Ch. 2 Discovering the Universe for Yourself

Motion of StarsSeasons

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Why do stars rise and set?

Earth rotates west to east, so stars appear to circle from east to west.

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Nightly Motion of the Stars

Celestial Sphere

Celestial Sphere Rotation

Celestial Sphere Rotation

Celestial Sphere

Star A

Star B

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Figure 2

Horizon

Earth rotates west to east, so stars appear to circle from east to west.

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Nightly Motion of the Stars

• For stars (& the Moon and planets) that appear in the southern sky: Stars first rise near the eastern horizon, move upward and toward the south, and then move down and set near the western horizon.

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Nightly Motion of the Stars

Celestial Sphere

Celestial Sphere Rotation

Celestial Sphere Rotation

Celestial Sphere

Star A

Star B

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Figure 2

Horizon

• Imagine looking toward the North. What do stars appear to do over the course of an evening?

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Our view from Earth:• Stars near the north celestial pole are circumpolar and

never set.• We cannot see stars near the south celestial pole.• All other stars (and Sun, Moon, planets) rise in east and

set in west.

Celestial Equator

Your Horizon

A circumpolar star never sets

This star never rises

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Quiz

What is the arrow pointing to?A. the zenithB. the north celestial poleC. the celestial equator

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Lecture-Tutorial (LT):Motion (pp. 3-6)

• Work with a partner!

• Read the instructions and questions carefully.

• Discuss the concepts and your answers with one another.

• Come to a consensus answer you both agree on.

• If you get stuck or are not sure of your answer, ask another group.

• If you get really stuck or don’t understand what the LT is asking, ask for help.

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What have we learned?• What does the universe look like from Earth?

– We can see over 2,000 stars and the Milky Way with our naked eyes, and each position on the sky belongs to one of 88 constellations

– We can specify the position of an object in the local sky by its altitude above the horizon and its direction along the horizon

• Why do stars rise and set?– Because of Earth’s rotation.

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Ch. 2.2 The Reason for Seasons

Our goals for learning:• What causes the seasons?• How do we mark the progression of the

seasons?• How does the orientation of Earth’s axis

change with time?

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QuizEarth is closer to the Sun in summer and farther from the Sun in winter.

A: TrueB: False

Hint: When it is summer in the U.S., it is winter in Australia.

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What causes the seasons?

Seasons depend on how Earth’s axis affects the directness of sunlight

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Direct light causes more heating.

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Axis tilt changes directness of sunlight during the year.

whydoesfluxsunlightvary clip14

Reasons for Seasons applet

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Sun’s altitude also changes with 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.

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Summary: 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.

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Lecture-Tutorial (LT):Seasons (pp. 91-96)

• Work with a partner!

• Read the instructions and questions carefully.

• Discuss the concepts and your answers with one another.

• Come to a consensus answer you both agree on.

• If you get stuck or are not sure of your answer, ask another group.

• If you get really stuck or don’t understand what the LT is asking, ask for help.

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Why doesn’t distance matter?• Variation of Earth-Sun distance is small — about

3%; this small variation is overwhelmed by the effects of axis tilt.

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How do we mark the progression of the seasons? • We define four special points: summer solstice winter solstice spring (vernal) equinox fall (autumnal) equinox

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We can recognize solstices and equinoxes by Sun’s path across 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.

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QuizWhen it is summer in the United States, in Australia it is:

A. WinterB. SummerC. It is always summer in Australia

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QuizWhat is the primary cause of the seasons?

A. In summer the (whole) Earth is closer to the SunB. In summer the tilt of the Earth’s axis makes the part of the

Earth we are on closer to the SunC. In summer the Sun is up for more hoursD. In summer the Sun climbs higher in the sky so its rays hit

the ground more directlyE. C and D

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QuizIf the tilt of the Earth’s axis to its orbital plane was 40 degrees, instead of 23 ½, but its distance from the Sun remained the same, what would happen to the seasons?

A. They wouldn’t change muchB. They would become less extreme–winter and summer

would be more alikeC. They would become more extreme–winter colder and

summer warmerD. The whole Earth would get colderE. The whole Earth would get warmer

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Quiz

A. True, because the Earth would be at the same distance from the Sun throughout its orbit, there would be no summer or winter.

B. True, it is the deviations from a circular orbit that create the seasons.

C. False, the seasons are due to the tilt of the Earth’s axis, not its distance from the Sun.

D. False, the poles would still be cooler than the equator and seasonal variations would therefore still exist.

If Earth’s orbit were a perfect circle, we would not have seasons.

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What have we learned?

• What causes the seasons?– The tilt of the Earth’s axis causes sunlight to

hit different parts of the Earth more directly during the summer and less directly during the winter

– We can specify the position of an object in the local sky by its altitude above the horizon and its direction along the horizon

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What have we learned?

• How do we mark the progression of the seasons?– The summer and winter solstices are when

the Northern Hemisphere gets its most and least direct sunlight, respectively. The spring and fall equinoxes are when both hemispheres get equally direct sunlight.

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