The year, unit of time
The year used here is the average year of the Gregorian calendar, exactly 365.2425 days or 31 556 952 seconds. It is a defined duration and not a lived one: no single year on a calendar lasts that long, since each is either 365 days or 366.
Two years compete, and they differ by 648 seconds. The one above comes from the Gregorian reform of 1582, which dropped three leap days every four hundred years to hold the calendar against the seasons, leaving an average of 365.2425 days. Astronomy kept the older Julian year of exactly 365.25 days, which the International Astronomical Union carried into its 1976 system of constants, and it is that one, not the calendar's, that defines the light-year. Over a century the two drift eighteen hours apart.
The year in other units
| 1 yr | 31,556,952 s |
| 1 yr | 31,556,952,000 ms |
| 1 yr | 31,556,952,000,000 µs |
| 1 yr | 525,949 min |
| 1 yr | 8,766 h |
| 1 yr | 365.242 d |
| 1 yr | 52.1775 wk |
| 1 yr | 12 mo |
Frequently asked questions
How many seconds are in a year?
31 556 952 in the calendar average used here, against 31 557 600 in the Julian year of astronomy. The gap of 648 seconds is why any long duration should say which year it means.
Why 365.2425 and not 365.25?
Because a quarter of a day a year is slightly too much. The Earth's orbit takes about eleven minutes less than 365.25 days, so the Julian calendar gained a whole day every 128 years; the Gregorian reform removed three leap days every four centuries to stop that drift.
Is this the year of the light-year?
No, and the difference is visible. A light-year is built on the Julian year of 365.25 days, which makes it exactly 9 460 730 472 580 800 meters. Computed with the calendar year it would fall short by about 194 billion kilometers.