Monday, Sept. 8
A100
Solar System
 Read Chapter 1 in text – History of Astronomy
 Quiz Today (essential facts and scientific
notation)
 Rooftop Session Sept. 10, 9 PM
 1st Homework due Friday (get it on Oncourse)
Today’s APOD
The Sun Today
First Quiz
• Bubble and write your name on the
Scantron form
• You may consult with your neighbors
• No books, notes, papers, internet, cell
phones
What can we see in the night sky?
With the naked eye,
we can see more than
2,000 stars, as well
as 5 planets, the
Moon, comets,
meteors, the Milky
Way, and a few other
special objects
The Milky Way is a
band of light that
makes a circle around
the celestial sphere
Orion
rising
The night sky is familiar, but few people look
closely at (or think about) the complex phenomena
we observe in sky.
The
Sky!
North
Sky chart for
9 PM TONIGHT
The brightness
of a star is
shown by the
size of its dot
South
Jupiter & the Moon in Sagittarius
Jupiter & the Moon in Sagittarius
Just look south… Jupiter is the brightest
object in the southern sky
Sept 8
Sept 9
Check out the just south of west,
just after sunset
• See Mercury,
Venus, and Mars
• You will need to
be able to see low
on the horizon to
the west
southwest
Constellations
A constellation
is a region of
the sky
Constellation
regions are
irregular in
shape
Do the Stars in a constellation
belong together?
• NO!
• The stars in a constellation
are not all at the same
distance from the Sun
• They do not form a real
group in space
The Big Dipper is part
of the constellation
Ursa Major
The stars of the Big Dipper
are not all at the same
distance from the Sun
The Constellations
The 88 official
(defined by the
International
Astronomical
Union)
constellations
cover the
celestial sphere
The Celestial Sphere
The stars all appear
to lie on a large
sphere surrounding
the Earth (the
celestial sphere)
Really, the stars
are all at different
distances
Angular
Measurements
• Full circle = 360º
• 1º = 60 (arcminutes)
• 1 = 60 (arcseconds)
We measure
the sky using
angles
Measure the Angular Size
Image taken by the
Galileo spacecraft
(Jupiter, 1995-97)
during its lunar flyby in December,
1992
The dark areas are lava rock
filled impact basins (mare)
Tycho impact basin
The “real” Moon is ½ degree across
Angular Size
distance
360 degrees
angular size = physical size 
2  distance
angular
size
physical size
An object’s
angular size
appears
smaller if it
is farther
away
The angular size of Mars changes depending
on where Earth and Mars are in their orbits
The
Local
Sky
An object’s altitude (above horizon) and direction
(along horizon) gives its location in your local sky
Definitions:
The Local Sky
Zenith: The point
directly overhead
Horizon: All points
90° away from
zenith
Meridian: Line
passing through
zenith and
connecting N and S
points on horizon
Our view from Earth:
• Stars near the north celestial
pole are circumpolar and never
set.
• All other stars (and Sun, Moon,
planets) rise in east and set in
west.
A circumpolar
star never sets
Celestial equator
This star never
rises in Indiana
Why do stars
rise and set?
Stars in the northern sky
circle the northern celestial
pole (in the southern
hemisphere stars circle the
southern celestial pole)
The Earth
spins under
Polaris
(Polaris is NOT
the brightest
star in the sky!)
Finding
Polaris
North
Big Dipper’s on
its side in the
northwestern
sky
Find an open
area away
from nearby
lights where
you have a
good view of
the sky
Find the Big
Dipper in the
northwestern sky
South
Sky chart for 9 PM TONIGHT
Finding
Polaris
To find Polaris:
• find the Big Dipper
• follow to Polaris
• the Big Dipper is ALWAYS up
in Indiana!
Altitude of the celestial pole
= your latitude
Why do the constellations we see
depend on latitude and time of year?
 They depend on latitude because your
position on Earth determines which
constellations remain below the
horizon.
 They depend on time of year because
Earth’s orbit changes the apparent
location of the Sun among the stars.
The sky changes as Earth orbits the Sun
As the Earth
orbits the Sun,
different
constellations are
visible at night
At midnight, the
stars on our
meridian are
opposite the Sun
in the sky
Where on the Earth is it
possible to see the most stars
through the year?
a) From the north pole
b) From latitude 23.5 degrees
c) From the equator
d) anywhere, since latitude makes
no difference.
Precession!
Over time, the
direction of the Earth’s
rotation axis drifts
around a circle.
At different times,
different stars (other
than Polaris) are close
to the pole
The location of the
celestial pole traces out
a circle in the sky every
26,000 years
Vocabulary Review
o Constellation
o Ecliptic
o Equator
o Celestial sphere
o Latitude
o Longitude
o Meridian
o Zenith
o Horizon
o Altitude
o North and south
celestial poles
One question I’ve always had
about astronomy is…
Dates
to
ASSIGNMENTS
Remember
this week
 Read Chapter 1, History of…
 Kirkwood Obs. open Weds.
 Rooftop Sky Viewing Weds.
 1st HW due Friday