Bulbous gnomon

Why is the gnomon of that sundial on Mars so round and bulbous? The
sundial also doubles as a camera calibrator, but the gnomon had to be
painted black after it was built because the original metal was too
shiny - therefore, the gnomon must not be part of the calibration
features of the sundial. So, why is it relatively so thick?

"Axel Harvey" wrote in message
om...
Why is the gnomon of that sundial on Mars so round and bulbous? The
sundial also doubles as a camera calibrator, but the gnomon had to be
painted black after it was built because the original metal was too
shiny - therefore, the gnomon must not be part of the calibration
features of the sundial. So, why is it relatively so thick?

According to http://www.planetary.org/mars/tpr_marsdial.html

"We also placed a post in the center of the plate to cast a shadow across
some of the materials; this allowed us to measure the contributions of
direct sunlight and of the diffuse skylight that fills in the shadow
region".

The ball acts as a nodus (specific position marker) on the gnomon for when
the sun is high, otherwise there is a 'daisy petal' marker lower down the
stem for use when the sun is lower.

The elements of the marsdial had to be fairly 'chunky' to allow the
calibration process to measure the differences between the direct sunlight
and diffuse skylight across the range of the marsdial background colours.

Thanks for the interesting question.