The Science of Astronomy Ancient Civilizations Ancient Greek European Renaissance Modern Science

Ancient Astronomical Knowledge Many of the surviving ancient structures have obvious astronomical purpose. These ancient structures clearly demonstrated that all ancient civilizations developed extensive knowledge of the celestial objects…most likely because of the need to predict the seasons due to the development of agriculture. Astronomical knowledge are also very useful tool for navigation. Usually knowledge of mathematics and geometry were usually developed at the same time.

Marking the Seasons In Hawaii, the first rise of the star cluster Pleiades is used to mark the beginning of the year. Sun rises over the heel stone at summer solstice Sun Dagger at summer solstics at Chaco Canyon, New Mexico

Navigating the World If you are sailing in the open sea from Tahiti to Hawaii in one of those voyaging canoe, how can you tell where you are at on Earth? Longitude? Latitude? A Polynesian navigational instrument

Ancient Greek Science The Ptolemaic Model of the Universe Earth is at the center of the universe All the objects move in perfect circular orbit Planets moves in small circles upon larger circles to explain the retrograde motion.

Minority Opinion Pythagoras (582–500 BC) The astronomy of the Pythagoreans marked an important advance in ancient scientific thought, for they were the first to consider the earth as a globe revolving with the other planets around a central fire. Aristarchus (310-230BC) Aristachus sought to explain the apparent retrograde motion of planet with a Sun-centered model.

Missed Opportunity Although Pythagoras and Aristarchus proposed heliocentric model of the cosmos, their ideas were not widely accepted by their contemporaries, probably because: Aristarchus model could not predict the retrograde motion any better than Ptolemaic model. If the Earth is revolving around the Sun, the stellar parallax must exist, but the ancient Greeks were not able to detect any stellar parallax. The ancient Greeks believed that the heavens must be geometrically perfect: heavenly objects must move in perfect circles and must reside on huge, perfect sphere encircling Earth.

What would you believe if you lived around 200 B.C. in Greek?

Although the majority of ancient Greek philosophers arrived at the wrong conclusion about the model of the universe, they did so based on sound logical reasoning processes, good (albeit crude in today’s standard) observational data, (no stellar parallax, apparent retrograde motion of planets), and good modeling efforts (Ptolemaic geocentric model and Aristarchus’s heliocentric model). They followed a very rigorous scientific method, and their failure was not the failure of the scientific method. It was due to the limited technology. They couldn’t have done better! The ancient Greeks were the first to rely on logical thinking to explain the natural phenomena. This is the same principle that was followed by the scientist of the 15th and 16th century to proof the validity of the heliocentric model of the solar system, and is the foundation of modern science.

The Dark Ages During the dark ages of Europe, the rest of the world continue to develop. But the knowledge of the Greeks were preserved in the city of Alexandria, in Egypt.

The Copernican Revolution Copernicus (1473-1543) revived the idea of a Sun-centered solar system model: However, like Aristarchus, Copernicus’s model was not accurate enough to convince many people. Tycho Brahe (1546-1601) made accurate (arc minutes) naked-eye measurement of planet motion Tycho believed that planets must circle the Sun, but his failure to detect stellar parallax forced him to put the Earth at the center of the system, with the Sun orbiting the Earth, and the planets orbit the Sun. Johannes Kepler (1571-1630) were able to make accurate prediction with his heliocentric model of planetary orbits, agreeing with Tyco’s observation Kepler’s initial failure (using prefect circular orbits) to match Tyco’s observation led him to adopt a model with elliptical planetary orbit. Galileo Galilei’s (1564-1642) telescopic observations helped solidify the heliocentric view of the solar system.

Kepler’s Reformation In attempting to explain Tycho’s observation of the planetary motion, Kepler concluded that planets do not orbit in perfect circles. Instead, the planets travel around the Sun in elliptical orbit. The ellipse The distance from one focus to a point on the ellipse to another focus is a constant

Kepler’s First Law The orbit of each planet about the Sun is an ellipse with the Sun at one focus.

Kepler’s Second Law As a planet moves around its orbit, it sweeps out equal areas in equal time.

Interesting Properties of Elliptical Orbits Kepler’s second law states that as a planet moves around its orbit, it sweeps out equal areas in equal time. It also means that the orbital speed is not constant like in a circular orbit. It is depends on its distance from the Sun. It is slower when it is further away for the Sun It is faster when it is closer to the Sun. Click image to start animation

Kepler’s Third Law More distant planets move more slowly in their orbit: The planets orbital period is related to the average distance to the Sun (Orbital period in years ) 2 = (average distance in AU) 3 or p 2 = a 3 where p is the orbital period measured in year, and a is the average from the Sun to the planet in AU.

A Sun-centered solar system model with the planets moving in elliptical orbits allowed Kepler to make accurate predictions of the planet’s positions in the sky. So, now the heliocentric view has a better model than geocentric view. But there were other questions/objections to the heliocentric model that need to be answered…

The Challenges Major Objections to the Sun-centered solar system model: If Earth is moving, then objects such as birds, falling stones, and clouds would be left behind as Earth moved along its path. The heavens must be perfect and unchanging. If the Earth is orbiting the Sun, then stellar parallax must be detectable. These objections must be addressed before the Sun-centered model can be accepted. Galileo’s observation with the new telescope helped to answer these questions…

Galileo’s Telescopic Observations Venus goes through the phases like the Moon Venus must be orbiting the Sun, not the Earth! This implies that not everything orbits Earth! The Four Moons of Jupiter Satellites can follow a moving planets This proofs that not everything orbits Earth! Sun has sunspots, and Moon has mountains and valley The heaven is not perfect!

Answering the Critics If Earth is moving, then objects such as birds, falling stones, and clouds would be left behind as Earth moved along its path. Galileo showed that a moving object remains in motion unless a force acts to stop it. Newton’s first law of motion Galileo saw through his telescope that there are four Moons orbiting Jupiter, not Earth. Objects can orbit a planet, thus the Moon can orbit the Earth without been left behind. 2. The heavens must be perfect and unchanging Tycho’s observation of supernova and comets Heaven can be changing. Galileo’s telescope showed that the Sun has sunspots, and the Moon has mountain and valleys Heaven can be imperfect. 3. If Earth is orbiting the Sun, then why couldn’t we observe any stellar parallax? Stars are too far away. We do measure it today!

In Hawaii, the linear speed of Earth’s rotation is about 1,566 km/hr = 0.435 km/sec, or 435 m/sec. If I drop a stone from a height of 1.25 meter above the ground, it is going to take approximately 0.5 second to reach the ground. The ground moves 217 m during the time it takes the stone to fall to the ground. How comes the stone does not get left behind? While we hold the ball before releasing it, the ball is also traveling with 1,566 km/hr. It is traveling with the same speed as the ground does while it is falling to the ground, because no force was applied to it to stop its motion in this direction. So, it does not got left behind! Newton’s First Law of Motion! Chapter 4

Summary The ancient Greeks were the first to use logical scientific method to try to explain the nature. The same scientific method was used by the scientists of the 15th and 16th century to finally establish the heliocentric model of the solar system. Tyco obtained very precise observations of planetary motion. Kepler was the first to device an accurate planetary model capable of predicting the position of the planets with great accuracy. Galileo’s telescopic observation helped to disprove many of the ancient believes, and firmly established the sun-centered model of the solar system .  It is interesting to note that up to this point, there were still no discussions on why the planets should move in elliptical orbits, or on what is keeping the planets from running away from the Sun?

Measures of Angles One complete circle can be divided into 360 degrees. One degree is divided into 60 ‘arc minutes’. One arc minutes is further divided into 60 ‘arc seconds’. One complete circle has 1,296,000 arc seconds.  Back