All calendars include the day as a basic unit. Days are defined by the Earth's rotation, as measured by the apparent motion of the sun across the sky. Our modern calendar (Gregorian) uses equal length days that begin and end at midnight, however other calendars may measure days from sunset to sunset (Hebrew calendar) or from sunrise to sunrise (Indian calendar), either of which gives rise to days that lengthen and shorten according to the seasons.
Other astronomically based calendar units include:
The table below gives the date of origin and astronomical basis of several different calendars.
| Calendar | Type | Year Introduced | Astronomical Basis | Still in use? |
|---|---|---|---|---|
| Roman | Lunar | 8th century BCE | Lunar month and week (originally). Later modified to a 304-day year of 10 fixed-length months with an uncounted winter gap between years. 2 additional months added in 7th century BCE, to give a 355-day year, with a leap month added every other year. This produced a pseudo-solar year that drifted by approximately 5 days over its 4-year cycle. | no |
| Julian | Solar | 46 BCE | Solar year. Replaced the ancient Roman calendar. Essentially identical to the modern calendar, but with leap years every 4 years, resulting in a drift from the astronomical year of 1 day every 128 years. Still used by the Russian Orthodox Church. | yes |
| Gregorian | Solar | 1582 CE | Solar Year. Our "modern" calendar, which has gradually replaced the Julian calendar in most of the world. Elimination of leap years for century years, unless they are millenium years corrected the mismatch between solar and calendar years. | yes |
| Chinese | Lunar | 14th century BCE, traditionally 2637 BCE | Lunar months, measured from astronomical new moon to new moon. An extra month is added in leap years to produce a pseudo-solar year. | yes |
| Hebrew | Lunisolar | 359 CE | Lunar months originally determined by observations of the new moon have been replaced by a fixed calendar calculated to keep the start of each month at the time of a new moon. In leap years, one of the months is repeated. Complex rules to keep the years and months in correspondence to astronomical events result in a calendar with a cycle of 689,472 years! | yes |
| Islamic | Lunar | 1622 CE | Based purely on lunar months. The 12 month year is approximately 354 days long, so the new year shifts by about 11 days per year. The month begins with the first observation of the crescent after the new moon, so it is impossible to determine the calendar in advance. | yes |
| Indian | Solar | before 14th century BCE, reformed 1957 CE | The Indian civil calendar is a solar calendar with 12 solar months. There are, however, more than 30 religious calendars in use in India, which can be either solar, lunar, or lunisolar. | yes |
| Egyptian | Solar | 4236 BCE | Egypt introduced the first solar calendar, which was began the year with the first observation of the rising of the star Sirius just before dawn. Ancient Egyptians actually used 3 calendars concurrently for a period of over 2000 years. One was a solar calendar, consisting of 12 months with 30 days with an extra 5 days that were not part of any month. They used a solar/stellar calendar to determine agricultural events (plantings and harvests), and a quasi-lunar calendar for determining festivals. | no |
| Mayan | Solar | 5th century BCE | The Mayans used 3 different calendars. Their main calendar was a solar calendar consisting of 18 months with 20 days each, with an additional 5 days that were not part of any month. They did not have leap years, but they were apparently aware that their calendar would cycle through the solar year over 550,800 days, so they had an excellent estimate of the length of an astronomical year. Their other calendars were non-astronomical, and were based on repeating fixed numbers of days. The Mayan calendar is unique in assigning the number zero to the first day of each month. | no |
For more information on various calendars, see Calendars from the Sky