We know that the year is 365 days long, but in a leap year there is an extra day (29 February). I'm quite pleased that a 29 February is so rare, as it is the only day that I dislocate my shoulder on.
The idea that every fourth year is a leap year isn't totally accurate, as the final year of a century is only a leap year if it can be divided by 400. The final year of last century (2000) was a leap year, but the final year of this century (2100) won't be.
Over a 400 year period, there will be 97 years with 366 days and 303 with 365 days. This is 146,097 days, so the average year length is 365.2425 days.
One year used by astronomers is the Tropical year, which is measured from vernal equinox (the start of the northern hemisphere spring) to vernal equinox.
The image - taken for today from Heavens Above shows the constellation of Pisces, in which we find the First Point of Aries.You can find the First Point of Aries by looking for the 0 hours along the x-axis and the 0 degrees along the y-axis and seeing where they cross. At that point - the First Point of Aries - you will see an inclined white line cross. That is known as the ecliptic and marks the path of the Sun in the sky. You will see that the Sun does, indeed, lie on the line, and there are 3 planets (Venus, Mars and Uranus) near the Sun and near that line. You can also make out Mercury, down in neighbouring Aquarius:
If you're in the tropics or the southern hemisphere then Mercury is an impressive object in the pre-dawn sky at the moment.
The Sun is at the First Point of Aries at the vernal equinox.
The tropical year is 365.2421897 days long. The old Julian calendar simply had every fourth year a leap year, but something happened when the average year was 365.25 days long.
We can see that the difference between the average Julian year and the tropical year is 0.0078103 days, which might not seem much (it's just over 11 minutes). Over a century this becomes 0.78103 days - which is 18 hours 45 minutes. So, each year, spring was starting earlier.
In May 1572 Ugo Boncompagni became Pope Gregory XIII and one task he set about was calendar reform. This might seem an odd thing for the Vatican to be worried about, but it has all to do with getting Easter right.
In 1582 he took the papal bull by the horns and decided to abolish the days from 5 to 14 October inclusive (for that year only), so 4 October was followed by 15 October.
Abolishing days is one of the perks of being Pope - yet not one of them has abolished Monday mornings.
Boncompagni also altered the rules concerning end-of-century years always being leap years, and so brought the average year length closer to the tropical year. The average year is now 0.0003103 days longer than the tropical year, and over 400 years this amounts to 0.12412 days - just under 2 hours 59 minutes. So, after 4 centuries, spring is starting a little bit less than 3 hours earlier.