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ASTR 100
Lecture 5: Seasons, Moon
phases, Tides, Eclipses, and
Transits
• Seasons
• Moon phases
• Tides
• Eclipses
• Transits
A) Persephone ate six pomegranate seeds while she was in Hades’ kingdom?
B) The Earth moves in an ellipse, getting closer to the Sun in Summer?
C) The Earth’s 23.5° tilt makes days shorter in Winter, less sunlight?
D) The Earth’s 23.5° tilt means sunlight hits the Earth at a shallower angle in Winter, weaker sunlight?
Why Seasons?
Primary Reason: Stronger\Weaker Sunlight
Intensity at surface is weaker at more severe angle
because of longer path….
Same idea behind sloped armor
Notice this on a daily basis. increased
scattering by more atmosphere at Sunset
http://astro.unl.edu/classaction/animations/coordsmotion/transitmovie.html
http://astro.unl.edu/classaction/animations/coordsmotion/eclipticsimulator.html
Tilt, shadow, and orbit in action….
Secondary (not as important) Reason:
More/Less Sunlight
South pole:
24 hr day
Hawaii:
12 hr day
If the Earth’s axis was tilted by ~90° what would the seasons feel like?
What about ~0°?
Lunar Phases
We’ll cover a lot of this in Next Week’s
exciting exercise on Lunar phases
Tidal locking.
Lunar “Day” =
Lunar “Year” =
28 days
Tidal locking of the Moon to the Earth:
The force of gravity depends on how
close matter is to the matter pulling it.
<---- <-
Over billions of
years, the
friction from
this
deformation
slowed the
Moon’s
rotation to the
length of its
orbit around
Earth.
Eclipses:
Eclipses are an alignment of the
Sun, Earth, and Moon
Solar eclipse: Moon gets in between
Earth and Sun
Lunar eclipse: Earth gets in between
Moon and Sun
Why don’t we have two a month?
Moon’s orbit is 5 deg tilted above ecliptic
Why don’t we have two a month?
Moon’s orbit does not lie on the ecliptic http://astro.unl.edu/classaction/animations/lunarcycles/mooninc.html
…An eclipse from Earth’s perspective
Shadows aren’t just shadows
Types of Lunar eclipses:
Total, partial, penumbral
Battle of Halys: May 28th, 585 B.C. …6:00 pm
Types of Solar eclipses:
Total, partial, annular
Why are lunar eclipses more frequent
than solar eclipses?
Scale model of Earth-Moon system
There was a transit of Venus 6/5/2012
Transits: “Baby solar eclipses” by a planet (an alignment of any three celestial bodies is
called a syzygy)
Key Terms:
partial/total/annular/penumbral solar/lunar eclipse,
Transit, tides, umbra, penumbra
Key Ideas:
What causes the seasons?
What causes eclipses and how are they classified?
Why causes tides?
What are transits and what needs to be true about the
transiting body?
The fact that Earth’s orbit is an
ellipse is not a reason for the seasons:
Distance to Sun Speed of Earth in orbit
Distance to Sun/variable
speed does produce effect
called the “analemma”
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