It's human nature to measure the passing of time, and one of the easiest ways to do so is by using astrology. The stars and moon have been used as a guide for thousands of years to help people monitor the seasons, although the exact methods vary amongst different religions and regions.
Most people around the world agree that a year in the Gregorian calendar has 365 days, but this isn't always the case. A year cannot be perfectly divided, which means that there is some variation in the number of days during certain years. In addition, some countries measure a year using different calendar systems, which results in more or fewer days.
The other planets in our solar system also have different orbital periods, which means that they have a different number of days in a year than we have in an Earth year. The length of a year will increase based on how close the planet is to the Sun and its orbital rate.
In this guide, we'll look at how we typically measure a year, along with the different types of calendars used worldwide.
A year (annus in Latin) is the orbital period of the Earth around the Sun. The planets in our solar system each have a different number of days in a year as they orbit the Sun at varying rates. A day is generally considered as the rotation of the Earth or other planets relative to fixed stars (including the Sun). This is called a sidereal day and takes the Earth 23 hours and 56 minutes to complete a full rotation.
In everyday life, however, a day refers to a 'solar day', which is the period between two solar noons when the Sun is at its highest point in the sky. A new day starts after a full 24-hour period. After approximately 365 days pass on Earth, a new year starts.
The table below shows the average length of time that it takes each planet in our solar system to orbit the Sun and, therefore, the number of days in each planetary year.
| Planet | Orbital period |
| Mercury | 88 days |
| Venus | 225 days |
| Earth | 365 days |
| Mars | 687 days |
| Jupiter | 4,333 days (12 Earth years) |
| Saturn | 10,759 days (29 Earth years) |
| Uranus | 30,687 days (84 Earth years) |
| Neptune | 60,190 days (165 Earth years) |
Although most people consider a year to have 365 days, this is a simplified view. The Western world currently follows the Gregorian calendar, which means that some years have an extra day. A day is also not exactly 24 hours long, which means some leftover hours need to be accounted for.
Continue reading for a further breakdown of the number of days in a year and why this can change.
A solar year, sometimes known as a tropical year, has approximately 365.25 days. This is the length of time that it takes for the Earth to orbit around the Sun. A full year is marked when the Sun returns to the same position in the sky as it was when the year started, which signals the full completion of seasons.
The Earth doesn't complete another full day before the Sun returns to its original position, which means that most years have 365 days, five hours and 48 minutes. However, the exact amount of days and hours in a year falls under some dispute. NASA currently states that an Earth year lasts for 365.2425 days, as does the International System of Units.
A year is usually rounded to 365.25 days, although it is sometimes suggested as 365.26 or 365.24 days. This is because the length of a true solar year varies on a regular basis. In 2027, the solar year will be 365 days, 5 hours and 39 minutes, but a solar year in 2032 will last for 365 days and 6 hours. It's difficult to calculate the exact length of a solar year, so it's common for scientists to be off by a few seconds.
Civilizations worldwide have observed different types of calendars and astronomical years throughout history. Although the Gregorian calendar is the most observed calendar in the world, there are other types of calendars that are used to mark the passing of time.
What are the Gregorian and Julian calendars?
Most people in western countries follow the standard Gregorian calendar year, which has 12 months and 365 days, with the exception of leap years.
Before the Gregorian calendar was first proposed in 1582, European countries used the Julian calendar, which was created by Julius Caesar in 46 B.C. The Julian calendar was replaced because it incorrectly measured the length of the Earth's orbit around the Sun.
The length of a year in the Julian calendar was overestimated by around 11 minutes, which meant that the dates of the seasons had shifted around 10 days by the 1500s. A year in the Julian calendar was approximately 365.25 days long, while a year in the Gregorian calendar is more accurately 365.2425 days long.
To correct the dates of the seasons, Pope Gregory XIII modified the calendar and based it on the vernal equinox. The calendar was advanced by 10 days after the end of 4 October 1582, so the following day was 15 October 1582. The main difference between the Julian and Gregorian calendars is that a leap year won't fall on a century year unless it is divisible by 400 (such as the year 1600 and 2000).
European countries and their colonies made the conversion from the Julian to the Gregorian calendar at different rates. Great Britain and its colonies did not use the Gregorian calendar until 1752, 170 years after it was first introduced. Eastern Orthodox countries in Europe started adopting the new calendar system in the 1910s and 1920s, although their national churches still use the Julian calendar.
What is the Chinese calendar?
This calendar is inspired by lunar cycles, as each month ends on the day of a new moon. Each year starts on the second or third new moon that follows the winter solstice.
Around 20% of the world's population observes the Chinese calendar. However, most Chinese people use the Gregorian calendar for everyday life and only use the lunar dates for traditional festivals.
A Chinese calendar year can for 355 days, although this varies with the cycle of the moon. A lunar calendar month could have either 29 or 30 days, which means that some years can have 353 or 354 days instead. The Chinese New Year will also change day each year. For example, the day fell on 1 February 2022 and 12 February 2021.
What is the Hijri/Islamic calendar?
The Islamic calendar, another lunar calendar, has 12 months with either 354 or 355 days. The calendar cycle repeats every 33 years. Although many Islamic countries observe the Gregorian calendar, the Islamic calendar is still used for religious purposes.
In 2022, the Islamic New Year was on 31 July (in the Gregorian calendar), while it fell on 9th August in 2021. A new month will only start when the new crescent moon is spotted, which means that it is difficult to predict the length of each month. Muslims look for the moon after sunset on the 29th day of each month. If they spot the moon, the month will have 29 days, but if they can't see it, the month will have 30 days. This can change the date that religious festivals fall at short notice.
What is the Hebrew calendar?
The Jewish calendar is based on the phases of the moon and divides the year into 12 lunar months. To make sure that religious events occur at the same time every year, a leap year system was devised to coordinate the lunar months with the solar year. This means that there is a leap month at the end of the year seven times in a cycle of 19 years.
Five of the months in the Hebrew calendar always have 30 days, and another five always have 29 days. The two remaining months alternate between 29 and 30 days. In total, a normal year in the Hebrew calendar may have 353, 354 or 355 days. In a leap year, there can be 383, 384 or 385 days.
The Earth's rotation and orbit around the Sun are not perfectly aligned. Although the Earth takes approximately 365.25 days to orbit the sun, this is rounded to 365 days in each calendar year. Every four years, a single day is added to the calendar to make up for the missing portion. This day is formed from the missing quarters from the previous three years, as well as the quarter in the fourth year.
The added day in a leap year is attached to the end of February. This means that the second month has 29 days instead of the usual 28 days. People born on the 29th day in a leap year usually celebrate their birthdays on 28 February outside of leap years, although they might celebrate it on 1 March instead.
Leap years are important to make sure that each calendar year aligns with the solar year. While 5 hours and 48 minutes don't seem like a large portion of time to lose each year, this will soon add up. Eventually, the seasons will shift completely, and traditionally warmer months would take place during cold seasons and vice versa.
As an Earth year lasts 365.242374 days rather than 365.25, the Gregorian calendar requires three leap days to be removed every 400 years. This means that years that are divisible by 100 aren't leap years unless they are divisible by 400. For example, the year 2000 was a leap year, but the year 2100 won't be.
Do other planets have leap years?
The other planets in our solar system have leap years in the same way that Earth does. This is because their orbit around the sun and rotation on their axis are not perfectly aligned. However, the frequency of leap years varies from planet to planet.
Mars, for example, has more leap years than standard years. A Martian year is 668 sols (days), but it takes Mars 668.6 sols to fully orbit the sun. To help maintain the right calendar, an extra sol has to be added to the year. Over the course of 10 years, Mars would have four years with 668 sols and six years with 669 sols.
Typically, a year in the Gregorian calendar is said to have 365 days. This is the approximate length of time it takes the Earth to orbit around the Sun. However, careful observations led scientists to understand that it more accurately takes the Earth 365.2425 days to orbit the Sun. This means that a small portion of a day is not counted each year, which could build up over time and disrupt the calendar.
To ensure that the seasons aren't misaligned, the solar calendar includes a leap year every four years. This means adding an extra day in February so that the missing hours from the previous three years are still counted. A leap year will therefore contain 366 days rather than the standard 365.
Other calendars, such as the Islamic and Chinese calendars, are observed in other countries around the world. These calendars are both based on the moon and so, therefore, divide a year into different months and days. Such a calendar is commonly used for religious events only, as it is not accurate enough to be used for agricultural purposes.
