Earth in Space

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Earth in Space

• Astronomy Science Explorer, Prentice Hall

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Why does the Earth have day and night?

• Astronomy the study of the moon, stars, and
  other objects in space
• Axis the imaginary line that passes through the
  Earths center and the North and South poles at
  an angle of 23.5º Polaris is the North Star.
• Rotation Earths spinning on its axis A point
  on the equator spin at 1600 km/hr
• Earths rotation causes day and night

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Whats in a day?

• In what cardinal direction does Earth rotate?
• Where does the sun appear to set?
• How many hours does one rotation take?
• How are daytime and nighttime defined?
• Think about it Why do the sun and moon appear to
  move each day?

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3rd Rock From the Sun

• In addition to rotating on its axis, Earth
  travels around the sun, our closest star.
• Revolution the movement of one object around
  another object
• Orbit the elliptical (or oval) path of
  revolution around the sun
• Think about it How long does one trip around the
  sun take?

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The Calendar

• 1500 B.C., British Isles
• Stonehenge build by ancient people
• Giant stones mark directions in which the sun
  rises and sets on the longest day of the year

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The Calendar

• 1300 B.C., China
• During the Shang dynasty, Chinese astronomers
  calculated the length of a year is 365.25 days.
• 300 B.C., Egypt
• Egyptian astronomers counted the numbers of days
  between the first appearance of the star Sirius.
  Sirius appeared before the Niles spring floods.
  There were about 365 days.

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The Calendar
Chichen Itza El Caracol (Mayan Observatory)

• 900 A.D., Mexico
• Mayan astronomers studied the sun, moon, and
  Venus. Their calendar had 365 days.
• The Mayans were also able to predict astronomical
  events 3000 years in the future.

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The Calendar

• A.D. 1450, Wyoming, USA
• Big Horn Medicine Wheel built by Native
  Americans. Individual stones are aligned with
  rising and setting of sun and stars. Used to
  determine migration times.

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The Calendar

• Earths orbit around the sun takes slightly more
  that 365 days 365 ¼ days
• Leap year was created to compensate. Every four
  years the years has 366 days.
• The time between two full moons is about 29 ½
  days. Hence the month (moonth).
• Romans altered Egyptian calendar of 12 30-day
  months (360 5 days) to the one currently used.

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Seasons on Earth
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Seasons on Earth

• The Earth is warmer at the equator and cooler at
  the poles. The equator receives the most direct
  sunlight. The poles receive sunlight at an
  angle.

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Seasons on Earth

• Earth has seasons due to the 23.5 degree tilt of
  its axis as Earth moves around the sun.
• When does the Northern Hemisphere have summer?
  Winter?
• Why do the Northern and Southern Hemispheres have
  opposite seasons?
• Which areas on Earth do not experience a change
  of season?

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Seasons Vocabulary

• Latitude measurement of distance from the
  equator, expressed in degrees north or south
  (equator 0º, North Pole 90º)
• Summer Hemisphere has more direct sunlight and
  more hours of daylight winter is the opposite.
• Solstice The two days each year when the noon
  sun is overhead at 23.5º south (winter) or 23.5º
  north (summer). For the Northern Hemisphere,
  summer solstice is June 21 and winter solstice is
  Dec. 21.
• Southern Hemisphere?

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Seasons Vocabulary

• Equinox Halfway between the solstices, the noon
  sun is directly over the equator for one day.
  Means equal night. Day and night are the same
  length.
• Vernal equinox (spring) Around March 21 for the
  Northern Hemisphere.
• Autumnal equinox (fall) Around Sept. 23 for the
  Northern Hemisphere.

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Phases, Eclipses, and Tides
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Motions of the Moon

• The moon revolves around Earth in 29.5 days.
• The moon also completes one rotation in 29.5
  days.
• The moon day is the same length as the moon year
  and is approximately one month on Earth.
• The moon has an elliptical orbit.

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Phases of the Moon

• Why can you observe the moon in the sky at night?
• The moon takes many shapes round circle, thin
  sliver, crescent shape.
• Phases Different shapes of the moon
• The moon goes through all of its phases each
  month (1 revolution).

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What Causes the Phases?

• Because the sun lights the moon, half of the moon
  is almost always in sunlight.
• However, the half of the moon facing the Earth is
  not always the half that is reflecting sunlight.
• The phase of the moon depends on how much of the
  sunlit side of the moon faces Earth.

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Phases of the Moon
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The Eight Phases

• New Moon (dark)
• Waxing Crescent
• First Quarter (half lighted)
• Waxing Gibbous
• Full Moon (fully lighted)
• Waning Gibbous
• Third Quarter (half lighted)
• Waning Crescent
• About 29.5 days later New Moon again

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1999, Salzburg - Austria
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Eclipses

• What causes an Eclipse?
• Eclipse an object (moon) in space comes between
  the sun and a third object (Earth), casting a
  shadow on the third object.
• Solar and Lunar Eclipses
• Why are eclipses rare?
• The moons orbit is slightly tilted with respect
  to the Earths orbit around the sun. The moons
  shadow rarely hits Earth and Earths shadow
  rarely covers the moon.

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Solar Eclipses

• Solar Eclipse the new moon passes between the
  Earth and the sun blocking the sunlight (during
  the day)
• Umbra Darkest part of moons shadow cone-shaped
• Small part of Earth experiences a total solar
  eclipse
• Penumbra Lighter part of moons shadow part of
  sun is visible
• Larger part of Earth experiences a partial solar
  eclipse (Do not look directly at a partial solar
  eclipse.)

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Lunar Eclipse sequence from 28th August 2007
taken with SkyWatcher ED100 refractor and Canon
PowerShot compact camera.by Steve Massey
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Lunar Eclipses

• Lunar Eclipse during a full moon the Earth
  passes directly between the moon and the sun
  Earths shadow blocks sunlight from hitting the
  moon
• Umbra total lunar eclipse occurs
• Penumbra partial lunar eclipse occurs
• A lunar eclipse can be observed from anywhere on
  Earth (unlike a solar eclipse)

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The Tides

• Tides The rise and fall of the water
  approximately every 12.5 hours The water rises
  for 6 hours and then falls for 6 hours twice a
  day, in a regular cycle.
• Tides occur because of differences in the force
  of gravity between the moon and different parts
  of Earth
• Force of Gravity Depends on the mass and
  distance between two objects

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A The moons gravity pulls the oceans more
strongly than the Earth. The oceans are drawn
towards the moon creating high tide.
C
A
B
B The moons gravity pulls the Earth more
strongly than the oceans. The oceans are left
behind creating a second high tide.
C D Water flowing towards the high tides and
away from points C and D creates low tides.
D
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The Tide Cycle

• There are always two high tides and two low tides
  occurring on Earth.
• Why does Ocean City Beach experience two high
  tides in one day?

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Spring and Neap Tides

• Spring Tides The highest high tide The tide
  with the greatest difference between high and low
  occurs when the sun, moon, and Earth line up.
  The gravitational pull of the sun and moon
  combine forces.
• Neap Tides The tide with the least difference
  between high and low occurs when the
  gravitational pull of the sun and moon are at
  right angles.
• How many times a month do Spring and Neap Tides
  occur? During which phases?

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Spring and Neap Tides
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