Understanding the Starry Sky

Ch. 01

Celestial Sphere

A model of the universeWe picture the sky as a hollow globe

surrounding earthAll stars are positioned inside the globe

facing earth

Celestial Sphere

http://www.astro.columbia.edu/~archung/labs/fall2001/lec02_fall01.html

Celestial Sphere Animation

Animation 1.1: Celestial Sphere

Celestial Sphere Features

“Sphere” appears to revolve around earth each day

Earth’s equator extended becomes celestial equator, earth’s poles (N&S) become celestial poles

Ecliptic is the path of the sun around the celestial sphere, requires one year for a revolution

Ecliptic & Celestial Sphere

http://www.utm.utoronto.ca/services/gallery/img/ecliptic.gif

Constellations

88 officialMany ancient, 48 Greek constellationsTypes

SeasonalZodiacCircumpolar

In order to more easily locate objects in the sky, we In order to more easily locate objects in the sky, we divide the sky into regions named after familiar divide the sky into regions named after familiar patterns of stars called patterns of stars called constellationsconstellations..

Ancient constellations were imaginary pictures outlined by familiar patterns of stars (stick figures).

Modern astronomers divide the sky into 88 official constellations or regions of space, many of which contain the ancient star patterns.

Sphere image of Constellation Boundaries

Zodiac

12 Constellations (traditional) along the eclipticPisces, Aries, etc.

Circumpolar Constellations

North and SouthNever setting (N), or never rising (S)In north, appear to wheel eternally about

Polaris, the North StarEx: Ursa Major (Big Dipper)

Circumpolar Stars

http://www.nmm.ac.uk/upload/img/circumpolar.jpg

“Star Trails”

N & S Circumpolar Stars

http://www.mhhe.com/physsci/astronomy/fix/student/images/01f19.jpg

Star Maps

Star Maps represent portions of the celestial sphere

Seasonal star maps provided in your textbook (appendix)

Horizon Star Map

http://www.drypen.net/Star_Map_3.jpg

Equatorial Star map

http://www.starlight-theatre.ca/images/MAP-C.GIF

Bright Star Names

About 50 of the brightest starsEx: Aldebaran, Sirius Mostly Arabic, some Greek & LatinBayer (1600), Greek letter names of stars

Ex: Alpha () Tauri means brightest star in constellation Taurus

26 Brightest Stars

http://www.astro.wisc.edu/~dolan/constellations/extra/brightest.html

Star Magnitude (Brightness)

Apparent Magnitude: Brightness of a star as seen from earth

Traditional: Greek Hipparchus, assigned 6 categories with 1 being the brightest (first magnitude)

About 100x brightness difference between 1 and 6

To calculate magnitude differences:2.5199(m2-m1)

Apparent Magnitude Scale

Terrestrial Coordinates

http://www2.shastacollege.edu/dscollon/images/Figures,%20Diagrams%20and%20Maps.htm

Celestial Coordinates

Celestial Equator: Extension of earth’s equatorial plane into space along the celestial sphere

N & S Celestial PolesVernal Equinox-Sky’s “prime meridian”Coordinates:

Declination (Dec)Right Ascension (RA)

Celestial Globe

http://www.onr.navy.mil/focus/spacesciences/observingsky/sphere1.htm

RA & Dec

http://abyss.uoregon.edu/~js/ast122/lectures/lec02.html

Summary

Earth Globe Celestial Sphere

North Pole North Celestial Pole

South Pole South Celestial Pole

Equator Celestial Equator

Prime Meridian Vernal Equinox

Latitude Declination

Longitude Right Ascension

Horizon System

Coordinate system based upon an observer’s horizon, not the celestial equator

GuidelinesHorizon-”Straight ahead”, line where earth and sky

appear to meetZenith-point straight upMeridian-Line “due south”, passes from north horizon

through zenith to south horizonNadir-point straight down

CoordinatesAltitudeAzimuth

Horizon System Diagram

http://www.yorku.ca/phall/P1070W05/L04/bea02-05.jpg

Altitude & Azimuth

http://www.nmm.ac.uk/upload/img/altaz-star2.jpg

Degrees in the Sky

Angular MeasureBased on degrees of a circle = 360ºFinger tip = 1ºFist = 10ºOutstretched hand = 20º

From horizon to zenith = 90ºThe moon and sun = 1/2º, your finger tip

will cover them (try this with the moon only)

Angular Measure

http://www.physics.capcollege.bc.ca/stan/Angular_measure.jpg

Polaris & Latitude

The height of Polaris (North Star) in degrees above the horizon is equal to an observer’s latitude (distance above the equator)

Our latitude 42-degreesPolaris will be about four fist-lengths

above horizonUseful for navigation

Geometric Explanation

p = Location

x° = Latitude

a° = Altitude of Polaris

x° = a°

http://homepage.mac.com/kvmagruder/images/polarislat.gif

Earth’s Motions

RotationRevolutionPrecession

Earth’s Motions: Rotation, Revolution

Revolution

Earth’s Seasons

Sun’s Apparent Yearly MotionSun travels around eclipticAbout 1° per day

360°/365 days 1° per day

Sun high in summer, low in winter

Seasonal Heights of Sun

Seasonal Sun Paths

http://www.madison.k12.wi.us/planetarium/sunpath.gif

SEPTEMBER MARCH

Seasonal Constellations

Earth’s Seasons

http://www.learn.londonmet.ac.uk/packages/clear/visual/daylight/sun_sky/images/seasons.png

Earth’s Seasons

Seasons Animation

Animation 1.2: The Seasons

Seasons & Celestial Sphere

Rotation: Solar and Sidereal Day

http://www.nmm.ac.uk/upload/img/sidereal-mean-day_20030415143827.gif

Sidereal Day

Precession

Earth’s other motionOne wobble takes 26,000 yearsAxis points in new directions during this

timeResult, the “North Star” changes over time

Thuban, 3000 BCPolaris, PresentVega, 14,000 AD

Precession

http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/milankovitch_2a_low.html

Earth’s Precession

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter18/graphics/precession.psd.gif

14,000 AD

Present

+ North Ecliptic Pole

+

Ancient Egyptian polestar

Inconstant North Star

“But I am as constant as the North Star…”

-Shakespeare, Julius Caesar

http://abyss.uoregon.edu/~js/images/bz990323.gif