December 17, 2024

Sundials are horological devices that indicate the time of day using the shadow cast by the apparent position of the sun. Used as early as 1500 BCE in Ancient Egypt and Babylon, sundials were among the only means to measure time prior to the advent of modern-day clocks. India has been a pioneer in the science and engineering of sundials, as evident from the 18th century time-keeping instruments in Jantar Mantars across the subcontinent, including the Vrihat Samrat Yantra of Jaipur, the largest stone sundial in the world. Although rendered practically obsolete by modern technology, sundials remain fascinating scientific tools, offering insights into not only the history and evolution of science, but also the celestial mechanics and orbital motions of the Earth.

In November 2024, a team consisting of members from SPL, TTDG and CMD of VSSC successfully designed and installed an accurate and fully functional sundial at the Rocket Garden of VSSC Space Museum, Thumba (8.53°N, 76.86°E). Unlike common sundials that only indicate the local solar time, this is a “Polar Sundial with Analemmatic Correction”, which accurately gives the Indian Standard Time (IST) as well as the date. Being among the exclusive few analemma-corrected sundials in the world, this system features a unique design that enables simultaneous time and date reading throughout the year using a single dial and pointer.

The sundial adopts a Polar configuration, with the dial plate (of size 4 ft x 4.2 ft) affixed on an acrylic wedge structure having a precise inclination of 8.53° as dictated by the local latitude of Thumba. Giving the quintessential feel of TERLS, a 3D-printed model of the RH200 sounding rocket (~1.6 ft tall; 1:7.4 scaled-down model) is used as the style for casting the shadow, fixed perpendicular to the inclined plate. The entire sundial structure is mounted on a 6 ft dia. concrete platform, ensuring exact alignment with the true North-South direction. The direction of the True Geographic North was geometrically determined by the Method of Perpendicular Bisectors (using the symmetric motion of the sun about the local meridian), avoiding errors from relying on the conventional magnetic compass for north determination. Thus, in this configuration the sundial plate is exactly parallel to the Earth’s rotation axis (polar axis), with the rocket-shaped style parallel to the Earth’s equatorial plane, pointing to the intersection of the celestial equator and local meridian.

Typical sundials have straight hour lines that can only indicate the local time at the installation site, which can be significantly different from the standard clock-time. This is not of much use today, with the Indian Standard Time (IST) being the accepted standard for civil time throughout the country. To obtain the actual time as seen on our watches, a systematic correction is incorporated to account for the sun’s apparent seasonal motions. During the course of the year, the sun exhibits seasonal variations in its path due to 2 reasons: (i) Earth’s axial tilt of (~23.44º with the ecliptic) results in an apparent north-south motion of the sun about the equator; (ii) Earth’s orbital eccentricity (~0.0167) results in an apparent east-west motion of the sun due to the speeding up or slowing down of the orbital motion. In combination, the two phenomena lead to an annual figure-of-8 motion of the sun in the sky, called the “Analemma” (discernible upon viewing the sun at the same time of day over one year), which manifests as the Equation of Time: the difference between local solar time measured by sundials and mean time measured by modern clocks.

To capture this systematic behaviour, the sundial design is enhanced by introducing inverted analemma curves instead of straight hour lines. This built-in “Analemmatic Correction” incorporates the Equation of Time into the sundial, automatically converting local solar time to mean solar time throughout the year. To further obtain the Indian Standard Time (defined at 82.5ºE longitude, passing through Mirzapur, UP), an additional longitude correction is applied, shifting the curves westward by ~22.53 minutes, as dictated by the local longitude of Thumba. This enables the sundial to give the exact Indian Standard Time (IST) as seen on clocks and watches all across India.

The sundial uses red and blue colours to indicate which halves of the analemma curves are to be used in each season, enabling round-the-year measurements. The added advantage of analemmatic correction is that the sundial is now capable of indicating the date: the horizontal declination curves indicate the start of each month, with the northern & southern extremes marking the solstices, and the central line marking the equinoxes. To improve readability, the analemma curves are limited to 15-minute intervals, and declination curves to the 1st of each month, though a simple visual interpolation can indicate the times and dates that fall in between.

The sundial design was validated using an in-house developed scaled-down prototype model, following which the finalized design was externally fabricated. The full-scale system was installed on 27 November 2024 and extensively tested under solar illumination, confirming a near-exact match of the indicated time with the IST (accurate to within one minute), as well as the date of the year.

On 29 November 2024, the sundial was demonstrated to Dr. S. Somanath, Chairman, ISRO, during his visit to the VSSC Space Museum, in the presence of Dr. S. Unnikrishnan Nair, Director, VSSC, along with Dr. Divya S. Iyer IAS, MD, Vizhinjam International Seaport, Shri T. P. Sreenivasan, former Indian Ambassador, and other dignitaries. Chairman appreciated the unique capability for simultaneous time and date measurements from such a simple system.

The sundial is open to visitors at the VSSC Space Museum, who are welcome to experience it, preferably during clear-sky sunny conditions. Presently, this is the only reported analemma-corrected sundial in India, that gives both the Indian Standard Time and date, with pointers indicating the polar & equatorial planes, and providing insights on the annual migration of the Sun and orbital motions of the Earth. The system is sure to strike a chord with curious minds, standing as yet another testimony to the marvels of our cosmos.

Team Members