Early History of Astronomy What is astronomy? Astronomy is the science that studies matter in outer space, especially the positions, dimensions, distribution, motion, composition, energy, and evolution of celestial bodies and phenomena. What is astrology? Astrology is the study of the movements and relative positions of celestial bodies interpreted as having an influence on human affairs. Until the ancient Greeks, astronomy was the same as astrology. Babylonian Astronomers • 1200 BC - 60 BC • Compiled star catalogs • First to divide circle into 360 degrees • Made calculations of daylength changes, planet motions and lunar eclipses Chinese Astronomers • 600 BC onward • Compiled star catalogs • Used for timekeeping • Observed and predicted comets and eclipses • First to record a ‘guest star’, a supernova, in 185 AD. Greek Astronomy • In the 4th century BC, Greeks treated astronomy as a branch of mathematics and developed geometric models to explain the motion of the known planets—Mercury, Venus, Mars and Jupiter. • Later Greek astronomers used philosophical arguments and reason to explain natural phenomena, while also using some observational data to support their explanations. • Some Greek astronomers believed that the sun, moon, and the known planets followed a geocentric model, others a heliocentric model. One of the objections to the heliocentric model was that, if the Earth was moving, why didn’t it leave the air and the moon behind? Aristotle • Greek mathematician and philosopher • 384 BC – 322 BC • He taught that the universe was a complex system of 55 concentric spheres with a stationary Earth at the center • Taught that the heavens were immutable (unchanging), made of a different (perfect) material than was Earth. Aristarchus of Stamos • Greek astronomer and mathematician • 310 BC - 230 BC • First heliocentric model of the solar system • Understood that the moon was much closer than the sun • Had the right geometric method to estimate the distance to the sun but had poor data so his calculation was incorrect Eratosthenes • Greek mathematician, head librarian at the Great Library in Alexandria • 276 BC – 195 BC • Calculated the circumference of Earth, and therefore its size, very accurately • Also measured the tilt of Earth’s axis • Founder of geography; developed system of latitude and longitude and created first map of the world • May have measured the distance to the sun • May have invented the leap day Eratosthenes (cont.) Measuring the sun’s angle at noon on the solstice at Alexandria (angle of a post’s shadow) and knowing that the sun’s angle at Syene (due north) was 0°, he calculated that this angle as 1/50th of a circle (Greeks knew Earth was a sphere). Knowing the distance between Alexandria and Syene, he multiplied by 50 to get the Earth’s circumference! Hipparchus of Greece • c. 190 BC – c. 120 BC • Mathematician, developed trigonometry • Discovered the precession of Earth’s axis (26,000 year cycle), leading to changes in which star is the pole star and dates of the equinoxes. • He calculated the distance to the moon and a method for predicting solar eclipses Ptolemy • Greek mathematician, astronomer, geographer • c. AD 90 to c. AD 168 • Wrote a treatise on astronomy that was translated into many languages • Using Aristotle’s model, he developed an epicycle model of a geocentric solar system used until the Middle Ages Ptolemy’s Model • Geocentric • Each planet moved in a circle (epicycle) along a circular orbit about the Earth • Order was: moon, Mercury, Venus, Sun, Mars, Jupiter and Saturn Retrograde Motion Ptolemy’s model of epicycles explained the retrograde motion of a planet, when it appears to move backward (westward) with respect to the stars. This apparent motion is caused by the different orbital speeds of Earth and the other planet. Let’s take a break: Write your answers on a piece of paper • What is the difference between astrology and astronomy? • Why don’t scientists accept astrological beliefs as scientific facts? • What’s the difference between a geocentric and a heliocentric model? • What is retrograde motion? • Why does Mars appear to have retrograde motion? Western Europe in the 4th through 12th centuries • After the Roman Empire fell, scientific studies declined. Almost no one could read ancient Greek, and the only writings available were simplified summaries. • In the 7th Century, an English monk named Bede wrote a text about how to compute the proper date of Easter. This was used until the 12th Century. • Under the Emperor Charlemagne in the 8th Century, there was a revival in learning and ancient texts were studied. • By the 10th Century word got around (after knights returned from Crusades) that the Arabic world had knowledge about astronomy. Western European scholars traveled to Spain and Sicily and translated the writings they found there from Greek and Arabic into Latin. • The strong control of the RC Church meant, however, that these writings had to be examined to see whether they agreed with Church theology. If they did, they were ‘reinterpreted’ through the Church’s viewpoint. Aristotle’s Model Aristotle’s and Ptolemy’s model of the universe was reintroduced to scholars, given the Church’s stamp of approval after tweaking it to agree with Church theology, and therefore accepted as completely true and supported by scripture: 1. The stationary Earth is at the center of the universe. 2. The planets and other celestial bodies travel in perfect circles around it. 3. The heavens are made of a perfect, unchanging substance different from substances on Earth. The Birth of Modern Astronomy • In a break from these philosophical and religious views, in the 16th Century, scientists began to develop more modern ideas about the universe. • They investigated and discovered the natural laws that govern the universe. • Major scientists who expanded astronomy were Copernicus, Brahe, Kepler, Galileo, and Sir Isaac Newton. Nicolaus Copernicus • Polish astronomer • 1473 - 1543 • Near death, finally published a book explaining heliocentric view of universe • Earth was a planet • Circular orbits of planets about the sun • Understood that the stars are very, very far away Tycho Brahe • Danish astronomer • 1546 - 1601 • Made extremely accurate stellar and solar system observations, esp. Mars • Proposed a model where sun and moon orbited Earth but the other planets orbited the sun • His observation of a supernova in 1572 gave evidence that the heavens could change Convinced that progress in astronomy meant more accurate observations, Brahe designed many sighting instruments like this wall quadrant • • He made nightly observations for many years and kept detailed records • Brahe ran his own printing press • His observatory was visited by many scholars Giordano Bruno (DON’T WRITE) • Italian friar 1548-1600 • Believed the sun was a star • Believed that there were Earth-like, inhabited planets orbiting other stars • For these and other beliefs, the Church convicted him of heresy and he was burned at the stake Johannes Kepler • German astronomer and physicist • 1571-1630 • Father of modern astrophysics, was the first to unite astronomy (math) and physics (natural world) • Originally an assistant to Brahe • Developed 3 laws of planetary motion using observations of Brahe • Supported the heliocentric model but with elliptical orbits instead of circles! Kepler’s Laws of Planetary Motion First Law: The orbit of every planet is an ellipse with the Sun at one focus. An ellipse is an OVAL. Kepler’s Laws of Planetary Motion Second Law: A line joining a planet and the Sun sweeps out equal areas during equal intervals of time. Planets revolve around the sun at a speed that varies depending on where it is in its orbit. Kepler’s Laws of Planetary Motion Third Law: There is a proportional relationship between a planet’s orbital period and its distance to the sun. Orbital period is the time it takes to make one full orbit around the sun. Astronomical Unit • A convenient way of measuring distances in the solar system is relative to the Earthsun distance. • An astronomical unit (AU) is the average distance between Earth and the sun; it is about 150 million kilometers. Galileo Galilei • Italian physicist, astronomer, mathematician • 1564 -1642 • Father of modern observational astronomy • Refined the refracting telescope into an instrument to observe the heavens Galileo Galilei’s Observations 1. Saw that planets were disks rather than points of light 2. Saw craters on the moon 3. Discovered 4 moons of Jupiter 4. Observed sunspots 5. Saw the Milky Way is made of stars not just nebulous (made of gas) Galileo observed that the phases of Venus as observed from Earth support the Copernican heliocentric model. Geocentric model Heliocentric model At long last, data to disprove one of the two conflicting models, geocentric and heliocentric! • Galileo defended his views in "Dialogue Concerning the Two Chief World Systems“ in which he said the Copernican model was ‘hypothetically’ true. • The Church felt he was attacking the pope, so Galileo was tried by the Inquisition, found “vehemently suspect of heresy,” and forced to recant. • Galileo spent the rest of his life under house arrest. Sir Isaac Newton • English physicist • 1643 -1727 • First to understand that planetary motion is due to gravity—same laws in heaven as on Earth • Explained why Kepler’s Laws worked by formulating the Universal Law of Gravitation • Explained that gravity between two bodies depends on both masses and the square of the distance between them. Let’s fire a cannonball from a cannon that is parallel to the ground (and neglect air drag). What happens if we increase the muzzle speed? Is there a speed that would have the cannonball NOT fall to the ground but stay in orbit? Yes, that speed is the minimum orbital speed at that particular height for the cannonball to achieve orbit around the Earth and stay there. As it orbits, a planet feels a pull directly toward the sun (blue arrow) while it is travelling along its path. Planetary Motion If somehow the sun could be turned off in an instant, the planet would fly off in the direction it had been This means the orbiting planet going (red arrow) at continually falls towards the Sun that instant. but also continually misses!