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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
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
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