Observing the Night Sky
• How many starts do you think you can see on
a crystal clear night?
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+
In ancient times, constellations only referred to the
brightest stars that appeared to form groups, representing
Asterism vs. Constellation
• An asterism is a star pattern that is not a
– Examples: The Big and Little Dipper, the Summer
Triangle, and the Winter Hexagon
Looking at the Sky
– 88 official
according to the IAU
• Many origins
– Greeks, Southern
Middle East, Asia, etc.
• Traced back as far as
5,000 years ago
• Stars define the celestial
• Question: Are the stars
of a Constellation close
to each other?
The stars of a
appear to be close
to one another.
Usually, this is only
a projection effect.
The stars of a
be located at very
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).
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
• 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
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
Paths of Stars • Earth’s counter-clockwise
– Stars rise in the East, set in the
• But some stars never set!
– Polaris stays nearly stationary
in the sky
– Stars near Polaris move
slightly, in circular,
– 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.
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
Polaris happens to be very near the North celestial pole.
Seasonal Stars and Constellation
• All other constellations and stars are seasonal
and can only be see at certain times of the
– Winter: Orion, Taurus, Canis Major
– Spring: Leo, Bootes, Virgo
– Summer: Cancer, Scorpio, Sagittarius
– Fall: Andromeda, Pisces, Gemini
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
constellations which the
Sun passes through
Because the Sun is bright, we can only see some constellations at
certain times of year.