Year

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Thirty Days Halth September
Thirty days halth September,
April, May, and November
All the rest have thirty-one,
But February has twenty-eight alone,
Except in leap year, that's the time
When February's days are twenty nine.

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The year is the time that it takes for the Earth to rotate once around the sun.

• 4 Seasons (Fall, Winter, Spring, Summer)
• 12 months (January, February, March, April, May, June, July, August, September, October, November, December)
• 365 1/4 days

It is due to this 1/4 day that we have a leap year. But what exactly is a calendar and what is its history?

Calendar year

All societies have calendars. They are first referenced in the Bible when the bible talks about certain rituals need to take place during certain seasons every year. But the original calendars were based on the cycle of the moon (lunar calendar) while today's common calendar is based on the cycle of the sun.

Jewish calendar or Hebrew calendar

The Jewish New year is Rosh Hashanna and it takes place either in September or October. The reason why the new year changes each year is due to the fact that the Hebrew calendar is a lunar calendar (based on the cycle of the moon) while the Gregorian calendar is based on the cycle of the sun.

During the Temple period a new moon would be declared in the Temple in Jerusalem (Bait Hamigdash). The fire would be lit on the mountain top. Then a person the next mountain top would see the fire and light his own fire. And this would continue to spread the word that a new month has been declared. This is why Jewish holidays last for two days outside of Israel, but are one day in Israel. Before the days of the internet, it could take a day for word to spread that a new month has begun.

Since the Hebrew calendar is based on the cycle of the moon, twelve cycles of the moon do not take 365 1/4 days. The solution to this problem is a leap month instead of a leap day.

In a cycle of 19 years, 7 years have 13 lunar months, while the remaining 12 have 12 lunar months. Each month begins on the date of the new moon, with some minor adjustments made for keeping certain holy days off certain days of the week. In the period before the destruction of the Temple in Jerusalem in 70 CE, the calendar was based on astronomical observations. After that time, rule-based systems for calculating the details of the calendar were adopted. The final version of the rules were described by Maimonides in 1178 CE, and have been maintained by Jewish communities around the world since that time.

Gregorian calendar

To continue in our story of the bible, the Gregorian calendar is based on the Birth of Christ. Year zero is the birth year of Christ. B.C. stands for "Before Christ", but some refer to this period as B.C.E. (Before common era). The time after year zero is C.E. (Common era) or A.D.

The Gregorian calendar used by most countries for legal purposes. The year is 365 days long, with a leap day inserted to adjust for the fractional day approximately every 4 years. A leap day is inserted in every year whose number is divisible by 4, unless the year is also divisible by 100 but not 400. Years with the leap day are called leap years Thus 1896 and 1904 were leap years, but 1900 was not; however, 1996, 2000, and 2004 are all leap years. This correction adjusts the year to be 365.2425 days long. As the tropical year is approximately 365.2422 days long, this will result in the calendar year gaining about one day in 3300 years. Various proposals have been made to adjust for the anomaly, but none have become official.

When the Gregorian calendar was adopted, an adjustment was made to return the vernal equinox to March 21, to allow the formula for the date of Easter to be based on the actual vernal equinox.

Islamic calendar

The Islamic calendar is a purely lunar calendar, with 12 months beginning on the date of the new moon in each year. This means that an Islamic religious holiday can happen during any season. There is no concept of a leap day or a leap month. As a result of the Islamic year averaging just over 354 days, the beginning of the year, and all dates fixed in the Islamic calendar, rotate through the solar year (and against solar calendars) in a cycle about 33 (solar) years long.

Julian calendar

The Julian calendar was developed by Roman astronomers and instituted by Julius Caesar in 46 BCE. The Julian year is normally 365 days long, with a leap day inserted every fourth year to adjust for the fractional day, making the average length of the year 365.25 days long. As the tropical year is approximately 365.2422 days long, this will result in the calendar year gaining about one day in 128 years, resulting in calendar drift.

When the Julian calendar was adopted, the equinoxes and solstices occurred on March 25, June 25, September 25, and December 25. When the formula for determining the date of Easter was adopted at the Council of Nicaea in 325 AD, the vernal equinox was on March 21, and that date was set as the base of the calculation of Easter.

Maya Calendar

The Maya calendar was based on a variety of cycles, including a 13-day and 20-day cycle which combined for a 260-day ceremonial "year" called the Tzolkin, and a 365 day year, called Haab, which had no compensation for the extra quarter-day in the solar cycle. The Tzolkin and Haab cycle in a 52 Haab cycle , and thus every date within a 52-Haab cycle could be uniquely identified by the combination of its Haab date and its Tzolkin date. This was considered sufficient for most people, as 52 years was longer than the average lifespan.

Astronomical years

The year is defined in several ways astronomically. The primary definitions are the tropical and the siderial years.

Tropical year

A tropical year is the time for the Earth to complete one revolution with respect to the framework provided by the intersection of the ecliptic (the plane of the orbit of the Earth) and the plane of the equator (the plane perpendicular to the rotation axis of the Earth). Because of the precession of the equinoxes, this framework moves slowly westward along the ecliptic with respect to the fixed stars (with a period of about 26,000 tropical years); as a consequence, the Earth completes this year before it completes a full orbit as measured in a fixed reference frame. Therefore a tropical year is shorter than the sidereal year. The exact length of a tropical year depends on the chosen starting point: for example the vernal equinox year is the time between successive vernal equinoxes. The mean tropical year (averaged over all ecliptic points) is 365.242 189 67 days (365d 5h 48min 45s) long.

Siderial year

The siderial year is the length of time required for the earth to return to the same position relative to the sun as seen by an observer outside the solar system. The siderial year is 365.256 363 051 days (365d 6h 9min 9s) long.