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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


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


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