Transcript Solar Eclipse - Cloudfront.net

Charting the Heavens:
Foundations of Astronomy
Learning Goals
• Describe the Celestial Sphere and how astronomers use
angular measurement to locate objects in the night sky.
• Account for the apparent motions of the Sun and the stars in
terms of the actual motion of the Earth. Explain why our
planet has seasons.
• Understand the changing appearance of the Moon and how
the relative motions of the Earth, the Sun, and the Moon
lead to eclipses.
The Earth's rotation axis is tilted with respect to its orbit around
the Sun => seasons.
Summer
Scorpius
Night
Tilt is 23.5o
Winter
Day
Sun high in
northern sky
Sun low in Day
northern sky
Night
Orion
The Motion of the Moon
The Moon has a cycle of "phases", which lasts about 29 days.
Half of the Moon's surface is lit by the Sun.
During this cycle, we see different fractions of the sunlit side.
Which way is the Sun here?
Cycle of phases slightly longer than time it takes Moon to do a complete
orbit around Earth.
Cycle of phases or
"synodic month"
Orbit time or
"sidereal month"
29.5 days
27.3 days
Eclipses
Lunar Eclipse
When the Earth passes directly between the Sun and the Moon.
Sun
Earth
Moon
Solar Eclipse
When the Moon passes directly between the Sun and the Earth.
Sun
Moon
Earth
Solar Eclipses
Diamond ring effect - just before or
after total
Total
Partial
Annular - why do these occur?
Lunar Eclipse
Moon's orbit tilted compared to Earth-Sun orbital plane:
Sun
Moon
Earth
5.2o
Side view
Moon's orbit slightly elliptical:
Moon
Distance varies by ~12%
Earth
Top view, exaggerated ellipse
Types of Solar Eclipses Explained
Certain seasons are more likely to have eclipses. Solar “eclipse
season” lasts about 38 days. Likely to get at least a partial eclipse
somewhere.
It's worse than this! The plane of the Moon's orbit precesses, so
that the eclipse season occurs about 19 days earlier each year.
Recent and upcoming total and annular solar eclipses
"Geocentric Model" of the Solar System
Ancient Greek astronomers knew of Sun, Moon, Mercury, Venus, Mars,
Jupiter and Saturn.
Aristotle vs. Aristarchus (3rd century B.C.):
Aristotle: Sun, Moon, Planets and Stars rotate around fixed Earth.
Aristarchus: Used geometry of eclipses to show Sun bigger than Earth
(and Moon smaller), so guessed that Earth orbits the Sun. Also guessed
Earth spins on its axis once a day => apparent motion of stars.
Aristotle: But there's no wind or parallax (apparent movement of stars).
Difficulty with Aristotle's "Geocentric" model: "Retrograde motion of the
planets".
Planets generally move in one direction
relative to the stars, but sometimes they appear
to loop back. This is "retrograde motion".
But if you support geocentric model, you must attribute retrograde
motion to actual motions of planets, leading to loops called “epicycles”.
Ptolemy's geocentric model (A.D. 140)
"Heliocentric" Model
●
Rediscovered by Copernicus in 16th century.
●
Put Sun at the center of everything.
Much simpler. Almost got rid of retrograde
motion.
●
But orbits circular in his model. In reality,
they’re elliptical, so it didn’t fit the data well.
●
●
Not generally accepted then.
Copernicus 1473-1543
Illustration from
Copernicus' work
showing heliocentric
model.
Planets generally move in one direction
relative to the stars, but sometimes they appear
to loop back. This is "retrograde motion".
Apparent motion
of Mars against
"fixed" stars
Mars
July
7
*
Earth
7
6
*
6
5
3
4
4
3
1
5
2
2
*
1
January
*
*
*
Kepler's First Law
The orbits of the planets are elliptical (not circular)
with the Sun at one focus of the ellipse.
Ellipses
distance between foci
eccentricity =
major axis length
(flatness of ellipse)
Kepler's Second Law
A line connecting the Sun and a planet sweeps out equal areas
in equal times.
slower
Translation: planets move faster
when closer to the Sun.
faster
Kepler's Third Law
The square of a planet's orbital period is proportional to the
cube of its semi-major axis.
P2
is proportional to
or
P2  a3
(for circular orbits, a=b=radius).
Translation: the larger a planet's orbit,
the longer the period.
a3
a
b