Transcript Astro 10 Lecture 1 - Intro to Astronomy

Astro 10-Lecture 3:
Seasons, Phases of the Moon and Eclipses
Dr. Eric Korpela
[email protected]
http://setiathome.ssl.berkeley.edu/~korpela/
(510) 643-6538
The Celestial Sphere
The Celestial Sphere
Concept Test
Another line on the celestial sphere
The Earth’s pole is tilted with respect to
the ecliptic
The sun appears to travels along the ecliptic
over the course of the year
The orbits of the other planets are also in
the ecliptic plane
The orbits of the other planets are also in
the ecliptic plane
Precession
Precession
Defining Seasons
•
•
•
•
Climate variations
Day length variations
Winter: short, cold days
Summer: long, warm days
Models of Possible Causes
•
Changing distance of Earth from Sun
(closer=warmer)
•
Variation in the Sun’s energy
(Sun changes)
•
Tilt of Earth’s spin axis relative to orbital axis
(tilt)
How do we choose between
them?
Choosing a Model
• Remember that any model must be supported
by evidence!
• Each of the models mentioned above makes
predictions that we can test
Preferring a Model
• Do they all explain the variations in day
length?
– NO! Variations in the Sun’s energy or
changing distance from the Sun do not cause
changes in the length of a day
– BUT – Maybe the day length variations are just
a coincidence, and the climate variations are
caused by one of these
How do we rule out a model FOR SURE?
Ruling out a Model
• If closer = warmer is the cause of the seasons, then
this model predicts that the size of the Sun
should vary over the course of a year
– Observation: But it doesn’t!
• Also, if seasons were caused by a changing EarthSun distance, summer here would occur at the
same time as summer in Australia!
– Observation: But it doesn’t!
• Seasons caused by changing Earth-Sun distance?
The orbit of the Earth
• Which of these is the most correct depiction
of the elliptical orbit of the Earth?
Ruling out another Model
• Sun changes model makes at least 2
predictions
– Celestial objects that shine by reflected light
should be brighter in summer
– Summer should be at the same time all over the
Earth
• Seasons caused by variations in Sun’s
energy output?
Tilt Model
• We’ve ruled out 2 of our 3 models!
• How does the tilt model explain the seasons?
– Distance to the sun is less for the hemisphere pointed
towards the Sun?
• NO! Remember the scale model!
– Tilt causes change in day lengths => more hours of
sunlight.
• YES!
– Tilt causes change in angle of sunlight => warmer days
• YES!
The size of the Earth
• Which of these is the most accurate
depiction of the relative sizes and distances
of the Sun, Earth, and Moon?
Tilt Model
Tilt Model
In the summer the sun is
higher in the sky. It
spends more time (>12
hours) above the horizon
than below it.
In the winter, the sun is low
in the sky, so it spends
less time above the
horizon (< 12 hours).
Tilt Model
Amount of light captured by a certain area of ground
decreases with angle
Tilt Model
Amount of light captured by a certain area of ground
decreases with angle
Tilt Model
Summer solstice, the sun reaches
(90-latitude)+23.5 degrees
above the horizon. (75.5
degrees for us)
Winter solstice, the sun only
reaches latitude -23.5 degrees
above the horizon (28.5
degrees for us)
Difference in illumination per
square meter of ground:
sin(28.5)/sin(75.5)=0.5
50% less sun per square
meter at noon.
Tilt Model
Length of the day also differs…
For us, longest day (summer
solstice) of the year is 15
hours.
The shortest (winter solstice) is 9
hours 50 minutes.
Sunlight difference
9.8 hours/15 hours=0.65
35% fewer daylight hours
The Moon: The Earth’s Companion
• The moon is about 1/6th
the size of the Earth
• Distance is many times
the diameter of the earth.
The Moon: The Earth’s Companion
Phases of the Moon
Phases of the Moon
Why do we see phases?
Moon rotates to keep same face toward
the Earth
Eclipses
Eclipses: Lunar
Eclipses: Lunar
Eclipses: Lunar
Eclipses: Solar (Partial)
Eclipses: Solar (Total)
Eclipses: Solar (Total)
Eclipses: Solar (Total)
• A lucky accident. The Sun is 400x as big as the moon, but
it’s also 400x as large. They appear almost the same size
on the sky.
Eclipses: Solar
Eclipses: Solar (Annular)
• Happen when Earth
is nearer the sun than
average (sun appears
bigger) or when the
moon is farther from
the earth than
average (moon
appears smaller).
Eclipses: Solar (Annular)
Eclipses can only happen at certain
phases of the moon.
• If the moon is
between us and the
sun, it must be a new
moon.
• If we are between the
sun and the moon, the
moon must be full.
Then why don’t we have eclipses every
month?
Concept Test
• Tonight the moon is at first quarter…
– About what time did the moon rise?
– About what time will it set?
– Could there be an eclipse tonight?
Then why don’t we have eclipses every
month?