The ball is the nodus, the post is the gnomon. A nodus is formed where the subcostal vein meets the leading edge of the wing. The Republican period and the "nodus" style was particularly common. Some sundials use both a style and a nodus to determine the time and date. This star was also known as "Nodus I" or "Nodus Primus". The mirror acts like a nodus, casting a single spot of light on the ceiling. It bore the traditional names "Aldib", "Altais" (the goat) and "Nodus Secundus". In the very similar beautiful demoiselle ("Calopteryx virgo"), the dark patch starts before the nodus. In Imperial iconography the nodus coiffure was associated predominately with the women of Augustus' household. The exterior of the building contains an example of a Nodus Sundial.