1. ## Shadow of sundial a straight line on equinox day?

Hi, I couldn't derive or visualize this using simple geometry.

Does the shadow of a simple sundial form a straight line plus/minus a
few hours around equinox (at all latitudes)? And a curve at all other
times?

Thanks.

3. ## Shadow of sundial a straight line on equinox day?

On Fri, 27 Mar 2009 20:04:41 -0700 (PDT), Peter Lewis
<kingkong88@gmail.com> wrote:

Only if your sundial is at the equator. While the Sun rises in the east
and sets in the west everywhere on Earth (on the equinox), it only
crosses the zenith at the equator. Consider an extreme example of a
sundial at one of the poles. The Sun makes a complete circle around the
horizon. That's going to produce a sundial shadow that's nearly a
perfect circle.
_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

5. ## Shadow of sundial a straight line on equinox day?

Sam Wormley wrote:

Here is the original reference: http://solarclockgarden.truman.edu/sundial.htm

6. ## Shadow of sundial a straight line on equinox day?

On Mar 28, 12:10 pm, Sam Wormley <sworml...@mchsi.com> wrote:

Thanks for anticipating my next question, and beating me to it.

7. ## Shadow of sundial a straight line on equinox day?

Mmm..., the puzzle deepens.

The example at http://solarclockgarden.truman.edu/sundial.htm is
clearly not at the equator. Seems to be in conflict with Chris'
explanation.

After thinking about it for a bit more, Chris, your polar example
would produce no shadow as the sun rays at the gnomon would be
parallel to the ground at the pole. Am I correct?

8. ## Shadow of sundial a straight line on equinox day?

Peter Lewis wrote:

Flat horizontal surface - Equinox Shadow tip follows a straight line.
http://www-groups.dcs.st-and.ac.uk/~.../Sundials.html

Other dates hyperbolic paths (between Arctic and Antarctic circles).

9. ## Shadow of sundial a straight line on equinox day?

On Fri, 27 Mar 2009 21:33:08 -0700 (PDT), Peter Lewis
<kingkong88@gmail.com> wrote:

Well, that's not a very typical sundial. It has a vertical gnomon, not
one pointing towards the pole. I don't see how the equinox shadow could
be as indicated (since on that day the Sun rises and sets very near
cardinal east and west), unless it has something to do with the shape of
the surface below the gnomon. The report says it is on some sort of
slope.

Well, it depends. Refraction actually brings the Sun higher, so it
circles a little above the horizon. And even if there were no
atmosphere, the Sun is an extended object, so there is a penumbral
shadow that is closer that the horizon.

The pole is a sort of singularity, and it might be confusing. But you
can imagine that you are just a little below the pole, where the Sun
will rise in the east, travel across the sky crossing the meridian just
a little above the southern horizon (for the north pole), and set in the
west. It seems pretty clear that the gnomon's shadow isn't going to
trace a straight line under those circumstances.
_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

10. ## Shadow of sundial a straight line on equinox day?

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