Lecture 3 -- Backyard Astronomy, from Ancient to Modern Announcemnets: HW #1 revised Spend time today on math & numbers -- will be important part of class Observations of early astronomers What did they see? Why? Intro to Motions of Sky Summer vs. winter constellations --------------- 4/3 pi r^3 -- how to do this on calculator 4 pi r^2 = 6.15725218 E 18 (7 EE 8) (X^2) * 4 * pi = How to do calcultor math what is a unit mm^3 vs. regular mm Units are reviewd in Appendix (A4) of book! all distances can be measured in ly and in cm! Numbers: used for counting (e.g., groundhogs) Units: special, standard way of counting and comparing things like weight, distance, time. We _could_ measure everything by length of a groundhog -- but what if we get a short groundhog? Or if we go to amazon: count by anacondas. But need to convert between them. `meter' is a standard unit, defined (for a long time) in paris. Everyone agrees on it. Common Units of Time ------ s yr build up units: microsec megayr (also week, day, etc.) 10^(-9) billionth, nano ('Nanobacteria on Mars are ~ 1 nm (nanometer) across') 10^(-6) millionth, micro 10^(-3) thousandth, milli 10^3 Thousand, Kilo 10^6 Million, Mega 10^9 Billion, Giga ('Universe is about 15 giga yr old') 10^12 Trillion Common Units of Size ----- cm m km ly ls (not common, but legal) lh (not common, but legal) pc (3.26 ly) Common Units of Volume ----- cm^3 m^3 km^3 ly^3 Velocity: Any combination of Size / Time ------------------------- Mi/hr cm/s Ly/s Other popular units, for measuring: ------------------- Angles (degrees) Temperature (K, F, C) Power (Watts) All of them are good! We can measure distance across room in meters, cm, or light years! room: 10 m 10 m * (100 cm / 1 m) = 10^3 cm 10 m * (100 cm / 1 m) (1 ly / 10^18 cm) = 10^-15 ly grains of sand on beach how much is a trillion? 10^10 v. 10^20! factor of 2? No factor of 10 billion! Grains of sand in bucket: guesses were 10^4, 10^10, 10,735, 10^6 r_grain = 1 mm (1 cm / 10 mm) = 0.1 cm V_grain = 4/3 pi r^3 = 4/3 pi (0.1 cm)^3 = 0.00418879 = 4 10^-3 !!! V_cup = 10cm x 5cm x 5cm = 250 cm^3 # of grains = v_cup / v_grain = 2.5 10^2 cm^3/ 4 10^-3 cm^3 = 6.2d4 ~ 1d5 ~ 62,000 Volume of earth's beaches: 10,000 km per continent, 10 contintnets, 100 m, 1 m V_continent = (10^4 km)(1000 m / 1 km)(100 cm / 1 m) (100 m)(100 cm / 1 m) (1 m)(100 cm / 1 m) = 1d15 cm^3 For whole earth, 10 continents: 1d16 cm^3 # of grains = v_beaches / v_grain = 1d16 cm^3 / 4d-3 cm^3 = 2.5 10^18 grains! Handout: worksheet over time & distance scales, in groups --------------------------------------------------------- History of Astronomy -------------------- Earliest cultures: Africans, 6500 bc Useful for practical reasons: crops, etc. Also useful for demons, and for security, ceremonies, fun, exploration, navigation, etc. Navigation: polynesians Follow patterns, develop generalizations (talked about on first day) What did they see? ------------------ Stars rising in E, Setting in W Sun rising in E, Setting in W Patterns of stars slowly shifting over a year, then coming back to start Position of sun changing: low in winter, high in summer Length of days changing Some patterns -- like big dipper -- up all year Some -- like Orion -- up in winter, not summer Moon -- comes up during day & night, changes shape, rises & sets All the stars look the same -- except for a few planets, which take `random' paths thru the sky Venus -- moves fast, always up in evening or morning, but not at night Saturn, Jupiter -- move slow, can be up at any time of night All planets -- mostly move forward, but sometimes move backwards Occasionally, long-tailed comet moves through the sky Or, shooting stars! Ptolemy (AD 100-170): Proposed set of spheres: Earth at center, then sun, moon, sun, etc. Proposed that stars were a long way away. Basically, this works! Worked for 1500 y. Solved problems of a) why Stars rise & set b) Why moon rises & sets, and why we have moon phases c) Why we have a North star d) Retrograde motions of the planets In Ptolemy's world, concept that stars were really little suns was not known. In fact, relatively new concept: o Fireflies o Light shining thru collander o Candles How far away are they? o Top of a tree o Mountaintop o Clouds o No concept of 'billions of miles' Need a demo: light in middle of room, with globe, and with constellations around room! Like CMSS. But, can't distinguish between Earth or Sun at center! Problem with Ptolemy: it was complex. Newton/Kepler/Brahe had a better model, which was simpler. Motions of Sky -------------- Earth goes around (revolves or orbits) Earth every 365 days (1 `year') Earth rotates every 24 hours (1 `day') Moon goes around Earth every 28 days (1 `month') Moon also rotates every 28 days (1 `month') Compared with these motions, the motions of the sun and the stars are slow: assume them to be completely, totally motionless in space! Rotation = spinning Revolution = moving around or orbiting Minute Essays ------------- Write a paragraph (4-5 sentences) about the key concepts covered in class and the reading today. Summarize the main point or points. Were there issues that were confusing? Interesting? Too complex? Too brief? Cool pictures or ideas? Feel free to add any questions or comments for me -- I'll read them the same day as class and get back to you. This is for both of us: so you can focus your ideas, and so I can get feedback as to what works, what doesn't, and the level to which concepts in class are getting communicated. Not graded individually, but will count toward Participation grade.