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

The Distance Scale

http://htwins.net/scale2/

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