The Sun crosses the dials on the days of equinox (March 21 and Sept 22). The dials can be used to read the time for six months each. Thus the instrument indicates the time of the year when the Sun is to the north or south of celestial equator.
Nadivalayas are seen in Jai Singh’s Observatories at Varanasi, Jaipur and Ujjain.
Nadivalaya is an equatorial sundial but of a different design. It performs the same function of indicating the local solar time.
It consists of two semicircular plates fixed on a masonry stand. The plates are oriented parallel to the equatorial plane. The gnomons are rods fixed to the centres of the semicircular plates. The gnomons are perpendicular to plates and hence one points towards the north pole and the other to the south pole.
Earth’s rotation axis is tilted by 23.50. As a result, the Sun does not rise exactly at East and set in the West on all the days of the year. Or in other words, the apparent path of Son in the sky will be in the planes parallel to the equator everyday. It crosses the equator only twice in an year on March 21st( Vernal Equinox) and September 22nd( Autumnal Equinox). Theses days are called equinoxes because the duration of the day and night are the same on those days.
Between March 21st and September 22nd , the Sun will be in the Northern hemisphere and between September 22nd to March 21st in the Southern hemisphere. Nadivalaya is an effective tool for demonstrating the above phenomenon.
After March 21st, the sun rays will illuminate the northern face and the shadow of the gnomon can be seen on this plate. This plate is called the Summer dial.
After September 22nd, the southern face will be illuminated an dthe shadow of the gnomon too falls on this plate. This plate is called the winter dial. On the days of the equinoxes, since the Sun moves exactly on the equator, we do not see the shadow of the gnomon on any plate. Therefore the instrument is very useful in fixing the days of equinox.