Hi Brent, The spiral is not at a constant angle that could be incorporated into a dial. It is true that the sun crosses the equator at only one point (time) during an equinox. The crossing can actually be noticed over the period of a day on an accurately aligned equatorial sundial. But at the solstices, the "spiral" which has been reducing its pitch, stops going up or down, and reverses itself. That event is so slow that it cannot be seen on a sundial. "Solstice" means the sun stands still; it is at the top or the bottom of a sine wave and as the Earth rotates under it, it seems to stay parallel to the equator. Equatorial dials with 360 degree disks cast a shadow on the gnomon. That shadow rotates around the gnomon for a couple of days around the solstices without a noticeable change in height; the spiral has stopped and is changing direction. Chuck Nafziger
When you see an eagle in flight, think of freedom. Remember the freedom we once had under the Constitution and hope that we will get the wisdom and courage to take it back. ---------------------------------------- > Date: Sun, 13 Mar 2011 16:37:04 -0700 > From: [email protected] > To: [email protected] > Subject: Re: Spiral cut earth > CC: [email protected] > > Ok great, now we are getting somewhere. > > So when we align our sundials to the north that also means > we are aligning it East and West. > Since the line of the sun from sunrise to sunset is not > directly East to West but slightly skewed (explained by the > spiral). Wouldn't it make for a more accurate sundial to > align it to account for that skew? > > Also, when you say on the equinox the sun shines directly on > the equator don't you mean the sun shines on one spot on > the equator before moving north or south on it's spiral > path? > > > thanks; > brent > > > On 3/13/2011 4:19 PM, Roger Bailey wrote: > > Yes, this experiment shows the line for any given date and > > declination follows the latitude. It is a circle but not a > > Great Circle as shown with the string. The plane of a Great > > Circle goes through the centre of the earth. Latitude > > circles do not. > > > > Yes, the sun moves with latitude as the solar declination > > changes. On the equinox, the sun shines directly on the > > equator. At the solstices, it shines directly on the > > tropics, Cancer and Capricorn. So yes. the suns path traces > > a spiral path up to one tropic line and down to the other > > and back each year. > > > > Regards, Roger > > > > -------------------------------------------------- > > From: "Brent" > > Sent: Sunday, March 13, 2011 3:17 PM > > To: "Roger Bailey" > > Cc: > > Subject: Re: Spiral cut earth > > > >> Hi Roger; > >> > >> well I did that experiment. > >> I'm not sure I did it right or understood what you are > >> trying to teach me. > >> > >> The light travels along latitude lines as the globe turns. > >> When I pull a string between two points it comes off the > >> latitude line and is straight. > >> > >> That's what I expected. > >> > >> So the line between sunrise and sunset would be straight > >> in this experiment. > >> However, the earth is moving around the sun and I didn't > >> move the globe. I think if I moved the globe while it was > >> turning my light would scribe the spiral line. > >> > >> Otherwise, how would the sun move from latitude to latitude? > >> > >> Maybe I am not missing something or maybe I am not > >> explaining myself. > >> > >> What am I confused about? > >> > >> thanks for your patience. > >> brent > >> > >> On 3/13/2011 12:38 PM, Roger Bailey wrote: > >>> Hi Brent, > >>> > >>> Just do it!. Get a globe and a cheap laser pointer from a > >>> dollar store. Shine the pointer on the globe and rotate the > >>> globe. Note several points. Then take a string and stretch > >>> it over the points. Is it the same line? A great circle? > >>> > >>> By the way, The shadow from any point is a straight line > >>> west to east on the equinox at all latitudes. Ok there is a > >>> slight shift as the declination changes a bit during the > >>> day. Again, just do it. Put a stick in the ground and mark > >>> the shadow on the equinox from dawn due east and sunset due > >>> west. OK this does not correct for refraction and > >>> semi-diameter. At theoretical sunrise or set, you can just > >>> place one full sun diameter between the sun and the horizon. > >>> > >>> Regards, Roger Bailey > >>> > >>> -------------------------------------------------- > >>> From: "Brent" > >>> Sent: Sunday, March 13, 2011 8:46 AM > >>> To: > >>> Subject: Re: Spiral cut earth > >>> > >>>> If the sun had a laser from its' center pointed directly at > >>>> the center of the earth it would scribe that spiral line I > >>>> was asking about. > >>>> So maybe that spiral line is a time line? > >>>> It would be local high noon under each spot the laser lit > >>>> and advance one day each revolution. > >>>> > >>>> Sorry for the flurry of emails. > >>>> > >>>> On 3/13/2011 8:24 AM, Brent wrote: > >>>>> Put another way; > >>>>> > >>>>> When they say on the equinox the sun is directly above the > >>>>> equator maybe that is true for only one particular > >>>>> longitude on the equator. > >>>>> > >>>>> The equinox is not a day, it is a precise moment. > >>>>> > >>>>> So if I was on the equator on the day of the equinox and > >>>>> marked the sunrise and marked the sunset and drew a line > >>>>> between them it would not be east to west but slightly > >>>>> skewed. > >>>>> > >>>>> As I watch the sun it moves slightly north or south > >>>>> throughout the day depending on the season. > >>>>> > >>>>> Doesn't this mean that our sundials should be adjusted > >>>>> accordingly? Wouldn't they work more accurate if we had > >>>>> them aligned not east to west but slightly skewed? > >>>>> Of course we would have to realign them each season. > >>>>> > >>>>> > >>>>> > >>>>> On 3/13/2011 7:49 AM, Brent wrote: > >>>>>> > >>>>>> I was thinking that the sun tracks a straight line across > >>>>>> each latitude line. > >>>>>> So at an equinox the sun will be directly above the > >>>>>> equator > >>>>>> for one earth revolution and the next day it will shift a > >>>>>> few degrees and be tracking a higher latitude. > >>>>>> > >>>>>> But that can't be right, we don't jump from one > >>>>>> latitude to > >>>>>> the next. The transition must be smooth from one > >>>>>> latitude to > >>>>>> the next. The latitude tracking lines would look more > >>>>>> like a > >>>>>> spiral cut ham. > >>>>>> > >>>>>> So if I track the sun across my sky on any day, it is not > >>>>>> moving exactly east to west but slightly skewed > >>>>>> depending on > >>>>>> the season. > >>>>>> > >>>>>> Is this right? > >>>>>> If so, what do you call that spiral line? > >>>>>> > >>>>>> brent > >>>>>> > >>>>>> --------------------------------------------------- > >>>>>> https://lists.uni-koeln.de/mailman/listinfo/sundial > >>>>>> > >>>>>> > >>>>> > >>>>> --------------------------------------------------- > >>>>> https://lists.uni-koeln.de/mailman/listinfo/sundial > >>>>> > >>>>> > >>>> > >>>> --------------------------------------------------- > >>>> https://lists.uni-koeln.de/mailman/listinfo/sundial > >>>> > >>> > >> > > > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
