Science
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Nov4_LunarPhasesandEclipses2.pptAspects of Lunar Phases
This diagram is looking down at the Earth’s
North Pole and the moon’s orbit
8 phases, but some are duplicates. What is the pattern?
Lunar Phases- basic concepts
3) Because of the rotation of the Earth, the Moon appears to rise and set in the same manner as the Sun does (east to west).
1) ½ of the moon will always be sunlit and ½ will always be dark.
The part of the sunlit side that we can see from Earth is called its phase. It depends upon the alignment of the Sun, Moon, and Earth at any given time as they revolve. Its geometry
2) The Moon revolves around the Earth counter-clockwise (looking down from the North Pole) while the Earth revolves around the Sun (also counter-clockwise from northern view).
Some are more on the day side
Earth’s dayside- towards sun
Earth’s nightside- away from sun
Crescent (2 and 8) and new moon
This kills misconception #1: The moon is up during the day!
Its just hard to see.
Some are more on the night side
Earth’s dayside- towards sun
Earth’s nightside- away from sun
Gibbous (4 and 6) and Full moon
When its on the nightside part of its orbit, the phases are big. Easier to see
Crossover points where we see half and half
First (3) and last (7) quarter
Called “quarter” because the moon at point #3 is ¼ the way around the earth
Summary of phases
1 new moon
2 crescents
1 1st quarter
2 gibbouses
1 3rd quarter
1 full moon
How do we tell the difference between the 2 crescents and the 2
gibbous’s?
Waxing Phases
At 2,3 and 4, the moon is growing: waxing
Note: the moon’s size stays the same, its just the bright part that we can see is
growing
Waning Phases
At 6,7 and 8, the moon is shrinking: waning
Note: the moon’s size stays the same, its just the bright part that we can see is
getting smaller
Animation of moon’s orbit and phases
Whats good about this is that Earth’s rotation is also accurately
simulated. Worth the 45 seconds!
*
What is the moon’s orbital period?
Moon takes about 4 weeks (28 -29 days) to orbit the earth.
So how far does it go in 1 week?
What is the moon’s orbital period?
So how far does it go in 1 week? You should be able to show this
Ans: 2 phases/week (e.g. from 1 to 3)
Examples of the Lunar Phases
Waxing crescent
Waxing crescent – 1st quarter – waxing gibbous
Waxing gibbous
Full – waning gibbous
Waning gibbous – waning crescent
Waning crescent
Photo from: http://aa.usno.navy.mil/graphics/Moon_phases.jpg
Why does the moon always look the same? Same basalt blotches
The misconception of the “dark side of
the moon”.
Its not dark, but we still can’t see it from
Earth………. Why not?
The “far side of the moon”
Rotation period = revolution period
Consider walking around a globe
Person does not rotate. Does
not always face the globe
This is what the moon does.
A result of Earth’s gravity
Person rotates once per revolution
Always faces the globe
Eclipses
- When the Sun or Moon is blocked to an observer on Earth over a short period of time (minutes to hours) an eclipse has occurred.
- If the Sun is blocked it is a solar eclipse.
- If the Moon is blocked it is lunar eclipse.
Eclipses: when the sun or moon appears blocked by a shadow
Lunar Eclipse
When the Moon is in the full moon position, the light from the Sun may be blocked by the Earth which casts a shadow. As the Moon travels into the shadow it is gradually covered. Ancients thought the Moon was being eaten.
http://www.hermit.org/Eclipse/why_lucnar.html
View from above the Northern Poles
Lunar Eclipse
Shouldn’t there be a lunar eclipse every time we have a full moon?
Lunar Eclipse
The Earth revolves around the Sun on a plane called the plane of the ecliptic.
The Moon orbits Earth on a plane but that plane is tilted 5o with respect to the plane of the ecliptic. This means that when the moon is in the full position it may be above or below the shadow of the Earth and no eclipse will occur.
Diagrams from http://www.ap.stmarys.ca/demos/content/astronomy/lunar_eclipse/lunar_eclipse.html
Lunar Eclipse Sequence
Passing into the shadow
Totality
Coming out of the shadow
Solar Eclipse
http://www.hermit.org/eclipse/why_solar.html
When the Moon passes directly between the Earth and the Sun, the Moon casts a shadow on the Earth. Where the shadow falls, observers will see an eclipse of the Sun, a solar eclipse. As with a lunar eclipse, the shadow of the Moon may fall above or below Earth and no eclipse occurs. If the entire Sun is covered we see a total eclipse; of only part if the surface is blocked we see a partial eclipse.
The moon and the sun seem to be exactly the same size! Just a total
coincidence.
Moon is 400 times smaller than the sun, but about 400 times closer than the sun
Mathematically: Ratio of distance to earth divided by size
Moon 239,000 miles Sun 93,000,000 miles
-------------- = ---------------
2159 miles across 865,000 miles across
Both these fractions equal about 108
108 has long been considered a sacred number in Hinduism and yoga
https://www.yogajournal.com/yoga-101/the-number-108
Something weird about solar eclipses
Consequence of the weird coincidence of the moon and sun’s size.
Its very hard for the moon to block out the sun. They have to line up just
Perfectly. Solar eclipses are very rare.
Also unique in the solar system- no other planet has a moon with this
apparent exact match up to the sun. Other planets moons can
block the sun but only earth has this exact matchup.
Another aspect of why Earth is unique.
Is this meaningful? No one knows.
Solar eclipses will end in 600 million years because the moon is moving
further from the earth by 1.5 inches/year. It will eventually be
too small to fully cover up the sun.
Pictures from DC in 2017
The moon’s shadow seen from space
https://www.theverge.com/tldr/2017/8/21/16180544/total-solar-eclipse-2017-best-photos-videos-nasa-gifs
Under the zone of totality
Closeup of the diamond ring effect
Sequence of photos: before during and after
Summary
Phases: new, cresent, quarter, gibbous full
waxing and waning
Eclipses: solar: S – M – E
lunar: S – E – M
Ecliptic plane
Rotation and revolution periods for Earth and moon and their significance
View from the Moon: for the homework problem
need to visualize what the light and dark pattern (phase)
of the Earth would be if you are on the Moon
(standing at point “x” – a few examples given) looking back at Earth
x
x
x
