# Why do you sometimes miss a leap year?

I'm not all that fond of leap years, so I don't really miss them when they aren't around.

The purpose of leap years is to keep the calendar in synchronization with the solar year and the seasons. The "year" - one orbit of the Earth around the Sun - isn't an exact multiple of "days" - the time that it takes for the Earth to rotate once. The "year" is actually 365.26 days long. The orbital period of the Moon is also not neatly divisible by either days or years.

Some cultures do not sync the year to the seasons; the Islamic calendar, for example. A holiday that happens in summer one year will happen in spring a few years later, and in winter a few years after that.

But most, especially in agricultural societies, do or did. If the calendar does nothing else, it should at least offer some standardization on when to plant, and when to harvest.

Because the year is not a precise integer number of days long, we will need to have "leap" years, or "intercalendary days" to tweak the calendar into correspondence with the year.

The current Gregorian calendar of 365 days per year adds one "leap" day every 4 years. If the year were exactly 365.25 days long, this would keep everything accurate forever, but the year is actually a bit over that; 365.26 days. So once per century, in century years, the Gregorian calendar skips a leap day. Even that isn't QUITE perfect, so on years divisible by 400, we do have a "leap day".

So 1900 (a century year) was NOT a leap year, even though it was divisible by 4. 1996, 2000, and 2004 were all leap years. But the year 2100 will not be.

The purpose of leap years is to keep the calendar in synchronization with the solar year and the seasons. The "year" - one orbit of the Earth around the Sun - isn't an exact multiple of "days" - the time that it takes for the Earth to rotate once. The "year" is actually 365.26 days long. The orbital period of the Moon is also not neatly divisible by either days or years.

Some cultures do not sync the year to the seasons; the Islamic calendar, for example. A holiday that happens in summer one year will happen in spring a few years later, and in winter a few years after that.

But most, especially in agricultural societies, do or did. If the calendar does nothing else, it should at least offer some standardization on when to plant, and when to harvest.

Because the year is not a precise integer number of days long, we will need to have "leap" years, or "intercalendary days" to tweak the calendar into correspondence with the year.

The current Gregorian calendar of 365 days per year adds one "leap" day every 4 years. If the year were exactly 365.25 days long, this would keep everything accurate forever, but the year is actually a bit over that; 365.26 days. So once per century, in century years, the Gregorian calendar skips a leap day. Even that isn't QUITE perfect, so on years divisible by 400, we do have a "leap day".

So 1900 (a century year) was NOT a leap year, even though it was divisible by 4. 1996, 2000, and 2004 were all leap years. But the year 2100 will not be.

I'm not all that fond of leap years, so I don't really miss them when they aren't around.

The purpose of leap years is to keep the calendar in synchronization with the solar year and the seasons. The "year" - one orbit of the Earth around the Sun - isn't an exact multiple of "days" - the time that it takes for the Earth to rotate once. The "year" is actually 365.26 days long. The orbital period of the Moon is also not neatly divisible by either days or years.

Some cultures do not sync the year to the seasons; the Islamic calendar, for example. A holiday that happens in summer one year will happen in spring a few years later, and in winter a few years after that.

But most, especially in agricultural societies, do or did. If the calendar does nothing else, it should at least offer some standardization on when to plant, and when to harvest.

Because the year is not a precise integer number of days long, we will need to have "leap" years, or "intercalendary days" to tweak the calendar into correspondence with the year.

The current Gregorian calendar of 365 days per year adds one "leap" day every 4 years. If the year were exactly 365.25 days long, this would keep everything accurate forever, but the year is actually a bit over that; 365.26 days. So once per century, in century years, the Gregorian calendar skips a leap day. Even that isn't QUITE perfect, so on years divisible by 400, we do have a "leap day".

So 1900 (a century year) was NOT a leap year, even though it was divisible by 4. 1996, 2000, and 2004 were all leap years. But the year 2100 will not be.

The purpose of leap years is to keep the calendar in synchronization with the solar year and the seasons. The "year" - one orbit of the Earth around the Sun - isn't an exact multiple of "days" - the time that it takes for the Earth to rotate once. The "year" is actually 365.26 days long. The orbital period of the Moon is also not neatly divisible by either days or years.

Some cultures do not sync the year to the seasons; the Islamic calendar, for example. A holiday that happens in summer one year will happen in spring a few years later, and in winter a few years after that.

But most, especially in agricultural societies, do or did. If the calendar does nothing else, it should at least offer some standardization on when to plant, and when to harvest.

Because the year is not a precise integer number of days long, we will need to have "leap" years, or "intercalendary days" to tweak the calendar into correspondence with the year.

The current Gregorian calendar of 365 days per year adds one "leap" day every 4 years. If the year were exactly 365.25 days long, this would keep everything accurate forever, but the year is actually a bit over that; 365.26 days. So once per century, in century years, the Gregorian calendar skips a leap day. Even that isn't QUITE perfect, so on years divisible by 400, we do have a "leap day".

So 1900 (a century year) was NOT a leap year, even though it was divisible by 4. 1996, 2000, and 2004 were all leap years. But the year 2100 will not be.