Naked-Eye Astronomy

The most important graph: 1/r 2 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 r 2

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?

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

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

Homework Typing in formulae in WebAssign: Area of circle is pi*r^2 Webpage: – WebAssign:

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

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

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 2 nd answer wrong to wrong How answers are revised in a typical question

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 20 th century –Success has been a result of development and exploitation of the scientific method

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)

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

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

Basic Observations in Astronomy Positions of objects (sun, moon, planets, stars …) Motion of objects – with respect to you, the observer - with respect to other objects in 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

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

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”

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

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

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”

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

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

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

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

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