In This Lesson:
The Celestial
Sphere, Seasons,
and Moon
(Lesson 2 of 2)
Today is Monday,
April 20th, 2015
Pre-Class:
How long is a year?
What causes the seasons?
http://deepskycolors.com/pics/astro/2011/11/mb_201
1-11_Aries.jpg
Dust clouds within the Aries constellation.
Today’s Agenda
•
•
•
•
•
The Celestial Sphere
Celestial Coordinates
Seasons
Detailed Moon Information
Eclipses
• Where is this in my book?
– Pages 19-43, and 189-212.
By the end of this lesson…
• You should be able to locate objects via a celestial
coordinate system.
• You should be able to distinguish between a
solar/synodic and sidereal time period.
• You should be able to astronomically explain the
seasons and the phases of the moon.
• You should be able to describe the moon via a
variety of properties.
• You should be able to identify the circumstances
under which a lunar or solar eclipse occurs.
Pre-Test
• Grab your computers – we’re going to see how
much middle school astronomy you
remember.
• Unit 1 Lesson 2 Pre-Test
– Username: [firstnamelastname]832
– Password: Gleicher[house/apt. #]
What’s your star sign?
• It may seem silly to you at this point, but
students in the past have sometimes confused
this course (Astronomy) with horoscopes
(Astrology).
• They’re so similarly named because they both
concern celestial bodies and space.
– All those “star signs” are literally stars – they’re
constellations like the one on the first slide.
• However, Astronomy is science while astrology
is…not.
The Astrological Signs
• So how’d astrologers pick those twelve star
signs?
• Well, they saw those constellations fairly
readily in the celestial sphere.
– The celestial sphere is the “sphere of stars” that
appears to rotate around the Earth.
• The “movement” of the stars (they really don’t move) is
called diurnal motion.
The Celestial Sphere
• Other things early astronomers noticed:
– The planets/Sun move through a band of the
celestial sphere called the zodiac.
• Those are the “star sign” constellations.
– The stars “rotate” around the North Star (Polaris).
• Key: Because the Earth rotates toward the EAST, the
stars appear to rotate WESTWARD through the sky.
– The Sun “moves” through a path called the
ecliptic.
The Astrological Constellations
The Celestial Sphere
• The earliest astronomers didn’t realize:
– The Earth is doing the rotation, not the
stars.
– The stars are at different distances.
• So stars within constellations are not “related.”
• In fact, some many be many light years apart
from each other.
• Now’s probably a good time to discuss
two reasons why the stars “twinkle.”
– There’s a huge distance between us.
– The Earth’s atmosphere and wind can
distort the light a little bit.
The Celestial Sphere
The Celestial Sphere
• How far are the other stars, anyway?
– UniverseToday – What is the Closest Star?
• We can get a good sense for that “moving”
celestial sphere if we just speed things up to a
more human-like time frame.
– Celestial Sphere Rotation video
• If we change the time lapse process a little bit,
however, we can reveal the rotations of the
Earth within the celestial background.
– Star Spin video
Aside: Edmond Halley
• Edmond Halley is most well
known for his comet.
• He also was the first to confirm
that the stars move.
• To do this, in 1718 he compared
the positions of stars to those
noted (rather accurately) by the
ancient Greek astronomer
Hipparchus circa 150 BC.
– Turns out, in 1800 years, the
position of the star Arcturus had
moved just a little bit, but enough
to confirm his hypothesis.
https://cbs3weather.files.wordpress.com/2014/04/wx-blog3.jpg
Edmond Halley
(1656-1742)
The Horizon System
• One way to locate stuff is the horizon system.
– Like a compass, the horizon system helps you find stuff
in the sky by specifying azimuth (direction, like
“Northwest”) and altitude (60°, for example).
• Zenith, by the way, is straight up (90°). That one IS useful.
• This doesn’t really work for astronomers since,
well, stars “move” in the sky.
– Astronomers (and telescopes with equatorial mounts)
use right ascension and declination.
The Equatorial System
• The equatorial system is a more logical method
because it projects the Earth’s equator onto the
celestial sphere.
– Instead of specifying points on a sphere via latitude and
longitude like Earth, this sphere is viewed from within.
– So the equator is called the celestial equator.
• Any star that is located on the celestial equator has a
declination (dec or δ) of 0°.
• A star located straight above the north pole (like
Polaris) has a declination of +90°.
• A star located straight above the south pole has a
declination of -90°.
Declination
http://www.astronomyforbeginners.com/astronomy/celestialsphere.php
The Equatorial System
• So declination is kinda like altitude in the sky,
or like latitude on Earth.
• The equivalent of azimuth (compass direction)
or longitude is right ascension (RA or α).
– Right ascension is named for the ascent of a star
from the east (or “right”) horizon.
• Like arcminutes, every “hour” of right
ascension across the sky is equivalent to 15° of
longitude.
– 24 hours of RA = 360° of longitude/sky.
Right Ascension
http://www.astronomyforbeginners.com/astronomy/celestialsphere.php
Right Ascension
• Right ascension gets a little tricky.
• Think about it: The stars “move” in the sky, so
shouldn’t the “longitude” lines of RA also
change?
– Sorta.
• Right ascension is a fixed point (more next slide).
• What changes is the hour angle, which is just a
relationship between the fixed right ascension point
and the current sidereal time.
Right Ascension Anchors
• Anyone know where 0° longitude is located on Earth?
– The Prime Meridian, which passes through Greenwich,
England.
• Anyone know where 0 hr RA is located on the celestial
sphere?
– The first point of Aries (which is technically now in Pisces),
which is at the intersection of the celestial equator and
meridian.
• So RA is anchored to a fixed point and all Dec/RA
calculations are made starting there.
• For astronomers, if you can spot a star with a known RA
value in the night sky, it doesn’t matter what time it is.
– Just use that star’s relative RA to find your object of choice.
The Equatorial System
Eeee…what do I need to know?
• I expect you to know the following:
– Astronomers locate objects on the celestial sphere
using a coordinate system.
– On Earth it’s latitude and longitude but on the
celestial sphere it’s declination and right
ascension, respectively.
– The reason for the change is because dec/RA are
fixed on the celestial sphere and thus consistent.
Extra Practice
• Simulation: Sidereal Time and Hour Angle
• Celestial Coordinates worksheet (in groups or
pairs)
– Still need help? Visit:
• http://www.astronomygcse.co.uk/AstroGCSE/Unit4/Decand
RA.htm
• http://www.astronomyforbeginners.com/astronomy/celesti
alsphere.php
Earth’s Rotation
• It took many years to firmly
establish that the Earth was
rotating, not the stars all rotating
around it.
– Humanity as a whole just seems to
have a really big ego.
• We now know that the Earth rotates
once about its axis…wait…axis?
– Just so we’re all clear, Earth’s axis is
the imaginary line that runs from
North Pole to South Pole through the
Earth.
• Fun fact: At the equator, the Earth
rotates at slightly over 1040 mph!
http://science.jrank.org/kids/article_images/space_p12.jpg
Earth’s Rotation
• So Earth rotates about its axis once every…?
– 23 hours, 56 minutes, and 4.09 seconds. Huh?
• If you were thinking 24 hours, you’re also
right…sorta.
– A solar (synodic) day is 24 hours – the type of day you
know.
• The Sun goes from a spot in the sky back to the same spot in
the sky.
– A sidereal day (“sid-EAR-ial”) is 23 hours, 56 minutes,
and 4.09 seconds.
• A distant star goes from a spot in the sky back to the same spot
in the sky.
http://astro.unl.edu/naap/motion3/sidereal_synodic.html
http://www.astro.cornell.edu/academics/courses/astro201/sidereal.htma
Let’s be clear here…
• “Sidereal” literally means “relating to distant
stars.”
• “Synodic” means “relating to conjunctions” of
celestial bodies.
– When in doubt, remember sidereal is all about the
background stars and not the Sun.
• UniverseToday – How Fast Does the Earth
Rotate?
Solar Day versus Sidereal Day
• So what’s the difference?
– The reason the sidereal day is
shorter than the synodic/solar
day is because the sidereal day is
how long the Earth takes to
rotate 360°.
– Since the revolution of the Earth
around the sun is a curved path,
it takes slightly longer for the
Earth to re-point toward the Sun,
but not as long toward what is
effectively a static background
star.
1  2 = Sidereal Day
1  3 = Solar Day
http://astro.unl.edu/naap/motion3/sidereal_synodic.html
Solar Day versus Sidereal Day
Leap Years
• The Earth takes 365.25 days, if we count in
solar days, to revolve around the sun.
– A solar year is also known as a tropical year, by
the way.
• We make up for the missing time in the
sidereal day by having a leap year every 4
years in the Gregorian calendar:
– Unless the year is evenly divisible by 100.
– Except when divisible by 400.
• So 1900 was not a leap year; 2000 was.
To Be Clear…
• You should know:
– The Earth rotates counterclockwise when viewed
from above the North Pole.
– The Earth revolves counterclockwise when viewed
from above the Sun.
Seasons
• So now for the big question:
• What causes the seasons?
– Seriously, what’s your answer?
• Let’s check with some Harvard graduates on
their big day.
• A Private Universe
A Private Universe
• Here’s the thing…all those Harvard grads are, well,
wrong.
• The Earth’s orbit around the sun is actually quite close
to circular.
• Even if it weren’t, the seasons can’t be caused by
proximity to the Sun.
– If that were the case, the whole planet would experience the
same season all at once.
– In reality, right now the Southern Hemisphere is experiencing
the opposite season we are.
• Let’s explain this a little bit better than an Ivy Leaguer.
Rotations and Revolutions
• So the Earth rotates about its axis once every
sidereal day (23 hr, 56 min, 4 sec).
• The Earth, as you know, also revolves around
the Sun once every 365.25 days.
– The orbit around the Sun is slightly elliptical,
which means it’s not quite a perfect circle.
– As it revolves, it points toward Polaris, which is
commonly known as the North Star.
Rotations and Revolutions
Tilt
• Did you notice the Earth isn’t pointing
perfectly North/South while it rotates?
– It turns out that tilt is kinda important.
• The Earth is tilted 23.5° relative to the
perpendicular of its orbital plane.
• That means, during different parts of
the year, certain parts of the planet
receive more sunlight per day and are
ever so slightly more directed at Sun.
– Hence, seasons.
Seasons Model
• [this is your cue, Mr. G – grab your props]
Seasons
• For more:
– TED: Rebecca Kaplan – Reasons for the Seasons
– UniverseToday – Why Are There Seasons?
– Veritasium – What Causes the Seasons?
– Veritasium – Why Does the Moon Orbit Earth?
Seasons
• On the equinoxes, the day length and night length
are equal.
– The vernal equinox is on/about March 20.
– The autumnal equinox is on/about September 23.
Note: This is Earth’s orbit
as viewed from the side.
Seasons
• On the solstices, the day/night lengths are at their
extremes (shortest or longest).
– The summer solstice is on June 21.
– The winter solstice is on December 21.
Note: This is Earth’s orbit
as viewed from the side.
Seasons
• What this means is that the Northern
Hemisphere gets the most direct solar radiation
(known as insolation) on the Summer Solstice.
• The Northern Hemisphere gets the least direct
insolation on the Winter Solstice.
• Insolation on the Spring/Fall Equinoxes is
relatively equal and “mid-range.”
– You even notice the Sun’s “path” across the sky (the
ecliptic) is higher in summer than in winter.
Sun Angle by Seasons
• In Boston, as an example:
http://scienceblogs.com/startswithabang/files/2012/06/sun_angle_seasonal.jpeg
Extreme Solar Angles
• Did you happen to notice something about
the angle of the Sun at the poles?
– When the Northern Hemisphere is in winter, there
are times when the Artic Circle gets no sunlight all
day.
Extreme Solar Angles
• Similarly, at the same time of year, the
Antarctic circle gets sunlight for all 24 hours.
– And vice versa, when it’s summer in the northern
hemisphere.
Aside: Total Sun?
• The 2002 movie Insomnia by
(Christopher Nolan) takes
place above the Arctic Circle.
• In reality, suicide rates in
Alaska are much higher than
other states and some people
cite the strange photoperiod
as a possible cause.
http://boltechai.net/wp-content/uploads/2014/08/52.jpg
Seasons
• Remember that Earth’s orbit about the Sun is slightly
eccentric (not perfectly circular).
• At aphelion (July 4), Earth is farthest from the Sun.
• At perihelion (January 3), Earth is closest to the Sun.
– More on this whole ellipse thing next unit.
Note: This is Earth’s orbit
as viewed from the side.
Seasons Practice
• Seasons Interactive
• The Seasons worksheet
– Can’t remember the Tropics and/or Circles?
Arctic Circle
Tropic of Cancer
Equator
Tropic of Capricorn
Antarctic Circle
Note that axis tilt is not shown.
Moon Phases
• Another product of Earth’s simultaneous rotation
about its axis and revolution around the Earth is
that the Moon has its own phases.
• Before we explore them, let’s get one thing
straight:
– Ever hear the phrase, “The dark side of the moon?”
– More accurately, it would be a “back side of the
moon,” because the same side of the moon always
faces us, though it isn’t always dark.
• Here’s what I mean…
Moon Phases
• The Moon rotates with the same period as its orbit.
– In other words, it takes 27.3 days to rotate once and 27.3
days to revolve once.
• This is called synchronous rotation.
Without Lunar Rotation
With Lunar Rotation
Another View
With Lunar Rotation
Without Lunar Rotation
http://upload.wikimedia.org/wikipedia/commons/5/56/Tidal_locking_of_the_Moon_with_the_Earth.gif
Moon Phases
Moon Phases – Another Look
• Video: One Year of the Moon in 2.5 Minutes
Moon Phases
• What’s evident from that Moon phase image?
– The Moon kinda wobbles.
– The Moon gets closer and further during orbit.
– Shadows and light pass across the same moon surface.
• So…we’ve got a lot to talk about. A summary:
1.
2.
3.
4.
General lunar characteristics
Lunar gravity effects
Lunar rotation/revolution effects
Lunar (and solar) eclipses
• To help you summarize this info, use the Moon Details
note organizer worksheet.
1. General Lunar Characteristics
Moon Size and Gravity
• The moon is about 25%
the size of the Earth but it
only has 17% of the
gravity.
– That low gravity means the
moon has no significant
atmosphere.
• It technically has a little.
– No atmosphere means no
protection from meteor
impacts, hence all the
craters.
http://www.astronomy.org/programs/moon/earth-moon-size.gif
1. General Lunar Characteristics
The Surface
• Light areas of the moon are highlands.
• Dark areas of the moon are lowlands
called maria (singular: mare).
– The name comes from the Latin for “sea,”
because early astronomers thought there
were oceans on the moon.
– Apollo 11 landed in the Sea of Tranquility
(Mare Tranquillitatis).
• Highlands are composed of anorthosite,
which is a different kind of rock from the
maria, which are made of basalts from
cooled lava flows.
• The dusty/rocky surface of the Moon is
overall called regolith.
http://csep10.phys.utk.edu/astr161/lect/moon/luna.gif
http://upload.wikimedia.org/wikipe
dia/commons/thumb/6/61/Moon_
names.svg/2000pxMoon_names.svg.png
1. General Lunar Characteristics
The Far Side of the Moon
• Interestingly, the far side of the moon
(the “dark” side) appears much
different from the side of the moon
we always see.
– It’s got virtually no maria and a whole
bunch of craters.
• Key: Any region of a celestial body
having a lot of craters means that
area is relatively old.
– This is because young surfaces haven’t
had time to get so many craters.
– Earth has few craters because we have
shifting tectonic plates and fresh soil.
• And speaking of craters…
http://csep10.phys.utk.edu/astr161/lect/moon/luna.gif
1. The Near Side and the Far Side
Photographed by the Clementine Spacecraft in 1994
Near Side
http://astrobites.org/wp-content/uploads/2014/06/NearAndFarMoonWiki.pngs
Far Side
1. General Lunar Characteristics
Craters and the Surface
• Many of the craters we can see have been named
after famous astronomers.
– One of the most prominent is Tycho, named after Tycho
Brahe. It’s the big one in the image below.
• See those faint “lines” running from it?
http://upload.wikimedia.org/wikipedia/commons/6/6b/Tycho_Crater.jpg
1. General Lunar Characteristics
Craters and the Surface
• Those faint lines coming from the crater are the
rays, made of ejecta from that meteor impact.
• Other surface features include rilles, which are
grooves in the surface in which lava once flowed.
http://upload.wikimedia.org/wikipedia/commons/6/6b/Tycho_Crater.jpg
http://upload.wikimedia.org/wikipe
dia/commons/thumb/6/61/Moon_
names.svg/2000pxMoon_names.svg.png
2. Lunar Gravity Effects
Tides
• Tides are caused by the Moon’s gravity as it tugs on
the water on Earth.
– The water on Earth “bulges” in two spots – the side nearest
the Moon and the opposite side (due to inertia).
– Two tides occur each day (high tide and low tide).
• Tides vary periodically as, on occasion, the Moon and
the Sun combine to exert a lot of gravitational force
on water.
– The spring tide.
• Sometimes, the Moon and Sun exert less-than-normal
pulls on water because they’re at right angles to one
another and cancel out each other’s gravity.
– The neap tide.
2. Lunar Gravity Effects
Daily Tides
• Note that we are rotated into the tidal bulge,
which remains constant.
http://kgortney.pbworks.com/f/1241198825/Tide-Animation.gif
2. Lunar Gravity Effects
Spring/Neap Tides
SUN
http://home.hiwaay.net/~krcool/Astro/moon/moontides/
For more…
• UniverseToday – How Do The Tides Work
3. Lunar Rotation/Revolution Effects
Lunar Elliptical Orbit
• Like Earth, the Moon also has an elliptical orbit.
– Also like Earth (and the other planets), the Moon
revolves counterclockwise around the Earth.
• You remember how perihelion is when the Earth
is closest to the Sun?
– Perigee is when the Moon is closest to the Earth.
• You remember how aphelion is when the Earth is
farthest from the Sun?
– Apogee is when the Moon is furthest from the Earth.
3. Lunar Rotation/Revolution Effects
Librations
• Even though we only see one side
of the Moon, it wobbles enough
that we actually get to see 59% of it.
– There are a number of different
“wobbles” and explanations for that
59%, but in short they’re all known as
librations.
• The “up and down” rocking is a result of
the moon’s ecliptic being inclined relative
to Earth’s orbital plane.
• The “side to side” rocking is a result of the
moon’s elliptical orbit and variable orbit
speed.
Aside: Libra Weight?
• Libra, a star sign and a
constellation, comes from the
Latin for “scale.”
• “Libra” also used to be the
word used for “pounds,” as in
the unit of weight.
– Ever wonder why “pounds” is
abbreviated “lbs?” Now you
know.
• Just the way old-timey scales
might oscillate, the Moon’s
movements were also called
librations.
http://www-spof.gsfc.nasa.gov/stargaze/Smoon4.htm
http://i.space.com/images/i/000/029/891/i02/libra.jpg?1371505868
Precession
• One quick thing while we’re on the topic of
wobbling.
• While not as much or as quickly as the Moon, the
Earth wobbles slightly on its axis, a movement
caused by the Moon.
– This is called precession, and it takes 26,000 years to
complete a cycle.
– It’s like seeing a top that’s soon going to stop
spinning.
Aside: Milankovitch Cycles
• Precession is one of three components
of something collectively known as
Milankovitch Cycles, named for
astronomer Milutin Milankovitch.
• The other two components are:
– Axial Tilt
• The tilt of the Earth on its axis; varies between
22.1° and 24.5°.
– Eccentricity
Milutin Milankovitch
• The Earth’s orbit varies between nearly perfectly (1879-1958)
circular and kinda elliptical as a result of the
gravity of Jupiter/Saturn and other planets.
"Milutin Milanković" by Unknown - http://www.sanu.ac.rs/. Licensed under Public Domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Milutin_Milankovi%C4%87.jpg#/media/File:Milutin_Milankovi%C4%87.jpg
Aside: Milankovitch Cycles
• Milankovitch Cycles are often cited by deniers of
climate change as a reason why climate change is not
anthropogenic (human-caused).
• In fact, they do correlate with significant shifts in
Earth’s climate.
• There’s just one problem in using them to explain the
recent changes in temperature:
– Precession cycles take 26,000 years.
– Axial tilt cycles take 41,000 years.
– Eccentricity cycles take 100,000 years.
• Sorry guys, believe it or not, climate change is still
reality.
Milankovitch Cycles and Climate Change
http://upload.wikimedia.org/wikipedia/commons/7/7e/Milankovitch_Variations.png
Back to the Phases of the Moon
• There are some vocabulary words associated with the
moon phases:
– Waxing means the visible part of the Moon is getting larger.
– Waning means the visible part of the Moon is getting
smaller.
– Gibbous means the Moon is nearly full (whether waxing or
waning).
• Key: The moon phases “move” right to left (backward
reading direction).
– You may want to sketch these…
Moon
Invisible
From
Earth
Moon Phases: Why?
• What causes the shadows and lights across
the Moon?
– Yep, it’s the positioning of the Sun and the Moon
relative to Earth.
SUN
Moon
Invisible
From
Earth
Synodic versus Sidereal Months
• Months are named for moon movements.
• A sidereal month is like a sidereal day.
– The movement of the Moon around the Earth 360°.
– Key: It takes 27.3 days.
• A synodic month is like a synodic/solar day.
– The time it takes to go from new moon to new moon.
– Key: It takes 29.5 days and needs to revolve 390° to
do that, not 360°.
• Why?
Synodic versus Sidereal Months
• Remember, in 27.3 days the Moon has
completed 360° (synodic month).
– However, in that time, the Earth travels a little under
30° around the Sun.
• So the Moon needs to “catch up” in order to get
in the right location to be a new moon again.
– That takes another 2.2 days, totaling 29.5 days and
390°.
Synodic versus Sidereal Months
Lunar Phases
• Lunar Phases Interactive
• Lunar Phases worksheet
The Moon
• So, uh, it turns out there’s a whole lot of people that
actually believe NASA faked the moon landing.
– Crazy, right?
• The skepticism boiled over in 2001 when FOX (surprise)
aired a show called Conspiracy Theory: Did We Land on
the Moon?
• As you might guess, all the “theories” presented can be
refuted…but let’s give ‘em a fair shake anyway.
–
–
–
–
First we’ll watch Conspiracy Theory.
Then we’ll watch two quick follow-up videos.
Then we’ll do a little of our own research to find the truth.
Then we’ll watch In the Shadow of the Moon for a
counterpoint.
Follow-Up Videos
• Buzz Aldrin Punches Bart Sibrel
• UniverseToday – How Do We Know the Moon
Landing Isn’t Fake
• In the Shadow of the Moon – Were the Moon
Landings Faked
• In the Shadow of the Moon
– With video questions.
Just for fun…
http://pbfcomics.com/248/
4. Lunar (and Solar) Eclipses
• Hopefully by now you’ve gotten a sense for the
weird dance thing the Moon and Earth are doing
around the sun.
– It takes three to celestially tango.
• As they move about one another, there are times
when they “intersect” or align in special ways.
• You know these intersections better as…?
– Eclipses! (both solar and lunar)
• Not “of the heart…”
Let’s start with Lunar Eclipses.
• Before we fully understand all the terminology
of eclipses, we need to know the names for
shadows.
– Yes, shadows.
• The Sun illuminates only part of Earth, causing
the Earth to cast a two-part shadow.
– The umbra is the main, darkest shadow.
– The penumbra is on the edges of the umbra and
isn’t quite as dark.
• Here’s what I mean…
¡Umbra y Penumbra!
• The penumbra is the partial shadow, while the
umbra is the complete shadow.
• Fun fact: The umbra shadow is 500,000 miles long.
Penumbral Lunar Eclipse
• A penumbral lunar eclipse occurs if the Moon
passes only through the penumbra (and none
of the umbra).
Partial Lunar Eclipse
• A partial lunar eclipse occurs if the Moon
passes through the penumbra and part-way
into the umbra.
Total Lunar Eclipse
• A total lunar eclipse occurs if the Moon
passes completely into the umbra.
Upcoming Lunar Eclipses:
http://www.mreclipse.com/Special/LEnext.html
Wait a second…
• If the Moon revolves around the Earth each month,
how come we don’t always see a lunar eclipse?
– Well, first, the Moon must be in the “full moon” phase.
– Then, the Moon’s orbit is not quite “level” with the Earth’s.
• It’s tilted about 5°, so it doesn’t always line up.
• Though it’s not really moving, the Sun appears to take
a “path” through the celestial sphere.
– That path is called the ecliptic, and for a lunar eclipse, the
Moon must cross into the ecliptic.
• As in, must “intersect” with the plane of the Sun/Earth’s orbit.
Aside: Blood Moons
• Sometimes, during a
total lunar eclipse, the
moon may take on a
reddish glow, known
commonly as the
blood moon.
– This is caused by light
from the Sun passing
through Earth’s
atmosphere before
illuminating the Moon.
Solar Eclipses
• Lunar eclipses are more common than solar eclipses
because the Moon is moving into Earth’s relatively large
shadow.
• On the other hand, solar eclipses are not nearly so
common.
– The Earth has to move into the relatively small shadow of the
Moon.
• Like the Earth, the Moon also casts a shadow with an
umbra and penumbra.
Solar Eclipses
• Because of the size discrepancy between the
Moon and Earth, there are 2.5 different kinds
of solar eclipse:
– Total Solar Eclipse
– Partial Solar Eclipse
– Annular Solar Eclipse
• Let’s take a look…
Total Solar Eclipse
• A total solar eclipse
occurs in the region of
Earth where the umbra
“lands.”
– The umbral shadow
location is known as the
region of totality.
– As you might guess, the
Moon’s shadow moves
across the Earth.
– During an eclipse, things
get quite eerily dark and
you can see the corona
(sun’s outer atmosphere).
Region of
total solar
eclipse.
Path of
total
eclipse.
Region of partial solar eclipse.
Total Solar Eclipse: August 11, 1999
Partial Solar Eclipse
• A partial solar eclipse
occurs in the region of
Earth just outside
totality.
– You may see the diamond
ring effect.
Region of
total solar
eclipse.
Path of
total
eclipse.
Region of partial solar eclipse.
http://www.dreamview.net/dv/new/photos/101917.jpg
Annular Solar Eclipse
• An annular solar eclipse
occurs if the Moon’s
umbra does not quite
touch the ground.
– Around the Moon is the
annulus or “ring of fire.”
Solar Eclipses
• Solar eclipses can make for very dramatic
images, like this one:
Total
Solar
Eclipse
From where would you
see a total solar eclipse?
From where would you
see a partial solar
eclipse?
Partial
Solar
Eclipse
Tell your neighbor…
Via Mir Space Station
Upcoming Solar Eclipses
Closure
• Eclipses Interactive
• Eclipses of Moon and Sun worksheet
– Note: If you’re confused about external/internal
tangent lines, see below.
External tangent lines
are in white.
Internal tangent lines
are in red.