We feel the heat from the Sun and we see it's light. It's energy cannot last forever, so what is the Sun's age? The first attempts at an answer were based on the age of the Earth - since the Sun and Earth are so closely connected, the age of the one gives clues as to that of the other. The first approaches assumed that the Earth was hot when made, from the material drawn out from the Sun. Yet this led on to the question of how long had it taken to cool? In 1779, the French naturalist Georges-Louis Leclerc experimented with a small-scale model of the Earth made from a small, hot, iron sphere, and in the second half of the nineteenth century, the British physicist William Thomson made theoretical calculations. Their estimates - at 75,000 years and 40 million years respectively - were gross underestimates of the true age of the Earth.
German astrophysicist Hermann von Helmholtz and the Canadian - American astronomer Simon Newcomb took a direct astrophysical tack by assuming that the source of energy for the Sun was the heat that it liberated as it settled under gravity. How long would the Sun take to contract to it's present size, starting from the nebula of gas from which it was born? The answer, 20 million years, was similar.
The modern technique to measure the age of the Earth originated in the first years of the twentieth century from work in Canada by the nuclear scientists Ernest Rutherford and Frederick Soddy on radioactive elements that break down over time into other, daughter elements. Radioactive radium and its daughter, helium, are found in the Earth's crust and Rutherford calculated how long it had taken for the one to decay to the other: it was 40 million years. Applying new, detailed knowledge about radioactive decay gathered in the first half of the twentieth century, geologist Arthur Holmes concluded from old rocks from all over the world, including Sri Lanka, that the Earth was 1.6 billion years old. The so - called Genesis rock retrieved by the Apollo 15 astronauts from the surface of the Moon is 4.5 billion years old. The most modern figure for the age of the Earth is 4.6 billion years, and that the Sun is a little older, being the Earth's parent body.