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Insights Measuring How Many Days Are in a Year - Comments

  1. May 22, 2015 #1


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  2. jcsd
  3. May 22, 2015 #2
    Great first entry @Janus! This is a topic I never knew could be so deep!
  4. May 22, 2015 #3
    There are so many rational ways to measure a year.

    But let's not forget our Muslim readers. Their year is based on how many months have passed. And their months are based on direct observation of the moon. The length of the month can change depending on the weather since clouds interfere with moon sightings. Because of this it is impossible to predict with certainty what the next Muslim year's dates will be. The future is spun out real time.

    So anyone who thinks these various astronomical measurements are complex, be glad you're not a Muslim. (Unless you are, in which case the future is as God wills it.)

    Humanity has measured time in lots of ways in history. Some of them will seem strange to us living in an era where calendars are common place technology.
  5. May 23, 2015 #4


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    This is a very cool topic. I recently read a book by Stephen Jay Gould called "Millenium" (or something like that) which discussed many of these points. The calendar is a formidably complicated beast to tackle when one really gets deep into the topic.
  6. May 24, 2015 #5


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    To make things worse, the rotation rate of earth is not constant. The effect can be up to one second per year.
    Over longer timescales, the days get longer by about 2.3 milliseconds each century, currently with a lower rate of 1.7 ms as some continental masses still move around related to the last ice age.

    To keep atomic clocks in sync with the position of the sun, we frequently need leap seconds, and we will need more and more in the future.
  7. May 24, 2015 #6
    Well thank god they realized the slowing rotation rate of the Earth in the 60's and changed the SI definition a second. :rolleyes:

    I guess we just have to wait for the Kilogram, Ampere and Kelvin to get a much needed change in definition to one in terms of (preferably dimensionless) fundamental constants.
  8. May 24, 2015 #7


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    Well atomic clocks would run more stable than Earth either way. But frequently tuning them would be really messy and make astronomic measurements weird ("and then we have to account for the longer seconds in 1994 to compare the results").
    You need constants with dimensions to fix units. Dimensionless constants (in SI) do not allow to define things like kilograms.
  9. May 24, 2015 #8
    Ahh, right, silly me. But incorporation of the fundamental constants in SI definitions is definitely the need of the hour.
  10. May 29, 2015 #9
    This is very interesting topic.I never knew that and never thought about that.
  11. Sep 21, 2015 #10


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    Great post, enjoyable read. Thank you.
  12. Sep 24, 2015 #11
    Really an interesting topic though I never thought it would have been so much elaborated. I know only the direct 365 days :p
  13. Oct 23, 2015 #12


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    "This also means that the position of the Sun with respect to the stars on a given date as seen from the Earth also changes over the years. In the time since the dates for the astrological signs were established, the Sun has moved an entire Zodiac sign. Thus, during the Summer solstice this year, instead of just entering Cancer as the astrological dates suggest, the Sun is just leaving Taurus and entering Gemini. (So if the the Tropic of Cancer had been named today, it would have likely been called the Tropic of Gemini and the Tropic of Capricorn would be the Tropic of Sagittarius)."

    So, at the time astrological signs were established, the vernal equinox pointed towards the constellation Aries. And that some point in Aries (the first point of Aries?) could be used to find the direction of the vernal equinox no matter what day of the year it was. And now the direction of the vernal equinox (the first point of Aries) lies in the constellation Pisces?

    Man, I can't wait until the direction of the vernal equinox (the first point of Aries) approaches the constellation Aquarius. It will be such a great occasion that we'll all dance and sing songs about it! "It is the dawning of the Age of Aquarius!"
  14. Apr 12, 2016 #13
    Three historical definitions of "day" are given, but the currently excepted definition is not included i.e.
    86,400 SI seconds where the SI second is based on the hyperfine transition frequency of the caesium atom.
  15. Apr 12, 2016 #14
    Well, try this. Figure out the EXACT way that they determine when Easter is each year. And, no, it has little to do with the Vernal Equinox. It has something to do with "The Golden Number" (which has a lot to do with astronomy). This was so confusing that the various churches (Catholic, Anglican, etc.) would print the day Easter falls on for a large number of years in the Common Prayer Book of the ordinary Christian (Easter is by far the most important Christian holiday).
  16. Apr 13, 2016 #15
    Excellent post. I'd like to point out that we live in a very special time during the 26,000 precession cycle where we have the brightest star that it's possible to have as the North Star which is very close to the actual north pole, this lasts about 200 years before drifting away.
  17. Apr 18, 2016 #16
    In the past they must have thought there was only 360 days in a year, that’s probably why we’ve got 360 degrees in a circle?
  18. Apr 18, 2016 #17


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    Nah, the 360 degrees thing is left over from the Babylonian number system: https://en.wikipedia.org/wiki/Babylonian_numerals

    The legacy of sexagesimal still survives to this day, in the form of degrees (360° in a circle or 60° in an angle of an equilateral triangle), minutes, and seconds in trigonometry and the measurement of time, although both of these systems are actually mixed radix.

    The Babylonian calendar used lunar months, which vary from 29-30 days each, along with an intercalary month when necessary. The years wouldn't have been 360 days long as far as I can tell.
  19. Apr 18, 2016 #18


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    It is quite easy to get the length of a year with a precision of a day over a human lifetime without any dedicated measurements, and getting 365.25 as approximation is possible with very simple astronomical observations. I don't think anyone ever thought a year would be 360 days long. The Babylonians certainly knew it better (see above).
    Getting more precise than 365.25 is challenging, and brings up all the complications mentioned in the insights article.
  20. Apr 19, 2016 #19
    Finding how many days in a year by using the sighting of the crescent moon must have been impossible?

    If you count 6 times 29 and 6 times 30, add them together, it comes to 354 days.

    That’s 11.25 days less than the accepted figure of 365.25…

    Did they know how to add fractions?

    It must have been after Newton and Kepler when they finally got it about right?
  21. Apr 19, 2016 #20


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    Yes, ancient civilizations knew how to add fractions, and I doubt they split the year up into 6 months of 29 days and 6 months of 30 days. Being 10+ days off in a single year was very, very noticeable to those who watch for these kinds of things.

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