Years are defined as the length of time in days that is required by our tilted Earth to make one complete revolution around the sun. This presents a choice. While traveling through its orbit Earth has five concurrent cycles, each of which is said to be a year in length. All five yearly cycles are completed in a different period of time, but most of them require between 365.24 and 365.26 days.
Perhaps the truest period of Earth’s revolution is the sidereal year. It is the number of days spent by the sun in its apparent passage from a fixed star and then back to the same position. Many calendars dating from the first millennium B.C. were based upon this method of determining when a new year began.
Some peoples still use the sidereal year to regulate their farming activities. The Tukano Indians of northwest Brazil, for example, reckon the seasons by watching the Milky Way’s Pleiades star cluster. They know that heavy seasonal rains will soon follow when these stars dip below the horizon just after sunset. Tukanos watch the Pleiades to know when to prepare their fields and plant crops.
A different Earth cycle than the sidereal year is used to determine year length for most societies. It is called the tropical or equinoctial year—the amount of time that passes between two vernal equinoxes. Because this cycle is more regular than the others, it is the one that was chosen to fix the length of our calendar.
With the vernal equinox as a reference point, the Earth makes one complete revolution around the sun in just a little under 365.25 days. This is currently about twenty minutes shorter than the sidereal year. The different amount of time required by the sidereal year results from the Earth’s wobbling like a top as it spins on its axis while orbiting the sun.
A tropical year’s length in excess of 365 days amounts to nearly a full day in every four years. As a result, the number of calendar days in every four-year period that includes a leap year is very closely matched with nature.
It has always been relatively easy to pattern the calendar’s day and year upon that of Earth’s motion, but months are different. Simulating lunar periods with predetermined month lengths presents more problems than that of modelling Earth’s rotation and orbit.
Month lengths formerly followed motions of the moon. But our Gregorian Calendar‘s irregular distribution of days in the various months no longer corresponds to either the lunar cycle or to that of any other object in the heavens.
As is true for the year, there are five ways to measure the length of a month. Historically, only two of these have been used to determine month length. The period in which the moon completes an orbit around the Earth and returns to the same apparent position in the sky is about 27.32 days. This is known as the sidereal month. Few peoples have ever used this period for calendrical purposes.
Because Earth is moving in its orbit around the sun in the same direction as the moon, time needed for the lunar body to return to the same phase (a synodic month) is longer; it averages just slightly over 29.53 days. This period has been known to within a second for well over two thousand years. It is the one used in lunar calendars today.
Month lengths had been fixed at thirty days for many centuries about three thousand years ago. Then, beginning as early as the eighth century B.C., calendar month lengths in many places became identical to that of the lunar cycle. In Rome, as late as 448 B.C., strictly lunar months were still in use. * By the fourth century B.C., month lengths in most calendars had again become fixed. But their number of days were assigned in what was apparently an arbitrary fashion.
While our calendar and the names of its months were derived from a long line of Roman calendars, the word “month” itself came from the Anglo-Saxon monath. ** To find details about each of our Gregorian Calendar months, visit Nuggets of Information about Months.
Had the Earth’s axis of rotation not been inclined to the plane of its orbit, there would have been less need for us to be aware of year length. This was said to have been the case in the so-called Golden Age as described by ancient writers. *** They claimed that in a time far preceding their own there were no seasons as we know them today. At any particular place the temperature was about the same one day as the next.
But the Earth’s axis is tilted to the plane of its orbit around the sun. Due to this tilt, a constantly changing amount of sunlight reaches each part of Earth’s surface as our planet travels from one day to the next on its yearly journey. As a result of this continuous change we have seasons—one of mankind’s original motivations for measuring the year’s span.
Many religious festivals have long been scheduled according to phases of the moon. Without correlating months with the year, these commemorations get out of step with seasons. But Easter will continue to be observed early in springtime because its date is calculated to occur each year shortly after the vernal equinox.
* Dionysius Antiq. x.59.
** Krythe, Maymie R. All about the Months.
New York: Harper & Row, Publishers, 1966. p. 60.
*** Ovid. Metamorphoses. See Saturn or Golden Age