Observational Astronomy
Astronomy from space
Hubble Space Telescope
Atomic Energy
Theory of Relativity
Nebulae – stellar nurseries
Spiral Galaxy and Nucleus
Hubble Deep Field: 10,000 Galaxies
Universe: Past and Present
Cosmic Epochs and Evolution
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
Parsec = 3.26 Lys (parallax angle unit)
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/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 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?
Parallax: Measurement of Distance
• Apparent movement due to viewing
position
• No actual motion of the viewed object
• Difference in angle between line of sight
from the Earth and from the Sun (half the
diameter or the radius of the Earth’s orbit,
1AU)
• Since the parallax of stars is very small, it
is measured in parsec (parallax second)
distance d= 1/p OR p = 1/d
• 1 parsec = 3.26 LY = 206,265 AU
Distance Measure in Astronomy:
The Parallax Method
Parallax is the change in
angle due to motion
Circle = 360o (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 / 
d
d


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  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 (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
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 23.5o
to each other
Vernal Equinox
Winter Solstice
THE CELESTIAL SPHERE:
Coordinates and Map of Objects in the Sky
Declination d:
“celestial latitude”
Right Ascension :
“celestial longitude”
Star at (,d)
“celestial coordinates”
Vernal Equinox:
Position of Sun
In the Sky
on the first day
of spring;
Day = Night
=0
Star
Apparent Rotation of Celestial Sphere