Download Topic 4: Earth-Moon

Topic 4
Earth-Moon-Sun
Earth’s Rotation – Causes daily
changes
• The spinning of earth on its AXIS – the
imaginary line through the planet from the
North Pole to the South Pole – is its
rotation
• The axis of the earth
is tilted 23.5° from a
line perpendicular to
the plane of its orbit
of the sun.
Earth’s Rotation
• As the earth revolves around the sun, Earth’s
axis remains tilted at 23.5°
• The north end of the axis points toward the
north star (Polaris)
• Earth rotates 360° from west to east (counter
clockwise looking down from the north pole)
in 24 hours, at an angular rate of l5° per hour
(360°/24 hours = 15°/hour)
Tilt of the axis always stays the
same – 23.5° from a line
perpendicular to the ecliptic plane
Evidence of Earth’s Revolution
around the Sun
• Seasonal Constellations: Because of the
Sun’s annual motion, some constellations
are visible at night only during certain
seasons.
• Constellations are visible when the dark
side of Earth (away from sun) faces
toward the constellation.
Zenith Position of the Sun
• Because of the earth’s spherical shape, on
any particular date, there is just 1 place
where insolation (incoming solar radiation)
is at an angle of 90°. All other places are
less than 90°.
• Does the 90° ever reach the United
States?
• An observer in New York State will never
see the Sun directly overhead.
90° Perpendicular Rays of the Sun
aka the ZENITH POSITION
• The noon perpendicular rays of the sun
travel southward from the Tropic of Cancer
(23.5° N) on June 21 passing the Equator
on September 21 and going to the Tropic
of Capricorn (23.5° S) where it turns
around and starts going back up passing
the Equator on March 21and arriving at
the Tropic of Cancer again on June 21.
Seasons
Seasons are caused by:
 The revolution of the Earth around the Sun
The tilt of the Earth on its axis

Parallelism of the Earth’s axis
Summer Solstice
• On June 21st , the Northern Hemisphere is tilted toward
the Sun and the Tropic of Cancer is receiving the Sun’s
vertical/direct ray which causes summer.
• The Southern Hemisphere is tilted away from the Sun at
this time causing it to have winter.
Winter Solstice
• On December 21st, the Southern Hemisphere is
tilted toward the Sun so the Tropic of Capricorn
gets the direct ray, giving them summer and us
winter.
Equinoxes
• On March 21st and September 22nd, neither
hemisphere is tilted toward the Sun, the direct
ray falls on the Equator, and every place on
Earth has equal day and night.
The tilt of the Earth on its axis is also
responsible for the day and night length:
What date is it here?
June 21st
What date is it here?
December 21st
Because of the tilt of the Earth’s axis, the sun’s
rays are only perpendicular (directly overhead) at
noon, between 23.5° N and 23.5° S during the
year
June 21 23.5° N
March/Sept. 21
Dec. 21 23.5° S
Tropic is from the Greek for "turning" describing the Sun's motion at the
solstices.
• The locations of the Arctic and Antarctic
Circles are determined by the positions of
the Sun…the maximum distance from the
Poles that can have 24 hours of daylight
“ALASKA MIDNIGHT SUN”
NORTHERN HEMISPHERE
FROM MARCH 21 UNTIL SEPTEMBER 21 –
SUN DOES NOT SET
TIMED PHOTOGRAPHY
The Sun’s apparent path is an arc that rises in the
east and sets in the west, traveling 15/hr and
reaching its highest point at solar noon
Summer maximum altitude of noon sun is 73.5°,
Winter is 26.5° Spring/Autumn?
Changing Altitude of the Noon Sun
LONG ISLAND, NEW YORK
Sun’s path for our latitude
HIGHEST
ALTITUDE AT
SOLAR NOON
AT 40° N
HOW DOES THE LOCATION OF SUNRISE
AND SUNSET CHANGE DURING THE
COURSE OF THE YEAR
NO MATTER WHERE YOU ARE ON
THE GLOBE, ON THE EQUINOXES
THE SUN WILL ALWAYS RISE DUE
EAST AND DUE WEST
WHEN THE SUN TRAVELS NORTH OF THE
EQUATOR TO THE TROPIC OF CANCER ON JUNE
21ST (23.5°N), THE SUNRISE AND SUNSET
POSITIONS WILL MOVE NORTH OF EAST.
WHEN THE SUN TRAVELS SOUTH OF THE
EQUATOR ° TO THE TROPIC OF CAPRICORN ON
DECEMBER 21ST (23.5°S), THE SUNRISE AND
SUNSET POSITIONS WILL MOVE SOUTH OF EAST
Apparent Motion of Sun due to
Rotation and Revolution
• Rotation: Sun – apparent daily path from
sunrise to sunset has the shape of an arc
moving l5° per hour.
• Revolution: However, the sun’s arc
changes both its position and its length
with the seasons
PROOF OF REVOLUTION – SEASONS
The apparent rising and setting of the sun is due to
rotation
Changes in the altitude of the path and horizon position
of sunrise and sunset are due to revolution
Season
Sunrise
Sunset
Summer
North of
east
North of
west
Equinox
Fall/Spring
Winter
Due east
Due West
South of
east
South of
west
EQUINOXES – DUE EAST
SUMMER SOLSTICE
NORTH OF EAST
WINTER SOLSTICE
SOUTH OF EAST
The Moon
The Moon appears to rise in the east and
set in the west, moving at a rate of 15/hr,
due to Earth’s rotation
• However, the Moon is also orbiting Earth at a
rate of 13/day. Since it takes the Earth 50
minutes to “catch up” with the Moon’s orbit, the
Moon rises 50 minutes later each day.
Moon Orbits the Earth – but appears to
rise in the east and set in the west due to
earth’s faster rotation
Moon’s Rotation = Moon Revolution
 rotation period =
27.3 days
 revolution period =
27.3 days
 Moon shows the same
“face” to the Earth
 Velocity: 360°/27.3
days = 13° per day
Moon Phases
Half of the moon is always receiving light
from the sun.
The “lit” portion that is visible to earth
varies as the moon REVOLVES around
earth. THIS CAUSES THE PHASES OF
THE MOON.
Visible portion of moon growing = waxing
Visible portion of moon shrinking = waning
Lunar Phases
Cycle of Lunar Phases
Takes 29.53 days
This is because when moon gets back to
its original position in 27.3 days, the earth
has moved 1°/day or about 27°.
The moon moving at l3°/day takes about 2
days to catch up with Earth and align with
it and the sun in a new moon phase.
What we observe from earth
Ocean Tides
• An ocean tide refers to
the cyclic rise and fall of
seawater. Tides are
caused by slight
variations in gravitational
attraction between the
Earth and the moon and
the sun. Tides are cyclic
primarily because of the
cyclical influence of the
Earth's rotation.
Moon 6.2 million km. from Earth
• St. Michel, N. coast of
France
• ~16.8 m highest tidal
range in Nova Scotia
Nova Scotia Tides
• Wolfville, NS (16 m tidal range)
• Diurnal Tides (one high and one low every 12
hours and 25 minutes
Without the Moon, all the water on the Earth
would be evenly distributed and we would not
experience tides.
• The figure below shows how the position of the moon
affects where high and low tides form.
Daily Cycle of Tides
 Driven by the gravitational pull of the moon and Sun
 Affected by nearness of the moon more than the size
of the Sun
 As earth rotates, the tides move around the Earth
alternating between high and low tides
approximately every 6 hours.
• Timing the Tides Tides occur at different times each
day because the Earth rotates more quickly than the
moon revolves around the Earth.
• Bay of Fundy high – low tide
• http://www.youtube.com/watch?v=YiHP8E
G_Wq4
Spring Tides – Neap Tides
 Tidal range: Difference
in height between high tide
and low tide
 Spring Tides: new and
full moon phase – greatest
range
 Neap Tides: 1st and 3rd
Quarter phases – smallest
range
Lunar and Solar Eclipses
The Moon orbits the Earth, and travels
with the Earth about the Sun.
Sometimes the Moon gets between the
Earth and the Sun, causing solar
eclipses, and sometimes the Moon goes
into the Earth's shadow, causing lunar
eclipses.
Solar Eclipses – can occur only
during the new moon phase
Lunar eclipses – can occur only
during the full moon phase
Solar Eclipses
Solar Eclipses Occur when moon’s
orbit intersects ecliptic during the
new moon phase
If Lunar eclipses happen in the full moon phase and
solar eclipses happen in the new moon phase, why
don’t they happen every month???
As the Moon orbits the Earth, its orbit is tilted slightly
(about 5 degrees) from earth’s orbital plane (ecliptic)
The moon crosses earth’s plane twice during its orbit. If
this crossing happens at the new moon phase, the Moon
will be lined up with the Sun and pass in front of it.
This alignment has to be perfect in order for the Moon to
completely cover the Sun for a total solar eclipse (the
sky will darken just like nighttime, and the stars will be
visible) Total solar eclipses last about 7 minutes and
occur in a very narrow range across Earth. Full moon
phase = total lunar eclipse if perfect.
Total Solar Eclipses only last about 7
minutes in a very narrow range across
Earth