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How long is a year? A calendar year is the amount of time that it takes the earth to move in one complete orbit (nearly a circle, but not quite) around the sun. A day on the other hand, is one complete rotation of the earth around it's own center.

The ancient Greeks were the first to realize that a year is not made up of a whole (1,2,3,...365...) number of days. Hipparchus, a great astronomer, mathematician and scientist of the day calculated (with amazing accuracy!) that a year consists of 365.246 days. That is to say that the Earth spins around itself like a top 365.246 times every time that it traces its ellipse through space around the sun.

Noting this fact, Julius Caeser established a system of time (now known as the 'Julian' calendar) which considered the year to be made of 365.25 days. If we wanted to make every year the exact same length, we would have two options. First, we could ignore the extra .25 days. The problem with this method is that the system of days and months would shift by roughly 6 hours per year. After 100 years, January 1st would come to be at the same time in the Earth's orbit of the sun that January 25th used to be! After 1000 years, New Year's Day would be near the end of the summer, on September 8th! Clearly unacceptable.

Our other option would be to have one roughly six-hour day at the end of every year. This would be even worse however, since dawn, daylight and dusk would all be shifted six hours on the clock! In the second year of this calendar, sunrise would occur around noon, the third year at 6 PM, the fourth year at midnight, before arriving again at roughly 6 AM. Every four years this cycle would repeat.

The system of adding one day per year every four years thus keeps the same daylight hours by adding a day as the four 0.25 days accumulate to one. This also keeps the total shift of days of the year to less than one day. A pretty good compromise, and certainly worth the oddness of the extra day every four years.

We no longer use the Julian calendar, but an improved version known as the "Gregorian" calendar. Because of rounding 365.246 to 365.25, the Julian calendar lost 10 days over the nearly 2000 years between its start date and 1582 when the Gregorian calendar was implemented. The 10 days were skipped entirely by the world, which jumped from October 4th to October 15th. After the skipped days, a new version of leap year was put into place: every four hundred years, three leap years are omitted. 1600 was a leap year, 1700,1800 and 1900 were not. 2000 was again a leap year, 2100, 2200 and 2300 will not include February 29th.

Even the Gregorian calendar is no longer as precise as scientific measurement. Over the course of about 500 days, this calendar currently loses one second because the earth's rotation is gradually slowing down!* Since the 1970s, leap seconds have been added occasionally to the time and calendar to correct for this change. This confusing practice may end soon, and the time we all use will become separate from the earth's rotation as best we can measure it. GPS satellites and other systems which require time that matched Earth's rotation would continue to use leap-seconds.

*Note: this video attributes the slowing to the drag of the ocean; in actuality, the slowing is mostly due to the change in angular momentum of the earth due to the moon's gravity deforming it. This is known as 'tidal acceleration'.

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