# Leap Year

A leap year is a year with one more day, therefore 366, instead of the usual 365, arbitrarily set at February 29 every 4 years. Leap years are years divisible by 4, except for years of the century whose number is not divisible by 400. For example, here is the complete list of leap years for the 21st century (from 2001 to 2100):

2004, 2008, 2012, 2016, 2020, 2024, 2028, 2032, 2036, 2040, 2044, 2048, 2052, 2056, 2060, 2064, 2068, 2072, 2076, 2080, 2084, 2088, 2092, 2096.

Leap year principle
The rule, established by the Gregorian calendar in 1582, is to add a day for every year whose value meets one of the following conditions:

• Multiple of 4 but not of 100;
• Multiple of 400.
• Thus, 1700, 1800 and 1900 were not leap (divisible by 100) but 1600 and 2000 were leap (divisible by 400).

This rule therefore consists of adding one day every 4 years, except 3 times every 400 years. It partly makes up for the difference between the number of calendar days and the number of real days, which is 365.2422 days, the Earth not performing a whole number of rotations on itself when it loops a revolution around the Sun (which would be an extraordinary coincidence).

#### Origin of the leap word

The name comes from the Roman appellation bis-sextilis, meaning “second sixth day”. This extra day is indeed an invention of the ancient Romans, who fixed it after the sixth day before the calends of March (a period of their calendar), at about the same time of the year as our 29th. February. This non-integral number of days in the year had been noticed by the Egyptians, who had already had to add 5 days to their 360-day calendar.

#### Why 366 days every 4 years?

The primary purpose of leap years is to align the calendar with the seasons. Indeed, a year does not last 365 days exactly, but rather 365.242199 days. To avoid creating a gap with the passage of time, we add a day every 4 years to resynchronize with the seasons.

## Who invented leap years?

Julius Caesar.

The leap year is an idea of ​​Julius Caesar. In the days of ancient Rome, more than 2,000 years ago, it was noted that the calendar was offset from the solar year. Normal: the Romans had divided the year into twelve months of 29 and 31 days, so that the calendar had only 355 days (while the Earth revolves around the Sun in 365 and a quarter days). The Roman calendar took precedence over the actual position of the Earth around the Sun. More and more in advance … Up to 90 days. When there is such a big shift, the seasons are shifted: it is still freezing winter in April!

To rebalance all that, the Romans organized small catch-ups by adding days here and there. But the citizens who lived far from the capital, Rome, often did not even know about it, because they had not been brought to the news! Nobody understood anything any more. The year -46 was such a mess that the Romans called it “the year of confusion”. So, it was decided to make up for all the lag at once. We had to invent two new months, completely artificial, and add them at the end of the year: October, November, “first leap month”, “second leap month”, December. It was a very long year: 445 days!

A leap year (bissextile year) starting on a Wednesday is a 366-day year with January 1 falling on a Wednesday, February 29 on a Saturday, and December 31 on a Thursday, such as: 1992 and 2020, the latter two. 2048, the next, 2076, the next…

Other leap years starting on a Wednesday
2nd millennium:
18th century: 1716 • 1744 • 1772
19th century: 1812 • 1840 • 1868 • 1896
twentieth century: 1908 • 1936 • 1964 • 1992
3rd millennium:
21st century: 2020 • 2048 • 2076
21st century: 2116 • 2144 • 2172
21st century: 2212 • 2240 • 2268 • 2296

## Why we created a leap year?

The calendar must correspond to the seasons, it is imperative that the rotation of the earth around the sun corresponds to a full year. However, the earth does not revolve around the sun in exactly 365 days but in 365.2422 days. In the long run, this slight difference is counted in days and the seasons no longer correspond to the calendar.

Julius Caesar asked the astronomer Sosigene of Alexandria to think about the problem. Thus, we created the Julian calendar which corrected it in part by adding a sixth bis “bissextus” day every 4 years. However, although the problem was diminishing, there was still a fairly significant lag of a dozen more days in 1,500 years and it was by creating the Gregorian calendar, our current calendar, that Pope Gregory XIII, further narrowed this gap. . By adding 1 day every 4 years except 3 times every 400 years, there is now only a gap of 3 days every 10,000 years, which is acceptable.

#### How do you know if a year is leap year?

To know if a year is leap year, it must obey one of the following two rules:

Be divisible by 4 but not divisible by 100
To be divisible by 400
Example: 2016 is divisible by 4 (2016/4 = 504) but not divisible by 100 (2016/100 = 20.16) -> 2016 is therefore leap.
On the other hand, 1900 is not leap because it is divisible by 4 but also by 100, so the 1st condition is not good and it is not divisible by 400.

#### Why we add a day in February for the leap year?

It will not have escaped your notice that the month of February has fewer days than the other months of the year. This results from the fact that the Emperor Augustus wanted the month of August to which he gave his name to have the same number of days as that of July of Julius Caesar. So he decided to take a day off February and add it to August. So July and August both have 31 days while February has only 28 and we add one in leap years.

There are 291 leap years between 1800 and 3000:

1804, 1808, 1812, 1816, 1820, 1824, 1828, 1832, 1836, 1840, 1844, 1848, 1852, 1856, 1860, 1864, 1868, 1872, 1876, 1880, 1884, 1888, 1892, 1896, 1904, 1908, 1912, 1916, 1920, 1924, 1928, 1932, 1936, 1940, 1944, 1948, 1952, 1956, 1960, 1964, 1968, 1972, 1976, 1980, 1984, 1988, 1992, 1996, 2000, 2004, 2008, 2012, 2016, 2020, 2024, 2028, 2032, 2036, 2040, 2044, 2048, 2052, 2056, 2060, 2064, 2068, 2072, 2076, 2080, 2084, 2088, 2092, 2096, 2104, 2108, 2112, 2116, 2120, 2124, 2128, 2132, 2136, 2140, 2144, 2148, 2152, 2156, 2160, 2164, 2168, 2172, 2176, 2180, 2184, 2188, 2192, 2196, 2204, 2208, 2212, 2216, 2220, 2224, 2228, 2232, 2236, 2240, 2244, 2248, 2252, 2256, 2260, 2264, 2268, 2272, 2276, 2280, 2284, 2288, 2292, 2296, 2304, 2308, 2312, 2316, 2320, 2324, 2328, 2332, 2336, 2340, 2344, 2348, 2352, 2356, 2360, 2364, 2368, 2372, 2376, 2380, 2384, 2388, 2392, 2396, 2400, 2404, 2408, 2412, 2416, 2420, 2424, 2428, 2432, 2436, 2440, 2444, 2448, 2452, 2456, 2460, 2464, 2468, 2472, 2476, 2480, 2484, 2488, 2492, 2496, 2504, 2508, 2512, 2516, 2520, 2524, 2528, 2532, 2536, 2540, 2544, 2548, 2552, 2556, 2560, 2564, 2568, 2572, 2576, 2580, 2584, 2588, 2592, 2596, 2604, 2608, 2612, 2616, 2620, 2624, 2628, 2632, 2636, 2640, 2644, 2648, 2652, 2656, 2660, 2664, 2668, 2672, 2676, 2680, 2684, 2688, 2692, 2696, 2704, 2708, 2712, 2716, 2720, 2724, 2728, 2732, 2736, 2740, 2744, 2748, 2752, 2756, 2760, 2764, 2768, 2772, 2776, 2780, 2784, 2788, 2792, 2796, 2800, 2804, 2808, 2812, 2816, 2820, 2824, 2828, 2832, 2836, 2840, 2844, 2848, 2852, 2856, 2860, 2864, 2868, 2872, 2876, 2880, 2884, 2888, 2892, 2896, 2904, 2908, 2912, 2916, 2920, 2924, 2928, 2932, 2936, 2940, 2944, 2948, 2952, 2956, 2960, 2964, 2968, 2972, 2976, 2980, 2984, 2988, 2992, 2996.

There are 243 leap years between 2000 and 3000:
2000, 2004, 2008, 2012, 2016, 2020, 2024, 2028, 2032, 2036, 2040, 2044, 2048, 2052, 2056, 2060, 2064, 2068, 2072, 2076, 2080, 2084, 2088, 2092, 2096, 2104, 2108, 2112, 2116, 2120, 2124, 2128, 2132, 2136, 2140, 2144, 2148, 2152, 2156, 2160, 2164, 2168, 2172, 2176, 2180, 2184, 2188, 2192, 2196, 2204, 2208, 2212, 2216, 2220, 2224, 2228, 2232, 2236, 2240, 2244, 2248, 2252, 2256, 2260, 2264, 2268, 2272, 2276, 2280, 2284, 2288, 2292, 2296, 2304, 2308, 2312, 2316, 2320, 2324, 2328, 2332, 2336, 2340, 2344, 2348, 2352, 2356, 2360, 2364, 2368, 2372, 2376, 2380, 2384, 2388, 2392, 2396, 2400, 2404, 2408, 2412, 2416, 2420, 2424, 2428, 2432, 2436, 2440, 2444, 2448, 2452, 2456, 2460, 2464, 2468, 2472, 2476, 2480, 2484, 2488, 2492, 2496, 2504, 2508, 2512, 2516, 2520, 2524, 2528, 2532, 2536, 2540, 2544, 2548, 2552, 2556, 2560, 2564, 2568, 2572, 2576, 2580, 2584, 2588, 2592, 2596, 2604, 2608, 2612, 2616, 2620, 2624, 2628, 2632, 2636, 2640, 2644, 2648, 2652, 2656, 2660, 2664, 2668, 2672, 2676, 2680, 2684, 2688, 2692, 2696, 2704, 2708, 2712, 2716, 2720, 2724, 2728, 2732, 2736, 2740, 2744, 2748, 2752, 2756, 2760, 2764, 2768, 2772, 2776, 2780, 2784, 2788, 2792, 2796, 2800, 2804, 2808, 2812, 2816, 2820, 2824, 2828, 2832, 2836, 2840, 2844, 2848, 2852, 2856, 2860, 2864, 2868, 2872, 2876, 2880, 2884, 2888, 2892, 2896, 2904, 2908, 2912, 2916, 2920, 2924, 2928, 2932, 2936, 2940, 2944, 2948, 2952, 2956, 2960, 2964, 2968, 2972, 2976, 2980, 2984, 2988, 2992, 2996.

## February 29th

In leap years, February is extended by a leap day . This gives these years a February 29 , which is then the 60th day of the Gregorian calendar, leaving 306 days until the end of the year.

In the Roman calendar, February was originally the last month and therefore the one to which the leap days were added. The actual leap day of the Gregorian calendar in Roman counting is, however, as in the Julian calendar , a second February 24, inserted after February 23. This fact was evident in the Roman Catholic Church until the church festival calendar was reorganized in 1969 when the festival of St. Matthias, which was always celebrated on February 24, was postponed to February 25 in the leap year. For the Evangelical Church in Germanythis rule still applies.

Today’s regular leap days exist since Pope Gregory XIII. in a papal bull on February 24, 1582 ordered the cancellation of ten accrued leap days. From then on, all are leap years whole numbers divisible by four, with the exception of the centuries that are not divisible by 400, i.e. 1700, 1800, 1900. In contrast, 1600 and 2000 were leap years. Similarly, 2100, 2200, 2300, and 2500 will not be leap years, but 2400 will be a leap year.