Ancient Greek Astronomy PDF

Aristotle (384 – 322 BCE) - A Greek philosopher and scientist Contributions - He believed the Earth is spherical. - He was a major proponent of the geocentric model. Scientific support - Aristotle argued that the earth is a round sphere, and as he observed the lunar eclips...

Aristotle (384 – 322 BCE) - A Greek philosopher and scientist Contributions - He believed the Earth is spherical. - He was a major proponent of the geocentric model. Scientific support - Aristotle argued that the earth is a round sphere, and as he observed the lunar eclipses, it showed a circular cast shadow of the earth to the moon. - In cultural beliefs, the Gods put the earth in the center of the universe because they see humans as high beings. - Aristotle believed there are 4 elements (earth, water, air, and fire), and the heaviest element is believed to be at the center of the universe and the other celestial bodies orbited around it. For earth as the earth element, he concluded the earth is in the center of the universe. Aristarchus of Samos (310 – 230 BCE) - A Greek mathematician and astronomer Contributions - He proposed the first known heliocentric model - He measured the sizes and distances of earth, sun, and moon - He stated the universe is vast and stars were distant suns Motivations - Aristarchus noted that the Sun is much larger than the Earth and the Moon, therefore he proposed the Sun must be in the center given it is much larger than Earth and Moon. - He supported his claims by measuring the size of the Earth, Sun, and Moon with geometric calculations using the phases of the moon. - He measured the Earth, Sun, and Moon to support his claims in his heliocentric model that the Sun is at the center of the universe Many astronomers opposed his arguments for several reasons. First, it is against their cultural beliefs as mentioned in Aristotle's claims. Also, they questioned if the Sun is at the center of the universe and the Earth orbited around the Sun the stars and other celestial objects must also show movement or shift of positions as the Earth orbited around the Sun. He supported his claims by applying the apparent stellar parallax to the movement of stars and celestial bodies. However, his observations of the stars in the first month and sixth months did not show any difference in location, thus, rejecting the heliocentric model. Parallax - The apparent shift in direction of an object as a result of the motion of the observer. Stellar parallax - The shift in the apparent direction of the star due to Earth's orbital motion. Eratosthenes of Cyrene (276 – 194 BCE) - A Greek astronomer, mathematician, geographer, and poet. Contributions - Measured the Earth's circumference Motivations - Eratosthenes wanted to make precise geographic knowledge and planned to make a map of the entire world. During the summer solstice or June 21, Eratosthenes observed sunlight striking a vertical well at noon in Syene, Egypt, indicating the Sun was directly overhead. In Alexandria, Eratosthenes observed the Sun's rays making an angle with the vertical equal to 1/50 of a circle (7°). He figured out the size of Earth by measuring the angle in Alexandria, which was 1/50 of Earth's circumference north of Syene. Alexandria was measured to be 5000 stadia north of Syene, and Eratosthenes concluded that Earth's circumference must be 50 x 5000, or 250,000 stadia. This information helped him determine the Earth's size and the location of the Sun's rays. Hipparchus of Nicaea (190 – 125 BCE) - A Greek astronomer, geographer, and mathematician. Contributions - He pioneered the catalog of stars - He proposed the apparent magnitude system of stars - He discovered the precession of the Earth When the Roman Republic was expanding its influence throughout the Mediterranean region. There he measured, as accurately as possible, the positions of objects in the sky, compiling a pioneering star catalog with about 850 entries. He designated celestial coordinates for each star, specifying its position in the sky, just as we specify the position of a point on Earth by giving its latitude and longitude. He also divided the stars into apparent magnitudes according to their apparent brightness. He called the brightest ones "stars of the first magnitude"; the next brightest group, "stars of the second magnitude"; and so forth. This rather arbitrary system, in modified form, remains in use today (although it is less and less useful for professional astronomers). By observing the stars and comparing his data with older observations, Hipparchus made one of his most remarkable discoveries: the position in the sky of the north celestial pole had altered over the previous century and a half. Hipparchus deduced correctly that this had happened not only during the period covered by his observations, but was happening all the time: the direction around which the sky appears to rotate changes slowly but continuously. Recall from the section on celestial poles and the celestial equator that the north celestial pole is just the projection of Earth's North Pole into the sky. If the north celestial pole is wobbling around, then Earth itself must be doing the wobbling. Today, we understand that the direction in which Earth's axis points does indeed change slowly but regularly motion we call precession. Claudius Ptolemy (100 – 170 AD) - A Greek astronomer, geographer, and mathematician. Contributions - The Almagest (The Greatest) - The geocentric model - He explained the retrograde motion The last great astronomer of the Roman era was Claudius Ptolemy (or Ptolemaeus), who flourished in Alexandria in about the year 140. He wrote a mammoth compilation of astronomical knowledge, which today is called by its Arabic name, Almagest (meaning "The Greatest"). Ptolemy's most important contribution was a geometric representation of the solar system that predicted the positions of the planets for any desired date and time. Ptolemy supplemented this material with new observations of his own and produced a cosmological model that endured more than a thousand years, until the time of Copernicus. Also, to solve the apparent retrograde motion of planets or the temporary apparent westward motion of a planet as Earth swings between it and the Sun is called retrograde motion, Ptolemy solved the problem of explaining the observed motions of planets by having each planet revolve in a small orbit called an epicycle. The epicycle's center then revolved around Earth in a circle called a deferent. However, we shall see in Orbits and Gravity that the planets, like Earth, travel about the Sun in orbits that are ellipses, not circles. Their actual behavior cannot be accurately represented by a uniform circular motion scheme. To match the observed motions of the planets, Ptolemy had to center the deferent circles, not on Earth, but at points some distance from Earth. In addition, he introduced uniform circular motion around yet another axis, called the equant point.