Year

What Is Year?

A year is a way to measure a long period of time. It is based on how long Earth needs to travel once all the way around the Sun. In everyday life, we use years to tell our age, plan school and work, and mark big events like birthdays and holidays.

Most people think of a year as 12 months or about 365 days. Because the real path of Earth around the Sun is a little longer than 365 days, our calendars use special rules, like leap years, to stay in step with the seasons.

Definition

In simple words, a year is the length of time it takes Earth to complete one orbit around the Sun.

There are several useful definitions of a year:

• Calendar year The period used in a calendar, usually 12 months, starting on a set date such as 1 January and ending on 31 December.
• Common year A calendar year with 365 days.
• Leap year A calendar year with 366 days. One extra day is added to keep the calendar close to the seasons.
• Gregorian year average In the modern Gregorian calendar, the average length of a year is 365.2425 days when you include leap years over a long time.
• Tropical year The time from one spring equinox to the next. This is closely linked to the cycle of seasons and is about 365.2422 days.
• Sidereal year The time it takes Earth to orbit the Sun once compared to the fixed stars in the sky. It is about 365.2564 days.

In science and engineering, a year is sometimes treated as a fixed number of seconds to make calculations easier.

History / Origin

Long before modern science, people noticed that certain events in nature repeat again and again. For example, the pattern of seasons spring, summer, autumn and winter returns in a regular cycle. Early farmers watched the Sun, stars and seasons to know when to plant and harvest crops. Over time, they learned that these changes repeat roughly every 365 days. This cycle became the idea of a year.

Different ancient cultures built calendars based on this yearly cycle:

• Ancient Egyptians used the yearly flooding of the Nile River and the rising of the star Sirius to mark a year.
• Babylonians and other Middle Eastern cultures created early solar and lunar calendars, trying to match the Moon months with the Sun year.
• Ancient Greeks carefully measured the length of the year and studied how the Sun and stars moved in the sky.
• Romans used the Julian calendar, started under Julius Caesar. It used a year of 365 days and added one extra day every 4 years, giving an average of 365.25 days.

The Julian calendar was a big improvement, but it was not perfect. It ran slightly faster than the real seasonal year by about 11 minutes each year. Over many centuries this small mismatch added up, and the dates of seasons and church holidays slowly shifted.

In 1582, Pope Gregory XIII introduced the Gregorian calendar. This new system kept the basic idea of a 365 day year but changed the leap year rules to be more accurate. Under the Gregorian rules, most years that end in 00 are not leap years unless the year number is also divisible by 400. This makes the average year length much closer to the real tropical year linked to the seasons.

Today, most of the world uses the Gregorian calendar year for civil life, while some cultures also keep traditional religious or cultural calendars with their own ways of counting years.

Symbol & Abbreviation

The word year can be shortened in several ways. The choice depends on the subject, for example science, finance or everyday writing.

• y or yr Common short form in science and technical writing. Example 5 yr.
• a From the Latin word annus, meaning year. It is used in some scientific contexts. Example m a for meters per year.
• yr or yrs Very common in informal English. Example 10 yrs old or 3 yrs later.
• p.a. Short for per annum, meaning per year. Often used in finance. Example 5 percent p.a. interest.

There is no single official symbol in the International System of Units, but y and a are the most common in scientific texts.

Current Use Around the World

Years are used everywhere as one of the main ways to measure time. Nearly every part of life is organized around the idea of a year.

In daily life

• Age People say their age in years. For example, I am 12 years old.
• Events Birthdays, New Year celebrations and anniversaries all repeat every year.
• Planning People make yearly plans, goals or resolutions, such as what they will do this year.

In calendars and culture

• The Gregorian calendar year is the main civil year used by most countries for government, business and schools.
• Some cultures also follow religious or traditional years, such as the Islamic lunar year, the Hebrew year, the Chinese lunar solar year and others. These calendars may have different year lengths or leap rules.
• Different eras count years from different starting points, such as years numbered from the supposed birth of Jesus in the Gregorian system, or from an important event in another culture.

In school and work

• School year The time from the start of one academic term to the start of the next year of study. It may or may not follow the calendar year exactly.
• Fiscal year The year used for budgets, taxes and company accounts. It can start in January, April, July or another month, depending on the country or organization.

In science and technology

• Astronomy uses different year types, like tropical year and sidereal year, to study planet motions and long time scales.
• Geology and cosmology describe very long periods in millions of years (Ma) or billions of years (Ga).
• Engineering and environment studies often look at yearly averages, such as average rainfall per year or energy use per year.

Example Conversions

The length of a year can change a little depending on what exact type of year you use. Below are some helpful approximate conversions that are often used.

Calendar year to days

• 1 common year 365 days
• 1 leap year 366 days
• 1 Gregorian year on average about 365.2425 days

Days to year (approximate)

• 365 days about 1 year
• 730 days about 2 years
• 1,000 days about 2.74 years

Year to months and weeks

• 1 year 12 months
• 1 year about 52 weeks plus 1 day
• Leap year about 52 weeks plus 2 days

Year to hours and seconds

Using a simple 365 day year:

• 1 year 365 days
• 1 year 365 × 24 8,760 hours
• 1 year 365 × 24 × 60 525,600 minutes
• 1 year 365 × 24 × 60 × 60 31,536,000 seconds

Using the average Gregorian year of 365.2425 days, you get about 31,556,952 seconds per year. Scientists may use this more exact value in calculations.

Related Units

A year is one of many units used to measure time. Here are some closely related ones, from shorter to much longer periods.

• Second The base unit of time in the International System of Units.
• Minute 60 seconds.
• Hour 60 minutes or 3,600 seconds.
• Day Usually 24 hours. About one rotation of Earth on its axis.
• Week 7 days.
• Month A part of the year, often based roughly on the Moon cycle. In the Gregorian calendar, months have 28, 29, 30 or 31 days.
• Decade 10 years.
• Century 100 years.
• Millennium 1,000 years.
• Ma or million years Used in geology and Earth history.
• Ga or billion years Used for very long cosmic and planetary times.

In astronomy, there are also special year related ideas such as the light year, which measures distance, not time. A light year is how far light travels in one year in empty space.

FAQs

Why is a year about 365 days long?

Because that is roughly how long it takes Earth to go once around the Sun. One full orbit takes a little more than 365 days, which is why we need leap years to keep the calendar aligned with the seasons.

Why do we have leap years?

Earth takes about 365.2422 days to orbit the Sun. If every year had only 365 days, the calendar would slowly drift, and seasons would move to different dates. By adding an extra day about every four years, the Gregorian calendar keeps the average year length very close to the real seasonal year.

How do I know if a year is a leap year?

In the Gregorian system, a year is a leap year if it can be divided by 4. However, if the year ends in 00, it is only a leap year if it can also be divided by 400. For example, 2000 was a leap year, but 1900 was not.

Is a year always the same length?

Not exactly. A common year has 365 days, while a leap year has 366. Different scientific definitions, like tropical or sidereal year, also differ slightly. For everyday use, people usually treat a year as either 365 or 366 days, depending on the calendar.

What is the difference between a calendar year and a fiscal year?

A calendar year usually runs from 1 January to 31 December. A fiscal year is the 12 month period a company or government uses for money planning and taxes. A fiscal year can start in another month, such as April or July, and still lasts one year in length.

What does per annum mean?

Per annum is Latin for per year. It is often used in business and banking. For example, 6 percent interest per annum means 6 percent interest every year.

Why do some calendars have different New Year dates?

Different cultures choose different starting points for the year. For example, the Gregorian calendar starts on 1 January, the Chinese New Year falls on a changing date based on the Moon, and other traditions have their own New Year days. All of them mark the beginning of a new year in that culture system.

Is a light year a unit of time or distance?

Even though it has the word year in it, a light year is a unit of distance. It is how far light travels in one year in empty space. Astronomers use it to describe huge distances between stars and galaxies.

How many years are in a century and in a millennium?

There are 100 years in a century and 1,000 years in a millennium.