD, theorized that this odd motion could be explained by complicated motions called "epicycles" of the planets as they orbited Earth.
This animation from McGraw-Hill Higher Ed Astronomy Interactives illustrates the true reason behind the seemingly erratic behavior. It arises because the planets' orbital speeds are not the same; they increase with distance from the sun. Inner planets overtake outer planets periodically. It was this retrograde motion of the planets that helped us to realize that Earth was not the center of everything.
The Polish astronomer Nicholas Copernicus realized that the planets orbited the Sun, but believed in a Heliocentric system, where everything orbited the Sun, including distant stars.
Galileao Galiliei (1564 - 1642) was credited with using an early telescope to discover that Jupiter had a system of moons. This proved that not everything orbited the Sun, and began the modern era of astronomy. We started to recognize that Earth was a planet that orbited a star, like many other stars in our galaxy. Later on, we realized that our galaxy was one of a multitude of galaxies. We progressed from seeing ourselves as the center of the universe to a citizen of the universe.