Dating on the earth
In the early twentieth century, it was discovered that some chemical elements decay into others at highly stable rates.
By measuring these rates, and the relative amounts of parent and daughter atoms in a rock, scientists could measure how long it had been since the rock solidified.
We take a star’s worth of gas held together by gravity, calculate its structure, and then follow its evolution over millions or billions of years.
The process relies on a lot of measurements and simplifying assumptions—from the temperature-dependent rates of many different nuclear reactions, to the absorbing and emitting properties of atoms under temperatures and pressures inaccessible on Earth, to the treatment of convection and rotation in the stellar interior.
Lord Kelvin calculated that the Sun could only have sustained its current luminosity for about 20–40 million years. It remained a puzzle until the discovery of nuclear fusion, the Sun’s actual energy source, in the 1930s.
Creation dating required careful accounting of the chronology given in Genesis and then matching it to historical events recorded elsewhere. Charles Lyell popularized the concept of uniformitarianism in the mid-1800s and argued that the Earth had to be very old indeed.“How big” is almost always an easier question to answer than “how old.” Though we can measure the sizes of animals and plants easily enough, we can often only guess at their ages. The ancient Greeks Eratosthenes and Aristarchus measured the size of the Earth and Moon, but could not begin to understand how old they were.