UNIT NINE: Matter and Motion in the Universe

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UNIT NINE Matter and Motionin the Universe

• Chapter 26 The Solar System
• Chapter 27 Stars
• Chapter 28 Exploring the Universe

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Chapter Twenty-Six The Solar System

• 26.1 Motion and the Solar System
• 26.2 Motion and Astronomical Cycles
• 26.3 Objects in the Solar System

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Section 26.2 Learning Goals

• Define the lunar cycle.
• Differentiate solar and lunar eclipses.
• Explain what causes seasons.
• Describe the impact of the Moon on tides.

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26.2 Motion and astronomical cycles

• The Sun is at a point called the focus that is
  offset from the center of the orbit.
• This causes the distance from the Sun to vary as
  a planet orbits.

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26.2 Rotation

• In addition to orbiting the Sun, the planets also
  rotate.
• An axis is the imaginary line that passes through
  the center of a planet from pole to pole.

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26.2 Revolution

• All of the planets orbit, or revolve, around the
  Sun in the same direction (counter-clockwise).
• A year is the time it takes a planet to complete
  one revolution around the Sun.

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26.2 Motion and keeping track of time

• A calendar is a means of keeping track of all the
  days in a year.
• Ancient civilizations developed calendars based
  on their observations of the Sun, Moon, and stars.

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26.2 Motion and keeping track of time

• The Egyptians adopted a calendar with 365 days in
  a year, divided into 12 months, each with 30
  days, and an extra five days at the end.

As early as 3500 BC, monuments called obelisks
were built to separate the day into parts.
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26.2 Motion and keeping track of time

• Early water clocks were stone containers that
  allowed water to drip at a constant rate through
  a small hole.

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26.2 Motion and keeping track of time

• The sundial is the oldest known clock.
• A sundial uses the shadow of a pointer that moves
  from one side of the base to the other as the Sun
  appears to travel from east to west during the
  day.

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26.2 Keeping track of time

• Today we divide each rotation of Earth into 24
  equal parts called hours.
• Each hour is divided up into 60 parts called
  minutes and each minute into 60 parts called
  seconds.

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26.2 The lunar cycle

• The gradual change in the appearance of the Moon
  is called the lunar cycle.
• The lunar cycle occurs because of the relative
  positions of Earth, the Moon, and the Sun.
• Although the lunar cycle is a continuous process,
  there are eight recognized phases.

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26.2 The lunar cycle

• The orbit of the Moon is tilted about 5 degrees
  from Earths orbit.
• The lunar cycle is caused by the angle the Moon
  makes with Earth and the Sun as it orbits Earth.

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26.2 The lunar cycle

• The Moon is not in Earths shadow except during
  rare eclipses.
• The Sun-facing side of the Moon is lit by
  sunlight almost all the time.

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26.2 Phases of the Moon

• The lighted side of the Moon faces away from
  Earth.
• How the moon appears to Earth dwellers at
  different positions in its orbit is shown below

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26.2 Lunar eclipses

• A lunar eclipse occurs when Earths shadow falls
  on the Moon.
• Because of its tilted orbit, in most months,
  Earths shadow does not block the sunlight from
  hitting the Moon.

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26.2 Lunar eclipses

• A lunar eclipse can be total or partial and all
  observers on the dark side of Earth can see it at
  the same time.
• A partial eclipse occurs when only part of the
  Moon falls in Earths shadow.

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26.2 Solar eclipses

• A solar eclipse occurs when the Moons shadow
  falls on Earth.

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26.2 Solar eclipses

• A total solar eclipse is caused by the Moons
  shadow blocking out the Sun.
• The Sun is completely blocked by the Moons
  shadow for about two or three minutes.

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26.2 The Seasons

• The seasons are caused by the 23-degree tilt of
  Earths axis of rotation relative to its orbit.

• During summer in the Northern Hemisphere, the
  north end of the axial tilt is facing toward the
  Sun.
• This results in more direct sunlight and higher
  temperatures.

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26.2 The Seasons

• The path of the Sun across the sky in the
  Northern hemisphere during the year shows the
  positions of the 1st day of each new season.

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26.2 Tides

• The daily cycle of rising and falling ocean
  levels is called a tide.
• If the Moon passes overhead once every 24 hrs.,
  you would expect the tide to rise only once.
• As Earth revolves around the common center of
  mass, the ocean on the opposite side from the
  Moon is flung outward a little every 12 hrs.

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26.2 Tides

• The Moon does not really orbit Earth as if Earth
  were fixed in place.
• Instead, Earth and the Moon orbit around a common
  center of mass.

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26.2 Tides

• As Earth revolves around the center of mass, the
  ocean on the opposite side from the Moon is
  flung outward a little by its own inertia.

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Investigation 26B
Earths Seasons

• Key Question
• What causes the seasons?