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Time taken for earth to orbit the sun?

  1. Apr 1, 2013 #1
    Are we just really lucky that the time it takes for earth to orbit the sun is 365.25 days? I mean if it was something like 365.71 days, then we couldn't have our simple leap year every four years. We would have to have a 71 leap years every century, or even if we were to simplify it a bit, 7 leap years every decade (with maybe two instead of one the last year). Does that make sense?

    Is it coincidence, or is there some reason behind this?
  2. jcsd
  3. Apr 1, 2013 #2
    It is not exactly 365.25 days.
  4. Apr 1, 2013 #3
    but it is close enough for us to be able to have leap year every 4 years for a long time
  5. Apr 1, 2013 #4


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    It's coincidental, which becomes obvious once you realise that the year is not only not 365.25 days long, but the actual value changes over time.
    365.25 is a rounded value used by the Julian calendar, which was inaccurate enough for the Gregorian calendar to supercede it. In it, there are 97 leap years in 400 years.
    It's still just a convenient approximation of whichever one of the astronomical years you might want to think of.
    Julian year still persists in many scientific contexts, but it's hardly an actual, precise value.

    Wikipedia has some more reading on the subject:

  6. Apr 3, 2013 #5
    The fractional part of days to years (solar days to tropical years, at the current era) has to be something between 0 and 0.999.... 0.25 (or 0.25219) isn't so surprising. But I doubt we'd still use "leap" years if the fractional part was bigger than one-half. If there were 365.71 days in a year, I imagine our calendar would have "anti-leap" years of 365 days per decade, and "normal" years of 366 days. That way, most years would be "normal". If the fractional part of days in a year was close to 0.5 (say 365.49 or 365.52 days per year), we might talk about "short years" and "long years". In any case, we'd take our calendar for granted; since we wouldn't know of a planet with a year of 365.25 (more or less) days.
  7. Apr 3, 2013 #6


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    What is your point?
  8. Apr 3, 2013 #7
    The original post talked about 7 leap years per decade. My point was that the idea of calling most years anything special (such as "leap") is rather silly.
  9. Apr 4, 2013 #8
    It's a coincidence. It's not all that lucky though. There is maybe a 2% chance that the remainder would be that close to a simple fraction.
  10. Apr 4, 2013 #9
    Thanks for all your answers, guys! ^^
  11. Apr 5, 2013 #10


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    It would be luckier if it only took 361.27632 days.

    Then the 360 degrees in a circle would be closer to matching how the stars actually move across the sky.

    They it is now, the stars really only shift by .9856 degrees per night - not the 1 degree per night that our 360 degree circles use. With a 361.27632 day year, the stars would shift by .9965 degrees per night - an inaccuracy of less than 4 thousandths of a degree!

    It would be really unlucky if it took 389.953 days. Can you imagine trying to create a numbering system that worked nicely with 390?! How many minutes would we put into an hour and how many seconds into a minute? (seeing as how our timekeeping system was designed to be compatible with our angle measuring system)

    The Babylonians probably would have had to go with 400 degrees in a circle, develop our time keeping system accordingly, and accept the fact their measurement of the stars would be off an whole 25 thousandths of a degree each night instead of the 14 thousandths inaccuracy we already have.

    Of course, actually, with a nearly 400 day year, we would have gone with a base 20 numbering system for angles and time instead of a base 60 system, and that would have actually made things a lot simpler, since base 20 systems have historically been a lot more common than base 60 numbering systems, so maybe we wouldn't have been so unlucky after all.
    Last edited: Apr 5, 2013
  12. Apr 27, 2013 #11
    Sidereal vs. Solar?

    Hi, I didn't want to start a new thread for the following question, and found this one to be the closest to what I need to know.

    In a planet where a "Sidereal Day" and a "Solar Day" differ by a large value, like Mercury or Venus, which term of the two represents the time between two sunrises (or sunsets) on the surface of the planet?

    I've gone through Wikipedia inputs for "day", "mercury", "earth", "sidereal time", "solar time" in addition to other links, and found no direct answer except for a hint that it might be "sidereal".

    So I'd like to make sure, thanks. :)
  13. Apr 27, 2013 #12


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    That'll be the solar day, or more precisely, the apparent solar day.

    You want the time it takes for the Sun to return to the same spot in the sky(e.g.the zenith), not the time for some background stars to do the same(which would be sidereal day).

    And welcome to the forum!
  14. Apr 27, 2013 #13
    Great, Bandersnatch.

    Thanks a million. : )
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