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Transcript
Observations of the Sky
Observations of the Sky
What patterns, motions, and events do we
notice in the sky?
What theories can we create to explain
them?
How can these theories be tested?
How does the sky change over…
A day?
A night?
A month?
A year?
A lifetime?
A millenium?
Why do we see these changes?
How does the sky change over…
A millenium?
Slow change in location of “North” star
How does the sky change over…
A millenium?
Slow change in location of “North” star
How does the sky change over…
A day?
The Sun rises in the East, sets in the West
It rises and sets at an angle
It takes about 24 hours from sunrise to
sunrise
Good Science => Careful observations?
How does the sky change over…
A day?
The Sun rises in the East, sets in the West
… for *most* observers, but not all!
… not always due East, nor due West
How does the sky change over…
A day?
The Sun rises and sets at an angle
… but angle isn’t the same everywhere!
Oakland
Canada
Alaska
Sunrise 9 February in the North America
Venezuela
Rio
Sunrise 9 February in the Southern Hemisphere
New
Zealand
How does the sky change over…
A night?
The Moon, planets, and most stars also
rise in the East, set in the West, at an angle
The sky seems to rotate around a fixed
point – Polaris.
How does the sky change over…
A night?
The sky rotates around Polaris (check out
http://www.youtube.com/watch?v=3V3rmDG5J8A )
Did you know?
 The ANGLE of Polaris above the horizon
indicates your latitude in the Northern
Hemisphere
Altitude angle
Did you know?
 The ANGLE of Polaris above the horizon
indicates your latitude in the Northern
Hemisphere
Did you know?
 Polaris is really *3* stars;
 Brightest is yellow “giant” ~ 5 x more massive
than our Sun
 Pulsates & changes in brightness (“Cepheid”)
 Distance 300-433 ly away!
How does the sky change over…
A night?
In the Southern Hemisphere, the sky
seems to rotate the other way! (check out
http://www.youtube.com/watch?v=XfRFCdYCcZA
How does the sky change over…
A night?
Some constellations always visible (circumpolar)
How does the sky change over…
A “moonth?”
The Moon goes through phases
We see slow change in which
constellations are visible at night
The Sun’s slowly shifts “in front of” different
zodiac constellations
Good Science => Careful observations?
How does the sky change over…
A “moonth?”
The Moon goes through phases
How does the sky change over…
A “moonth?”
We see slow change in which
constellations are visible at night
Pisces!
Changing constellations at sunset…
Aquarius!
Pisces!
Aquarius!
How does the sky change over…
A “moonth?”
The Sun’s slowly shifts “in front of” different
zodiac constellations
 H. A. Rey
(Author of
Curious
George!)
“The Stars”
1980,
from Houghton
Mifflin Co.
How does the sky change over…
A year?
Constellations we see at night return to the
same pattern
Sun’s rising/setting points, and noon-time
height return to the same points.
Sunrise Planting Calendars –
Southwestern US Peoples
Walpi (Hopi) horizon calendar. McCloskey, after Stephen.
Observations from Bear Clan House.
Sunrise Planting Calendars –
Southwestern US Peoples
From Zeilik (1985) Hopi planting calendar. Village of Shungopavi.
How does the sky change over…
A year?
The Sun’s height changes slowly
Photo of the sun, taken
@ same time every day,
over a year
How does the sky change over a year?
How does the sky change over a year?
Sun’s “speed” across the sky varies
Upper loop = Summer,
Earth moves slower in
its orbit
lower loop = Winter,
Earth moves faster
in its orbit
The analemma
Bergamo, Italy
Photos by John Nystuen, University of Michigan
How does the sky change over…
A year?
Summer
Sun higher in the sky, shining more DIRECTLY
Sun up longer, more hours in the day
Winter
Sun lower in the sky, less direct light
Sun not up as long
TRUE OR FALSE?
Earth is closer to the Sun in summer and
farther from the Sun in winter.
TRUE OR FALSE?
Earth is closer to the Sun in summer and
farther from the Sun in winter.
(Hint: When it is summer in the United
States, it is winter in Australia.)
TRUE OR FALSE!
Earth is closer to the Sun in sun summer
and farther from the Sun in winter.
• Seasons are opposite in the N and S
hemispheres, so distance cannot be the
reason.
• The real reason for seasons involves Earth’s
axis tilt.
What causes the seasons?
Seasons depend on how Earth’s axis affects the directness of sunlight.
Direct light causes more heating.
Directness of Light
Axis tilt changes directness of
sunlight during the year.
Why Does the Flux of Sunlight Vary
Sun’s altitude changes w/ seasons
Sun’s position at noon in summer:
higher altitude means more direct
sunlight.
Sun’s position at noon in winter:
lower altitude means less direct
sunlight.
The Real Reason for Seasons
 Earth’s axis points in the same direction (to
Polaris) all year round, so its orientation
relative to the Sun changes as Earth orbits
the Sun.
 Summer occurs in your hemisphere when
sunlight hits it more directly; winter occurs
when the sunlight is less direct.
 AXIS TILT is the key to the seasons; without
it, we would not have seasons on Earth.
Why doesn’t distance matter?
• Variation of Earth–Sun distance small ~ 3%
• Small variation overwhelmed by effects of
axis tilt.
How do we mark the progression of the
seasons?
• We define four special points:
summer solstice
winter solstice
spring (vernal) equinox
fall (autumnal) equinox
Recognize solstices and equinoxes by
the Sun’s path across the sky.
Summer solstice: Highest path,
rise and set at most extreme
north of due east
Winter solstice: Lowest path,
rise and set at most extreme
south of due east
Equinoxes: Sun rises precisely
due east and sets precisely
due west.
Seasonal changes are more
extreme at high latitudes.
Path of the Sun on the summer solstice at the Arctic
Circle
How does the sky change over…
A year?
Constellations return to the same pattern
Sun’s rising/setting points, and noon-time
height return to the same points.
(5) Planets “wander” in front of the zodiac
at different rates, and even dance!
How does the sky change over…
A year?
(5) Planets “wander” in
front of zodiac at different
rates, & even dance!
Tunc Tezel, apod031216
How does the sky change over…
A lifetime?
Annual patterns repeat: Moon Phases,
Seasons, constellations
Longer patterns apparent: Planetary
motions, “syzygies”, solar eclipses
How does the sky change over…
A millenium?
Slow change in location of “North” star
How does the sky change over…
A millenium?
Slow change in location of “North” star
Slower, even smaller change in direct
positions of stars themselves