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How is 12 am defined?

  1. Aug 9, 2012 #1
    People tell 12am is midnight but then how is midnight defined? 12am must be defined with respect to some natural event like when earth has rotated this much degrees or whatever. Anyone knows?
     
  2. jcsd
  3. Aug 9, 2012 #2

    Ryan_m_b

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    Wikipedia is your friend
     
  4. Aug 9, 2012 #3

    BobG

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    And note that they're talking about solar midnight, solar noon, etc. which is a geometric arrangement between the Earth (your location, specifically) and the Sun.

    Your clocks run on mean solar time - in other words, your clocks are based on the average solar day; not this particular solar day. Solar days vary depending on where the Earth is in its orbit around the Sun, while the average solar day is just that - an average that doesn't change each day.

    So, midnight on your clock is just the change of day.

    http://flhsgeoscience.blogspot.com/2012/01/latest-sunrise-of-year.html
     
  5. Aug 9, 2012 #4
    Yes i always check wikipedia before PF. Actually my question is that if all the clocks and record of time is lost can we again resume? For example - we lost all clocks and its afternoon. How will we know if it is 2 or 3 pm?
     
  6. Aug 9, 2012 #5

    Ryan_m_b

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    Midnight is roughly half the distance between sunrise and sunset, so simply build some form of measuring device and after the first night you'll know when midnight will be on the second. E.g. I started my homemade counter at sunset, at sunrise it registered X so tonight I'll know it's midnight when it reads 1/2X.

    Alternatively use a planisphere in reverse.
     
  7. Aug 9, 2012 #6
    Ok thank you
     
  8. Aug 9, 2012 #7
    That would work 'roughly', because, 12:00 don't always equal = (Sunrise-Sunset)/2
    Just for the sake of knowledge, I would like to ask, is there a way to do that with extreme precision?
     
  9. Aug 9, 2012 #8

    Ryan_m_b

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    The reason it is roughly is because we have arbitrary time zones it can be two seperate times for people on the same line of longitude and the same time for people in two different lattitudes. To work out true solar midnight would require knowing at what point the sun is at nadir.
     
  10. Aug 9, 2012 #9
    I have a book somewhere… and if you really want me to, I might try and dig it out and give you a more detailed summary of the technique it outlines. As best I remember it, you have to put a stick in the ground, and you have to ensure that it is perfectly vertical (use a plumb line). Then you mark where the shadow is at dawn and where it is at dusk. Unfortunately, you then have to repeat that exercise right through the year. At the end, for your precise location on earth, you will be able to define midnight, midday, the summer and winter solstice and the two equinox. I suppose you do need to be somewhere sunny! And there is an angle that you can measure, between the two extremes of any of your lines – the two extremes of your dawn line, or your dusk line or your midday line or whatever. The angle you measure corresponds exactly to the angle that earth’s orbit is from perfect horizontal, because it is caused by it.
     
  11. Aug 10, 2012 #10

    BobG

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    You can keep time by following the location of the stars using a merkhet.

    One of the oldest methods, but there are several methods of keeping time with the stars provided you know your precise longitude. Or, if you had an accurate clock, then the stars could be used to provide your precise longitude. (Developing an accurate clock that would work at sea was one of the big challenges for naval navigation.)

    If you don't need an incredible amount of precision, the stars are accurate enough. They'll shift about 1 degree per night, meaning they'll pass your string four minutes earlier per night. That's actually just slightly less than 1 degree, since it will take 365+ days to shift 360 degrees. Close enough.

    And if you wanted to develop a numbering system that was compatible with your timekeeping/navigation methods, 360 degrees in a circle works much better than a number like 365.25. 360 is divisible by more different numbers. In fact, you can develop a base 60 system. Both your degrees and your time work with each other easily. In fact, it's common for astronomers to measure the angle of stars in 'hours' instead of degrees, while satellite operators measure the Local Sidereal Time for a tracking station in degrees (hour angle times 15). They're so compatible that you can use the units almost interchangeably.
     
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