some basic facts 12 constellations of the zodiac: groups of stars in an 18 degree wide belt around...
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Some Basic Facts• 12 Constellations of the Zodiac: groups of stars
in an 18 degree wide belt around the Ecliptic -apparent path of the Sun in the Sky among the stars
• No physical relationship among stars in a constellation, except apparent 2-D location
• Ancient Greeks Geocentric Model
• Modern (Copernicus) Heliocentric Model
• 9 8 Planets: 4 inner Terrestrial and 4 outer Jovian (Pluto is neither)
Relative Sizes in astronomyFrom very small to very large
(meters)
Some Essential Numerical Figures
• Radius of the Earth = 6500 Km
• Speed of light – 300,000 Km/sec
• Astronomical distances are so large that we use the speed of light to measure them
• Mean Earth-Sun Distance – 150 million Km
= 1 Astronomical Unit (AU) = 8.3 Light Minutes
• 1 Light Year (Ly) = 9.5 trillion Km = 63,240 AU
The Milky Way100,000 Lys Across
Stellar Constellations
Connect bright stars to discern some shape
Ancient Figures and Constellations
The Orion Constellation
The North Star (Polaris)
The Summer Triangle
Winter Triangle of Bright Stars
LINEAR AND ANGULAR SIZE OF OBJECTS
angle subtended by the object at the observer; the farther the object, smaller the
Angular size of moon = 30’1 Degree = 60’ (minutes) = 60 x 60 = 3,600 ‘’ (seconds)
What is the angular size of the Sun? How large does the Sun appear ?
Angular ‘distance’ between stars
While angular distance can be measured by observations, actual distances aredifficult to measure (What do we need?)
Orbital and angular motion of the Earth
The Earth moves one degree in its orbit around the Sun each day. Why?
Distance Measure in Astronomy:The Parallax Method
1 parsec (pc) = 3.26 Light Years (Ly)
Circle = 360o (degrees)1 degree = 60’ (minutes)1 minute’ = 60” (arcseconds)
Measure of distances in angles:The distance d of an object thatmakes an angle of 1” as the Earthmoves to opposite sides of the Sun
d
Object at a distance of 1 pc
d (pc) = 1 /
Parallax is the change in angle due to motion
90
1 AU
d
Stellar and Astronomical Distances
• 1 parsec (pc) = 3.26 LY = 205,000 AU• The stars are very far away• Nearest Star Alpha Centauri 4.3 LY,
more than 1 pc ! The parallax angle is less than one arcsecond (“)
• That’s why the Greeks could not see the stars move
• Galaxies have been seen up to more than 10 billion Lys away
Night Sky ExposureGeocentric or Heliocentric ?
Earth’s rotation and the Sky
Daily Rotation of the Earth and Stars
Annual Revolution of the Earth around the Sun and position of stars
Location of Heavenly Objects• How do you locate places on the Earth?
• Latitude and Longitude
• Latitude: angle measured from the Equator (0o), up or down, N-S
• Longitude: angle measured from the Prime Meridian, E-W, 0o – 180o
• How would you find location in mid-ocean ?
• First rule of navigation: Lookup angle of Polaris Latitude
• How do you find the longitude? Clock ?
Celestial Map and Celestial Coordinates
Celestial PolesAnd Equator –Extension of theEarth’s polesAnd equator
Celestial Equator is the extension of the Earth’s Equator up to the CS
Analogous toLatitude and Longitude on The Earth –Measured inDegrees
Ecliptic and the Celestial Equator
The Ecliptic and theCelestial Equator Intersect atVernal (Spring) andAutumn Equinoxes
At an angle of 23.5o to each other
Ecliptic is the apparentPath of the Sun on theCelestial Sphere
Autumn Equinox
Vernal Equinox
Winter Solstice
Summer Solstice
THE CELESTIAL SPHERE:Coordinates and Map of Objects in the Sky
Declination “celestial latitude”
Right Ascension :“celestial longitude”
Vernal Equinox:Position of Sun In the Skyon the first day of spring;Day = Night = 0
Star at (“celestial coordinates”
Star
Apparent Rotation of Celestial Sphere
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