Empirical Earth · The Sky — Sun, Moon & Stars

Sundials — a globe in the garden

The claim

“A sundial only works with a local Sun over a flat Earth.”

What is measured

A working sundial is a globe in miniature. Its shadow edge points along the planet’s axis, and the tilt you set it to is your own latitude.

What would show this is wrong

a sundial whose gnomon is set at some angle other than the local latitude and which still keeps correct time through the year; or a single dial, unadjusted, that reads correctly at two widely separated latitudes. Every dial ever cut has had to be cut for its own latitude, and a dial carried north or south stops telling the truth. That is the sphere, in the garden, in brass.

Sources

  1. Sundial design: the gnomon and the celestial pole. On a horizontal sundial the gnomon (the shadow-casting edge) is set at an angle equal to the site’s latitude and points at the celestial po link
  2. Horizontal sundial hour-line formula. The angle of each hour line from the noon line is θ = arctan(sin φ × tan H), where φ is the latitude and H is the Sun’s hour angle at 15° per hour from link
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