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The Motions of the Earth and Moon: 

The Motions of the Earth and Moon


Yesterday, Sunday September 23, 2007 5:51 am FALL EQUINOX

Earth Rotates about its Axis: 

Earth Rotates about its Axis Description … Causes EVERY object in the sky to appear to move from East to West during the day This called apparent daily motion EVERY object Sun Moon Planets Stars

Sun rises and sets: 

Sun rises and sets Looking South

(most) Stars rise and set : 

(most) Stars rise and set

Apparent Daily Motion: 

Apparent Daily Motion Sun rising and setting On any given day the Sun is in a certain direction from the Earth. When Earth’s rotation brings Columbus around so we can see the Sun: sunrise! When we are rotated away from Sun: sunset. Stars -Same explanation. Now rotation takes Columbus around so that we are in the direction of the night time stars

Watching Stars from the North Pole: 

Watching Stars from the North Pole

The North Celestial Pole (NCP): 

The North Celestial Pole (NCP) Special spot on the sky that the Earth’s rotation axis points to (headed north) Why special? During the night the stars appear to rotate around this spot Where in the sky the NCP is depends on your latitude

Watching Stars from Arizona: 

Watching Stars from Arizona

Circumpolar Stars: 

Circumpolar Stars Some stars don’t rise or set Reason: they are so close to the NCP that the circle they make going around it never dips below the horizon Called Circumpolar Stars

Polaris and NCP: 

Polaris and NCP

Big Dipper: 

Big Dipper

A Lucky Break!: 

A Lucky Break! The star Polaris is amazing close to NCP (now) The “pointer stars” in the Big Dipper line up directly with Polaris!!!! Big Dipper is circumpolar constellation See it all night and all year Polaris also called Pole Star, North Star

Why care about finding Polaris?: 

Why care about finding Polaris? The direction of Polaris is due North The angle between Polaris and the horizon is YOUR LATITUDE Celestial Navigation …

Big Dipper as Star Guide: 

Big Dipper as Star Guide

Precession of the Earth: 

Precession of the Earth The NS rotation axis of the Earth slowly rotates Like a top wobbling as it spins down Due to the Earth rotating and the gravitational force from the Sun & Moon Precession Period ~ 24,000 years Affects where NCP is among constellations Along with everything else!



Constellation Facts: 

Constellation Facts Group of stars all in roughly the same direction from Earth, BUT …. Each has its own different distance from the Earth Therefore, NOT grouped together is space From Greek astronomers have inherited the names of the constellations: mythological figures or animals Different cultures on Earth each have different names for the constellations

Constellation Stars at Different Distances from Earth: 

Constellation Stars at Different Distances from Earth

Orion’s Story: 

Orion’s Story Orion’s father: Neptune the Sea-God Orion’s mother: Amazon Queen Eurayle Orion inherits mom’s skill as a hunter. In fact, greatest hunter in the world Thus producing a great ego. Orion boasted that he could kill any animal on Earth “Pride goeth before a fall” Orion killed by a wee scorpion So the constellations Orion and Scorpius were placed on opposite sides of the sky. Never visible at the same time

A Question for the Ages: 

A Question for the Ages How do we know the Earth is rotating causing the apparent daily motion of sky objects? What if the Earth was not rotating and the Sun, stars, (and planets) were actually moving around us? Either way, we would see the same movements of celestial objects in the sky…

Proof that the Earth is Rotating: 

Proof that the Earth is Rotating Not connected to Astronomy Foucault Pendulum Coriolis Effect

Foucault Pendulum: 

Foucault Pendulum Swinging Pendulum cannot change the plane it is swinging in unless an outside torque (“twist”) is applied Foucault Pendula designed to have no torque on them, BUT you see plane of swinging slowly rotating anyway Change is due to Earth rotating underneath the pendulum every 24 hours

Coriolis Effect: 

Coriolis Effect Anything moving object headed North or South in the in the Northern Hemisphere will be deflected to the right due to the Earth’s Rotation Deflection is largest if object flying faster and farther Long-range artillery shells Hurricane winds Opposite in the Southern Hemisphere: deflection is to the left

Effect causes aim to be off: 

Effect causes aim to be off

Effect causes hurricanes to rotate: 

Effect causes hurricanes to rotate Hurricanes Rotation: N. Hemishpere: Counter-Clockwise S. Hemishpere: Clockwise

Textbook Reading: 

Textbook Reading Chapter 2.1 and 2.2

Earth Orbits the Sun: 

Earth Orbits the Sun

Sun’s Path in the Sky: 

Sun’s Path in the Sky As Earth orbits the Sun, every day the direction of the Sun – from the Earth’s point of view – changes. So, Sun’s position among the stars changes as well Sun’s direction changes about 1 per day Over a year, the Sun will trace out a circular path through the constellations. This circle is called the Ecliptic. Ecliptic is an important reference path on the sky map

Look at an Entire Year: 

Look at an Entire Year

Visible Constellations change over a year: 

Visible Constellations change over a year May have heard … “Orion is a winter constellation” “You can see Cygnus most of the summer” Above comments arise because you cannot see the Constellations near where the Sun is in the sky Sun so bright it washes out rest of stars Since Sun moves along the Ecliptic over a year the constellations “away” from the Sun change over a year.

Constellations on opposite side of Sun (Opposition): 

Constellations on opposite side of Sun (Opposition)

The Zodiac: 

The Zodiac Band of constellations closest to the Ecliptic Correspond to Horoscope “Signs” Astrology used to make predictions Useful for backyard astronomy: Planets and the Moon can always be found in the Zodiac Width of the Zodiac belt ~ 18°

Earth’s Rotation Axis is Tilted: 

Earth’s Rotation Axis is Tilted


Vivaldi Why do we experience seasons on the Earth? The Earth’s rotation axis is tilted by 23.5º Because of the axis tilt, the Sun is higher in the sky (higher altitude at noon) in June then it is in December. Thus, in June, sunlight strikes the ground in a direction closer to vertical, the absorbed energy is diluted over just a small area, and the ground and air are warm. Seasons are opposite in the Southern Hemisphere Seasons are NOT due to the Earth getting closer and then farther away from the Sun during a year Earth’s orbit is nearly circular (E-S distance stays the same) Earth is slightly closer to Sun in January!



Solstices & Equinoxes: On Earth: 

Solstices & Equinoxes: On Earth These special times (obviously) mark the astronomical beginning of the seasons. How could ancient races know when a season is about to start?

Solstices & Equinoxes: 

Solstices & Equinoxes

Another Question for the Ages: 

Another Question for the Ages How doe we know that the Earth is going around the Sun and not vice versa? As before, motions in the sky would look the same to the naked eye either way Proof of the Earth’s orbit around the Sun was the measurement of Aberration of Starlight Continuing change in the direction of a star from Earth because of the Earth’s yearly orbit around the Sun Measurement of Starlight Aberration 1720s by James Bradley & Samuel Molyneaux

Analogy: Moving in the Rain: 

Analogy: Moving in the Rain

Small Angles: 

Small Angles 1 arc-minute = 1/60th of 1 degree Smallest angle you can see with your naked eye 1 arc-second= 1/60th of 1 arc-minute Change in the direction of star due to stellar aberration ~ 20 arc-seconds Cannot be seen by the naked eye!!!

Textbook Reading: 

Textbook Reading Chapter 2.3

Moon orbits the Earth: 

Moon orbits the Earth

Moon has Phases (Crescent): 

Moon has Phases (Crescent)

All The Moon’s Phases: 

All The Moon’s Phases

Explanation of Moon Phases: 

Explanation of Moon Phases The Moon’s orbital period is ~ month. Orbits counter clock-wise (looking down from N). Moon is always in Zodiac Moves FAST, 12° per night!! The Moon does not generate light but it can reflect light Very poor reflectivity: 10% At a given point in its orbit around the Earth, half the Moon’s surface is illuminated by the Sun The phase of the Moon at that point is determined by what fraction of the illuminated side we can see from Earth The phases changes because we see different fractions depending on where the Moon is in its orbit. Examples: Full Moon  See 100% of illuminated hemisphere New Moon  See 0% of illuminated hemisphere

“Earthshine” and Explanation: 

“Earthshine” and Explanation

Does the Full Moon ever look any different than this?: 

Does the Full Moon ever look any different than this?

The “Man in the Moon”: 

The “Man in the Moon” No, we always see the same side of the Moon Full moon: see all of the side pointed at us Other phases only see part of the same side pointed at us Different culture’s see different “images”: rabbit, etc. This simple fact has two amazing consequences The Moon itself must be rotating The Rotation Period of the Moon equals its Orbital Period (both one month) Called “tidal locking” of a moon Caused by gravitational force between Moon & Earth Just about all moons in the SS are tidally locked

The Moon Must Rotate: 

The Moon Must Rotate

The Far (not Dark) Side of the Moon: 

The Far (not Dark) Side of the Moon

Textbook Reading: 

Textbook Reading Chapter 2.4

Lunar & Solar Eclipses: 

Lunar & Solar Eclipses S07


Eclipses Need exact alignment of Earth, Sun, & Moon Lunar Eclipse: Moon moves into the shadow of the Earth Solar Eclipse: Tip of Moon’s shadow seen from Earth Earth rotates “underneath” the Moon’s Shadow

Lunar Eclipse: 

Lunar Eclipse

Lunar Eclipse Facts: 

Lunar Eclipse Facts Last a few hours, from Moon first entering shadow to Moon completely out of Earth’s shadow. Can only happen at time of Full Moon Can be seen (weather – permitting) anywhere on the “Night-Side” of the Earth Total Lunar Eclipse: Moon completely covered by Earth’s shadow Partial Lunar Eclipse: Moon not completely covered by Earth’s shadow Occur much more often than Solar Eclipses

Earth’s Shadow: 

Earth’s Shadow

Effect of Earth’s Atmosphere: 

Effect of Earth’s Atmosphere During a Lunar Eclipse Moon can have a red-orange color … …but no light should be hitting the Moon Explanation: Earth’s atmosphere bends light into the shadow due to refraction Red light is refracted much better by atmospheric molecules than blue light Red light bounces off Moon, seen on Earth Same process explains why the sky in blue and why sunsets are red

Rayleigh Scattering: 

Rayleigh Scattering Sunlight

Solar Eclipse: 

Solar Eclipse

Fate takes a hand …: 

Fate takes a hand … REMARABLE COINCIDENCE : Angular size of the Moon equals the angular size of Sun. So, Moon can completely block out the light from the Sun!! Sun 400x farther away from Earth than Moon, but also 400x larger than Moon Angular size of both ~ 1 Allows Total Solar eclipses Was not true in the past, will not be true in the future Not true of any other moon orbiting any other planet in our Solar System

Solar Eclipse Facts: 

Solar Eclipse Facts Total Solar Eclipse can be seen only from a small circular area on the Earth (~ 200 miles big) Due to Earth’s rotation this area “moves” over different places on the Earth. Called Path of Totality Duration of Totality ~ few minutes (point on the surface of the Earth moves FAST) Places outside of the POT see a Partial Solar Eclipse Solar Eclipse can only happen at New Moon During Totality the Corona of the Sun pops into view Weird events during Totality …

Path of Totality: 

Path of Totality

Solar Eclipse Beginning …: 

Solar Eclipse Beginning …

… Continues …: 

… Continues …

… to Totality: 

… to Totality

Path of Totality (From Space!): 

Path of Totality (From Space!)

Textbook Reading: 

Textbook Reading Chapter 2.5

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