Motions of the Earth and Moon

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Motions of the Earth and Moon

• Reading Ch 2.1-2.4

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Earth Motion 1 Rotation

• Rotational Axis through North and South Poles
• Rotational Period (How long it takes to go once
  around) is 24 hours
• Counter-Clockwise direction as viewed from above
  North Pole
• 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

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Rising and Setting
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Apparent Daily Motion

• Sun rising and setting
• On any given day the Sun is in a certain
  direction from the Earth. When Earths 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

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Watching Stars from Arizona
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The North Celestial Pole (NCP)

• Special spot on the sky that the Earths rotation
  axis points to
• Why special?
• During the night the stars appear to rotate
  around this spot
• Also special (and useful) in other ways

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Watching Stars from the North Pole
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Circumpolar Stars

• Some stars dont 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

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Polaris and NCP
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Big Dipper Points to Polaris

• http//www.astropix.com/HTML/C_SPRING/URSAS.HTM

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Lucky Breaks!

• 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

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Why care about finding Polaris?

• The direction of Polaris is due North
• The angle between Polaris and the horizon is YOUR
  LATITUDE
• Celestial Navigation

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Big Dipper as Star Guide
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Earth Motion 2 Precession

• 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!

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A Question for the Ages

• How do we know the Earths rotation causes 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

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Proof that the Earth is Rotating

• Not connected to Astronomy
• Foucault Pendulum
• Coriolis Effect

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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

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Coriolis Effect

• Observed fact An object flying through the sky
  (say a cannonball) headed North or South in the
  Northern Hemisphere will drift to the right as
  seen from the surface of the Earth
• The drift will be to the East if flying object
  headed North
• The drift will be to the West if flying object
  headed South
• Coriolis Drift due to the Earths Rotation

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Effect causes aim to be off
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Coriolis Effect II

• Amount of Drift is larger if object flying faster
  and farther
• Long-range artillery shells
• Hurricane winds
• Opposite in the Southern Hemisphere Coriolis
  drift is to the left

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Effect causes hurricanes to rotate

• Hurricanes Rotation
• N. Hemisphere Counter-Clockwise
• S. Hemisphere Clockwise

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Interlude Constellations
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Astronomy Picture of the Day (APOD)

• http//antwrp.gsfc.nasa.gov/apod/ap090627.html
• Summer Triangle
• http//antwrp.gsfc.nasa.gov/apod/ap090912.html

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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

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Constellation Stars at Different Distances from
Earth
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Earth Motion 3 Orbits the Sun
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Suns Path Among the Stars

• As Earth orbits the Sun, every day the direction
  of the Sun from the Earths point of view
  changes. So, Suns position among the stars
  changes as well
• Suns 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

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Ecliptic Constellations and Months
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Look at an Entire Year
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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.

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Constellations on opposite side of Sun
(Opposition)
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The Zodiac

• Band of constellations closest to the Ecliptic
• Correspond to Horoscope Signs
• Astrology used to make predictions (not science!)
• Useful for backyard astronomy Planets and the
  Moon can always be found in the Zodiac
• Width of the Zodiac belt 18

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Mars
Saturn
Mercury
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Return to the Sun

• Observations from studying the apparent Daily
  Motion of the Sun more carefully
• Noon altitude of Sun changes from day to day
• Higher in Summer Lower in Winter
• Never directly overhead in Columbus
• Duration of daylight changes from day to day
• Longer in Summer Shorter in Winter
• Direction of Sunrise/Sunset changes from day to
  day
• North of E/W in Summer South of E/W in Winter
• WHY?...

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Earths Rotation Axis is Tilted

• Causes the Suns light to hit Columbus at
  different angles throughout the year

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December
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March September
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June
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Axis Tilt is 23.5º from perpendicular to Earths
orbital plane
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Reason for the Seasons

• Why do we experience seasons on the Earth?
• The Earths 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
• Earths orbit is nearly circular (E-S distance
  stays the same)
• Earth is slightly closer to Sun in January!

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Energy Concentrated
Energy Diluted
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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?

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Solstices Equinoxes
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Another Question for the Ages

• How do 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 Earths orbit around the Sun was the
  measurement of Stellar Parallax
• Change in the direction of a star from Earth due
  to the changing position of the moving Earth
• We see parallax every day

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Everyday Parallax

• Change in relative position of nearby and distant
  objects due to motion of the observer

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Stellar Parallax

• First measured in 1838. Direction shift 1.52
  arcsecond

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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
  parallax is arc-seconds
• Cannot be seen by the naked eye!!!

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Motion 1Moon orbits the Earth
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Causing the Moon to show Phases
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Moons Orbital Motion

• The Moons orbital period is 29.5 days
• 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 (poorly, 10 reflective)
• At a given point in its orbit around the Earth,
  half the Moons 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 phase 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

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All The Moons Phases
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Earthshine
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Does the Full Moon ever look any different than
this?
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The Man in the Moon

• No, we always see the same side of the Moon
• This simple fact has two amazing consequences
• The Moon itself must be rotating (Motion 2)
• 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

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The Moon Must Rotate
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The Far (not Dark) Side of the Moon