Astronomy 311: Historical Astronomy

Transcription

1 Astronomy 311: Historical Astronomy Greek scientific thought started with a school of Philosphers in Ionia in the sixth century BC. The most prominent was Thales. Examples of this early thought was that Earth floats in water (Thales), a periodic universe (Heraclitus, BC), comets are formed by planets colliding, the Earth is solid and flat, that eclipses are caused by the shadow of the earth and other bodies and that eclipses of the Sun are caused by the Moon ( Anaxagoras: BC), The Classical Greeks After 450BC, the Greeks began to keep astronomical and meteorolgoical diaries (parapegmata). Two astronomers in particular Meton and Euctemon. Euctemon gave the lengths of the seasons as 90, 90, 92 and 93 days. Notice the idea of measurement was already there. Equinox: those times in the Earth s orbit around the Sun when the Sun shines equally on both hemispheres i.e. 12 hours day/night for everyone. Directly overhead at noon at the equator. Sets and rises exactly precisely due west and east. Callipus, about 330BC found more accurate numbers of 92, 89, 90, 94 days. Notice the Sun s speed is different in different quadrants. This is Kepler s second law - conservation of angular momentum. Eudoxus ( BC) placed astronomical objects like the stars and planets on spheres which rotated and linked so as to try to reproduce salient features of the motion of the planetsa He assumed all objects a fixed distance from us. Know of Eudoxus through Aristotle ( BCa and Simplicius (sixth century AD)). Constructed spheres to deal with irregular motion of planets/objects around the ecliptic. For planets, this is rethrograde motion: planets generally appear to move slowly eastward through the zodiac but occasionally some planets appear to move westward relative to the stars. Eudoxus gave each planet 4 spheres: 2 to reproduce regular motion across the eciptic and two to reproduce retrograde motion. System modified by Callipus ( BC). The theory does not represent the observed motion very well - only the qualitative features. 1

2 Aristotle ( BC) Eudoxus and Callipus used the sphere system merely as a mathematical construct. Did not believe them to be real. But Aristotle believed them to be real. Aristotle gave several reasons for believing the Earth to be spherical: the edge of the dark part of the moon in an eclipse is always convex plus others. Aristotle thought the natural tendency of heavy bodies was to fall to the center of the Earth. This should apply to the Earth also and since it doesn not fall, it must be at the center. Ancient seafarers knew that a ship a long way away was below sea-level with the same effect in all directions (only possible on a sphere). Being hull-down on the horizon. This argument appears in writing around 10BC by Strabo. Aristotle quoted the radius of the Earth to be stades - dont know how he got this. Eratosthenes ( BC) made the first well documented measurement of the circumference of the Earth. He knew that at Syene (Egypt) the sun was overhead at midday in midsummer: it shone straight down a well. He measured the angle between the zenith and the Sun at midday in Alexandria (due north of Syene): one fiftieth of a complete revolution. He quoted a difference of 5000 stades as the distance between Syene and Alexandria. So the circumference is = stades. Difficult to know how accurate this estimate was. Posidonius (135-50BC) used a similar method as Eratosthenes. Generally these estimates were underestimates for the size of the Earth. Thats why Columbus, looking for India, called the inhabitatnts of the land he found Indians because he believed he had already got to India going on classical Greek estimates of the Earth s circumference. Arsitotle thought the Earth was at rest. Herclides was the first person to think the Earth is in the center and moves in a circle while the heavans are at rest. Aristarchus ( BC) Though the Earth revolved in a circular orbit around the Earth. Wrote a book On the sizes and distances of the Sun and Moon. 2

3 He assumed that the moon gets its light from the Sun, the Earth is at the center of the Moon s sphere, when the moon appears halved, the great circle which separates the lit part from the unlit part points to our eye, when the moon appears to be halved, its angular distance from the Sun is 1/30 of a right angle less than a right angle, the width of the Earth s shadow is twice the width of the moon, the apparent size of the moon is 1/15 os a sign of the zodiac. Got estimates for the ratio of the Sun-Earth to Earth-Moon distance, Sun/Moon relative sizes and Moon/Earth relative sizes. Right math, but wrong numbers. Also thought that stars were distant and showed no parallax because they were distant. Hipparchus combined accurate data with clever geometrical constructions. Connected to Babylonian Astronomy. Born in Nicaea in Bithynia (northern Turkey) - spent much of his life in Rhodes. One of most influential of ancient astronomers. Know about him through the Almagest written by Ptolemy. Made an instrument called a dioptra to measure the apparent diameter of the Sun. Used numerical data based on observations. Developed one of the first star catalogues - recorded observations span the years BC. Measured the length of the year by timing solstices and equinoxes. How do you measure the passage of time? Sidereal day: time between successive crossings of the meridian of a given star: 23 hrs 56 minutes. Solar day: time between successive crossings of the meridian by the Sun: 24 hours. Synodic month: time between new moons:29.5 days. Sidereal month: moon returns to its position relative to the distant background of stars:27.3 days. Time between successive spring equinoxes is the tropical year: days - 20 minutes. Sidereal year: time for the Earth to complete one rotation relative to the fixed stars: days. Modern calendar based on the tropical year - hence leap year. Earth s precession means tropical year is shorter than sidereal year. 3

4 earth s rotation around Sun means synodic month and solar day longer than sidereal month and day. Nutation is the variation of the rate of precession. Speed of Earth around Sun varies - Sun moves around ecliptic irregularly. Reaches its maximum speed at the same time of year. The time from least speed to average speed is less than the time from average speed to greatest speed. Felt he had to account for this only with uniform circular motions. Did this not by combining several circular motions as Eudoxus had done but by saying that the center of the Sun s orbit is not the Earth but some other point - this was pretty effective. Formulated a similar theory for the motion of the Moon, except that the center of the eccentric orbit is not fixed but also moves in a circle around Earth. Hipparchus is credited with the precession of the equinoxes: motion of the equinoxes relative to the background stars in the zodiac. This is about 50.3 arc seconds/year or about 1 degree every 71.6 years. Hipparchus found the star 6 degrees west of the autumn equinox. But 150 years earlier Timocharis had found Spica 8 degrees wets of the Ptolemy Lived and worked in Alexandria and wrote a substantial compendium of early Astronomy, the Almagest, a corruption of the Arabic for Megiste syntaxisa Alexandria was the place to be for culture and knowledge at this time. Dates unknown but perhaps AD. Almagest was perhaps the first synthesis of astronomy into a single work. By the fourth century AD this was the definitive work on mathematical astronomy and dominated astronomy till the 1500 s. Thought that the Earth was a sphere: the further west the earlier the recorded time for an eclipse. So the Sun rises later for obsservers further west. Observers further north cannot see the southerly stars. Hull down phenomenon. But thought the Earth was at the center - Aristotlean reasoning. Synthesized and extended Hipparchus system of epicycles to explain the motions of the Moon, Sun and planets. Geocentric. At least 80 epicycles to explain the motions of the Sun, Moon and the five planets known at the time - the Ptolemaic system. 4

5 Predicts positions of planets accurately for naked eye observations. Aryabhata Indian astronomy is a substantial subject with many treatises written. Aryabhata, born in AD 476, lived after Ptolemy but there is evidence that his astronomy developed out of pre-ptolemaic Greek ideas. The Aryabhatiya is concisley written in 121 stanzas of Sanksrit verse. 5