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AST 208 Topics. Time and celestial coordinates. Telescopes. And instruments. The Solar System. The Moon. Celestial Mechanics. Time and the Seasons. Celestial Sphere. What is a day?. A day is defined as the time between two successive upper transits of a given celestial reference point

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AST 208 Topics

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Ast 208 topics

AST 208 Topics

  • Time and celestial coordinates



And instruments

And instruments

The solar system

The Solar System

The moon

The Moon

Celestial mechanics

Celestial Mechanics

Time and the seasons

Time and the Seasons

Celestial sphere

Celestial Sphere

What is a day

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

Apparent solar time

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

    • Earth’s orbit is not a circle

    • Earth does not orbit in the plane of the equator, but the plane of the ecliptic

A sun dial can measure apparent solar time

A sun dial can measure apparent solar time

Mean solar time

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

The analemma

The analemma

Time zones

Time Zones

  • 24 hours in 360 degrees

  • Each 1 hour time zone is 15 degrees wide

Ast 208 topics

  • 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

Solar calendars

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

Precession of the equinoxes

Precession of the equinoxes

Tropical year

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

Julian calendar

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

Gregorian calendar

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

Change from julian to gregorian

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”

Calendar riots

Calendar Riots

Lunar calendars

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

Cathedrals as observatories

Cathedrals as Observatories

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