Naked-Eye Astronomy

1. Naked-Eye Astronomy

2. The most important graph: 1/r2 • Force of gravity (and EM) falls off like that • Brightness of stars goes down like that • Simple reason: • things spread out over a sphere • dilutes over the surface of the sphere • surface scales like r2

3. Example: Two 100W light bulbs • One APPEARS nine time brighter than the other. How far away is it? • We can’t compute how far away it is in absolute terms, but it must be √9=3 times farther than the other one. • Which one is brighter?

4. Appearances • Don’t be fooled by appearances! • The sun and the moon appear to be the same size in the sky (0.5 degrees) • Alpha Centauri appears to be much dimmer than the Sun • Alpha Centauri and Vega appear to be equally bright

5. Think! • The moon and the sun COULD be at different distances • Alpha Centauri and Vega COULD be different types of stars • Find out  Measure! • That’s where science starts

6. Homework • Typing in formulae in WebAssign: • Area of circle is pi*r^2 • Webpage: • http://faculty.otterbein.edu/utrittmann/is2403-02 • WebAssign: www.webassign.net

7. Peer Instruction: How it works • Peer instruction is learning by instructing your fellow students and being instructed by them • The process involves 6 steps: • Mini-lecture by course instructor • Conceptual multiple-choice question is put up • Flash-cards are used to “poll the audience” • A few minutes of discussion between students • “Final answer” via flash-cards • The instructor explains the correct answer

8. Concept Questions • Concept questions maybe easy to answer, but are not simple • You need background knowledge to answer them • They teach you how to use facts and knowledge to find the answer to a problem • They test if you got the concept rather than just knowing facts

9. Why it works • Carefully chosen questions • It is easier to be convinced and to convince if the reasoning is sound and hence the answer correct Right to right Wrong to right Right to wrong No 2nd answer wrong to wrong How answers are revised in a typical question

10. What is Astronomy? • The science dealing with all the celestial bodies in the Universe • Cosmology is the branch of astronomy that deals with the cosmos, or Universe as a whole • The medieval list of the Liberal Arts: grammar, rhetoric, logic (trivium); arithmetic, music, geometry and astronomy (quadrivium) • Is an “exact science” for ~5000 yrs • Most rapid advancements in astronomy have occurred during the Renaissance and the 20th century • Success has been a result of development and exploitation of the scientific method

11. Why study Astronomy? • Practical reasons: seasons, tides, navigation, space technology, satellite communication • Idealistic reasons: cosmological questions (“Where do we come from?”), aesthetics, curiosity “Two things fill the mind with ever new and increasing admiration and reverence, the more frequently and enduringly the reasoning mind is occupied with them: the star spangled sky over me and the moral law in me.” (I. Kant)

12. Astronomy and Culture • Astronomy had and has an enormous influence on human culture and the way we organize our lives • For example: • The year is the rotation period of the Earth around the Sun • The year is subdivided into months, the period of the Moon around the Earth • The weeks seven days are named after the seven bodies in the solar system known in antiquity: Sunday, Monday, Saturday (obv.), Tuesday=Mardi, Wednesday = Mercredi, Thursday=Jeudi, Friday=Vendredi

13. Stars nebulae molecular clouds star clusters Solar System black holes pulsars Sun planets moons comets meteors asteroids dust terrestrial jovian clusters and superclusters voids galaxies like the Milky Way quasars THE UNIVERSE What’s in the Universe?

14. Basic Observations in Astronomy • Positions of objects (sun, moon, planets, stars …) • Motion of objects – with respect to you, the observer - with respect to other objectsin the sky • Changes (day/night, seasons, etc.) • Appearance of objects (phases of the moon, etc.) • Special events (eclipses, transitions, etc.)  All “in the sky”, i.e. on the Celestial Sphere

15. Position: Angles vs. Distances • Locations in the sky are easy to measure: 2 angles • Distances from observer are hard (one length)  Together they give the location of an object in three-dimensional space

16. Angles and Angular Size Angles measured in degrees full circle = 360; right angle = 90 1 = 60' (minutes of arc or arc minutes) 1' = 60" (seconds of arc or arc seconds) Typical angular sizes: Moon 0.5, Sun 0.5,Jupiter 20”, Betelgeuse (α Ori) 0.05”

17. The Trouble with Angles Angular size of an object cannot tell us its actual size – depends on how far away it is Sun and Moon have very nearly the same angular size (30' = ½) when viewed from Earth

18. Without Distances … • We do not know the size of an object • This makes it hard to figure out the “inner workings” of an object • We can’t picture the structure of the solar system, galaxy, cosmos

19. The most important measurement in Astronomy: Distance! • The distances are astronomical! • The distance scales are very different • Solar system: light minutes • Stars: light years • Galaxies: 100,000 ly • Universe: billions of ly • Need different “yardsticks”

20. Yardsticks and the Expanding Universe • Realizing (measuring) the distances to objects means realizing how big the universe is: • We realized that the solar system is not the universe • We realized that our galaxy is not the universe • We realized that the universe is not static

21. What’s up in the night sky? The Celestial Sphere • An imaginary sphere surrounding the earth, on which we picture the stars attached • Axis through earth’s north and south pole goes through celestial north and south pole • Earth’s equator Celestial equator

22. Celestial Coordinates Earth:latitude, longitude Sky: • declination (dec) [from equator,+/-90°] • right ascension (RA) [from vernal equinox, 0-24h; 6h=90°] Examples: • Westerville, OH 40.1°N, 88°W • Betelgeuse (α Orionis) dec = 7° 24’RA = 5h 52m

23. What’s up for you? Observer Coordinates • Horizon – the plane you stand on • Zenith– the point right above you • Meridian – the line from North to Zenith to south

24. …depends where you are! • Your local sky – your view depends on your location on earth