1. Observing the Night Sky
• How many stars can the unaided human eye
see on a crystal clear night?

2. Visible stars on a clear night
• Unaided human eye – 6,000 stars
• At any one time – 3,000 stars
– ½ above horizon, ½ below horizon
• Reality – light pollution and other factors limit
visible stars to a few hundred
– Suburban: 500+
– Inner-city: 300+

3. The Magnitude Scale
• The apparent brightness of celestial objects is
measured on a scale of apparent magnitude (mv).
• The brightest stars are ~ 1st magnitude or less
• Faintest stars (unaided eye) are ~ 6th magnitude.
• This is a logarithmic scale with a base of 2.512
– A 1st magnitude star is 2.512 times brighter than a 2nd
magnitude star
– How many times brighter is a 1st magnitude star compared
to a 6th magnitude star?

4. Intensity Ratio (how many times
brighter)
• 2.512 ^ (mv1 – mv2)
• 2.512 ^ (6-1) = 100
• How many times brighter is Sirius (mv = -1.42)
than Polaris (mv = 1.97)

5. Constellations
In ancient times, constellations only referred to the
brightest stars that appeared to form groups, representing
mythological figures.

6. Today, constellations are well-defined regions on
the sky, irrespective of the presence or absence of
bright stars in those regions. They serve as a way to
break up and organize the sky (celestial sphere).

7. Constellations
Stars are named by a Greek letter ( ) according to their
relative brightness within a given constellation + the
possessive form of the name of the constellation:
Orion Betelgeuse = Orionis
Betelgeuze Rigel = Orionis
Rigel

8. How can we use the Constellations?
• Think of the constellations as a map of the sky.
The more constellations you can recognize the
better your ability to navigate the night sky
will be.
• Every celestial object can be found within a
constellation. Knowing the constellations tells
you where to look for objects and phenomena
like planets, galaxies, comets, and meteor
showers.

9. Looking at the Sky
• Constellations
– 88 official
constellations
according to the IAU
(International
Astronomical Union)
• Many origins
– Greeks, Southern
European cultures,
Middle East, Asia, etc.
• Stars define the celestial
sphere
• Question: Are the stars
of a Constellation close
to each other?

10. Projection Effect:
The stars of a
constellation only
appear to be close
to one another.
The stars of a
constellation may
be located at very
different distances
from us.

11. Asterism vs. Constellation
• An asterism is a star pattern that is not a
Constellation
– Examples: The Big and Little Dipper, the Summer
Triangle, and the Winter Hexagon

12. Our View of the Sky
• Our view changes because of the motions that
occur in the solar system.
– Earth is spinning (rotating)
– Earth is orbiting (revolving) the Sun
– The Moon is orbiting Earth
– The other planets are also orbiting the Sun

13. Paths of Stars • Earth’s counter-clockwise
rotation
– Stars rise in the East, set in the
West
• But some stars never set!
– Polaris stays nearly stationary
in the sky
– Stars near Polaris move
slightly, in circular, counter-
clockwise paths
– Some objects are never visible
from the north
• The Southern Cross (SH)
• Star positions change from night
to night as the Earth revolves
about the Sun – rise and set 4
minutes earlier each day.

14. Risings and Settings (apparent motion)
The spin of the earth causes the stars to appear to rotate
about the celestial pole. Some stars are therefore
circumpolar and never set, while others dip below the
horizon. Which stars are which depend on where you are
on earth.

15. Circumpolar Constellations

16. The Yearly Motion
In addition to rotating, the Earth alsorevolvesabout the Sun.
One revolution = 365.242 days.
As the earth revolves the
Sun is projected in front
of different constellations
at different times of year.
The path the Sun takes
across heavens is called
the ecliptic.The
constellations which the
Sun passes through
arezodiac constellations.
Because the Sun is bright, we can only see some constellations at
certain times of year.

18. Misconceptions about yearly motion:
The cause of the seasons…
Misconception #1: Misconception #2:
• The distance between Earth • Because we are tilted
and the Sun create seasons. towards the Sun in the
Month Earth-Sun Summer we are closer to it
Distance and therefore it is warmer.
December 147.2 million
km
• If 4.8 million km doesn’t
March 149.0 million
drastically affect the
km seasons, then the fact that
June 152.0 million we are slightly closer to the
km Sun when tilted towards it
September 150.2 million has no effect at all…
km

19. Seasons
• Seasonal change is the result of the Angle of
Incidenceof the Sun’s rays and the length of
day (seasons animation).