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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.

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ast 111 lecture 7

AST 111 Lecture 7

Eclipses, Solar and Sidereal Days, Precession

eclipses
Eclipses

An eclipse is when one celestial object passes in front of another.

eclipses2
Eclipses

Transit: Small object in front of large

Occultation: Large object in front of small

lunar 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 eclipses1
Lunar Eclipses
  • Why don’t we see a lunar eclipse during every full moon?
lunar eclipses2
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 eclipses5
Lunar Eclipses
  • Moon in umbra
  • Earth’s atmosphere “lenses” light onto the moon
lunar eclipses6
Lunar Eclipses
  • Partially in umbra, partially in penumbra
lunar eclipses7
Lunar Eclipses
  • Moon in penumbra
  • Almost can’t tell it’s an eclipse
solar eclipses
Solar Eclipses
  • Sun, Moon, Earth in a straight line
  • The Moon gets between the Sun and the Earth
  • Must be a New Moon
solar vs sidereal day
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 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
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 day1
The Solar Day
  • The length of time for the Sun to start at the Meridian and return to the Meridian
  • 24 hours on average
solar and sidereal days
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 days1
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 days2
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
Earth’s axis precesses.
  • Just like a wobbling, spinning top
  • Every 26,000 years
the north star
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