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AST 111 Lecture 7 Eclipses, Solar and Sidereal Days, Precession Eclipses An eclipse is when one celestial object passes in front of another. Eclipses Eclipses Transit: Small object in front of large Occultation: Large object in front of small Eclipses Eclipses Lunar Eclipses • Sun, Earth, and Moon in a straight line – The Earth gets between the Sun and the Moon – Must be a Full Moon Lunar Eclipses • Why don’t we see a lunar eclipse during every full moon? 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 Lunar Eclipses Lunar Eclipses Lunar Eclipses • Moon in umbra • Earth’s atmosphere “lenses” light onto the moon Lunar Eclipses • Partially in umbra, partially in penumbra Lunar Eclipses • Moon in penumbra • Almost can’t tell it’s an eclipse Solar Eclipses • Sun, Moon, Earth in a straight line • The Moon gets between the Sun and the Earth • Must be a New Moon Solar Eclipses Solar Eclipses 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! 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 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? The Solar Day • The length of time for the Sun to start at the Meridian and return to the Meridian • 24 hours on average The Sidereal Day The Solar Day 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 Solar and Sidereal Days • If Earth did not orbit the Sun (just stayed stationary), would the solar day equal the sidereal day? 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 Earth’s axis precesses. • Just like a wobbling, spinning top • Every 26,000 years 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