Sun, Earth, Time, Day and Year

Probe and Ponder

Close your eyes and picture yesterday morning. How did you know it was morning and not midnight? Most likely the Sun was already up, warm light pouring through the window, birds calling. But suppose you had no clock, no phone, no watch — how would you tell that three hours had passed? Or that it was almost time for lunch?

In this chapter we step outside and look at the sky — the oldest clock and calendar ever invented. The Sun, the Moon and the stars have been ticking above our heads since long before humans existed, and ancient Indian observers learnt to read them with astonishing precision.

11.1 The Earth Spins — and a Day is Born

Stand still in your school ground at sunrise. In a few minutes you will see the Sun climb a little higher. By noon it is overhead. By evening it slides down to the west and disappears. For most of history people believed the Sun was moving around the Earth — a reasonable guess, because that is exactly what our eyes show us.

We now know the truth is the opposite. The Earth rotates on its own axis — an imaginary line running from the North Pole to the South Pole — completing one full turn in about 24 hours. As our part of the Earth swings round to face the Sun, we call it day. When it swings away, we call it night. The Sun has barely moved; it is we who are turning.

Why Does the Sun Appear to Move from East to West?

The Earth spins from west to east, so the edge of our view slides eastwards. Whatever is in the sky — Sun, Moon, stars — therefore seems to drift in the opposite direction, from east to west. The rising Sun always appears in the east; the setting Sun always in the west. Many Indian languages even capture this: Pūrva (east) comes from "in front", where the day begins.

11.2 The Sundial — The First Clock

Long before mechanical clocks, ancient people noticed that the shadow of a stick in the ground changed direction and length as the Sun moved across the sky. A pointed stick stuck upright in level earth could, with a little marking, be turned into a clock. This simple device is called a sundial.

At sunrise the shadow is long and points westwards. As the morning advances, the shadow swings towards the north (in the Indian subcontinent) and shortens, reaching its shortest length near noon. By afternoon it grows again and points eastwards, lengthening until sunset. Markings drawn on the ground turn these changes into readable "hours".

Jantar Mantar — India's Giant Open-Air Clocks

In the early 18th century, Maharaja Sawai Jai Singh II of Jaipur built a series of stone observatories called Jantar Mantar at Delhi, Jaipur, Ujjain, Varanasi and Mathura. Each houses giant sundials — stone triangles and circles built so large that they can measure time to within a few seconds. The Samrat Yantra ("Supreme Instrument") at Jaipur is the tallest sundial on Earth.

11.3 The Earth Revolves — and a Year is Born

While Earth spins on its axis, it also travels along a wide oval path around the Sun. This orbital journey is called revolution. One complete trip around the Sun takes approximately 365 days and 6 hours.

The Problem of the Quarter Day — and the Leap Year

If a year were exactly 365 days, everything would be simple. But it is 365 days plus roughly 6 hours. If we ignored those 6 hours every year, after 4 years the calendar would be 24 hours behind the Sun — our summer would start shifting into spring. Within a century, seasons and months would fall completely out of step!

The clever solution: ignore the 6 extra hours for three ordinary years, then add all the saved hours (6 × 4 = 24 = one full day) as a single extra day in the fourth year. That extra day is February 29, and such a year is called a leap year.

The Saur Maas — India's Solar Month

Ancient Indian astronomers divided the Sun's yearly path through the sky into 12 equal parts, one for each zodiac sign (rashi). The time the Sun takes to travel through one zodiac sign is the Saur Maas — the solar month — and it lasts about 30 days. The day the Sun crosses from one sign into the next is called a Sankranti. Makar Sankranti, celebrated in January, is the day the Sun enters Makara (Capricorn).

11.4 The Sun Across the Year

Because Earth's axis is tilted by about 23.5°, the Sun does not rise at exactly the same point on the eastern horizon every day. In summer it rises a little to the north of due east; in winter a little to the south. Ancient observers who watched sunrise from a fixed spot over many months could track the year almost as precisely as a modern calendar.

This is also why days are longer in summer and shorter in winter: the tilt means the Sun's path through the summer sky is high and long, while in winter it is low and short. The two special days when day and night are exactly equal — the equinoxes — occur around 21 March and 23 September each year.

11.5 What We Owe to the Sun