The universe was originally believed to be all life on Earth existing on a flat sphere that was orbited by the Sun, Moon, a handful of planets, and a fixed sphere of stars. Today, we know that we all live on a spherical planet that is in orbit around the Sun along with many other planets and bodies within our solar system, which is one of many solar systems containing planets in a dynamic galaxy in a constantly changing universe. So what made our understanding of the heavens alter so drastically?
A Spherical Earth
Many of us learned in school that it was Christopher Columbus who proved that the Earth was in fact a sphere. Many feared that when he set sail in 1492, the explorers would sail off the edge of the Earth never to be seen again. But this is a myth. Columbus was simply looking for a shortcut to reach India without going all the way around Africa.
The earliest known recordings of the Earth being known as a sphere date back to around the 6th century BCE. It is generally accepted that Pythagoras was the first to discover that Earth was round and that Aristotle was the first to prove it.
There were several reasons that the ancient Greeks used to prove that the Earth was round.
- The Earth casts a circular shadow on the Moon during lunar eclipses.
- Some stars that are low in the southern sky in Egypt are not visible at higher latitudes.
- When a ship sails away, the tops of the sails are the last to disappear beyond the horizon and if the Earth is flat then the entire ship would always be in view, albeit practically invisible when viewed from a great distance.
Eratosthenes was the first to measure the circumference of the Earth. He did this by measuring the angle of the Sun hitting sticks set in the ground at noon at two locations separated by about 5000 stadia (the stadium was a common unit of measurement at that time).
It was reported that in Syene, the sun appeared directly overhead at noon casting no shadow on the stick. In Alexandria, the shadow cast by the Sun on the stick was at an angle of approximately 7.2 degrees. These two measurements led Eratosthenes to calculate the circumference of the Earth to be approximately 250,000 stadia.
Now you may be asking: how big is a stadium? There were two different versions of the stadium measurement that Eratosthenes could have used: 1 Attic (Greek) stadium = 185 meters, 1 Egyptian stadia = 157.5 meters.
It is generally assumed that Eratosthenes used the Greek measure as he was a Greek, but since he was living in Egypt it leads to some debate between historians. Using the Greek system, the circumference of the Earth was measured to be about 46,620 km (28,968.3 miles) while the Egyptian system leads to an approximate circumference measurement of 39,375 km (24,466.5 miles). The accepted value for the circumference of the Earth is 40,075 km (24,901 miles), so while the Egyptian definition of a stadium gives a closer value, either way gives a fantastic estimation of the circumference of the Earth all while using sticks and some trigonometry.
The Earth Orbits the Sun
If you were able to use a time machine to travel back to anytime before the late 1600s and polled the population, pretty much everyone you would ask would say that the Earth was indeed the center of the universe (known as geocentrism) and anything else was considered blasphemy!
Heliocentrism, where the Earth and all other solar system bodies orbit around the Sun, was actually theorized by several ancient astronomers and philosophers beginning with the earliest known recordings in the 9th century BCE by Yajnavalkya, an Indian philosopher, although none were able to gain a large enough following to be generally accepted by the general public until after the invention of the telescope in 1608 plus an additional several decades of observations and research. Aristarchus, Plato, Aristotle and Ptolemy were also known to have researched and believed in the heliocentric model of the solar system.
Nicolaus Copernicus is generally credited with laying the foundation to prove that the Sun lay at the center of our solar system. Galileo Galilei was the first to use a telescope to prove Copernicus’ heliocentric model, but was viewed as a heretic when his works were published in 1623 and was put under house arrest for the rest of his life after several hearings by the church. Further studies by Isaac Newton proved that the Earth did travel around the Sun with the other planets and while public opinion was moving in favor of heliocentrism, it took the church until 1835 to drop its opposition of heliocentrism and Galileo was officially forgiven in 1992, nearly 350 years after Galileo published his findings.
The Universe is Expanding
Questions about the universe have been asked for as long as humans have been gazing towards the sky. Two of these questions that weren’t answered until the early 1900s, are whether the universe is static or not and whether it is infinite or not?
Wondering about either the finite or infinite nature of the universe posed several paradoxes. The ancient Greeks couldn’t fathom what an infinite universe would look like, but at the same time, if the universe was finite and you reached the edge and stuck your hand through, where would your hand go?
Another paradox brought up in the early 1800s by Heinrich Olbers indicated that if the universe were to be infinite then the night sky should be as bright as a star due to the fact that if the universe were infinite, then in any direction your line of eyesight would eventually land on a star. By this reasoning, the sky should be completely covered by stars with no gaps. But because our sky is mainly dark with a few stars scattered across it, then the universe must be finite.
On the flip side, Isaac Newton had brought up in the late 1700s that the universe must be infinite. The law of gravity, discovered by Newton, stated that gravity was always attractive. It should then follow that if a finite universe followed the laws of physics, it should collapse on itself due to the attractive forces. But because we are all still here, the universe must be infinite.
Even Albert Einstein had run across the problem when working on his theory of general relativity. The equations he had developed showed that the universe should either be expanding or collapsing, and general consensus at the time was that the universe was static. Einstein had introduced a cosmological constant into his equations to cancel out the effects of gravity.
The development of larger telescopes and higher quality spectrographs combined with research by Edwin Hubble led to the discovery in 1929 that the universe was expanding, ending the centuries-long debate. By observing a large number of galaxies, Hubble showed that the spectrum of each galaxy was redshifted, and the greater the distance the galaxy was, the greater the redshift. This matched results given by Einstein’s theory of gravity and led Einstein to say his famous quote that the introduction of his cosmological constant was his “greatest blunder.”
Far away worlds had often been wondered, dreamed, and written about all throughout recorded history and probably before. The world of science fiction contains countless examples of alien civilizations on planets that are both similar and very dissimilar to our own Earth. Proof of the existence of planets outside our solar system came in 1995, turning science fiction into science fact.
A good deal of scientific inquiry that took place in the fifteenth and sixteenth centuries were aligned with the values of the church and any discovery that had gone against their teachings could be found to be punishable (such as with Galileo). To many in these times, there wasn’t even room to question the existence of other worlds, no less populated ones, with the Earth being the center of the entire universe. Giordano Bruno had postulated the existence of other worlds and accepted the heliocentric model of the universe only to receive the death penalty from the church.
The first exoplanet discovery came in 1994 and we now know of at least 3,375 with nearly another 4,700 awaiting confirmation. Several hundred of these planets are approximately Earth mass and a few even reside in the habitable zone (where liquid water could exist) of their parent star.
Now that we know that there are Earth-mass planets within the habitable zones of their suns, we can start trying to look for signs of life. Just because a star is within their habitable zone, it does not mean that the conditions on the planet are able to support life, such as with Venus. With our current technology, we just are beginning to be able to detect the compositions of exoplanet atmospheres, assuming that the planet does have an atmosphere. Looking for spectral markers including water, carbon dioxide, and oxygen could lead to a discovering of a planet with the conditions needed for life, as we know it.
The next big discovery that could change our way of thinking could be discovering the proof that intelligent life does exist on another planet. Perhaps it is about time to start planning for a United Federation of Planets and sharing a friendly drink with our neighbors.