Don’t Panic
• We are both learning how to use Astronomy
Place.
– Remember to “Join the Class”
– There have been problems submitting answers
• In the future I will set up Angel to accept answers in
the cases where you can’t submit them to
Astronomy Place
• If problems persist come to my office so we can
work together
For this time deadline is extended until Monday
Spaceship Earth
What are the motions of spaceship Eearth?
• Earth SPINS on its axis towards the EAST
once every 24 hours
• Earth ORBITS around the Sun
once every 365.25 days. Speed ~ 1x105 km/hr.
• Earth, the Sun & the Solar System all orbit around
the center of the Milky Way galaxy once every
2.3x108 years. Speed ~ 8x105 km/hr.
• The Milky Way is moving toward the Andromeda
galaxy, speed ~ 3x105 km/hr
• The Milky Way (along with the local group)
partakes in the expansion of the universe.
Earth Spins (Rotates) on its axis
toward the East, once every 24 hours
Earth ORBITS around the Sun
once a year
What
keeps the
Earth
in orbit
and stops
it from
flying off?
The Solar System ORBITS the
center of the Milky Way galaxy
Period =
2.3x108 years
What keeps it
in orbit so it
doesn’t fly off?
Our motion through the universe
The Milky Way is moving through the
universe ~ 600 km/s
Expanding Universe Model Balloon
The Seasons
What could produce our seasons - the
difference between summer and winter?
• Changing energy output by the Sun
• Changing amount of energy we receive
from the Sun
•
Distance from the Sun
•
Orientation of the ground re. Sun’s light
(Tilt of Earth’s axis of rotation)
Sun’s heating & Distance
Sun’s heating & Orientation
Sun’s heating & Rays
The Cause of the Seasons
Why doesn’t Distance Matter?
• Small variation for Earth — about 3%
• Distance does matter for some other planets,
notably Mars and Pluto.
• Surprisingly, seasons are more extreme in
N. hemisphere, even thought Earth is closer
to Sun in S. hemisphere summer
(and farther in S. hemisphere winter)
because of land/ocean distribution
Moon’s Phases
Phases of the Moon
A Solar Eclipse
Eclipses
• The Earth & Moon cast
shadows.
• When either passes
through the other’s
shadow, we have an
eclipse.
• Why don’t we have an
eclipse every full & new
Moon?
Planetary Motion
Usually planets appear to move toward the East re the stars.
Occasionally, they appear to move backward - toward the
West, called Retrograde Motion
Retrograde Motion
View from Earth: explained by big circles + epicycles
big circle=‘deferent’.
Explains most of
planets motion
Epicycle, rolls on big
circle. Explains
retrograde loop.
Retrograde Motion
• Like the Sun, planets usually
appear to move eastward relative
to the stars.
• But as we pass them by in our
orbit, they move west relative to
the stars for a few weeks or
months.
 Noticeable over many nights; on
a single night, a planet rises in
east and sets in west…
Next Week
• I will be away Monday and Wednesday.
Classes will occur in BPS 1410 as usual
both days.
Dr. Horace Smith will take over for me.
Assignment for Next Week
Make sure you pick up homework due
Wednesday Jan 28
(If you miss a Wednesday class, homework
is linked to the course web page
http://www.pa.msu.edu/courses/isp205/sec-3)
• Pre-class assignment for Monday Jan 26, is
linked to the course web page and is to be done
on Angel.