Hi! I did a simulation using Starry Night on this question, and the result can be viewed here: http://nordnorsk.vitensenter.no/himmel/solursida/DecChanges.php This simulation is done standing on the North Pole, but as the Earth is close to being a point when viewed from the Sun, this is about the same as being at the centre of the Earth standing parallell to the rotation axis. The images show the sky with zenith (= polar point) in the centre. The horizon is a bit in from the edge.
The delination of the Earth is changing abot 0.4 degree per 24 hour at equinoxes and the paht of the sun makes a slow spiral up to higher declinations. At solstice the declination hardly changes at all and the sun seems to make a circle. In reality this circle is a tight spiral working up to the top then slowly down again. On the other hand, there is no need for adjusting the sundial for this as it is the hour angle of the sun that is important, not the declination - as long as your gnomon is parallell to the rotation axis of the Earth. Atmoshperic refraction might influence the hour angle close to sunset at lower latitudes, but this changes with air preassure and humidity of the air (if I remember correctly) and might wary from day to day depending on the weather. (Hm - if I had the time I should do some calcullations on the effect of atmospheric refraction on sundial time - but I don't have the time.) Best Anne Bruvold 69:40N 18:56E -----Opprinnelig melding----- Fra: [email protected] [mailto:[email protected]] På vegne av Brent Sendt: 14. mars 2011 00:37 Til: Roger Bailey Kopi: [email protected] Emne: Re: Spiral cut earth 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" <[email protected]> > Sent: Sunday, March 13, 2011 3:17 PM > To: "Roger Bailey" <[email protected]> > Cc: <[email protected]> > 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" <[email protected]> >>> Sent: Sunday, March 13, 2011 8:46 AM >>> To: <[email protected]> >>> 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
