History of Astronomy
Astronomy is one of the humanity’s oldest science. Its basic activity is to learn about what we see in the universe. Astronomy is considered both as a hobby and as subject to study in depth.
Observational astronomy is an activity that amateur observers enjoy as a hobby and pastime and was the first type of astronomy humans did. There are millions of people in the world who stargaze regularly from their backyards or personal observatories. Most aren’t necessarily trained in the science, but simply love to watch the stars. Others are trained but do not make their living at doing the science of astronomy. On the professional research side, there are more than 11,000 astronomers who are trained to do in-depth studies of the stars and galaxies.
From them and their work, we get our basic understanding of the universe. It’s such an interesting topic and raises many astronomy-related questions in people’s minds about the cosmos itself, how it got started, what’s out there, and how we explore it.
In this series of blog, we will explore this interesting topic both as a hobby and a subject. We will explore from how we started looking up at the sky to questioning the almighty universe and how it works.
Before we begin our study, take a moment to ponder where we are in space and what we live on; a very special planet. “That pale blue dot”
We have a very little information on early human’s impression of the heavens, mostly some drawings of eclipse, comets etc. One of the earliest recorded astronomical observations is the Nebra sky disk from northern Europe dating approximately 1,600 BC. This 30 cm bronze disk depicts the Sun, a lunar crescent and stars (including the Pleiades star cluster).
The earliest written records were astronomical observations produced by the Babylonians around 1600 B.C. who recorded positions of planets, times of eclipses, etc. Of the many philosophers before the time of Socrates (the Presocratics) was Thales (~ 480 B.C.). His combination of math and Babylonian data allowed him to predict eclipses.
For a long time, it was realized that the earth’s surface was curved by people familiar with the behavior of incoming and outgoing ships. For it was obvious that as a ship passed over the horizon, the hull disappeared first, then the topmost sailing masts (although one could argue this is an effect of refraction in the atmosphere). Ancient astronomers could see with their eyes that the Sun and the Moon were round. And the shadow of the Earth, cast on the lunar surface during a lunar eclipse, is curved. A sphere is the simplest shape to explain the Earth’s shadow (a disk would sometimes display a shadow shaped like a line or oval).
Eratosthenes was the first one to calculate the Earth’s circumference which is pretty close to modern day value. Eratosthenes used a spherical Earth model, and some simple geometry, to calculate its circumference. He knew that on a special day i.e summer solstice at noon in the Egyptian city of Syene, a stick placed in the ground will cast no shadow as it is parallel to sun’s rays. At the same time, a stick on the ground in Alexandria, to the north will cast a shadow at an angle of 7 degrees. Eratosthenes realizes that the ratio of a complete circle (360 degrees) to 7 degrees is the same as the ratio of the circumference of the Earth to the distance from Alexandria to Seyene which was approximately 4900 stadia (784 Kms)
C/784 = 360/7
Which gives C (Circumference of Earth) as 40320 kilometers, which is amazingly close to the modern value of 40,030 kilometers. Not bad for a more than 2,000-year-old estimate made with no modern technology!
With this calculation, Eratosthenes becomes the father of geography eventually drawing up the first maps of the known world and determining the size of the most fundamental object in the Universe, our own planet.
During the times before the invention of the telescope, there were only seven objects visible to the ancients, the Sun and the Moon, plus the five planets, Mercury, Venus, Mars, Jupiter and Saturn. Plato first proposed that the planets followed perfect circular orbits around the Earth (for the circle is the most perfect shape). Later, an astronomer named Eudoxus created the first model of a geocentric universe around 380 B.C. and the beginning of the geocentric versus heliocentric debate started.
Unfortunately, as the Greeks continued to explore the motion of the sun, the moon, and the other planets, it became increasingly apparent that their geocentric models could not accurately nor easily predict the motion of the other planets.
Take the apparent motion of Mars from an observer on the Earth, for example. As the Earth and Mars orbit around the sun, Mars appears to advance forwards, and then stop and start moving backwards, and then stop and change direction once again to start moving forwards (shown in the picture below). You can see in the picture that this phenomenon is easily explained by a heliocentric universe (“heliocentric” meaning the sun is the center of the universe), but why Mars would follow such an unusual orbit (when, according to them, it was supposed to have a circular orbit) if the Earth was the center of the universe.
Aristarchus (310 BC – 230 BC) was the first to propose a “new” Sun centered cosmology.
One of the primary objections to the heliocentric model is that the stars display no parallax (the apparent shift of nearby stars on the sky due to the Earth’s motion around the Sun). However, Aristarchus believed that the stars were very distant and, thus, display parallax’s that are too small to be seen with the eye.
But there were few oppositions related to heliocentric model proposed by Aristarchus by the geocentric believer. Later on Aristarchus’ model was ruled out by the philosophers at the time for three reasons:
- Earth in orbit around Sun means that the Earth is in motion. Before the discovery of Newton’s law of motion, it was impossible to imagine motion without being able to `feel’ it. Clearly, no motion is detected
- If the Earth undergoes a circular orbit, then nearby stars would have a parallax. A parallax is an apparent shift in the position of nearby stars relative to distant stars. Of course, if all the stars are implanted on the crystal celestial sphere, then there is no parallax.
- Lastly, geocentric ideas seem more `natural’ to a philosopher. Earth at the center of the Universe is a very ego-centric idea, and has an aesthetic appeal
The Earth’s motion, as a simple matter of dynamics, was extremely perplexing to the medieval thinker. The size and mass of the Earth was approximately known since Eratosthenes had measured the circumference of the Earth (thus, the volume is known and one could simply multiple the volume with the mean density of rock to obtain a rough mass estimate). The force required to move the Earth seemed impossible to the average medieval natural philosopher
Later in 200 AD, Ptolemy created the mathematical model (Ptolemaic system) of geocentric theory and thus proving earth is at center of the universe. Ptolemy’s system is one of the first examples of scientists attempting to “save the phenomena”, to develop a combination of perfect circles to match the irregular motion of the planets, i.e., using concepts asserted by pure reason that match the observed phenomenon.
Ptolemy accepted the following order for celestial objects in the solar system: Earth (center), Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn. However, when the detailed observations of the planets in the skies is examined, the planets undergo motion which is impossible to explain in the geocentric model, a backward track for the outer planets. This behavior is called retrograde motion.
Ptolemy explained the phenomenon of retrograde motion by giving a theory that the planets move around the circumference of their own epicycles and moving around the earth at the same time.
Deferents were large circles and epicycles were small circles whose centers moved around the circumferences of the deferents. The Sun, Moon, and planets moved around the circumference of their own epicycles.
The motion of planets and sun were around center of distances (+) but their speed was decided by center of motion – Equant. This model looked complicated but was complete description of solar system and predicted motion of planets and sun in the sky and geocentric model was widely accepted for hundreds of years.
Later in 1500s, Copernicus reinvented the heliocentric model. Although, he was not the first astronomer who challenged Ptolemy system but he was the first one to successfully formulate it. The heliocentric model had a greater impact than simply an improvement to solve retrograde motion. By placing the Sun at the center of the Solar System, Copernicus forced a change in our worldview. According to Copernicus, a heliocentric planetary orbit is a combination of two circular motions. The first is motion of the planet around a small circular epicycle, and the second is the motion of the center of the epicycle around the sun on a circular deferent. Both motions are uniform, and in the same direction.
In fact, Copernicus was forced to use more epicycles than Ptolemy, i.e. a more complicated system of circles on circles.
In 1600s, Kepler derived the laws of planetary motion which corrects the problems of epicycle in heliocentric model by using elliptical orbit instead of using circle for orbit of planets.
In 1680, Newton developed the law of Gravitation, laws of accelerated motion, invented calculus (math tool), the 1st reflecting telescope and theory of light.
In 18-20th century, with discovery of the outer planets, Expansion of universe. Albert Einstein’s theory of relativity and discoveries in stellar and galactic areas helped us to understand the working model of universe never like before.
It was a great journey from looking up at the stars and noticing its motions to know the true nature of the universe. Still there are many mysteries which modern science is unable to explain, but the most important thing was and will be that we never stopped questioning and always looked for alternatives to answer those mysteries and some alternatives turned into theories.