1) On a clear night, the unaided human eye can see about 6,000 stars, though light pollution limits this to a few hundred in urban/suburban areas. 2) Constellations originally referred to patterns of the brightest stars representing mythological figures, though today they are defined regions of the sky regardless of star brightness. 3) The apparent motion of stars is caused by the Earth's rotation on its axis and revolution around the Sun, making some stars appear to rise and set daily or be visible only seasonally.

1. Observing the Night Sky
• How many starts do you think you can 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. Constellations
In ancient times, constellations only referred to the
brightest stars that appeared to form groups, representing
mythological figures.

4. 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

5. 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.
• Traced back as far as
5,000 years ago
• Stars define the celestial
sphere
• Question: Are the stars
of a Constellation close
to each other?

6. The stars of a
constellation only
appear to be close
to one another.
Usually, this is only
a projection effect.
The stars of a
constellation may
be located at very
different distances
from us.

7. 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).

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. 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 around) the Sun
– The Moon is orbiting Earth
– The other planets are also orbiting the Sun

10. 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,
counterclockwise 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.

11. 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.
Polaris happens to be very near the North celestial pole.

12. Apparent Motion

13. What would the apparent motion of
the stars be if…
1. You were standing at the North Pole.
2. You were standing at the equator
3. You were standing at the South Pole

14. Circumpolar Stars (from our
latitude)

15. Circumpolar Constellations

16. Seasonal Stars and Constellation
• All other constellations and stars are seasonal
and can only be see at certain times of the
year.
– Winter: Orion, Taurus, Canis Major
– Spring: Leo, Bootes, Virgo
– Summer: Cancer, Scorpio, Sagittarius
– Fall: Andromeda, Pisces, Gemini

17. The Yearly Motion
In addition to rotating, the Earth alsorevolvesabout the Sun.
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.