Astronomy Lecture 05/06

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This motion of the Sun on the sky traces out a path on the celestial sphere known as the

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This motion of the Sun on the sky traces out a path on the celestial sphere known as the

Ecliptic

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2

Ecliptic

The apparent path of the Sun, relative to the stars on the celestial sphere, over the course of a year

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Zodiac

The 12 constellations through which the sun moves as it follows the path on the ecliptic

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4

What angle does the ecliptic from on the celestial sphere?

23.5

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5

What causes seasons?

Depend on the orbit (revolution) of the earth around the sun and how the tilt of the Earth’s axis affects the directness of sunlight and length of days

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What is Summer Solstice?

Sunlight falls more directly on the Northern Hemisphere, making solar energy more concentrated and making the Sun’s path longer and higher through the sky

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What is Winter Solstice?

Sunlight falls more directly on the Southern Hemisphere than the Northern hemisphere

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Spring/Fall Equinox

The sun shines equally on both hemispheres

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When is Summer solcitce?

June 21

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When is Winter solstice?

December 21

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When is Spring Equinox?

March 21

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When is Fall Equinox?

September 22

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What does the Spring equinox mark?

The transition from the Southern Hemisphere being tipped towards the sun to the Northern Hemisphere being tipped towards the sun

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What does the Fall equinox mark?

Marks the transition from the Northern Hemisphere being tipped towards the sun to the Southern Hemisphere being tipped towards the sun

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On equinoxes, are the days equal length?

Yes

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Crossing points

Mark the Spring Equinox and Fall Equinox

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Why doesn’t distance matter?

Only small variation of Earth-Sun distance about 3%

This small variation overwhelmed by effects of axis tilt

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Because of its relationship to changing seasons, ancient cultures attached great significance to the ecliptic

Marks the center line of the Zodiac, a band of the sky divided into 12 parts of 30 degrees. (No longer of special importance in astronomy)

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Evening star

Any planet in the sunset sky

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Morning star

Any planet in the dawn sky

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Does Venus seasonally switch from morning to evening star?

Yes

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22

When does the axis of the Earth tilt?

Every 26,000 years

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23

How close is the Moon to the Earth?

380,000 km

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When does the Moon orbit the Earth?

Every 27.32 days

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Where does the Moon set and rise?

Rise in the east and set in the west every night

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What causes the phases of the Moon?

The moon reflects light from the sun

Half of the Moon is illuminated by the Sun and half is dark

We see a changing amount of the bright and dark faces as the Moon orbits

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New Moon

Occurs when the Moon lies in the same direction as the sun

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Full Moon

Occurs when the Moon is opposite the sun

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Synchronous rotation

The Moon rotates exactly once with each orbit (why only one face is visible from the Earth)

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What causes eclipses?

The Earth and Moon cast shadows

When either passes through the other’s shadow

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Why don’t we have an eclipse at every new and full moon?

The Moon’s orbit is tilted 5 degrees to ecliptic plane

So we have about 2 eclipse seasons each year, with a lunar eclipse at full moon and solar eclipse at new motion

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How much of the moon’s surface is at all times illuminated by the sun?

Half towards the sun, the other half is dark

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What is a Lunar Eclipse?

  • Definition: A lunar eclipse occurs when the Earth passes between the Sun and the Moon, casting a shadow on the Moon.

  • Types: There are three types of lunar eclipses: total, partial, and penumbral.

  • Appearance: During a total lunar eclipse, the Moon appears reddish-orange due to the Earth's atmosphere bending sunlight towards it.

  • Frequency: Lunar eclipses occur about twice a year, but may not be visible from all locations.

  • Can only occur at full moon when the moon is on the other side of the Earth

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What causes eclipses?

When the Moon passes between the Sun and Earth, casting the Moon’s shadow on Earth

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Solar Eclipse

can only occur at new moon, when the moon is in between the sun and the earth

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annular eclipse

when the moon crosses in front of the sun it is too small to fully cover the sun

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2 Conditions to have an eclipse

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

  2. the Moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (its nodes)

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38

What’s the phase of the moon needed for a lunar eclipse?

Full Moon

The sun, earth, and moon all have to be aligned

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What’s the difference between a lunar and solar eclipse?

  • A lunar eclipse occurs when the Earth passes between the sun and the moon, and the Earth's shadow falls on the moon

  • A solar eclipse occurs when the moon passes between the sun and the Earth, blocking the sun's light from reaching the Earth

<ul><li><p>A lunar eclipse occurs when the Earth passes between the sun and the moon, and the Earth&apos;s shadow falls on the moon</p></li><li><p>A solar eclipse occurs when the moon passes between the sun and the Earth, blocking the sun&apos;s light from reaching the Earth</p></li></ul>
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