Dot Physics

When is noon?

I was at the zoo this weekend, it was fun but hot. Somewhere near the middle, there was a sun dial.

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The cool thing (well, I think sun dials are cool anyway) was that the shadow was pointing about to the “12″. I looked at my watch and it said 1:05 PM. What is up with that? The answer: Daylight Saving Time.

What does noon traditionally mean? In this day of the digital watch, most people associate noon with 12:00, you know – lunch time. But before clocks, noon referred to the time of day where the Sun was at the highest point in the sky. It is not too difficult to find this time. Just put a stick in the ground and watch the shadow. Here is an old video I did of that.

Note: It rained during that day, and then at the end the clay got too soft so that the stick fell over. You get the idea – right? When the shadow was pointing North and at its shortest, that is when the Sun was at its highest point. Noon. Also, that is how a sun dial works.

There are two problems with the sun dial. Daylight Saving Time is the first problem. For DST, the clocks are shifted ahead by one hour so that it is still sunny later in the evening. The other problem is time zones. Here is an image from Wikipedia’s page on time zones.

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In each of these time zones, they have the same time. However, the Sun is not at the highest point at all places in the time zone at the same time. Before the establishment of the time zones, each location would set the clocks to the local noon time. Of course this is a problem when you start traveling (especially with something fast like a train) and every location is its own time zone.

For me, I am in the Central Time zone. It just so happens that New Orleans (and thus the zoo) are very close to 90 degrees from starting time zone (Greenwhich). This means that we are very close to being just 1 hour off local noon. But take Dallas. They have the same time as New Orleans, but are much further West. When their clocks say 1:00 PM (DST), the Sun will not yet be at its highest point.

Comments

  1. #1 James McCann
    May 25, 2010

    I think a discussion of the equation of time would be appropriate here – Earth’s orbit is not circular after all.

    http://en.wikipedia.org/wiki/Equation_of_time

  2. #2 Rhett Allain
    May 25, 2010

    @James,

    I was considering that, but decided to leave it out for simplicity. Of course, you are right though. Technically, I should include that also.

  3. #3 Dave Gill
    May 25, 2010

    I wrote a piece on the EoT and the analemma back in ’06 – it is on pg. 4 of the linked PDF newsletter.

    http://www.twcac.org/onlinehorizon/horizonpdf/horizon_winter06.pdf

  4. #4 Lyle
    May 25, 2010

    To figure out when noon happens take the center of the time zone meridian (multiples of 15 starting with 0) and the longitude you are at. Find the difference and multiply by 4 mins per degree and you can figure out local noon is. For example I live about 99 w so you take 9 (the difference from central time) multiply by 4 and so local noon is 12:36 standard time or 1:36 daylight time.

  5. #5 James McCann
    May 25, 2010

    Some really terrific photographic analemmas are linked at the bottom of the wikipedia article on Analemma:

    http://en.wikipedia.org/wiki/Analemma

    Ancient Greece seems to be heavily favored.

  6. #6 Coturnix
    May 25, 2010

    Very nice! I wrote about (human) biological consequences of time zones here. And don’t get me started about Daylight Saving Time…that becomes a prickly issue ;-)

  7. #7 Keith Harwood
    May 25, 2010

    When you do get around to discussing the equation of time, could you describe how it was first measured. It was known long before there were accurate clocks, but I don’t see how to do it without one.

  8. #8 Ahcuah
    May 26, 2010

    I notice that you very carefully refrained from a pet peeve of mine: calling 12 noon “12 p.m.” “P.m.” of course stands for “post meridien”, or “after mid day” or “after noon”. It is also obvious that noon can not be “after” noon. If one is going to use an abbreviation, 12 noon would be simply “12 m.”

    But I realize that this is a battle I have lost.