Article of the month October

Equator projection sundials 3

In the previous two months, we described projections onto the horizontal plane which resulted in an elliptic or circular hour scale.
The present projection is parallel to the equatorial plane, resulting in a linear hour scale.
The gnomon is in the equatorial plane, and meets the horizontal at an angle of 90 - phi degrees.
A disadvantage of this projection is that the sundial cannot be read during the equinoxes, because then the shadow of the entire gnomon coincides with the hour scale.

In this sundial, there occurs retrograde motion. During the early morning and late evening hours, the shadow moves in the opposite direction.
The points where the shadow changes direction lie at 6 and 18 hours apparent time.

Retrograde motion also occurs with elliptic or circular hour scales, if the date scale extends outside the ellipse or circle.