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  • #16
    Originally posted by Neil View Post
    I still don't quite get it because I see the earth spinning on a central axis therefore 15 degrees = 1 hour don't it? I would have thought that even with the 23 or 24 degrees of axix tilt that the sun would appear to shift 15 deg/hour.
    It's the orientation/rotation of the spin axis you have to measure, not of it's projection as you are doing. Your method would work only if you were standing on the north (or south) pole.

    Get a long stick, stick it in the ground at an angle so it points to the north celestial pole (the star Polaris is close, it is there even in the daytime). The elevation angle should be the same as your latitude, at azimuth zero degrees true north. Let the stick be able to freely rotate in it's hole but not otherwise move relative to the ground. The stick is now parallel to the earth's rotation axis.

    Now fasten a short stick to the side of the long one with a single screw so that it can pivot, such that the short stick is free to point at the sun (there will be only a point shadow on the ground from the short stick). At a later time measure the rotation angle you have to rotate the long stick to again point the short stick at the sun. Divide that angle by 15 to get the elapsed time in hours. It's not the projected angle on the ground you need, it is the rotation angle about the earth's axis that measures time. If you do this using a star (any star) instead of the sun, divide by 15.04.

    You could calibrate the projected shadow along the ground to read out in hours if you wanted, it would be different for different latitudes. That is the principle of the sundial.
    Last edited by Nessmuk; 12-11-2006, 04:14 PM.
    "Leave the beaten track behind occasionally and dive into the woods. Every time you do you will be certain to find something you have never seen before." - Alexander Graham Bell

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    • #17
      Originally posted by TFR View Post
      The Sun does move 15 degrees per hour (roughly), but in its direction of travel, which is not the same as straight across the sky, except briefly around local noon. Your method will work ok then, but not well at other times.

      Consider late in the day in the summer. The sun is setting almost vertically, so your shadow isn't moving much, it's just getting longer.
      So there must be a mathematical equation for determining degree of shadow shift per hour according to latitude, date and time of day.
      1111111111

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      • #18
        Wow, I wonder if my mom ever would've believed she'd someday be responsible for starting this conversation.
        Scooting here and there
        Through the woods and up the peaks
        Random Scoots awaits (DP)


        Eat, sleep, hike, repeat.

        It doesn't have to be viewtiful to be beautiful. (NL)

        "Pushing the limits of easy."

        Comment


        • #19
          This thread is awesome.

          Originally posted by Nessmuk View Post
          Therefore the stars appear in the same positon 1/365th of a circle (4 minutes) earlier as measured relative to our 24 hour solar day.
          Alright, I think it's coming back to me (from years ago reading H.A. Rey's "The Stars"). I definitely goofed and conflated leap years with sidereal time, but I think I'm now up to speed there. Do I remember correctly, though, that the long term effects of sidereal time result in "the precession of the equinoxes," which is why astrological signs no longer line up with their corresponding constellations? Apologies once again if I'm way off base here. I'm far from any kind of expert on these things. Also apologies if I'm way too far OT.
          46/46, 12/48, 58/115
          46-R #6866

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          • #20
            Originally posted by AdkWalrus View Post
            Do I remember correctly, though, that the long term effects of sidereal time result in "the precession of the equinoxes," which is why astrological signs no longer line up with their corresponding constellations? Apologies once again if I'm way off base here. I'm far from any kind of expert on these things. Also apologies if I'm way too far OT.
            Imagine the earth as spinning like a top, a top spinning on the floor but tipped slightly to one side. In the absence of other forces, the gyroscopic effect wants to keep the axis of the top always pointed in the same direction, no matter how you otherwise move the top around on the floor. However, other forces, such as gravity coming through the floor, try to make it fall over. Being a spinning top, it resists tipping directly over, but it instead wobbles a bit in response. Notice that even though the spin axis of the top wobbles in a circular motion, it keeps the same tip angle with respect to vertical. This wobble is called "precession".

            The same is true of the earth, it is a top tipped 23.5 degrees with respect to the plane of its orbital path. It's axis always points in the same direction, the northern axis pointed toward a spot of sky close to the star Polaris. 24 hours a day, 365 days a year, year after year it points in the same direction. That 23.5 degree tip is what causes our seasons, with the apparent path of the sun going from north to south back to north over the course of a year, causing our seasons.

            At 2 points in the path, the equinoxes (first day of spring and first day of autumn) the sun crosses the mid point, the plane of earth's equator - heading north in spring and south in autumn. That happens around March 21 and September 21 each year because the earth's axis always points toward the same direction in space.

            Approximately, that is. Just like the spinning top, the earth precesses due to the action of gravity from the sun and moon. The direction of the earth's axis wobbles (precesses) in a 23.5 degree circle, completing one wobble in 28,500 years. The wobble causes the apparent date (place in the earth's orbit) where the the sun's path crosses over from north to south to change ever so slightly each year - the "precession of the equinoxes".
            Last edited by Nessmuk; 12-12-2006, 09:39 AM.
            "Leave the beaten track behind occasionally and dive into the woods. Every time you do you will be certain to find something you have never seen before." - Alexander Graham Bell

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            • #21
              Brilliant. Thanks so much, Nessmuk. I'm sure I'll never fully understand this stuff, but you've at least restored most of what I once knew. Much appreciated!
              46/46, 12/48, 58/115
              46-R #6866

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              • #22
                Originally posted by i12climbup
                I'm not normally in the habit of reviving old threads, but this one is too good not to bring back to the top. I thought maybe the newcomers (cory, klips, cows, etc. ) would find it interesting. Thanks to Randsomscooter for starting it.
                This really is a good one AFAIC. There are a lot of folks out there who struggle this time of year and can't wait for winter solstice because the days start getting longer. It's a huge psychological boost. Well, they don't really have to wait that long, as explained in the initial post.
                Scooting here and there
                Through the woods and up the peaks
                Random Scoots awaits (DP)


                Eat, sleep, hike, repeat.

                It doesn't have to be viewtiful to be beautiful. (NL)

                "Pushing the limits of easy."

                Comment


                • #23
                  Haha! I was just thinking of this thread when I got up to go for my run in the dark yesterday morning!
                  46/46, 12/48, 58/115
                  46-R #6866

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                  • #24
                    Originally posted by i12climbup
                    Resurrection time ..... again.
                    I've been enjoying the later sunsets for the past 11 days now. Getting a lot more work done. But it's the sunrise over Hurricane (closer to Knoblock this time of year) that I see while sipping my morning coffee. Still getting later.
                    Scooting here and there
                    Through the woods and up the peaks
                    Random Scoots awaits (DP)


                    Eat, sleep, hike, repeat.

                    It doesn't have to be viewtiful to be beautiful. (NL)

                    "Pushing the limits of easy."

                    Comment


                    • #25
                      Lake Placid's latest sunrise for this year occurred on November 3 at 7:42. On January 3, 2013* the sun will rise at 7:36, then somewhat earlier from then on.

                      *Assuming the world as we know it still exists.
                      Nature we have always with us, an inexhaustible storehouse of that which moves the heart, appeals to the mind, and fires the imagination - health to the body, a stimulus to the intellect, and a joy to the soul. - John Burroughs

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                      • #26
                        Thread resurrection time again.


                        Winter hiking season begins on December 21, which this year does coincide with the winter solstice.

                        December 21 is not only the day with the least daylight this year, but is also a new moon. No moonlight.

                        The good news is that sunsets start getting later after December 10th!!

                        2014, Keene, NY:

                        Earliest sunset - 4:16pm on December 10th.
                        Latest sunrise - 7:31am on January 3rd.
                        Scooting here and there
                        Through the woods and up the peaks
                        Random Scoots awaits (DP)


                        Eat, sleep, hike, repeat.

                        It doesn't have to be viewtiful to be beautiful. (NL)

                        "Pushing the limits of easy."

                        Comment


                        • #27
                          Originally posted by randomscooter View Post
                          2014, Keene, NY:

                          Earliest sunset - 4:16pm on December 10th.
                          Latest sunrise - 7:31am on January 3rd.
                          Actually the latest sunrise is now the day before DST ends!
                          Tom Rankin - 5444W "In the depths of Summer, I finally learned that there lay within me an invincible Winter"

                          Proud Member #0003 of ADKHP Foundation
                          Volunteer Balsam Lake Mountain
                          Past President Catskill 3500 Club
                          CEO Views And Brews!

                          Trail maintainer for the Dry Brook Ridge trail from Mill Brook Road to just past the Lean-to

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                          • #28
                            Thread resurrection. :-)

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