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