astronomy
DESCRIPTION
Astronomy. Chapter Eighteen: Earth, the Moon, and the Sun. 18.1 The Earth and Its Moon 18.2 Earth Cycles 18.3 The Sun. Investigation 18A. Days and Months. What are days and months?. 18.2 Earth’s cycles. A calendar is a means of keeping track of all the days in a year. - PowerPoint PPT PresentationTRANSCRIPT
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Astronomy
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Chapter Eighteen: Earth, the Moon, and the Sun
• 18.1 The Earth and Its Moon
• 18.2 Earth Cycles
• 18.3 The Sun
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Investigation 18A
• What are days and months?
Days and Months
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18.2 Earth’s cycles
• 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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18.2 Keeping track of time
• The ancient Egyptians divided the day into parts that were similar to hours.
• As early as 3500 BC, monuments called obelisks were built to separate the day into parts.
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18.2 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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18.2 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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18.2 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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18.2 The lunar cycle
• The orbit of the Moon is tilted about 5 degrees from Earth’s 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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18.2 The lunar cycle
• The Moon is not in Earth’s shadow except during rare eclipses.
• The Sun-facing side of the Moon is lit by sunlight almost all the time.
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18.2 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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18.2 Lunar eclipses
• A lunar eclipse occurs when Earth’s shadow falls on the Moon.
• Because of its tilted orbit, in most months, Earth’s shadow does not block the sunlight from hitting the Moon.
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18.2 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 Earth’s shadow.
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18.2 Solar eclipses• A solar eclipse occurs when the Moon’s
shadow falls on Earth.
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18.2 Solar eclipses• A total solar eclipse is
caused by the Moon’s shadow blocking out the Sun.
• The Sun is completely blocked by the Moon’s shadow for about two or three minutes.
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18.2 The Seasons• The seasons are caused by the 23-degree tilt of
Earth’s 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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18.2 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.