it go through several equinoxes. The shadow from the gnomon on the disk
gets faint several days before the equinox and stays that way several days
afterward. On my dial, the reflected light from the base makes the dial
unreadable for a longer time on the bottom side. Holding a piece of paper
by the time numbers, perpendicular to the disk, allows one to read the time
during those few days before and after the equinox.
The shadow from the edge of the disk onto the gnomon is the indicator for
the equinox. That shadow moves rapidly around the time of the equinox. I
have marked representative dates on the gnomon so the sundial shows the
approximate date. The shadow of the disk edge moving from the part of the
gnomon sticking out of the top of the disk, disappearing, and showing on the
gnomon on the bottom of the disk (vernal equinox) can be followed on the day
of the equinox. Any discrepencies in the flatness of the disk or alignment
errors can be seen on that day. My dial has some small errors in flatness
that become apparent, but I can still ascertain the day of the equinox if
the sun is shining.
Chuck Nafziger
47 deg 40.7 min N
122 deg 23.7 min W
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