Chapter 2 Knowing the Heavens

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Guiding Questions

• What role did astronomy play in ancient
  civilizations?
• Are the stars that make up a constellation
  actually close to one other?
• Are the same stars visible every night of the
  year? What is so special about the North Star?
• Are the same stars visible from any location on
  Earth?
• What causes the seasons? Why are they opposite in
  the northern and southern hemispheres?
• Has the same star always been the North Star?
• Can we use the rising and setting of the Sun as
  the basis of our system of keeping time?
• Why are there leap years?

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Naked-eye astronomy had an important place in
ancient civilizations.
Chichén Itzá in Yucatán Stonehenge in British
Isles Medicine Wheel in Wyoming Casa Grande in
Arizona
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Eighty-eight constellations cover the entire sky.
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Eighty-eight constellations cover the entire sky.
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Eighty-eight constellations cover the entire sky.

• 6000 stars visible to unaided eye (only half are
  above the horizon).
• 88 semi-rectangular groups of stars called
  constellations.
• Some stars in the constellations are quite close
  while others are very far away.

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Eighty-eight constellations cover the entire
sky. Constellation names are derived from the
myths and legends of antiquity.
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The appearance of the sky changes during the
course of the night and from one night to the
next.
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Diurnal Motion of the Night Sky

• Each night, most stars appear to rise in the
  east, move across the sky, and set in the west
  because of Earths rotation.

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Looking toward the North

• The North Star (Polaris) does not appear to move.
• Stars in the northern sky seem to move in a
  counter-clockwise sense.
• Northern stars that never set are called
  circumpolar stars.

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As Earth orbits our Sun, different constellations
are visible at different times of the year.
The circumpolar constellations are always the
same because they are visible no matter where
Earth is in its orbit.
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It is convenient to imagine that the stars are
located on a celestial sphere.
Celestial equator splits the sky into a
northern half and a southern half. North
celestial pole the point directly above Earths
rotation axis.
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Celestial Coordinates pinpoint positions on the
celestial sphere.
Right Ascension how far objects are to the east
of the vernal equinox (Suns position on March
21). Declination how far objects are above or
below the celestial equator.
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The point directly overhead is called the
ZENITH.The line that splits the sky into
eastern and western halves is called the MERIDIAN.
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The celestial sphere seems to spin around the
Earth. The horizon is the line that separates
what can be seen in the sky and what cannot. For
observers on Earth at a latitude of 35º, the NCP
is located at an altitude of 35º above the
horizon. Stars near the SCP are never visible.
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Over the course of a year, the Suns position in
the sky changes.
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The Suns daily path across the sky
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Dec. 21 Winter SolsticeMarch 21 Vernal
EquinoxJune 21Summer SolsticeSept.
21Autumnal equinox
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The ecliptic is the Suns apparent path around
the celestial sphere over the course of a year.
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The ecliptic is the Suns apparent path around
the celestial sphere over the course of a year.
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The seasons are caused by the tilt of Earths
axis of rotation.
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The seasons are caused by the tilt of Earths
axis of rotation.
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The Moon helps to cause precession, a slow,
conical motion of Earths axis of rotation.
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12,000 years from now, the bright star Vega will
be the new North Star because of precession.
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Positional astronomy plays an important role in
keeping track of time.

• Key Question When is the Sun on the meridian
  (directly in the south)?

• Apparent solar day the interval between two
  successive meridian transits of the Sun (varies
  around 24 hrs as Earth orbits the Sun at varying
  speeds).
• Mean solar day the interval between two
  successive meridian transits of the Sun IF it
  moved at a constant rate (exactly 24 hrs).
• Sidereal time the interval of time between two
  successive meridian transits of a star (23 hrs 56
  min).

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When is the Sun overhead at my location?
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Astronomical observations led to the development
of the modern calendar

• Our Sun takes 365.24220 days to move around the
  celestial sphere once (one year).
• 0.24220 fractional days is 5 hours, 48 minutes,
  and 46 seconds a fraction that has caused
  endless headaches for calendar makers who would
  rather the year was exactly 365 days long!
• In 45BC Julius Caesar decreed that years are 365
  days with one extra day added in February, every
  four years (good to one day in 128 years).
• In 1582, Pope Gregory XIII introduced the
  currently used Gregorian calendar that does not
  allow leap years in Centuries unless the year is
  evenly divisible by 400 (good to one day in 3300
  years).

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Guiding Questions

• What role did astronomy play in ancient
  civilizations?
• Are the stars that make up a constellation
  actually close to one other?
• Are the same stars visible every night of the
  year? What is so special about the North Star?
• Are the same stars visible from any location on
  Earth?
• What causes the seasons? Why are they opposite in
  the northern and southern hemispheres?
• Has the same star always been the North Star?
• Can we use the rising and setting of the Sun as
  the basis of our system of keeping time?
• Why are there leap years?