Ast 208 topics
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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


Telescopes

Telescopes


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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