Observations of the Sky. What patterns, motions, and events do we notice in the sky? What theories...

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?

Observations of the Sky

How does the sky change over…

A day?A night?A month?A year?A lifetime?A millenium?

Why do we see these changes?


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?


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


A day?The Sun rises and sets at an angle

… but angle isn’t the same everywhere!

Oakland

Venezuela

Rio New Zealand

Canada Alaska

Sunrise 9 February in the North America

Sunrise 9 February in the Southern Hemisphere


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.


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


A night?In the Southern Hemisphere, the sky

seems to rotate the other way! (check out http://www.youtube.com/watch?v=XfRFCdYCcZA


A night?Some constellations always visible (circumpolar)


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?


A “moonth?”The Moon goes through phases


A “moonth?”We see slow change in which

constellations are visible at night


A “moonth?”The Sun’s slowly shifts “in front of” different

zodiac constellations

What have we learned? Why do the constellations we see depend

on latitude and time of year?

— Your location determines which constellations are hidden by Earth.

— Time of year determines the location of the Sun in the sky.


A year?Sun’s rising/setting points, and noon-time

height return to the same points.

Constellations return to the same pattern

But planets “wander” in front of the zodiac at different rates, and even dance!

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.


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?

Sun’s “speed” across the sky varies

lower loop = Winter, Earth moves faster in its orbit

Upper loop = Summer, Earth moves slower in its orbit

How does the sky change over a year?

The analemma

Bergamo, Italy

Photos by John Nystuen, University of Michigan


A year?Summer

Sun higher in the sky, shining more DIRECTLYSun up longer, more hours in the day

WinterSun lower in the sky, less direct lightSun 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.)

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.

TRUE OR FALSE!

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


A year?Constellations return to the same pattern

But planets “wander” in front of the zodiac at different rates, and even dance!

Tunc Tezel, apod031216


A lifetime?Annual patterns repeat: Moon Phases,

Seasons, constellationsLonger patterns apparent: Planetary

motions, solar eclipses


A millenium?Slow change in location of “North” star


A millenium?Slow change in location of “North” star

Slower, even smaller change in direct positions of stars themselves

Observations of the Sky. What patterns, motions, and events do we notice in the sky? What theories...

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