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Lec 2: 29 August 2011 Chapter 1: Powers of 10, Angles, The Sky TODAY - The Layout of the Sky

•  The Appearance of the Sky (from Earth) •  Angular Size and Angular Distance •  Describing “Location” on the Sky

NEXT - The Daily Motion of the Sky •  Chapter 2 Pre-Quiz •  Celestial Coordinate Systems •  Spherical, Rotating Earth •  Begin: Daily Motion of the Sky

ANNOUNCEMENTS: First Labs: This Tuesday and Thursday

Chapter 2 PreQuiz on Wednesday

The 2 Greatest Challenges for You 1.  SCALE: How Big? How Far?

–  astronomy deals with the LARGEST and SMALLEST scales •  e.g. spatial, mass, time, temperature •  single set of physical laws describes it all !

–  how do you deal with a huge range of scales? •  visualize & compare •  conceptualize •  use Scientific Notation (review Appendix)

–  Video: “Powers of 10”

The 2 Greatest Challenges (continued) 2.  PERSPECTIVE: (egocentric->heliocentric)

–  3-dimensional universe projected onto a 2-dimensional sky

–  spherical geometry (Earth and Sky) –  everything is moving, including us! –  you must learn to shift your point of view

•  imagine how things will appear when you look up at the sky from the Earth

•  get used to looking at the sky; take every opportunity early in the semester to become familiar with the sky and how it “moves”

•  mostly it is due to the motion of the Earth (and us)

The Sky (viewed from Earth) •  The Sky APPEARS to us as if all the “celestial” bodies

(Sun, Moon, planets, stars, etc.) are on the inside of a sphere (2-Dimensional surface)

•  Really, they are distributed in a 3-Dimensional space –  demo: class photos –  demo: Orion in 3-D

•  We see one half of this sphere at any given time •  The Earth blocks out the other half •  The Sky is dark about half the time, when the Sun is

“below” the HORIZON •  The Sky is bright when Sun is above horizon (Why?) •  What celestial objects are visible in the daytime?

Are You Really 2-Dimensional? It Looks That Way.

Our 2-D view of the constellation Orion

The 3-D layout of the stars that appear to us as Orion

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The Sky (viewed from Earth) •  We see one half of this sphere at any given time

–  the Earth blocks out the other half •  The Sky is dark about half the time

– when the Sun is “below” the horizon •  The Sky is bright when Sun is above horizon

– Why? – What celestial objects are visible in the daytime?

•  Everything in the sky moves in a regular, predictable way, but you’ve got to watch it night after night, year after year to fully grasp the patterns (or just take this class :-)

Our Goals for Today...

•  How can we describe LOCATION on the sky?

•  How can we describe DISTANCE between objects on the sky?

•  How can we describe SIZE of objects on the sky?

•  In-class Exercise: Describing location on the sky

Location in the Sky. I. “Local” •  “Altitude-Azimuth” Coordinate System

–  depends on your location on Earth (different positions on Earth see same thing but at different altitudes or azimuths and at different times)

•  ALTITUDE (aka “Elevation”) –  angle measured up from horizon (0 to 90 degrees)

•  AZIMUTH (aka “heading” or “bearing”) –  angle measured along the horizon circle –  same as compass heading – measured from North toward the East –  examples: North=0o, East=90o, South=180o

•  HORIZON - circle where Earth and Sky appear to intersect •  ZENITH - point straight overhead •  MERIDIAN - circle (or semi-circle) on sky dividing through N, zenith, and S

These depend on your location on Earth, time of day, & time of year

Angular Units •  Subdivide complete CIRCLE into 360 degrees

•  Subdivide one degree into 60 arcminutes – minutes of arc –  abbreviated as 60 arcmin or 60´

•  Subdivide one arcminute into 60 arcseconds –  seconds of arc –  abbreviated as 60 arcsec or 60”

1° = 60 arcmin = 60´ 1´ = 60 arcsec = 60” “Angular Size” of Moon : ~1/2 degree

in this example, Moon is about 50 degrees “above the horizon”

Angular Size and Angular Distance

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Pointer stars in Big Dipper about 5o apart

[ so you could fit 10 Full Moons between them! ]

Location in the Sky. II. “Geographic” •  CONSTELLATIONS - names given to patterns

of stars in the sky; have boundaries analogous to state boundaries

•  analog: describe locations on Earth in terms of accepted boundaries; e.g. Charleston, South Carolina, United States, N. America

•  how did they get their names? •  why constellations aren’t a good way to describe

location (at least not accurately) •  how are constellations used now?

Eighty-eight constellations cover the sky

•  Ancient peoples looked at the stars and imagined groupings made pictures in the sky

•  Different patterns named by different cultures; modern names come from a variety of sources

ORION (the hunter)

Modern Constellations •  Entire sky divided into 88

“constellations”

–  internationally agreed upon names and boundaries

–  boundary lines drawn on the sky so that all stars are in only one constellation

–  different shapes and sizes; fit together like a jigsaw puzzle

–  many star names come from the constellation they are in (e.g. Alpha Centauri)

the stars in a constellation only appear to be close together (on the sky), because they are in nearly the same direction as seen from Earth

Looking North