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Earth's axial tilt and the shadow from the Sun

  1. May 15, 2013 #1
    I have watched the Sun's shadow make its trek from the solstice moments for many years. I have it marked and like to observe it as Earth's tilt reaches it's maximum point, from Summer to Winter and back. I used a heliodon to built a large structure (80 feet long/12 feet tall=roofline of my house) that gives me a long shadow line. It is not perfect, but reliable for my purposes. I have been observing this shadow-progression for the past 7 years and have the solstice shadows marked for record.


    Is there someplace/website I can visit to see a real-time recording of the Sun's shadow this year, as compared to years previous? Thanks.
  2. jcsd
  3. May 15, 2013 #2

    jim mcnamara

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    That is really interesting. You should post a picture if you get the chance.

    I do not quite understand what you want to compare. I assume you are are talking about the angle (length of the shadow) of the sun at "solar noon" (Astronomical noon) on a given day? Those angles will change slightly when compared year to year.

    For one reason - local clock time and sun's time do not match up perfectly, if you use a local clock. For example: Astronomical noon could be at 12:20 one year on March 21, and then 12:30 the next year on the same date.

    Or look at it this way. Let's use March 21 as our day to compare.

    Our calendar uses days. Because the the orbital period of the Earth is not evenly divisible by 24 hour days, the precise angle at solar noon on March 21 will be slightly different from year to year.

    So, tell us what you want, please.
  4. May 15, 2013 #3
    Jim, thanks for your interest in my interest.

    I live near Houston, TX. My home faces as near to True North as possible. My roofline casts a shadow across my front yard at 'high noon' that is clearly observable. I have marked this shadow line (100 feet from east to west) at both the Winter and Summer solstice moment at high noon for the past 7 years. It is a fun project of anticipation for me and I have the line marked for observation. Each year, at the exact solstice moment, I have watched the shadow reliably appear at the exact locations as last years'. So I basically have a line running across my yard for the Winter moment and a different line marked for the Summer moment, and watch it progress from one line to the next over the time it takes the shadow to travel from one line to the next and back.

    Ill take a picture and post it for you asap.
  5. May 15, 2013 #4


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    Are you using local time for your noon? You may want to try to find the actual maxima points, which should not happen at local noon. How much does it vary?
  6. May 15, 2013 #5

    It varies alot. I used 'high noon' in my earlier comments, which is a common term. But the time I use is Astronomical Noon. The beauty of this moment is the shadow cast confirms the moment, using angles of the shadow pre-noon and post-noon helps visually see the time approach and pass, but I like to use the Naval Observatory time as backup. Today, though rainy here, astronomical noon is roughly 1330ish hours. Much different than what we commonly call noon.
  7. May 15, 2013 #6

    jim mcnamara

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    I get 13:18 local time as astronomical noon for today in Houston.

    one hour of that 1 hour 18 minutes difference is due to CDT/CST. The latitude/longitude for Sam Houston Park in downtown Houston( N 29.76 / W 95.372) also is well into the CDT, so the analemma and longitude and latitude also play games with solar noon vs the local clock. http://en.wikipedia.org/wiki/Analemma

    But I still don't see what you want to compare.
  8. May 16, 2013 #7
    A Plumb-bob placed under an overhang or similar shadow casting structure will give you Astronomical Noon, as the ancients knew it. You actually don't even need a plumb-bob as you become familiar with the object's shadow progression each day. A sundial is the perfect time keeper as well. I've grown accustomed to watching the times of day through the shadows cast around my home and use these observations to determine the 'exact' (give or take a couple minutes) moment of high noon. I have also grown accustomed to watching the times of year through the shadow progression between the two solstice maximums.

    Jim, I am, basically, wondering if there are any sites that follow the progression of the Astronomical Noon shadow from one solstice to the next, as I have been doing. I have found lots of sundial sites, but not one I can compare my technique with. Having toured Mexico's ancient sites, I have seen works built to follow this shadow, but can't find any internet spots to enjoy.

    Anyway, I hope this isn't in the wrong category of the forum. I figured it is related to Earth's axial tilt and Astronomical Noon so I opened the thread here. Moderators, my apologies if it is not. Just curious, and thanks for your interests and for this wonderful site.
  9. May 16, 2013 #8

    D H

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    Not that I know of. Your project isn't nearly as interesting as ones that use a fixed clock time rather than a fixed solar time. You can find lots of sites that show time-lapsed pictures the Sun or its shadow with a picture taken every day at the same UTC (formerly known as GMT) time. For example, just pick a time of day and plop a stone down at the end of your shadow. Do this at the same time (clock time, not solar time; the only adjustment is for daylight savings time) every day for a year. You'll have a neat figure eight of stones. To see such images, just go to google images and search for analemma.

    Here's one site that used shadows to trace out an analemma on the ground: http://www.austintek.com/astro/analemma/analemma.html.
  10. May 16, 2013 #9
    DH, most interesting indeed. Thanks for the link.
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