In Hindu astronomy, Nadivalaya Yantram is an instrument based on the meridian and then positioned on a particular latitude equivalent angle. The center of the semicircle of this contrivance contains a nail, the shadow of which shows the local time. By applying the true difference correction (of local time and standard time), the Indian Standard Time can be obtained.
From March 21 to September 22, if we position our body to
the south of this instrument, the sun will not be visible for six months at a
stretch. Between September 23 and March 20, if we stand on the northern side,
the sun will be invisible for these six months. On both sides semicircles are drawn
and nails are positioned. On whichever semicircle the shadow of the nail falls,
that particular hemisphere (northern of southern) will be the hemisphere of the
sun. Thus, it will be confirmed that from March 21 to September 22 the sun is visible
in the northern hemisphere, and from September 23 to March 20 it is visible in the
southern.
At night, the hemispherical position of the other heavenly
bodies can be ascertained. If one stands to the south side of the naidvalaya
yantra and observes a particular planet/star, the it is in the southern hemisphere.
Similarly, if one stands to the north side of the naidvalaya yantra and observes
a particular planet/star, the it is in the northern hemisphere.
Nadivalaya Yantram is therefore very important for fixing
the hemispherical position of the sun and other heavenly bodies.