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
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