Calendar

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The calendar is a way to reckon time, specifically the passage of days in relation to months and years. The choice of calendar determines when the anniversaries of events will take place. Hence, it determines when and how long their celebrations—the feasts and fasts—will be, and even whether those take place at all.

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Historical origins

Observational calendars

Lunar calendars

Solar calendars

The Church calendar

Main article: Church calendar.

The Julian calendar

The date of Easter

The Paschal moon

Gregorian reforms

Main article: Gregorian Calendar.

Origins

Controversy

Adoption

The New Calendar

Main article: Revised Julian Calendar.

Origins

Controversy

Adoption

Schism and persecutions

Astronomical appendix

The Ptolemaic model

The Copernican revolution

The Copernican model

The Keplerian model

Classical physics

The Newtonian model

The relativistic model

Perturbative models

Computational models

Isotropy and geocentrism

The isotropic principle

Coordinate systems

Cosmic microwave background radiation

Modeling limitations

Chaos theory

Chaos theory is a field of mathematics which studies unpredictability. In chaotic systems there is a compounding of errors over time, which makes it impossible to predict the future even if those systems are behaving totally in accordance with predictable rules.

Suppose that you are driving a car in a straight line at exactly 100 km/h on an infinite flat plane. After an hour you are 100 km away, and after 100 hours, 10 000. If there is an uncertainty of 1% in your speed, then after an hour you would be 99 to 101 km away, an error of 1%. After a million hours you would be 99-101 million kilometers away, which is still an error of 1%. If instead you drove on the equator of a spherical earth with a circumference of 40 000 km, an error of 1% would still result in you being after an hour 99 to 101 km away, or 1%. After 400 hours you would be back where you started, except that you could be ahead or behind by 400 km. After 800 h you would have completed two revolutions and could be ahead or behind by 800 km. After 20 000 hours you would have completed 50 revolutions, and now you could be ahead or behind by up to 20 000 km. However, 20 000 km is the distance to the opposite end of the earth. This means that your distance that you could be ahead or behind have met, and you could be anywhere in the system (equator of the earth in this case). This concept is called Lyapunov time, and it limits the precision with which astronomical calculations can be performed into the future because even arbitrarily small measurements result in eventually zero knowledge of where objects will end up.

Computational effects

Geological effects

Other astrophysical effects

Conclusion: Apparent time in the heavens

References