Earth’s Motions
Unit 4
I. Rotation
1. The earth spins on an
axis
2. The axis of rotation is an
imaginary line through the
earth from north to south
pole
• The earth rotates around this
axis
• The axis is tilted
23½º from vertical
3. The axis always points
towards
Polaris
This is called
Parallelism of the axis.
A. Results of Rotation
1.
2.
3.
4.
5.
Sunrise and set
Moonrise and set
Planet rise and set
Coriolis effect
Motion of the foucault
pendulum
6. Circumpolar motion of stars
7. Location of sunrise/set
is due to the direction of
rotation…
the earth rotates from
west to east or
counterclockwise
B. Rate of Rotation
1. Angular rate:
360º
24 hours
15º/hr
2. Velocity is dependent upon
latitude
As latitude increases
velocity decreases
Rate of
Rotation
Latitude
Circumference
Equator
40,080 km 1670 km/hr
42º
31,200 km 1300 km/hr
60º
20,140 km 839 km/hr
90º
0 km
0 km/hr
C. Time, the Sun and
Earth’s Rotation
• We use the apparent motion
of the sun to tell time.
• Time is based upon the
actual movement of the
earth
• The earth’s rotation causes
the apparent rise and set of
the sun
• The apparent path of the
sun is an arc
• Local solar noon is always
the time at which the sun
reaches its highest point in
its arc
–Solar noon in NYS is
never directly overhead!!
Z
II. Revolution
1. The earth travels around
the sun
2. The orbit of the earth is
an ellipse with
the sun at one of the foci
3. The direction of revolution
is counterclockwise
A. Results of Revolution
1. Seasons
2. Seasonal changes in visible
constellations
3. Parallax
4. Retrograde motion
B. Rate of Revolution
1. Angular rate:
360º
365.25 days
~1º/day
2. Velocity
8
radius of revolution is 1.5 x 10
therefore
v=2(pi)r
T
v = ~ 2,580,822 km/day
(1548493 mi/day)
• Comparative velocity:
• The Earth travels fastest
when we are closest to the
sun and
slowest when we are
farthest from the sun
III. Seasons in the
Northern Hemisphere
Seasons and seasonal changes are
due to
the revolution of the earth
combined with
the tilt of the earth on its axis
and parallelism of the axis
A. Summer
1. Summer solstice
2. June 21st
3. Northern hemisphere is tipped
towards the sun
4. Earth is at its greatest distance
from the sun (aphelion)
5. Altitude of solar noon is highest
6.
7.
8.
9.
Days are longest
Shadows are shortest
The sun appears smallest
Sunrise occurs north of
east
10. Sunset occurs north of
west
11. The zenith sun is found at
23½ºN…Tropic of Cancer
12. The north pole experiences
24 hours of daylight
13. The south pole experiences
24 hours of darkness
B. Fall
1. Autumnal equinox
2. September 23rd
3. Neither hemisphere is
tilted towards the sun
4. The distance to the sun is
decreasing
5. The altitude of solar noon
is decreasing
6. The length of day is
decreasing
7. Shadow length is
increasing
8. The sun is looking bigger
9. Sunrise occurs due east
10. Sunset occurs due west
11. Zenith sun is found at the
equator…0º
12. We experience 12 hours of
daylight and 12 hours of
darkness
C.
1.
2.
3.
Winter
Winter solstice
December 21st
The northern hemisphere is
tilted away from the sun
4. The earth is closest to the
sun
5. The altitude of solar noon is
lowest
6. The length of day is
shortest
7. Shadow length is longest
8. The sun appears biggest
9. Sunrise occurs south of
east
10. Sunset occurs south of
west
11. The zenith sun is found at
the Tropic of
Capricorn…23½ºS
12. The north pole experiences 24
hours of darkness
13. The south pole experiences
24 hours of daylight
D. Spring
1. Vernal equinox
2. March 21st
3. Neither hemisphere is
tilted towards the sun
4. The distance to the sun is
increasing
5. The altitude of solar noon
is increasing
6. The length of day is
increasing
7. Shadow length is
decreasing
8. The sun is looking smaller
9. Sunrise occurs due east
10. Sunset occurs due west
11. The zenith sun is found at
the equator…0º
12. We experience 12 hours of
daylight and 12 hours of
darkness
Shadows in the northern hemisphere
A shadow cast
by an object at
solar noon in
NYS will
always point
north
June
July
Aug
Sept
Oct
Nov
Dec
May
Ap
Mar
Feb
Jan
S
O
A
N
J
J
D
J
M
F
A
M
D
S/M
J
IV. The Moon
A. Movements
1. Revolution
The moon travels a path that
is an ellipse with
the earth at one of the foci
(the other foci is called the
barycenter)
Its period of revolution is
27 1/3 days and is called a
sidereal month
2. The moon’s period of
rotation is
27 1/3 days …
its rotation is synchronous
with its revolution
• Because of this
synchronicity, the same side
of the moon always faces
the earth.
• Until we began exploring
space, the far side of the
moon remained unobserved.
B. Lunar Phenomena
1. Phases of the moon:
the changing shape of the
visible moon due to the
revolution of the moon
around the earth
• Because the earth is revolving
around the sun as the moon
revolves around the earth, one
complete cycle of phases takes
longer than one complete
revolution
• One cycle of phases takes
29½ days and is called a
synodic month
waning
quarter
¾ or gibbous
crescent
Full moon
New moon
crescent
¾ or gibbous
quarter
waxing
Waxing…
when the visible portion of
the moon is becoming larger
Waning…
when the visible portion of
the moon is becoming
smaller
2. Eclipses
a. Terms:
i. Umbra:
The darkest part of the shadow
cast by the earth or the moon.
The surface is completely
blocked from view.
ii. Penumbra:
The area of partial shadow
surrounding the darkest part
of the shadow cast by the
earth or the moon. The
surface is only partially
blocked from view.
b. Lunar eclipse:
when the moon is in the
shadow of the earth
The moon must be in the
full moon phase:
S–E–M
c. Solar eclipse:
when the earth is in the
shadow of the moon
The moon must be in the
new moon phase:
S–M–E
d. We don’t experience
eclipses with every new and
full moon because the plane
of the moon’s orbit is tilted
5º to the plane of the earth’s
orbit
e. When the moon is at perigee
and the sun is at aphelion, the
surface of the sun is
completely “blocked”
When the moon is at apogee
and the sun is at perihelion, we
see an annular eclipse in which
the sun is seen as a ring
around the moon’s shadow
3. Tides
• The cyclic rise and fall of
ocean water on the earth.
• They are caused by the
gravitational attraction
between the moon and the
earth’s waters…and, to a
lesser degree, the sun and
the earth
• Tidal events occur every
6 hours
• The position of the moon
affects the degree of the
tides
a. S – E – M = Full Moon
or
S – M – E = New Moon
the sun and the moon
work “together” to create
extreme tides…Spring tides
very high, high tides and
very low, low tides
M
b.
or
S
S
E
E
M
Quarter Moon
The sun and the moon work
against each other to create
minimal tides
• These tides are called
neap tides
very low, high tides and
very high, low tides
• Without the moon, the earth
would still experience tides,
but certainly not to the
extent that we do with the
moon’s influence.
4. Moonrise and
Moonset
• Caused by the
rotation of the earth
• Occurs 50 minutes later each
day because of the
revolution of the moon