Benefit of astronomy to ancient cultures

Transcription

1 Benefit of astronomy to ancient cultures Usefulness as a tool to predict the weather (seasons) Usefulness as a tool to tell time (sundials) Central Africa (6500 B.C.)

2 Alignments Many ancient cultures built structures to mark the seasons The structures were often aligned North-South, East- West Purpose - astronomical and social (rituals)? Stonehenge Sunlight pierces the center of carved spiral only at noon on summer solstice

3 Modern Science and the Greeks The Greeks are credited for developing the scientific method Instead of using superstition, they were the first to use logic and geometry to explain the motions of astronomical objects

4 Thales: the first astronomer First to ask What is the universe made of? Model: Universe is mostly water, and the Earth is a flat disk floating on the infinite ocean Even wrong ideas are good - they get people thinking and coming up with (hopefully!) better models

5 How do we know the Earth is round? Ships sailing out to sea disappear from the bottom up. Were the Earth flat they would just get smaller The edge of the Earth s shadow on the Moon is always part of a circular arc. Only a sphere always casts a circular shadow The altitude of the constellations changes as one moves north-south. This cannot happen if the Earth is flat (Anaximander)

6 Pythagoras: the Earth is a sphere within a celestial sphere. The reasoning was that the sphere is geometrically perfect Aristotle: the Earth s curved shadow during lunar eclipse proves that the Earth is spherical

7 Aristarchus: the Earth orbits the Sun 1) Distance to the Moon 2) The moon is closer to us than the Sun 3) The Size of the Earth 4) The distance of the Sun from the Earth

8 Brief trigonometry review -

9 1) Distance to the Moon Parallax - the apparent displacement of an object caused by the motion of the observer

10 2) How do we know the Moon is closer to us than the Sun? Solar eclipses I.e., the moon, being closer, blocks the light from the Sun

11 3) Size of the Earth Because the Sun is so far away, the shadow of the Earth is more-or-less cylindrical. Thus, the size of the Earth can be estimated from the size of its shadow The Earth s radius is 3 times that of the Moon s

12 Aristarchus (280 b.c.) used geometry When the moon is half illuminated, the Earth-Moon- Sun angle is 90 o 4) Size of the Sun By measuring then the Moon- Earth-Sun angle, the relative distances between the Moon and the Sun, and thus relative size can be calculated.

13 Size of the Sun (cont ) Aristarchus answers: the Sun is 20 times farther away than the Moon, and thus 20 times its size. Since the Earth is 3 times the size of the Moon, the Sun must be 7 times the Earth s size Real numbers: The Sun is 400 times the size of the Moon and 100 times the size of the Earth The Moon-Earth-Sun angle is extremely difficult to measure!

14 Size of the Sun (cont ) Punchline: Aristarchus was right in principle, but wrong in detail. He did, however, conclude that the Sun, the largest (and brightest) object in the known universe, must be at the center, and that the Earth must orbit the Sun

15 Eratosthenes Measured Earth s Size Syene: Sun passes directly overhead at summer solstice Alexander: Sun comes within 7 o of zenith at summer solstice Thus, Alexandria is 7 o in latitude to the north of Syene Syene-Alexander distance = 5,000 stadia Thus, The equivalent value of his estimate in km is 42,000 km. The actual circumference of the Earth is 40,000 km.

16 Apollonius and Hipparchus laid the foundation for the Ptolemaic, geocentric model of the Universe

17 Ptolemaic System Old belief Geocentric: Earth-centered (Ptolemy, 2 nd century A.D.) New belief Heliocentric: Sun-centered (Copernicus, A.D.)

18 Modeling of observational data helped to confirm the Heliocentric Model Tycho Brahe ( ): Made accurate measurements of the positions of stars & planets Johannes Kepler ( ): interpreted Tycho s data

19 Phases of Venus Old model. Problem Venus would always be in a crescent phase Galileo ( ): Made use of a telescope To discover: Phases of Venus 4 brightest moons of Jupiter

20