I think we all agree the sun rises (appears to rise) due east on the
equinox.
If the earth stops and the sun stops and I start walking towards the sun
(on the surface of the earth)
I believe I would be walking a straight line - due east.
And I would not be adjusting my direction each step, I just keep
marching towards the sun.
So would anyone argue that I am not walking due east?
And I am not walking along a latitude line.
I also understand the argument about always changing your direction as
you move by using a compass.
So maybe there are/two /due easts.
One due east is a local due east, a compass due east, a magnetic due east.
The other due east is kind of a solar due east, the
relationship/position between the earth and the sun.
Which has lots to do with our sundials.
So maybe we should forget about magnetic due east when discussing sundials.
ha!
brent :)
On 9/15/2015 10:12 AM, Steve Lelievre wrote:
Brent,
My 2 cents worth...
If by tilt you meant the obliquity of the ecliptic, that doesn't
affect things.
Your question is about walking on a globe (3D), but I suspect you're
imagining it like a flat map (2D). A map (assuming the map is a
Mercator Projection) represents latitude as a straight line but when
you move on the surface of the Earth you're not really following a
line that is straight in 3D terms.
The line of latitude is a really a circle representing a sectional
slice through the globe. To compensate for unbending a circle in 3D to
be a straight line on a 2D map, something else has to get distorted:
shape. That's why on a map Greenland looks huge compared to other
countries and is so wide at the top. On a globe it looks smaller and
more like a rectangle than a very wide triangle. The extreme cases are
the North and South Poles which are points on a globe but on a map get
stretched out to be a line - they would take up the whole top or
bottom edge of the map.
When you move on the surface of the Earth, following a true 3D
straight line would take you off into space. You can see that by
placing the middle of a ruler on the surface of a globe. The ends of
the ruler are not touching the globe, are they? If the point of
contact is at a Pole, then gradually tipping the end of the ruler down
towards the globe's surface will always make the point of contact
follow a line of longitude. If you start elsewhere on the globe and
initially point your ruler East, then rolling the point of contact
will make it cross lines of latitude as varying angles as you get
closer to the equator.
Thus, to walk due East (i.e. perpendicular to a North-South line) on
the Earth, after each step you have to recalculate where East is and
make a minute adjustment in your direction. A magnetic compass does
that for us automatically; as well, maps show East-West as linear. The
consequence is that we're used to thinking that we're moving in a
straight line, but really we're turning slightly after each step.
Cheers,
Steve
On 15/09/2015 07:10, Brent wrote:
I'm confused maybe.
I live in the northern hemishpere and anticipating the equinox on the
23rd.
Supposedly the sun will rise due east.
So if due east is a right angle from north south and I traveled due
east I would not follow my line of latitude.
I would get further and further south of my latitude the further I
traveled.
So either the lines of latitude are not east west lines or due east
is not a straight line but curved.
I suspect lines of latitude are not east west lines?
They would work fine if the earth was not tilted, but it is.
Wouldn't it make sense to coordinate the globe so lines of latitude
(or call them something else) are straight and a right angle
from north south?
brent
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