As you walk into the observatory, towards the region where all the instruments are located, turn left and walk towards the Laghu Samrat Yantra, ... the smaller of the sundials in the observatory.
This is a sundial that can give time to an accuracy of 20 seconds.
For the Laghu Samrat Yantra as well as the Brihat Samrat Yantra, a triangular wall acts as the Gnomon of the instrument. A gnomon is any device whose shadow would help us know the position of the Sun in the sky. The shadow of this wall, falls on the arcs placed on either side of the wall, and tells the time. These arcs are known as the Quadrants.
The triangular wall is placed exactly in the North South Direction. Many of the observatory instruments, you will notice, have such a North-South alignment. The angle in the inclined wall of the instrument is meant to be equal to the latitude of this location.
The shadow of the triangular wall, falling on these circular arcs, moves equal distances in equal time intervals. It is for this reason that the sundial is known as an equinoctial (or an equal hour) sundial.
Let us now, look at the way that the Laghu Samrat Yantra tells the time.
When the shadow of the triangular gnomon falls on the upper end of western quadrant, it is 6:00 AM local solar time. At Solar Noon, the shadow disappears from the western quadrant and reappears on the eastern quadrant. At 6:00 PM Solar time, the shadow hits the topmost point of the scale marked on the eastern quadrant.
The quadrant arcs, therefore, are divided into 6 hours each, for the morning and the afternoon segments. The hours are sub divided into 1 minute divisions that are further subdivided into three 20 second divisions. The instrument can therefore read solar time to 20 second accuracy.
Solar time, however, is not the same as our clock time. There is a correction factor that has to be added to the solar time to convert it to the clock time. This correction factor is different for different dates of the year. The black board placed under the Gnomon of the instrument, gives the correction factor for the day.
So, go ahead, observe the shadow on the quadrant carefully, note the solar time, add the correction factor and then check with the correct clock time. You will be surprised by the accuracy of this 300 year old instrument!
The Laghu Samrat Yantra has one other functionality. It can measure the angular position of the Sun in the sky, with respect to the Equator. This angular position with respect to the equator, for any celestial object, is known as its Declination.
Through the year, the Sun goes through the Uttarayana (the Northward) and the Dakshinayana, (the Southward), movements, as we know. On the days of the equinoxes, the Sun is perpendicular over the Equator and its angular distance from the Equator would then be zero. It moves Northward in summer, reaching a maximum angle of 23.5 degrees North of the Equator, on the day of the Summer Solstice. In winter, it moves southward, reaching a maximum angular distance from the equator, of 23.5 degrees South.
These Uttarayana and Dakshinayana movements of the Sun can be easily measured, using the Laghu Samrat Yantra, using the scales marked on its triangular wall.
A simple and elegant instrument that can easily tell the time and measure the Uttarayna and the Dakshinayana movements of the Sun this is the Samrat Yantra!
We discussed that the angle in the inclined wall of the Laghu Samrat Yantra is meant to be equal to the latitude of this location, isnt it?
What is achieved by such a special geometry?
Such an inclination of the wall of the instrument ensures that it is parallel to the axis of rotation of the Earth. ... The stairs that seem to lead nowhere, actually point towards the North Celestial Pole this is the region of the sky reached when we extend the axis of rotation of Earth, in the direction North. ... Stand at the bottom of the stairs and look along the surface of the wall at night, ... you should see Pole star or the Dhruvtara in that direction.
The wall is placed exactly in the North South Direction. ... Many of the observatory instruments, ... you will notice, ... have such a North-South alignment. ... The circular arcs that are placed on either side of the Laghu Samrat Yantra, ... are perpendicular to the wall ... and ... are parallel to the Equator of the Earth. ... The shadow of the triangular wall, ... falling on these circular arcs, moves equal distances in equal time intervals. It is for this reason that the sundial is known as an equinoctial (or an equal hour) sundial.
Such a shadow movement equal distances in equal hours - happens only when the Gnomon is pointing to the North Celestial Pole and the shadow falls in an arc parallel to the equator. When you place a pole in an arbitrary direction, its shadow does not move equal distances in equal intervals of time. You can sense that by studying the shadow movement around a cricket wicket or any vertical pole.
Let us now come to the other functionality of the Laghu Samrat Yantra, the Declination measurement. Declination is the angular distance of an object in the sky, from the Celestial Equator. (Celestial Equator is the projection of the equator of the Earth, on to the sky).
Declination is measured by holding a small rod over the scale projecting outwards, in such a way that its shadow coincides with one of the edges of the quadrant below.
The markings on this triangular wall are in degrees, which have been divided into 6 parts that are further subdivided into 5 parts. The Declination can, therefore be read to an accuracy of 2 minutes of arc.
There are two zero markings on the scale, towards the middle of the instrument. For measurements during winter, use the zero marking on the upper part of the instrument. For measurements in summer, use the zero marking on the lower part of the instrument. Place a small rod (or a pen) jutting outwards from the surface of the scale, so that, its shadow falls on the quadrant of the instrument. Move the rod around, until the shadow falls on the Northern edge of the quadrant (for winter measurements) or the Southern edge of the quadrant (for summer measurements). The reading on the Declination scale, where the rod rests, gives the Declination of the Sun, on that day.