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Astro110-01 Lecture 5

Eclipses of the Moon and theSun, and other odd events

Lunar eclipseSolar eclipse

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Brief Review

• Why do we see phases of the Moon?• What causes eclipses?

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Why do we see phases of the Moon?

• Lunar phases are aconsequence of theMoon’s 27.3-dayorbit around Earth.

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Phases of Moon

• Half of the Moon isilluminated by theSun and half is dark.

• We see a changingcombination of thebright and darkfaces as the Moonorbits Earth.

How to Simulate Lunar Phases

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Phases of the Moon

Phases of the Moon

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Moon Rise/Set by Phase

Time the Moon Rises and Sets for Different Phases

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Phases of the Moon: 29.5-day cyclenew

crescent

first quarter

gibbous

full

gibbous

last quarter

crescent

waxing• Moon visible in afternoon/evening• Gets “fuller” and rises later each day

waning• Moon visible in late night/morning• Gets “less illuminated”

and sets later each day

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Thought Question

A. First quarter

B. Waxing gibbous

C. Third quarter

D. Half moon

It’s 9 A.M. You look up in the sky and see amoon with half its face bright and half dark.What phase is it?

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A. First quarter

B. Waxing gibbous

C. Third quarter

D. Half moon

It’s 9 A.M. You look up in the sky and see amoon with half its face bright and half dark.What phase is it?

Thought Question

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We see only one side of the Moon

Synchronous rotation:The Moon rotates exactlyonce with each orbit.

This is why only one sideis visible from Earth.

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What causes eclipses?

• The Earth and Moon cast shadows.

• When either passes through the other’s shadow, wehave an eclipse.

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Lunar Eclipse

Lunar Eclipse

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When can eclipses occur?

• Lunar eclipses canoccur only at fullmoon.

• Lunar eclipses canbe penumbral,partial, or total.

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Solar Eclipse

Evolution of a Total Solar Eclipse

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When can solar eclipses occur?• Solar eclipses can occur

only at new moon.• Solar eclipses can be partial,

total, or annular.

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Why don’t we have an eclipse at every new and fullmoon?—The Moon’s orbit is tilted 5° to ecliptic plane.

—So we have about two eclipse seasons each year, with a lunareclipse at new moon and solar eclipse at full moon.

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Summary: Two conditions must be metto have an eclipse:

1. It must be a full moon (for a lunar eclipse) or anew moon (for a solar eclipse).

AND2. The Moon must be at or near one of the two

points in its orbit where it crosses the eclipticplane (its nodes).

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Predicting Eclipses• Eclipses recur with the 18 year, 11 1/3 day

saros cycle, but type (e.g., partial, total) andlocation may vary.

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What have we learned?

• Why do we see phases of the Moon?

—Half the Moon is lit by the Sun; half is inshadow, and its appearance to us isdetermined by the relative positions of Sun,Moon, and Earth.

• What causes eclipses?

—Lunar eclipse: Earth’s shadow on the Moon

—Solar eclipse: Moon’s shadow on Earth

—Tilt of Moon’s orbit means eclipses occurduring two periods each year

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2.4 The Ancient Mystery of the Planets

• What was once so mysterious about themovement of planets in our sky?

• Why did the ancient Greeks reject the realexplanation for planetary motion?

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Planets Known in Ancient Times• Mercury

— difficult to see; alwaysclose to Sun in sky

• Venus— very bright when visible;

morning or evening “star”

• Mars— noticeably red

• Jupiter— very bright

• Saturn— moderately bright

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What was once so mysteriousabout the movement of planets in our sky?

• Planets usually move slightly eastward from night tonight relative to the stars.

• But, sometimes they go westward relative to the starsfor a few weeks: apparent retrograde motion.

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We see apparent retrograde motion whenwe pass by a planet in its orbit.

Mars Retrograde Motion

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Explaining Apparent Retrograde Motion

• Easy for us to explain: this occurs when we“lap” another planet (or when Mercury orVenus laps us).

• But it is very difficult to explain if you thinkthat Earth is the center of the universe!

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Why did the ancient Greeks reject the realexplanation for planetary motion?

• Their inability to observe stellar parallax was a major factor.

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The Greeks knew that the lack of observableparallax could mean one of two things:

1. Stars are so far away that stellar parallax istoo small to notice with the naked eye.

2. Earth does not orbit Sun; it is the center ofthe universe.

With rare exceptions, such as Aristarchus, theGreeks rejected the correct explanation (1)because they did not think the stars could be thatfar away

Thus the stage was set for the long, historical showdownbetween Earth-centered and Sun-centered systems.

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What have we learned?

• What was so mysterious about planetary motion in oursky?

— Like the Sun and Moon, planets usually drift eastward relativeto the stars from night to night; but sometimes, for a few weeksor few months, a planet turns westward in its apparentretrograde motion.

• Why did the ancient Greeks reject the real explanationfor planetary motion?

— Most Greeks concluded that Earth must be stationary, becausethey thought the stars could not be so far away as to makeparallax undetectable.

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In class exercises

New PlanetSuppose we discover a new planet in another

solar system that has a circular orbit and anaxis tilt of 35º.Would you expect this planet to haveseasons?If so, would you expect them to be moreextreme than the seasons on Earth? If not,why not?

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View from the MoonSuppose you lived on the Moon, in which case you would

see Earth going through phases in your sky. Assumeyou live near the center of the face that looks towardEarth.

a. Suppose you see a full Earth in your sky. Whatphase of the Moon would people on Earth see?Explain

b. Suppose people on Earth see a full Moon. Whatphase would you see for Earth? Explain

c. Suppose people on Earth see a waxing gibbousMoon. What phase would you see for Earth? Explain

d. Suppose people on Earth are viewing a total lunareclipse. What would you see from your home on theMoon? Explain

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A Farther Moon

Suppose the distance to the Moon weretwice its actual value.

Would it still be possible to have a totalsolar eclipse? Why or why not?

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A Smaller Earth

Suppose Earth were smaller.

Would solar eclipses be any different? Ifso, how?

Would it still be possible to have a totalsolar eclipse? Why or why not?