The name “calendar” comes from Roman word “kalendae”, which is used to refer the first day of each month. The name “calendar” is also related to the verb “calare”, which means “to call out”, referring to the calling of the new moon, when it was seen for the first time. The name “calendar” also may has come from Latin word “calendarium”, which means “account book, register”, as debts were collected and accounts were settled on the “calends” of each month.

Calendar is a time counting system and a system of organizing days for different purposes. This is done by giving names to periods of time, for example, day, week, month and year. The astronomy is the key method of calculating a date and time. Stars create the stellar day, but the rotation period of planets regulates the seasons. The combination of these processes is monitored and recorded by calendar.

Generally calendar is based on movement of such celestial bodies as Sun and Moon. There are special calendars for Sun and Moon as well, which are called Solar calendars and Lunar calendars. It should be mentioned, that there are Lunisolar calendars too. Of course, there are calendars based on movements of other objects as well, for example, calendar of ancient Egypt, where a year is a period of time which passes between two rises of Sirius, but calendars like this are rare.

Solar calendar is a calendar whose dates indicate Earth’s position on its revolution around the Sun. In other words, Solar calendar is a calendar whose dates indicate Sun’s position on the celestial sphere. We can look at this question from two aspects – from the viewpoint of the Earth or from the viewpoint of the Sun. Solar calendars assign a date to each solar day. Solar day is a period of time between the Sun reaching its highest point in the sky two consecutive times. Solar day is also the 24-hour interval from one midnight to the following midnight. One example of Solar calendar is the Gregorian calendar, also called the Western calendar and the Christian calendar. The Gregorian calendar, which is widely accepted as standard in the world, is internationally the most widely used calendar. The Gregorian calendar was introduced in the 16th century as a modification of the Julian calendar.

Lunar calendar dates the months and numerates the days, basing on cycles of the lunar phases. Taking into account, that the length of the lunar month differs from the length of the tropical year, Lunar calendar quickly drifts against the seasons, therefore, there is need for intercalary months. In order to synchronise the year a 13th month is needed every 2 or 3 years. As there are more than 12 lunations or, in other words, synodic months in a solar year, the period of 12 lunar months is often called a lunar year. One example of Lunar calendar is the Islamic calendar, which is used mainly for religious purposes, but in Saudi Arabia it is the official calendar.

Lunisolar calendar is a Lunar calendar which has an extra month to realign the months with the seasons, therefore, it can be regarded as Solar calendar as well. Dates of Lunisolar calendars indicate not only solar days, but also moon phases.

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