Transcript Chapter 1

Chapter 3
Earth, Moon, and Sky
How do we locate
objects in the sky?
How are seasons and
tides related to
astronomy?
What goes into making
a modern calendar?
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3.1 Earth and Sky
Directions on Earth:
North (N) and South (S) are the directions
to the corresponding poles.
East (E) is the direction in which the Earth
rotates and West (W) is opposite.
Locations defined by latitude and
longitude.
Meridian: an imaginary circle around the
Earth passing through the poles.
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Latitude and Longitude
Used to locate positions on Earth.
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Seasons Arise from the Earth’s
Tilt
The Earth’s axis of rotation is tilted 23 degrees to its
plane of rotation. At different times of the year more
solar radiation (light) strikes the northern or southern
hemisphere.
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The Sun’s Path Changes with
the Seasons
The Sun is above the Cel. equator in summer, and
below in winter (in the Northern hemisphere).
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Illumination in June
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Illumination in December
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3.3 Keeping Time
• This is just a summary of the history of time.
• Our fundamental measure of time is the length of a
day.
– Our normal day is called a solar day – the Earth rotates
once w.r.t. the Sun, or noon-to-noon.
– The Earth moves from one sunrise to the next by about
1°, so stars appear to shift.
– A sidereal day is the time for the Earth to rotate once
w.r.t. the stars, and is about 4 minutes shorter.
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Sidereal Versus Solar Day
During one day, the Earth moves
around the Sun by about 1/365
since a complete orbit is 365 days.
This causes the time for the Earth
to rotate back to face the Sun to be
longer by about 4 minutes compard
to the time to rotate back to face the
same star.
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3.3.2/3 Solar Time
• Apparent solar time is the time at your location
based on the position of the Sun.
– It varies from city-to-city (Detroit to Baltimore).
– It varies day-to-day because the speed of the Earth varies.
• Mean solar time averages out the day-to-day
changes.
• Standard time is the same everywhere in a time
zone, and changes by 1 hr. or ½ hr. between zones.
• Daylight saving time is standard time + 1 hr. Shifts
daylight to align with work hours.
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The International Date Line
Traveling westward, a new time zone appears every 15° of
longitude, each 1 hr. earlier. You lose 24 hrs. (1 day) when
returning home. The problem is solved by the use of the
international date line. It passes thru the Pacific Ocean, by
agreement.
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3.4 The Calendar
• The challenge is that the obvious measures of day,
month, and year don’t mesh.
– 1 lunar month = 29.5306 days
– 1 solar year = 365.2422 days
• Many early calendars, as evidenced by ruins found
around the globe.
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Stonehenge
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Mayan Observatory at Caracol
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The Gregorian Calendar
• The Romans used the leap year, counting 1 extra day
every fourth year
– 1 year  365.25 days
– 11 minutes of error every year
• By 1582 the calendar was off by 10 days.
• Pope Gregory XIII introduced a new calendar, and
skipped 10 days so that Oct. 4, 1582 was followed
by Oct. 15, 1582!
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3.5 Phases and Motions of the
Moon
• The Moon is the second brightest object in the sky,
yet it’s all reflected sunlight.
• The Moon goes through phases every month.
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–
–
–
–
New Moon
First quarter
Full Moon
Third quarter
Back to new
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Phases of the Moon
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3.5.2 The Moon’s Revolution
and Rotation
•
•
•
•
•
•
Revolution – to go around in the orbit.
Rotation – to spin around an axis.
The Moon revolves once in about 29 days.
The Moon rotates once in exactly the same time.
We see only one side of the Moon.
The side we don’t see is called the “dark side”.
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The Moon Rotates Once per
Revolution
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3.6 Ocean Tides and the Moon
• Tides are due primarily to the Moon, with some
influence from the Sun.
• All points on the Earth are not equally distant from
the Moon.
– The pull of the Moon’s gravity is different.
– Forces result that push water and raise tides.
• Each day there are 2 high tides and 2 low tides.
Why?
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The Moon’s Gravity Pulls
Differently at Different
Locations
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Tides Come from Water Moving
in Response to the Moon
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The Sun’s Gravity Also
Contributes: Alignment of Sun
and Moon
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3.7 Eclipses of the Sun and
Moon
• An eclipse occurs when one object passes between
the Sun and another object, blocking the light from
the Sun.
– Solar eclipse: the Moon moves between the Earth and
the Sun.
– Lunar eclipse: the Earth moves between the Moon and
the Sun.
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What’s a Solar Eclipse
On Earth, the apparent size of the Moon and Sun are
almost identical. Total solar eclipses are dramatic.
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The Moon’s Shadow Falls on
the Earth
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A Lunar Eclipse: the Earth’s
Shadow Falls on the Moon
Lunar eclipses only occur during a full moon.
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