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AST 208 Topics • Time and celestial coordinates

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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 - PowerPoint PPT Presentation

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Page 1: AST 208 Topics

AST 208 Topics

• Time and celestial coordinates

Page 2: AST 208 Topics

Telescopes

Page 3: AST 208 Topics

And instruments

Page 4: AST 208 Topics

The Solar System

Page 5: AST 208 Topics

The Moon

Page 6: AST 208 Topics

Celestial Mechanics

Page 7: AST 208 Topics

Time and the Seasons

Page 8: AST 208 Topics

Celestial Sphere

Page 9: AST 208 Topics

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

Page 10: AST 208 Topics

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

Page 11: AST 208 Topics

A sun dial can measure apparent solar time

Page 12: AST 208 Topics

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

Page 14: AST 208 Topics

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

Page 16: AST 208 Topics

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

Page 17: AST 208 Topics

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

Page 19: AST 208 Topics

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

Page 20: AST 208 Topics

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

Page 21: AST 208 Topics

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”

Page 22: AST 208 Topics

Calendar Riots

Page 23: AST 208 Topics

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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