even with leap year every 4 years the year cannot be exact. after 4 years how far off is the true year?

Answer 1

Hi Len,

Leap years do not precisely recalibrate the solar year to the calendar. One leap year in a four year cycle corrects for exactly six hours per year, but the actual solar year is only 5 hours 49 minutes longer than the calendar year.

These 11 or 12 extra minutes a year begin to pile up. In a hundred years, the solstice will be happening almost 19 hours too early, and over 400 years the discrepancy will reach more than three days.

When Julius Caesar established our modern calendar, his Egyptian astronomers had no way of measuring the 11-12 minutes per year overage. It was not until the Eighth Century that scientists began to realize that the calendar was falling out of sync with the solar year.

By the 1500s, the first day of spring was falling on March 11th and Christmas was happening in the late autumn. To correct this, in 1582 Pope Gregory XIII decreed that three leap days must be taken out of the cycle every four hundred years. In the Gregorian calendar, years divisible by 100 but not by 400 (such as 1900 and 2100) are not leap years.

(The Pope also ruled that, to make up for the error since the time of Caesar, ten days had to be dropped from the calendar. The change took effect in October 1582. The day after October 4, 1582 became October 15. In the American colonies, the change did not take place until September 2, 1752, by which time still another day too many had accumulated. The next day after September 2nd was September 14th, 1752.)

The Gregorian reform is still not precise. There are still three left-over hours every four hundred years, and over many centuries, they will start to add up. As long ago as the time of astronomer John Herschel in the early 1800s, it was realized that the Gregorian calendar with its 400-year rule will add one leap year too many to the calendar every 4000 years, 970 instead of the number which is closer to accurate, 969.

In 3000 years the Spring equinox will be running one day late. To correct this, it will eventually be necessary to make a further modification and drop a leap year. One way to modify the rule would be to make millennial years divisible by 4000 not leap years. (Currently, millennial years that are divisible by 2000 are leap years, such as the years 2000 and 4000.)

An alternative is the way chosen by the Orthodox Church when it adopted the Gregorian calendar. Under the Orthodox rule, century years which, when divided by 900 yield a remainder that is either 200 or 600, are leap years. In other words, the Gregorian rule of two leap years in eight century years becomes two leap years in nine century years. In both versions, the years 1900, 2100, 2200, 2300, 2500, 2600 and 2700 are not leap years, but 2000 and 2400 are. The year 2800 is a leap year under the Gregorian calendar, but not in the Orthodox. Conversely, 2900 will be an Orthodox leap year.

The Orthodox rule has been adopted in Russia and likely will be picked up by the rest of the world in centuries to come. Although less elegant than the 4000-year rule, it has the advantage of making the calendar correction more gradual than an abrupt jolt after letting the discrepancies pile up for 4000 years.

Answer 2

what campbell said. some leap days are omitted to keep the calendar as closely tuned to the earth's revolution as is practical

years divisible by 100 will have no leap day
years divisible by 400 (such as 2000) will have the leap day

this keeps discrepancies to within a day. even so, by omitting the leap day every 100, 100, then 200 years, rather than 164, 168, 168 years, funny things still happen. for example, the vernal equinox usually occurs on march 21 (universal/greenwich time), but for 2008 and most of the 21st century, it will fall on march 20

Answer 3

There's one leap year every 4 years except in years that end in "00". As for those, there is one leap year every 400 years.

Answer 4

i believe that after ten leap years or something they have a leap year without the leap and that keeps things fairly close to exact, i don't know what the exact numbers are though.

Answer 5

The rules on leap years are more complicated to account for the difference.

Answer 6

The cycle of the seasons, technically known as the tropical year, is approximately 365.2422 days.

The average Gregorian calendar year is 365.2425 days in length. This agrees to within a half a minute of the length of the tropical year. It will take about 3,300 years before the Gregorian calendar is as much as one day out of step with the seasons.

Answer 7

Every year that ends in '00 that is NOT divisible does not have a Feb. 29th.

Leap 'seconds' are added regularly to keep the clocks accurate. For more on this see the web site below:

Answer 8

I think every four hundred years, the leap year is skipped.