Year (yr)
The year is the time it takes the Earth to complete one orbit around the Sun, and it is the largest unit of time in common everyday use. We express our ages in years, describe historical events in years, plan retirements and investments in years, and measure the age of fossils and stars in millions or billions of years. This converter uses the Gregorian average year of exactly 365.2425 days, which equals 31,557,600 seconds. This is the value most widely used in scientific and engineering contexts, based on the Gregorian calendar's rule of 97 leap years in every 400 years. It is close to but not identical with the tropical year (365.24219 days), which is the true period of Earth's seasons.
Definition
For this converter, one year equals 31,557,600 seconds, or exactly 365.2425 days, derived from the Gregorian calendar average. One year equals 12 average months, approximately 52.18 weeks, or exactly 8,765.82 hours. The tropical year (time from vernal equinox to vernal equinox) is approximately 365.24219 days. The Julian year used in astronomy is exactly 365.25 days.
History
The year as a unit of time is as old as agriculture. Knowing when to plant and harvest crops required tracking the seasons, and seasons are defined by the year. Ancient Egyptians determined the length of the year by observing the annual flood of the Nile and the heliacal rising of the star Sirius, arriving at an estimate of 365 days. Julius Caesar's reform of 46 BCE introduced the Julian calendar with a 365.25-day year, achieved by adding a leap day every four years. This was an excellent approximation but still slightly too long by about 11 minutes per year. Over centuries, this small error accumulated. By the sixteenth century, the calendar had drifted about ten days out of alignment with the astronomical year. Pope Gregory XIII introduced the Gregorian calendar in 1582, refining the leap year rule by skipping century years not divisible by 400. The Gregorian calendar gives a year of exactly 365.2425 days, accurate to within about 26 seconds of the tropical year.
Common Uses
Years are the standard unit for expressing human lifespan and age. Legal systems use years to define sentences, statutes of limitations, and age-of-majority thresholds. Financial instruments use years for interest rate calculations, bond maturities, and investment horizons. In science, radioactive decay rates and geological timescales use the year, often with prefixes: kiloyears (kyr), megayears (Myr), and gigayears (Gyr). The age of the universe is approximately 13.8 gigayears. In astronomy, distances are sometimes expressed in light-years, the distance light travels in one year: about 9.461 trillion kilometers.
Did You Know? Facts About Year
- The Earth's year is not getting longer, but the day is. Because the Earth's rotation is gradually slowing, the year contains slightly more seconds each century. When the dinosaurs roamed the Earth, a year lasted the same amount of time but contained more days — each day was shorter because Earth rotated faster.
- A light-year is the distance light travels in one year, not a measure of time. At the Gregorian year of 365.2425 days, one light-year is exactly 9,460,730,472,580.8 kilometers, or about 5.879 trillion miles.
- The word year comes from the Proto-Germanic *jǣra, related to similar words across Indo-European languages. The Latin annus (giving us annual, anniversary, etc.) and the Greek eniautos both mean year and share ancient roots.
- Not all planets have years we would recognize. Mercury's year is 88 Earth days. Venus's year is 225 Earth days. Mars's year is 687 Earth days. Jupiter's year is nearly 12 Earth years. Neptune's year is 165 Earth years, meaning it has completed fewer than one full orbit since its discovery in 1846.
- In radiometric dating, scientists use the symbol 'a' for year, from the Latin annum. A million years is written as 'Ma' (mega-annum). This notation is standard in geology and paleontology and avoids ambiguity with the various definitions of the year.