AST 111 Lecture 7 Eclipses, Solar and Sidereal Days, Precession

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

Eclipses

Transit: Small object in front of large Occultation: Large object in front of small

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

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 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 BC – We will have a new north star in AD 3000