three-minute-long Surya Tilak ceremony — where a beam of sunlight was projected onto the forehead of Lord Ram’s idol — was performed at noon on April 17 in Ayodhya’s newly-built Ram temple, marking the occasion of Ram Navami. This was achieved using a special mirror-lens arrangement, designed by astronomers at the Indian Institute of Astrophysics (IIA), Bengaluru, that has been installed in the building by a team from the Central Building Research Institute, Roorkee. THE CHALLENGES There were two major challenges in front of the team enabling it: CHALLENGE 1: Predict the sun’s position every year. CHALLENGE 2: To channelise Sun’s Rays on the forehead. CHALLENGE 1: PREDICT THE SUN’S POSITION EVERY YEAR When following the Gregorian calendar, the date of Ram Navami (the Hindu festival celebrating the birth of Lord Ram) varies each year. This is because the Gregorian calendar is a solar calendar (based on Earth’s revolution around the Sun — one revolution equals to one year), while the Hindu calendar is a lunar calendar (based on the Moon’s revolution around Earth — one revolution equals to one month). A solar year has roughly 365 days, whereas a lunar year is roughly 354 days long. CHALLENGE 2: CHANNELISE SUN’S RAYS ON FOREHEAD The IIA team, which began designing the apparatus three years ago, proposed a four-mirror-and-four-lens array for this purpose. Surya Tilak On Ramnavmi explained for UPSC HOW WILL THE SYSTEM ADJUST TO SUN’S POSITION EVERY YEAR? Given that the sun’s position vis-à-vis the first mirror will vary each year, a specially designed 19-gear system was built to make requisite adjustments. Once a year, one tooth of the gear will have to be manually turned so that the angle of the pickup mirror (first mirror) is adjusted, and it is ready to capture the Sun’s rays. WHY 19 TOOTH GEAR BOX? The number 19 was chosen to correspond to the Metonic cycle — a period of 19 years in which there are 235 lunar months, after which the Moon’s phases recur on the same days of the solar year. This means that every 19 years, the system will effectively reset, and the cycle will then begin again.