Age of Earth

The **age of the Earth** is estimated at a little over 4.5 billion years. Ca-Al-rich inclusions&nbsp;– the oldest known solid bits in meteorites formed in the solar system&nbsp;– are 4.567 billion years old. This gives an age for the Solar System  and an upper limit for the age of Earth.

## Early science 
The first facts about the Earth were worked out by the Ancient Greeks. A good estimate of the Earth's size was made by Eratosthenes (276 BC–194 BC), using trigonometry. 

The first estimate of the Earth's age based on evidence was by Benoît de Maillet (1656–1738), a French diplomat, philosopher and naturalist. He thought the Earth must have developed by slow, natural forces. He studied geology in the field. He could see signs of erosion on land and sedimentation in the sea. He hid his ideas under the guise of a talk with an Indian philosopher, to avoid conflict with the Catholic Church. His work stayed in manuscript until after his death, when it was published after its editor had made changes which damaged it. Now there is a better modern edition based on the manuscripts. de Maillet estimated that the Earth was older than two billion years. He also recognized the true nature of fossils, and had early ideas about evolution.

## 19th century 
In the last quarter of the 19th century there was a long-running debate on the age of the Earth. In Charles Lyell's *Principles of Geology* (1830–33), he showed that the Earth had changed slowly, and that what we see is the result of gradual changes. This uniformitarianism clearly meant that the Earth was , though Lyell did not try to work out how old.

His younger friend Charles Darwin believed this, too. Darwin saw that if evolution had taken place, it would have required a long time. Also, a huge amount of sedimentary rock lies between the early fossils in the Cambrian strata, and the present land surface. Darwin and Lyell agreed that it would have taken a very long time for so much rock to be deposited.

In the first edition of *On the origin of species* (1859), Darwin had estimated that the erosion of the Sussex Weald must have taken 300 million years.p314 Both he and Lyell were surprised when the physicist William Thomson (Lord Kelvin) said the Earth could not be as old as they thought. He made the calculation based on how long it must have taken the Earth to cool down to its present temperature, given a starting point of 2,000oC. Kelvin's result was based on the idea that the near-surface geothermal gradient reflects the conductive cooling of the solid Earth.

He did this calculation a number of times, making various s. In 1862 his estimate was between 20 and 400 million years; but in 1866 he reduced the top estimate to 100 million years, and attacked Darwin and Lyle for not taking notice of his calculation. We know Darwin was anxious and worried that this would not be long enough to allow for evolution.

In Kelvin’s model, heat on the Earth’s surface is derived from the cooling of a shallow outer crust, assuming a solid Earth. But if the conductivity inside the Earth were much higher than at the surface, then the Earth's core and lower mantle would also cool. This would provide a huge store of energy to the surface. In that case, Kelvin’s estimate of the age of the Earth would be too low by many times.

Perry's main reason was that convection in the fluid, or partly fluid, interior of the Earth
would transfer heat much more effectively than would conduction:
:"… much internal fluidity would practically mean infinite conductivity for our purpose".

Kelvin stuck by his estimate of 100 million years, and later reduced the estimate to about 20 million years.

We now know that the existence of a viscous fluid under a thin crust is a much greater factor than the discovery of radioactivity, In 1903, he shared the Nobel Prize in Physics with Pierre and Marie Curie "in recognition of the extraordinary services he has rendered by his discovery of spontaneous radioactivity".

Eventually, it was realized that radioactivity was a major source of heat inside the Earth. Basically, the whole system developed in the same period of time.

## Religious views 
Use of the scientific method assumes uniformitarian methods for calculating Earth's age. The Hindu religion comes closest to the present-day scientific estimate. Some Christians and Jews believe the Genesis creation narrative is  true, which would mean that the Earth was created between 5000 and 10,000 years ago, and the methods used to calculate age have not been consistent throughout Earth's history. However, these days most people think such questions are best answered by scientific methods.

## Related pages 
* Geochronology
* History of the geologic time scale
* Nature timeline

## References