Astro*Dictionary
by Michael Erlewine

1 article for
"Placidian Arcs"

The first modern system of primary arcs or directions. It was developed by Placidus di Titus, an Italian monk of the 17th Century. It is basically a reworking of Ptolemy's "modes of prorogation", using the then recently discovered methods of spherical trigonometry. It is an accurate mathematical representation of the formula "one degree = one year". He called these equatorial arcs Primary Arcs or Arcs of Direction, a terminology which is still in use.

 

See also: ♦ Arc of Direction ♦ Directions ♦ Prorogator ♦ Equatorial Arc