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Every
four years we have a leap year and an extra day is added to the
calendar in February. That's because the time it takes for the Earth to
orbit the Sun is approximately 365.25 days. Considering the Earth's
orbit as approximately a circular path around the Sun, If we didn't
compensate for the extra quarter day, the Earth would end up being a
little short of a full 360 degrees of its way around the Sun on the 31st
December (a quarter day short). Eventually 31st of December would fall
earlier and earlier in the seasons until it ended up in the summer. By
adding an extra day every four years, the Earth is allowed to "catch up"
in its orbit. Similarly, a day, or the time it takes the Earth to
revolve on its axis isn't exactly 24 hours long and is influenced by
environmental and geological events. In fact in 2025, the length of time
between the longest and shortest day was approximately 1.6 mS. It
mightn't seem like a lot, but to keep everything in sync, leap seconds
have to be added onto Coordinated Universal Time (UTC) every so often.
The leap second was introduced in 1972. The most recent leap second
adjustment was in December 31, 2016. However leap seconds are
controversial and cause issues with critical systems that rely on
accurate timekeeping.
More info here.
