Leap 年 and Leap 秒
Why We Adjust Our Clocks
Learn About Time AdjustmentsEarth's orbit doesn't fit neatly into whole days, and its rotation gradually slows. Leap years and leap seconds are the corrections that keep our calendars aligned with seasons and our clocks synchronized with Earth's actual rotation.
Leap 年 解説
Why We Need Them
Earth orbits the Sun in approximately 365.2422 days—not exactly 365. Without correction, the calendar would drift by about 1 day every 4 years, eventually putting summer months in winter.
The Gregorian Leap Year Rule
- 年 divisible by 4 are leap years
- EXCEPT years divisible by 100 are NOT leap years
- EXCEPT years divisible by 400 ARE leap years
Examples
| Year | Divisible by 4? | Divisible by 100? | Divisible by 400? | Leap Year? |
|---|---|---|---|---|
| 2024 | Yes | No | No | Yes |
| 2100 | Yes | Yes | No | No |
| 2000 | Yes | Yes | Yes | Yes |
| 1900 | Yes | Yes | No | No |
Accuracy of the Gregorian Calendar
| Calendar | Average Year Length | Error vs. True Year |
|---|---|---|
| 365-day (no leap) | 365.0000 days | ~1 day every 4 years |
| Julian (leap every 4) | 365.2500 days | ~1 day every 128 years |
| Gregorian | 365.2425 days | ~1 day every 3,236 years |
| Actual tropical year | 365.2422 days | — |
The Gregorian calendar is accurate enough that it won't need further correction for millennia.
Leap 秒 解説
Why We Need Them
Earth's rotation is gradually slowing (about 1.4 milliseconds per century) due to tidal friction. Atomic clocks, however, keep perfect time. Leap seconds reconcile these:
- Atomic time (TAI) counts perfect seconds
- Universal Time (UT1) tracks Earth's actual rotation
- UTC adds leap seconds to stay within 0.9 seconds of UT1
How They're Added
When needed, a leap second is inserted at 23:59:59 UTC on June 30 or December 31, creating a moment when clocks show 23:59:60 before rolling to 00:00:00.
Leap 秒 History
| Period | Leap 秒 Added | Notes |
|---|---|---|
| 1972 | +10 seconds | Initial UTC adjustment |
| 1972-1979 | +9 seconds | Nearly annual additions |
| 1980-1999 | +15 seconds | Regular additions |
| 2000-2016 | +4 seconds | Slowing need |
| 2017-2024 | 0 seconds | None added recently |
| 変換先tal (through 2024) | 27 seconds | UTC is 27s behind TAI |
Earth's rotation varies unpredictably; negative leap seconds may eventually be needed.
Born on February 29?
"Leap day babies" (leaplings) have birthdays that only occur every 4 years. Legal conventions vary:
- Most jurisdictions: Age advances on March 1 in non-leap years
- Some contexts: February 28 is used
- The person has still lived the same number of days as anyone else their age
Famous leaplings: Ja Rule, 変換先ny Robbins, Dinah Shore.
まとめ
Leap years keep our calendar aligned with Earth's orbit around the Sun, adding a day every 4 years (with century exceptions). Leap seconds keep atomic clocks aligned with Earth's gradually slowing rotation. While leap years will continue indefinitely, leap seconds are being phased out by 2035 due to the problems they cause for computing systems.