Download Motion of the Moon Phases of the Moon

Key Concepts: Lecture 5
Motion of the Moon
Phases of Moon
Solar Eclipses
Angular Size
Lunar Eclipses
Motion of the Planets
Motion of the Moon
• The Moon rises in the east and
sets in the west moving across the
sky in an arc
• The Moon moves slowly eastward
against the stars (half a degree per
hour)
• The motion of the Moon relative to
the stars causes it to rise 50
minutes later every day
• The Moon returns to the same
position amongst the stars every
27.3 days (its orbital period or
sidereal period)
Phases of the Moon
• Why does the Moon shine?
– It emits no visible light of its
own
– The Moon shines by
reflecting light
– The half of the Moon that
faces the Sun is lit up
• The lit portion of the Moon’s
surface which is visible from
Earth is called the phase
• Phases change in a regular
sequence over a 29.5 day
period (synodic period)
•
Phases of the Moon
The changes in
the Moon’s
phase are due
to changes in
the angle
between the
Sun, Moon and
the Earth
Eclipses
• A solar eclipse occurs when the Moon moves
between the Earth and the Sun
• A lunar eclipse occurs when the Earth moves
between the Sun and the Moon and the Moon moves
through the Earth’s shadow
• In ancient cultures eclipses were bad omens
Question
• If the Sun sets at 6pm, when
does a first quarter Moon rise?
Angular Size of Sun and Moon
The Moon Moves Over the Face of the Sun
• The Sun and Moon have very different
physical sizes
•
– Radius of Sun is 7x105 km
– Radius of Moon is 1.7x103 km
– So the Sun is 400 times bigger than the
Moon!!
• How can they appear to be nearly the
same size during an eclipse?
Angular Size
Solar Eclipse Video
• Answer: the Moon and Sun, coincidentally, have
nearly the same angular size
• Angular size of an object depends on two things
Sketch the sequence of a solar eclipse...
– The physical size of the object
– The distance to the object
Angular size (measured in radians) =
Physical Size
Distance
or we write it as:
Earth
θ
Moon
r
s
Sun
(2π radians = 360 degrees)
Shadows & Eclipses
Question
• Based on their physical and angular
sizes, how much further from us is the
Sun than the Moon?
•
A total solar eclipse can seen from only a small region on the Earth
– in order to see a total eclipse the Moon must appear to cover
the entire disk of the Sun
– you observe this when you are in the inner shadow (umbra) of
the Moon
•
Partial solar eclipses are seen over a larger area
– to see a partial eclipse, only part of the Sun needs to be
covered by the Moon
– you observe this when you are in the Moon’s outer shadow
(penumbra)
An Annular Solar Eclipse
• Why does an
annular eclipse
look different?
• The Moon’s
shadow moves
over the Earth
during a solar
eclipse
(see also
p18 C&M)
Text
When and How Often Are Solar Eclipses?
Next solar eclipse visible from USA
http://sunearth.gsfc.nasa.gov/eclipse/eclipse.html
Aug. 21st 2017
• Solar eclipses occur at new
Moon
• They do not occur every
month
– The Moon’s orbit is tilted
5 degrees to the ecliptic
• They occur when the Moon
crosses the ecliptic when it
is new Moon
– A partial, total or annular
eclipse can occur some
place on Earth between
3 and 5 times each year
– A total may occur 0 to 3
times a year
Lunar Eclipses
Question
• Why is the Moon’s surface still visible
during a total lunar eclipse?
• We see a total lunar eclipse when the Moon moves
through the Earth’s inner shadow (umbra)
• We see a partial lunar eclipse when the Moon moves
through the Earth’s outer shadow (penumbra)
• Lunar eclipses are visible anywhere on the night-time
side of the Earth
A Lunar Eclipse
• Why does the Moon then have a
reddish color?
When & How Often?
• Lunar eclipses occur at FULL Moon PHASE
• Lunar eclipses do not occur every month
because the Moon’s orbital plane is tilted by
5 degrees with respect to the ecliptic
– a total lunar eclipse occurs when the
Moon crosses the ecliptic at full Moon
– since the Earth’s shadow is much bigger
than the Moon, total lunar eclipses occur
more often than solar eclipses
• Lunar eclipses occur 2-5 times per year
Motion of the Planets
• The bright planets (Venus, Jupiter, Mars, Saturn) are the brightest
objects in the night sky (with the exception of the Moon) & they rise
in the east and set in the west
• “Planet” derived from Greek for “Wanderer”
– The planets move slowly among the stars staying near the ecliptic
– Different planets move at different speeds relative to the stars (of
the visible planets, Mercury is the
fastest, Saturn is the slowest)
– They move in complex
patterns, changing their
direction of motion
The “Inferior” Planets
• Venus and Mercury
• Stay near the Sun on the sky
• Are visible only near sunrise
and sunset (“morning &
evening stars”)
• Move away from the Sun and
then move toward it
Mercury - (ca. 15th century).
Mercury moves fast - the messenger
Venus depicted in the Aztec
Codex: Aztec god, Xolotl
(evening star)
Inferior Planets show Phases
Can you ever see Venus at midnight?
Why do inferior planets always appear near the Sun
(i.e. in the evening or morning)?
Why is it hard to see Mercury?
Motion of the Outer Planets
Planetary Transits:
The Transit of Venus
•
Outer (superior) planets
generally move eastward
relative to the stars
•
Outer (superior) planets
undergo retrograde motion
– motion relative to stars:
• slows
• reverses
• moves westward
relative to stars
• slows again
• reverses again
• resumes its general
eastward motion
•
This confused & perplexed
people for centuries & led
people of many cultures to
attribute superior powers to
the planets
You do not need to recall this slide for HW or Exam
Planets, Gods & Days of Week
The “Superior” Planets
• The naked-eye visible
superior planets are Mars,
Jupiter and Saturn
• Can appear far from the
Sun but remain near the
ecliptic
Mars (& Venus)
Fresco from Pompeii
(ca. 1 A.D.)
Saturn (ca. 18th century) - Arabic
illustration showing agricultural activities
under direction of Saturn
• English names for most of
the days of the week come
from Norse gods
• Tuesday : Tiwes - god of
war (Mars)
• Wednesday: Woden - god
of day & night (Mercury)
• Thursday: Thor - god of
thunder - head god
(Jupiter)
• Friday: Frega - goddess of
spring (Venus)