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Year



The Galactic year is the time it takes Earth's solar system to revolve once around the galactic center. It comprises roughly 226 million Earth years.

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Variation in the length of the year and the day

The exact length of an astronomical year changes over time. The main sources of this change are:

  • The precession of the equinoxes changes the position of astronomical events with respect to the apsides of Earth's orbit.
    An event moving toward perihelion recurs with a decreasing period from year to year; an event moving toward aphelion recurs with an increasing period from year to year.
    But this effect don't change the average value of the length of the year.
  • The gravitational influence of the Moon and planets changes the motion of the Earth from a steady orbit around the Sun.
    The Earth orbit varies by a chaotic way, but in a interval quite more reduced than the orbits of the nearest planets.
  • Tidal drag between the Earth and the Moon and Sun increases the length of the day and of the month (by transferring angular momentum from the rotation of the Earth to the revolution of the Moon); since the apparent mean solar day is the unit with which we measure the length of the year in civil life, the length of the year appears to change. Tidal drag in turn depends on factors such as post-glacial rebound and sea level rise.
  • Changes in the effective mass of the Sun, caused by solar wind and radiation of energy generated by nuclear fusion and radiated by its surface, will affect the Earth's orbital period over a long time (approximately an extra 1.25 microsecond per year[1]).

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Summary of various kinds of year

  • 346.62 days - a draconitic year in some septenary calendars.
  • 353, 354 or 355 days — the lengths of common years in some lunisolar calendars.
  • 354.37 days/12 lunar months - the average length of a year in lunar calendars.
  • 365 days — a common year in many solar calendars.
  • 365.24219 days — a mean tropical year near the year 2000.
  • 365.2424 days — a vernal equinox year.
  • 365.2425 days — the average length of a year in the Gregorian calendar.
  • 365.25 days — the average length of a year in the Julian calendar.
  • 365.2564 days — a sidereal year.
  • 366 days — a leap year in many solar calendars.
  • 383, 384 or 385 days — the lengths of leap years in some lunisolar calendars.
  • 383.9 days/13 lunar months - a leap year in some lunisolar calendars.

An average Gregorian year is 365.2425 days = 52.1775 weeks, 8,765.82 hours = 525,949.2 minutes = 31,556,952 seconds (mean solar, not SI).
A common year is 365 days = 8,760 hours = 525,600 minutes = 31,536,000 seconds.
A leap year is 366 days = 8,784 hours = 527,040 minutes = 31,622,400 seconds.
The 400-year cycle of the Gregorian calendar has 146,097 days and hence exactly 20,871 weeks.
See also numerical facts about the Gregorian calendar.

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Numeration or designation of year numbers

A calendar era is used to assign a number to individual years, using a reference point in the past as the beginning of the era. In many countries, the most common era is from the estimated date of the birth of Jesus Christ; dates in this era are designated anno Domini ("in the year of the Lord", abbreviated A.D.) or, more neutrally, C.E. (common era). Other eras are also used to enumerate the years in different cultural, religious or scientific contexts.

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Notes and References

  1. ^ Solar mass is ~2×1030 kg, decreasing at ~5×109 kg/s, or ~8×10−14 solar mass per year. The period of an orbiting body is proportional to \frac{1}{\sqrt{M}}, where M is the mass of the primary.
  2. ^ ~300 W of radiation produces ~9.5×109 J orbital energy decrease per year; this varies as 1/R, and period varies as R1.5

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See also

Look up Year in
Wiktionary, the free dictionary.



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