Some Basic Facts 12 Constellations of the Zodiac

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 astronomy From 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 Way 100, 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/scale 2/

LINEAR AND ANGULAR SIZE OF OBJECTS a angle subtended by the object at the observer; the farther the object, smaller the a

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 are difficult 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 Parallax is the change in angle due to motion Circle = 360 o (degrees) 1 degree = 60’ (minutes) 1 minute’ = 60” (arcseconds) 1 AU 90 Measure of distances in angles: The distance d of an object that makes an angle of 1” as the Earth moves to opposite sides of the Sun d (pc) = 1 / a d d a a 1 parsec (pc) = 3. 26 Light Years (Ly) Object at a distance of 1 pc

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 a 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 Exposure Geocentric 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 (0 o), up or down, N-S • Longitude: angle measured from the Prime Meridian, E-W, 0 o – 180 o • 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 Analogous to Latitude and Longitude on The Earth – Measured in Degrees Celestial Poles And Equator – Extension of the Earth’s poles And equator Celestial Equator is the extension of the Earth’s Equator up to the CS

Ecliptic and the Celestial Equator Ecliptic is the apparent Path of the Sun on the Celestial Sphere Autumn Equinox Summer Solstice The Ecliptic and the Celestial Equator Intersect at Vernal (Spring) and Autumn Equinoxes At an angle of 23. 5 o to each other Vernal Equinox Winter Solstice

THE CELESTIAL SPHERE: Coordinates and Map of Objects in the Sky Declination d: “celestial latitude” Right Ascension a: “celestial longitude” Star at (a, d) “celestial coordinates” Vernal Equinox: Position of Sun In the Sky on the first day of spring; Day = Night a=0 Star

Apparent Rotation of Celestial Sphere