The year is the main unit of time used in astronomy. It is the time it takes for the Earth to orbit the Sun.

There are 365 days in a year. However, every four years, there is a leap year, which has 366 days.

The sidereal year is the time it takes for the Earth to orbit the Sun once, relative to the fixed stars. The tropical year is the time it takes for the Earth to orbit the Sun once, relative to the vernal equinox. The sidereal year is shorter than the tropical year because the Earth's axis precesses, which means that the vernal equinox moves westward over time.

The length of a sidereal year is 365.2563 days.

The length of a tropical year is 365.2422 days.

A calendar year is a period of 365 or 366 days, beginning on January 1 and ending on December 31. A fiscal year is a period of 12 months, beginning and ending on dates that are specified by law or regulation. In most countries, the calendar year and the fiscal year are the same length.

A solar year is the time it takes for the Earth to orbit the Sun once. A lunar year is the time it takes for the Moon to orbit the Earth 12 times. The solar year is longer than the lunar year because the Moon's orbit is not perfectly circular.

The length of a solar year is 365.2422 days.

The length of a lunar year is 354.367 days.

A Julian year is a period of 365.25 days. A Gregorian year is a period of 365.2425 days. The Julian year is longer than the Gregorian year because the Gregorian calendar has a leap year every four years, except for years that are divisible by 100 but not by 400.

The Gregorian calendar was adopted in 1582.

Italy was the first country to adopt the Gregorian calendar in 1582.

Greece was the last country to adopt the Gregorian calendar in 1923.

A civil day is a period of 24 hours, beginning at midnight and ending at midnight. A solar day is the time it takes for the Earth to rotate once on its axis relative to the Sun. The civil day is longer than the solar day because the Earth's axis is tilted.

The length of a civil day is 24 hours.

The length of a solar day is 23 hours and 56 minutes.