sundial & Earth's tilt questions

On 26 Aug 2003 14:25:20 -0700, (cgbusch) wrote:

Horizontal Dials
The plane of the shadow-receiving surface is horizontal.

Equatorial
The plane of the shadow-receiving surface is parallel to the equator.

Analemmatic
Time is told by the sun's azimuth on a specific date.

With horiz dials the face is a flat plane cut through a cylinder at
the angle of the latitude. With an equatorial dial, the dial is at
the angle of the latitude. My question is this, should the tilt of
the Earth along its rotational axis (23.45°) be included? With an
Equatorial dial, if you are at 45 deg lat, and it is winter, shouldn't
the dail be set up varying from 68.45 deg in the winter to 21.55deg in
the summer?
No, just keep it like that. To stay accurate, any SunDial should be slightly
moved everyday... This is the "equation of time" caused by the fact that
Earth's orbit around the Sun is an ellipse. That equation is a bit "complex"
but your Sundial will be accurate on april 16, june 15, september 2 and december
26. At most, it will be late about 14 minutes on february 16, then fast by 4
minutes on may 15, late again by 6.5 min. on july 27 and finally fast by 18 min.
on november 4. Of course, these are only approximations as orbitals elements on
Earth changes constantly...
Benoît Morrissette