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While Gregorian calendar is a solar year it always start on 00:00 each year. But for sure the Earth completes each orbit around the Sun on different times each year.

So if we put January 1 as the start of the year we should calculate the exact time when the Earth completes its orbit around the Sun the Jan 1 of the next year.

So why does Gregorian calendar insists on celebrating each year on 00:00? Doesn't it cause a huge error in the future?

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    $\begingroup$ The Gregorian Calendar year is based on a calculated mean year The main concern was that the March 20th be the date that the March Equinox happens on, year over year., and a reasonable, human-computable formula for the number of days in each year. It was more important that the length of years be regular and predictable, than a specific astronomical event be used to pin the start of the year. $\endgroup$
    – notovny
    Sep 15, 2023 at 20:18

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The Gregorian calendar is designed to approximate the average tropical year, that is the average length of time from one spring equinox to the next.

The actual average length of a tropical year is 365.2422 days. The average length of a Gregorian year is 365.2425 days

So although it isn't perfect, it would take many thousands of years for the discrepancy to become noticeable. From year to year, the date of the spring equinox will move a little, it will always stay close to March 21st. (for generations to come) This is not a huge error; it is a tiny error that you will never notice in your life time.

It would be possible to design a calendar that started the year exactly on the spring equinox, (or winter solstice, for example) But then the clocks would not be aligned to the day time, which would be very inconvenient.

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  • $\begingroup$ FWIW, the March equinox tropical year is (currently) slightly longer than the mean tropical year. See en.wikipedia.org/wiki/… $\endgroup$
    – PM 2Ring
    Sep 16, 2023 at 9:33
  • $\begingroup$ For any calendar reformers here, the shortest-cycle approximation accurate to 7 significant digits is 31 leap years every 128 years, giving an average of 365.2421875 days per year. $\endgroup$
    – dan04
    Sep 19, 2023 at 0:09
  • $\begingroup$ You mean 11111₂ leap years every 10000000₂ years (clearly the solar system is a simulation) $\endgroup$
    – James K
    Oct 27, 2023 at 23:44

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