Transcript AST 111 Lecture 7

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