The natural science of astronomy is the study of celestial objects, observations and phenomena in the night sky. The ancient discipline of astronomy is the key method of calculating a date and time. A star in a solar system is the source of light which creates the stellar day, the rotation period of orbiting planets regulates the seasons on a planet surface, this combination is monitored and recorded by a calendar.
Tropical solar calendars
If the position of the earth in its orbit around the sun is reckoned with respect to the equinox, the point at which the orbit crosses the celestial equator, then its dates accurately indicate the seasons, that is, they are synchronized with the declination of the sun. Such a calendar is called a tropical solar calendar.
The following are tropical solar calendars:
- Gregorian calendar
- Iranian calendar (Jalāli Calendar)
- Indian national calendar (Saka Calendar)
- Julian calendar
- Coptic calendar
The Baha'i calendar begins the year on the vernal equinox and (as of 2015) sets its intercalary days so that the following year also begins on the vernal equinox. The moment of the vernal equinox in the northern hemisphere is determined as at Tehran "by means of astronomical computations from reliable sources".
The Zoroastrian calendar, the earliest solar calendar, is a religious calendar used by adherents of the Zoroastrian faith, and is an approximation of the tropical solar calendar.
Sidereal solar calendars
If the position of the earth (see above) is reckoned with respect to the fixed stars, then the dates indicate the zodiacal constellation near which the sun can be found. A calendar of this type is called a sidereal solar calendar.
The mean calendar year of such a calendar approximates the sidereal year.
Indian calendars like the Hindu calendar, Tamil calendar, Bengali calendar and Malayalam calendar are sidereal solar calendars. The Thai solar calendar, based on the Hindu solar calendar is also a sidereal calendar. They are calculated on the basis of the apparent motion of the sun through the twelve zodiacs rather than the true tropical movement of the earth.
Lunisolar calendars may be regarded as solar calendars, although their dates additionally indicate the moon phase. Because a typical lunisolar calendar has a year made up of a whole number of lunar months, it can't indicate the position of Earth on its revolution around the sun as well as a pure solar calendar can.
- The Universal House of Justice (2014-07-10). "To the Bahá’ís of the World". Retrieved 2014-07-10.