Transcript winter

The Motions of the Earth
Evidence of Movement
and the Consequences
How do we know that the
Earth is Round?
• First, the sky is curved. This is a
reflection of the Earth’s curvature.
• We can see the Earth’s shadow during
a lunar eclipse.
• Different constellations are visible in
Southern Hemisphere than here.
• Sun’s height changes with latitude.
Lunar Eclipse: Earth’s
shadow on the Full Moon
We see different constellations
than South Africa
The closer to the equator you are, the higher the
sun is in the sky. The Sun’s rays are most direct
on the equator, and most slanted in polar
regions.
Sun Height March 21
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Ht. of Sun (°)
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Rochester
Key West
Fairbanks
Kampala
Buenos Aires
Rays on Sun on Earth
Paths of Sun Across the Sky
0 °lat.
43 °N
lat
80 °N lat
east
south
west
What proof is there that the
Earth rotates in 24 hours?
• The Sun, the Moon, the Planets, and the
“fixed” stars all move from east to west.
• The stars leave trails across the sky that
go around the celestial poles.
• Physical evidence comes from the
Foucault pendulum.
• When it’s noon here, it’s midnight in
Japan.
Star Trails With Polaris
Near the Center
20°
33°
Similar Pattern
in the
Southern
Hemisphere
…except in the opposite
direction to the North!
South Celestial Pole
Foucault Pendulum
All dominoes fall down in a 24 hour period.
6 PM
12
Mid
North Pole
12 noon
6 AM
Different Time Zones: one of
the results of Earth’s rotation
43rd
Parallel
Portsmouth
Rochester
Muskegon
Madison
Sun Height 43 °N
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30
Ht. of Sun (°)
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Portsmouth, NH
Rochester, NY
Muskegon, MI
Madison, WI
The farther west you go, the later the sun rises and
sets. Solar noon is later too.
Sundial - Tracking the Sun
What is wrong
with this
picture?
(It’s not the
watch!)
Time Zone Maps
The Earth’s Revolution!
How do we know that the
Earth revolves around the
Sun in 365 days?
• The Sun completes one trip “through”
the zodiac in one year.
• The sky looks the same on Sept. 23,
2010 as it did on Sept. 23, 2009.
• The sun returns to the same place at
the beginnings of each season, which
can easily be marked.
The Sun appears to move
through the 12 signs of the
Zodiac in one year cycle.
Animation
Orion dominates our southern
sky every February.
For centuries people have
marked sun rise positions for
midsummer and midwinter.
The Sun
Dagger
• In Chaco Canyon, New
Mexico, there is a rock
painting aligned with the
equinoxes and solstices.
• This gave the Anasazi
people a way to check their
calendar and make
corrections as needed.
• Other cultures had similar
monuments to help them
keep track with the yearly
cycle of the sun.
The Effects of the Earth’s Tilt
• The Earth tilts 23.5° but it always points toward
the North Star (Polaris).
• Because the two poles tilt in the opposite
direction, seasons are reversed in Northern and
Southern Hemispheres.
• The height of the Sun changes with the seasons.
• Length of daylight is long in summer and short
in winter.
Seasons Animation
The poles
always point in
the same
direction,
which means
the Earth
either leans
toward the
sun or away
from it.
Seasons are reversed in the
two hemispheres
summer
winter
winter
summer
The higher the sun’s path, the
higher the intensity…
WINTER
SUMMER
Ht. of Sun - Rochester, NY
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21-Dec
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Time of Day
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Altitude of Sun (°)
60
Here in
Rochester, the
Sun is high in
the sky in
summer, and
low in winter.
(The top of the
sky is 90°)
Rochester
Alaska
If you live in Arctic or Antarctic
regions the effect of the tilt is
even greater
Ht. of Sun - Fairbanks, AK
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21-Mar
21-Jun
21-Dec
In Alaska, days are very long in summer, and very
short in winter. The same is true for Antarctica.
For locations in the Southern Hemisphere, the seasons
are reversed, so Dec. 21st is the longest day, and
June 21st is the shortest.
Ht. of Sun Buenos Aires, Arg.
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The Earth’s Precession
The Earth slowly wobbles like a top…once every 26,000 years.
By an incredible coincidence, Polaris is almost
directly over the North Pole. So it can be used
to help us navigate.
This is a temporary situation, however. Since the
Earth is wobbling like a top, the pole slowly
moves. This is called precession.
The Precession Cycle
Now the North Celestial Pole
is near Polaris (off by about 1
degree).
In the days of the Egyptian
pharaohs, it was near Thuban
in Draco.
In around 12,000 years the
wandering pole will point
toward Vega, but there will be
centuries when there is no
North Star.
Quick Quiz…
• How do we know the earth is round?
• What proof do we have that the Earth rotates
once every 24 hours?
• If we lost all our calendars, how could we
determine the length of a year?
• Why are seasons reversed in the southern
hemisphere from the northern hemisphere?
• What is meant by precession? What does
that have to do with Polaris?