1. NATS 1311 - From the Cosmos to Earth Our first exam will be Tuesday, September 23 at the regular class time. We will have a review today at 1:00 PM in the room next to where the regular lab is held. The exams will cover everything that we have covered in class up to the end of class today.

2. NATS 1311 - From the Cosmos to Earth Early Astronomy Most ancient societies recorded astronomical observations for religious or practical purposes - enabled them to keep track of time and seasons - crucial for agricultural societies. Africans used the Moon to predict the annual rainfall pattern - orientation of crescent “horns” relative to the horizon told time of year and thus expected rainfall. Why does the horn orientation vary with seasons?

3. NATS 1311 - From the Cosmos to Earth Over the course of the year, the movement of the Moon slowly changes its place relative to the horizon (because the Earth is gradually pointing further either away from or more toward the Moon's orbit around the Earth due to its tilt ). During the northern hemisphere summer, when the northern hemisphere points in the direction of the Sun, the night side of the Earth is tipped away from the Moon's orbit. This makes the Moon's path appear closer to the southern horizon. During the northern hemisphere winter, the northern hemisphere points away from the Sun and night observers are tipped toward the Moon's orbit. As a result, during the winter, the Moon's path is almost vertical to the western horizon. This seasonal effect also causes the Moon to appear higher in the night sky during the winter and lower in the night sky during the summer and changes the look of the waxing crescent moon.

4. NATS 1311 - From the Cosmos to Earth Many ancient cultures built structures to mark the seasons. Stonehenge - Sun rose directly over the Heel Stone on the summer solstice The Sun Dagger - New Mexico - built by Anasazi to mark summer solstice.

5. NATS 1311 - From the Cosmos to Earth Greek Science First people to propose explanations for motions of astronomical objects that relied on logic and geometry instead of supernatural forces - constructed models of nature. Thales (c. 624 - 546 B.C.) - Earth a flat disc floating on a giant ocean of water Anaximander (c. 610 - 546 B.C.) - Earth a cylinder that floats in empty space surrounded by a sphere of stars (the Celestial Sphere) with two separate rings that the Sun and Moon travel along Pythagoras (c. 560 - 480 B.C.) - things in nature governed by geometrical/mathematical relationships, i.e. musical notes - length of plucked strings - underlying rules of universe mathematical - most perfect shape is a sphere - Earth a sphere - all other heavenly bodies divine perfect spheres moving in perfect circles - create “music of the spheres”

6. NATS 1311 - From the Cosmos to Earth Plato (c. 428 - 348 B.C.) - all heavenly objects move in perfect circles at constant speed - must reside on huge circles encircling the Earth - contrary to observations - i.e. retrograde motion of planets Eudoxus (c. 400 - 347 B.C.) - heavenly spheres - Sun, Moon, and planets all attached to own sphere nested within several other spheres - varying rotation speeds and axes - explained many of the observed motions

7. NATS 1311 - From the Cosmos to Earth Aristotle (c. 384 - 322 B.C.) - universe can be divided in 2 parts: 1. Imperfect, changeable Earth, 2. Perfect Heavens (described by spheres) - expanded Eudoxus’ model of 27 nested spheres to 55 nested spheres - spheres transparent and connected like gears on giant machine - Earth’s position at center consequence of gravity - heavier things - rock, dirt, water - pulled to center forming Earth - lighter things floated towards the heavens - could not be moving or objects such at birds, falling stones, or clouds would be left behind as Earth moved along its way - Earth is round - shadow of Earth on Moon always curved - no stellar parallax observed - stars at same distance in heavens - celestial sphere

8. NATS 1311 - From the Cosmos to Earth Stellar Parallax Parallax Animation parallax - the change of angular position of two stationary points relative to each other as seen by an observer, due to the motion of said observer, I.e., the apparent shift of an object against a background due to a change in observer position.

9. NATS 1311 - From the Cosmos to Earth Stellar Parallax

10. NATS 1311 - From the Cosmos to Earth Heracleides (c. 388 - 315 B.C.) - Earth rotates - not all heavenly bodies circle Earth - observed that Mercury and Venus are always close to Sun in sky - must orbit the Sun Aristarchus (c. 310 - 230 B.C.) - proposed Sun-centered system - suggested that absence of stellar parallax because stars very far away - estimated relative size of Earth and moon from shadow on Moon during lunar eclipse - about 1/3 - Sun must be larger than the Earth - based on geometric argument - measured angle between Moon and Sun at first and third quarters - estimated Sun about 7 times larger in diameter (actually about 100 times)

11. NATS 1311 - From the Cosmos to Earth Eratosthenes (c. 276 - 196 B.C.) - accurately measured circumference of Earth At noon on the summer solstice, the Sun is directly overhead at Syene - at Alexandria comes within 7 degrees of zenith - so 7 degrees of latitude must separate Syene from Alexandria - or 7/360 of Earth’s circumference. Estimated Earth’s circumference to be about 42,000 km (actually 40,000 km)

12. NATS 1311 - From the Cosmos to Earth Hipparchus (c. 190 - 120 B.C.) - developed a star catalog - method to predict positions of sun and moon - discovered precession of earth’s axis of rotation Ptolemy (c. A.D. 100 - 170) - combined all available astronomical information into 13 volume work, the Almagest - a geometrical model of solar system with Earth in center that predicted positions of Sun, Moon and planets - used for 1,500 years - used idea of Apollonius (c. 240 - 190 B.C.) that planets move on small circle that turns upon larger circle around the Earth explained retrograde motion of planets - fully developed by Hipparchus - accurately predicted planetary positions within a few degrees of arc

13. NATS 1311 - From the Cosmos to Earth Retrograde Motion of Planets Apparent retrograde motion - reversal of planetary motion through the zodiac from eastward to westward Apparent retrograde motion of Jupiter

14. NATS 1311 - From the Cosmos to Earth Ptolemaic Model Deferent - main circle of planet path Epicycle - small circles on deferent along which planet moved

15. NATS 1311 - From the Cosmos to Earth Ptolemaic Model Animation

16. NATS 1311 - From the Cosmos to Earth Ptolemy's model of the motions of the Sun, Moon, and planets around Earth.

17. NATS 1311 - From the Cosmos to Earth Mars Retrograde Motion Earth’s orbital speed is faster than Mars’. When Earth catches up with Mars, Mars appears to move backwards against the background of the stars

18. NATS 1311 - From the Cosmos to Earth Mars Retrograde Motion Animation

19. NATS 1311 - From the Cosmos to Earth Nicholas Copernicus (1473-1543) - wanted better way to predict planetary positions - adopted Sun-centered planetary system suggested by Aristarchus - worked out simple geometric relationships that allowed calculation of planet’s true orbital period and distance from Sun in terms of Earth-Sun distance (AU) - still believed that heavenly motion must still be perfect circles - had to add circles upon circles as in the Ptolemaic model - did not make substantially better predictions and was not accepted for another 50 years Heliocentric World View - The Copernican Revolution