AST 208 Topics

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AST 208 Topics

• Time and celestial coordinates

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Telescopes
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And instruments
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The Solar System
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The Moon
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Celestial Mechanics
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Time and the Seasons
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Celestial Sphere
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What is a day?

• A day is defined as the time between two
  successive upper transits of a given celestial
  reference point
• An upper transit occurs when the reference point
  crosses the meridian moving westward

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Apparent solar time

• One can use the Sun to measure the length of a
  day. However, compared to a constant rate clock,
  the length of the day measured in this fashion
  changes during the course of the year
• Earths orbit is not a circle
• Earth does not orbit in the plane of the equator,
  but the plane of the ecliptic

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A sun dial can measure apparent solar time
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Mean Solar Time

• Imagine a fictitious point (the mean sun) that
  moves at a constant rate along the celestial
  equator at the average rate of the true sun
• Equation of time

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The analemma
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Time Zones

• 24 hours in 360 degrees
• Each 1 hour time zone is 15 degrees wide

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• Greenwich Mean Time 5 hours later than Eastern
  Standard Time
• Universal Coordinated Time (UT)
• Based on atomic clocks
• Leap seconds added when the difference between
  atomic clock time and earth rotation time becomes
  too big
• Close to GMT

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Solar Calendars

• A sidereal year is the time the earth takes to
  orbit the sun with respect to a stellar reference
  point 365.2564 mean solar days

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Precession of the equinoxes
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Tropical Year

• Year of the seasons orbital period with respect
  to the vernal equinox, that precesses about 50
  seconds of arc per year 365.2422 mean solar days

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Julian Calendar

• Cycle of 3 years of 365 days followed by one year
  of 366 days
• Gradually gets out of sync with the seasons
  because the tropical year is not exactly 365.25
  days long

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Gregorian calendar

• Modified Julian system. Only those century years
  divisible by 400 are leap years, except century
  years divisible by 4000 are not leap years
• Builds an error of 1 day per 20,000 years

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Change from Julian to Gregorian

• 1582 for much of Catholic Europe
• 1700 Protestant German countries
• 1752 Great Britain and its colonies
• Sept 2, 1752 was followed by Sept. 14, 1752
• Early colonial dates may be given as old style
  or new style

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Calendar Riots
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Lunar Calendars

• Based on cycle of the lunar phases rather than
  the apparent motion of the sun in the sky
• From one full moon to the next takes about 29.5
  days (one synodic period)
• This does not go evenly into 365 days

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Cathedrals as Observatories