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Earth and Other Planetary
Motion
(4.01-06)
Reading together
• As you look up at the sky, you will notice that
the stars and planets are not always in the
same position each night.
• “Polaris”, however, is always directly above the
North Pole of the Earth.
• The Earth rotates underneath Polaris each day.
This happens at the same time as Earth
revolves around the Sun.
Figure it out:
Revolve or Rotate, which is
which?
Reading Together
• These motions of the Earth are Responsible
for;
–
–
–
–
our Day and Night,
Seen in
animation
our 24 hour day,
below.
sunset and sunrise,
movement of the stars and constellations through
the sky,
– and our four seasons in the Northern Hemisphere.
Why do we
have sunrise?
.
What about
sunset?
1. Effects of Earth’s Rotation
• Rotation means the movement of the Earth
around its axis. (like a wheel rotates on an
axel)
• The Earth rotates every 24 hours
• Gives us morning and night
• Makes the sun, moon, stars and constellations
appear to rise in the East and set in the West
• Stars close to Polaris will not rise and set, just
rotate in the sky.
Imagine you lived on this ball, why
would the sun rise in the East?
An Aside, What’s up with
Polaris?
• One star that has become
extremely useful to travelers in the
Northern Hemisphere has been
Polaris.
Polaris lies at the tip of the little
dipper and is directly above the
earth’s northern axis of rotation.
Because of this, Polaris always seems
to stay still, while the other stars seem
to move across the sky.
Eyes on Polaris
More Aside
Star Shapes - Constellations
• When people looked up at the
night sky they looked for meaning
and patterns, but also for practical
tools.
• They began to develop the concept
of the constellations.
1. Some constellations were used for
story telling (Hercules, Orion’s Belt).
2. Some constellations were used for
giving directions and navigating both
the sea and land at night.
2. Effects of Earth’s Revolution
• Revolution means the orbit or revolving of the
Earth around the Sun.
• The Earth’s orbital period is 365 ¼ days or 1
year.
• Reasons why we see different stars and
constellations in winter and summer (What is
above us in the night changes).
Why do we have those pesky leap years?!? Can
you SEE the reason above?
• Our 4 seasons
are due to the
tilt of the Earth
on its axis as it
revolves or
orbits around
the sun.
Correcting ideas about seasons.
In your margin notes
• The reason for summer is not that the Earth is any closer to the
sun.
• The reason for summer is that when the earth is tilted
towards the sun, the sun’s light strikes the Earth directly.
• During winter the light is spread out over the Earth’s surface
more since the earth is tilted away from the sun. The same
amount of light spread out over a larger surface means the
surface is colder.
Northern winter
Northern Summer
Building Up a Diagram of the 4 Seasons in the
Northern Hemisphere
Fall Equinox
12 hr. Day = 12hr. Night
Tilted towards
Tilted away
Day of year with
most sunlight
Day of year with
least sunlight
Spring Equinox
Day length = Night length
3. Effects of other Planets’ Orbits
• All the other planets orbit around the sun,
at different speeds and distances from the
Sun
• They APPEAR to move past one another
and sometimes fall back relative to one
another (this is called retrograde motion)
but this is due to their different speeds and
positions in our Solar System.
• They APPEAR to move through different
constellations, but they are actually much
closer than the stars.
Mars is a great
example of a planet
that shows
retrograde motion.
The word Planet is
derived from a
Greek word that
called these “stars”
with “weird” motion,
THE WANDERERS.
• Retrograde motion
basically happens
because planets “lap”
other planets or
“pass” them in their
orbital race around
the sun.
Reminder of basic ideas
Rotation of the Earth on its axis
1. Time to complete one rotation is 24 hours
2. Causes day and night
3. Sun rises in the east and sets in the west
4. Polaris is above the axis of rotation
Revolution of the Earth around the sun
1. Time to complete one revolution is 365.25 days
2. Causes the seasons
3. Causes retrograde motion
Questions:
SUN