Transcript The Reasons for Seasons on Earth

Retrograde motion of Mars, June to November, 2003
Who is this guy?
Use SkyGazer or StarCal to
find out!
The Four Seasons
Is the changing seasons caused by the change
in the distance between the Sun and the Earth?
No. If it is, then
– The northern and southern hemisphere should have
the same season, not opposite season like we have.
– We should experience real seasonal changes in
Hawaii also.
–…
The Four Seasons: The Effect of the Tilt
of Earth’s Rotation Axis
Factors Affecting Earth’s Temperature
Distance to heat source
Web Project: tutorial on
www.astronomyplace.com ,
Chapter 2, Time and Seasons
Illumination Angle
Click on the Sun to start animation
Noon, Summer Solstice
Sunrise/Sunset,
Winter Solstice
Solstices and Equinoxes
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Equinox: An equinox is one of two opposite points
on the celestial sphere where the celestial equator and
ecliptic intersect.
Solstice: A solstice is either of the two times of the
year when the sun is at its greatest distance from the
equator.
Winter Solstice
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•
Spring Equinox ~ March 21
Summer Solstice ~ June 21
Fall Equinox ~ September 22
Winter Solstice ~ December 21
Ecliptic Plane
Spring Equinox
Celestial Equator
The dates of the equinoxes and solstices are only
approximate dates.
– The actual length of a year is about 365 ¼ days (365
days, 5 hours, 49 minutes), not exactly 365 days. We
have to add an extra day to a year every four years to
keep the seasons synchronized with the seasons (leap
year). Over a longer period of time, we need to skip a
leap year to compensate the extra minutes we add in
every leap year to keep the calendar in sync.
Fall Equinox
Fall Solstice
The Reasons for the Seasons on
Earth
• The seasons on Earth are caused by the 23.5º tilt of its
rotation axis, and its revolution around the Sun.
• Although the distance of the Earth to the Sun changes
slightly as it orbits around the Sun, its effect is not big
enough to cause the four seasons.
• The seasons for other planets may be due to the
changing distance to the Sun, not the tilt of their
rotation axis…For example, the orbit of Pluto has a
very large eccentricity.
The Effects of Precession
What changes do you expect in these things (or phenomena)
13,000 years later when the Earth’s rotation axis is pointed
toward Vega?
A. World Atlas (map of Earth)? No change.
B. Star chart (map of sky)?
 Location of celestial north would be different
C. Seasons?
− We will still have four seasons, since the tilt of the rotation axis of
Earth with respect to the ecliptic plane is not changed. But the time of
the season would be changed. Summer would be in January, and
winter would be in July.
D. Constellations?
− The patterns of stars in the sky would not change much…but their
position with respect to celestial north would be different).
 homework#2
The Moon
Facts about the Moon…
• We see the moon changes its
appearances and position in
the sky with approximately
30-day cycle.
• Unlike the stars, Moon can
also be seen during the day.
• We also see that the Moon is
always involved in eclipses,
and
• Its position seems to be well
correlated with the tide of the
ocean.
• Understanding the motion of
the Moon with respect to the
Sun and the Earth will explain
these phenomena.
The moon revolves
around the earth
with a period of
about 29 ½ days ~
one month!
Motions of the Sun-Moon-Earth System
1. Earth orbits the Sun every 365 ¼ days
2. Moon orbits Earth every 29 ½ days
3. The orbit of the Moon around Earth is
tilted about 18.5 degrees with repect to
the ecliptic plane
4. Moon’s orbit around Earth precesses with a
period of 18.5 years
Click on the image to start animation:
This is what the orbit of the Moon looks
like…
Click on the image to
start animation
• The changing phase of the Moon…
• The un-changing face of the Moon…
• Eclipses
– Lunar Eclipse
– Solar Eclipse
The Face and Phases of the Moon
• The phase (how much and which side of the Moon is
illuminated) of the Moon changes with about one month
period…
• The face (the features on the surface of the Moon) does not
change at all…
The Phase of the Moon
The phase of the
Moon depends on
the relative position
between the Sun,
the Earth, and the
Moon…
Phases of the Moon
Click on the image to start animation
Moon Rise/Set by Phase
Click on the image to start animation
Why do we Always See the Same Side
of the Moon?
The rotation period of the Moon with respect to the universe is exactly the same as
the rotation period of the Moon around the Earth.
Is this a coincidence?
No!  Tidal Lock, Chapter 5.
• The changing phase of the Moon…
• The un-changing face of the Moon…
• Eclipses
– Lunar Eclipses
– Solar Eclipses
Solar and Lunar Eclipses
Eclipse:
The total or partial obscuring
of one celestial body by
another…
The obscuration can be either
• One celestial body blocking
the view to the other:
– Solar eclipse---Moon
blocking Earth’s view to the
Sun…
• One celestial body is in the
shadow of another:
– Lunar eclipse---Moon is in
the shadow of the Earth…
Lunar eclipse image from http://www.mreclipse.com
Solar Eclipses
The solar corona is
revealed during a total
solar eclipse
• The corona is about one
millionth times fainter
than the disk of the Sun.
• Similar to observing stars
next to the Sun, the light
from the disk must be
blocked (by the moon, or
by special occulter in the
telescope) before we can
see the solar corona.
What Causes Eclipse?
• The Earth and Moon cast shadows.
• When either passes through the other’s shadow, we have an eclipse.
• Because the Sun is an extended bright object, there are two different
regions of the shadow:
– Penumbra is partially illuminated
– Umbra is completely dark
Click on the image to start animation
Lunar eclipses
• Lunar eclipses happens
when the Moon passes
through the shadow of the
Earth
– Everybody on the night side
of Earth can see the lunar
eclipse.
• Lunar eclipses can be
partial, penumbral, or
total.
• Lunar eclipses can occur
only at full moon.
Click on the image to start animation
Solar eclipses
• Solar eclipses occur when
the shadow of the Moon
falls on the surface of
Earth
– Only people in the shadow
can see the eclipse.
• Solar eclipses can be
partial, annular, or total.
• Solar eclipses can occur
only at new moon.
Click on the image to start animation
Eclipse Path
When total or ring solar eclipse happens…
• The diameter of the umbra of lunar shadow is no more than
270 km…you can see the total eclipse only if you are in a very
narrow and long eclipse path.
– In the 1990 total eclipse that passed through the big island of Hawaii,
people on Maui and Oahu cannot see the total eclipse!
• The diameter of the penumbra of the lunar shadow is about
7000 km (Earth’s diameter is about 13,000 km). So, the region
that partial eclipse can be seen is quite large…and people on
Maui and Oahu could see partial eclipse!
 Eclipse Prediction
Total Solar Eclipse
What’s the difference between a total and ring eclipse?
The distance between the Earth and the Sun.
You don’t see the Sun at all if
you are in the umbra
Sun
Click on the Sun to
start animation
Moon
You see the partial Sun if
you are in the penumbra
You see the whole Sun
outside of the shadow
Surface of the Earth
Ring Eclipse
• Ring eclipse happens when the Moon does not
completely block the Sun, like partial eclipses…
• The umbra is NOT
completely dark!
• You see a ring if you are
in here
Sun
Click on the Sun to start
animation
Surface of the Earth
Moon
You see the partial Sun if
you are in the penumbra
You see the whole Sun if you are
outside of the shadow
Distance Between the Moon and Earth
• Like the Earth’s orbit
around the Sun, the orbit of
the Moon around the Earth
is not a perfect circle.
– Eccentricity of Earth’s orbit is
~ 0.016
• The distance between the
Moon and Earth change.
This is why we have total
and ring eclipses
– Eccentricity of Lunar orbit is
~ 0.05
Solar Eclipses: A Coincidence?
• What if the Moon is closer (say twice as close)
to the Earth?
• What if the Moon is farther (say twice as far)
away from the Earth?
• What if the Moon is two-time bigger?
• What if the Moon is two-time smaller?
How Often Does Eclipse Occur?
• Do we have one solar and one lunar eclipse every
month, since we have a full and a new Moon every
month?
Click on the image to start animation
How Often Does Eclipse Occur?
• Do we have one solar and one lunar eclipse every six month,
when the nodes line-up with the Earth-Moon line?
– NO! Because of the precession of the lunar orbit!
Nodes: the two
points when the
Moon crosses the
ecliptic plane
• Only when the
nodes are
located right
between the
Earth-Sun line
can solar
eclipses occur.
The Precession of Lunar Orbit
• The orbital motion of the
Moon around Earth slowly
precesses with an 18.6 year
cycle as the Earth orbit the
Sun…
Click on the image to start animation
How Often Does Eclipse Occur?
•
The combination of these motions of the Moon
1. 29 ½ day orbital motion around Earth,
2. 5º tilt of the orbit w.r.t. the ecliptic, and
3. Precession of the lunar orbit w.r.t. the ecliptic,
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Eclipse occurs with a period of about 18 years and
11-1/3 days  Soras cycle
But these eclipses don’t happen at exactly the same
location over the Soras cycle…
Solar Eclipse Forecast
Solar eclipses from 2004 to 2030
Knowing the orbit
of the Earth and
the Moon, we can
now calculate the
time and path of
solar eclipses with
great accuracy.
Back to Eclipse Path
Eclipses: Summary
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The parties involved: Sun, Moon, and Earth
Motion of the Moon around Earth:
1. 29 ½ day revolution of the Moon around the Sun
2. Tilt (~5º) of the lunar orbit (around the Earth) w.r.t. the Ecliptic plane (Earth’s
orbital plane around the Sun)
3. The precession of the lunar orbit w.r.t. Earth-Sun direction
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Solar eclipse happens when the Moon is between the Earth and the Sun.
The size and distance of the Moon need to be just right for us to see total
eclipse.
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The changing distance between the Earth and the Moon explains the
occurrences of the total and ring solar eclipses.
The changing distance between the Earth and the Sun, and the Earth and the
Moon explains the difference in the duration of the solar eclipses.
Lunar eclipse happens when Earth is between the Moon and the Sun.