Calendar
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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Contents
- 1 Historical origins
- 2 The Church calendar
- 3 Gregorian reforms
- 4 The New Calendar
- 5 Schism and persecutions
- 6 Astronomical appendix
- 7 References
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.
