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AST 111 Lecture 7 Eclipses, Solar and Sidereal Days, Precession
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Eclipses An eclipse is when one celestial object passes in front of another.
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Eclipses
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Transit: Small object in front of large Occultation: Large object in front of small
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Eclipses
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Lunar Eclipses Sun, Earth, and Moon in a straight line – The Earth gets between the Sun and the Moon – Must be a Full Moon
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Lunar Eclipses Why don’t we see a lunar eclipse during every full moon?
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Lunar Eclipses The plane of the Moon’s orbit is inclined by 5 degrees to the ecliptic If Earth orbits the Sun in a pond, the moon spends half its time above and half its time below the pond’s surface
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Lunar Eclipses
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Moon in umbra Earth’s atmosphere “lenses” light onto the moon
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Lunar Eclipses Partially in umbra, partially in penumbra
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Lunar Eclipses Moon in penumbra Almost can’t tell it’s an eclipse
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Solar Eclipses Sun, Moon, Earth in a straight line The Moon gets between the Sun and the Earth Must be a New Moon
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Solar Eclipses
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Solar vs. Sidereal Day Imagine you’re where Earth is – but there’s no Earth. – You’re rotating in place. You see the Celestial Sphere rotating. – How many degrees do you need to rotate through to get back to the same view? Yes, this is as simple as you think it is!
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The Sidereal Day The length of time for Earth to complete one full rotation about its axis Also equal to the length of time it takes for a star (not the Sun) to come back to the same position in the sky 23 hours 56 minutes
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The Solar Day Say it’s noon, and the Sun is on the meridian. If Earth rotates 360 degrees: – Is the Sun back on the meridian? – Why or why not?
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The Solar Day The length of time for the Sun to start at the Meridian and return to the Meridian 24 hours on average
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The Sidereal Day
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The Solar Day
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Solar and Sidereal Days So… why are they different? Earth’s orbit around the Sun causes the Sun to move in the sky – Earth must rotate a little extra to bring the Sun to the Meridian
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Solar and Sidereal Days If Earth did not orbit the Sun (just stayed stationary), would the solar day equal the sidereal day?
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Solar and Sidereal Days Length of solar day varies throughout a year – This is due to the ellipticity of Earth’s orbit – This causes the horizontal motion of the analemma Length of sidereal day does not – The time it takes Earth to rotate once does not vary
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Earth’s axis precesses. Just like a wobbling, spinning top Every 26,000 years
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The North Star Precession of Earth’s axis causes the North Star to change after long periods of time – Current North Star: Polaris – Vega was the north star in 12000 BC – We will have a new north star in AD 3000
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