Take a look at a globe or a map of the world. You may be impressed by what you see—towering mountains, deep oceans, vast continents, and sprawling glaciers. These features make Earth what it is today.

Even more impressive, by some people's standards, is how old Earth is. Scientists have calculated the age of our planet to be approximately 4.5-billion years. But how did scientists determine that age? The answer is complicated: It involves everything from observation to complicated mathematics to understanding the elements that make up our planet.

First Calculation Made Was Too Young To Be Part of Our Universe

As 1800s-era scientists tried to determine the age of the planet, they made a few mistakes. In 1862, a famous Irish physicist and mathematician, Lord Kelvin, estimated that Earth was between 20-million and 400-million years old. But even an age of 400 million years would make the planet quite young compared to the rest of the universe.

Lord Kelvin based his conclusion on a calculation. He calculated how long it would have taken Earth to cool if it had begun as a molten, or hot liquid, mass. Kelvin's estimate was wrong by a significant amount, but his technique of drawing conclusions based on observations and calculations was an accurate scientific method.

Layers only Relative to One Another but No Specific Age

Scientists also tried to use relative dating techniques to determine Earth's age. Stratigraphy compares the configuration of layers of rock or sediment in order to determine how old each layer is in relation to one another. This technique can reveal which layers are older or which events happened before others if the layers of sediment have remained in sequential order. Layers can be rearranged, bent, or contain inconsistencies. However, stratigraphy yields no exact age for those layers or events.

Stratigraphy did not give scientists the exact number they were looking for, but it did suggest Earth was most likely billions of years old—not just millions as was previously thought.

Using Decay of Radioactive Elements

Advances in chemistry, geology, and physics continued, and in the early to mid-1900s, scientists found a way to determine the absolute age of a rock or mineral sample. The absolute age of a sample is its actual age in years. The method of determining absolute age is called radiometric dating, and it involves the decay, or breakdown, of radioactive elements. Using radiometric dating techniques, scientists could determine the actual age of a sample.

Radiometric dating requires an understanding of isotopes. Isotopes are different forms of the same element, which have a different number of neutrons—tiny particles inside the nucleus, or core, of an atom.

The isotopes of unstable radioactive elements are known as parent isotopes. They will decay into other, more stable elements known as daughter isotopes. The parent isotopes decay into daughter isotopes in a predictable way.

The amount of time it takes for a parent isotope to decay is called a half-life. The half-life of an element is the amount of time required for exactly half of a quantity of that element to decay. The age of a sample can be determined based on the ratio of parent to daughter isotopes within the sample.

Original Rocks No Longer Present

One problem with the radiometric dating approach is the presence of the rock cycle. During the rock cycle, rocks are constantly changing forms. Old rocks may even be destroyed as they slide back into Earth's mantle, to be replaced by newer rocks formed by solidified lava. This makes finding an exact age for the planet difficult, because the original rocks that formed on Earth at the earliest stages of its creation are no longer here.

The oldest rocks that have been found are about 3.8-billion-years old. However, some tiny minerals have been dated at 4.2 billion years.

To get around the difficulty presented by the rock cycle, scientists have looked elsewhere in the solar system for even older rock samples. They have examined rocks from the moon and from meteorites, neither of which have been altered by the rock cycle. The same techniques of radiometric dating have been used on those rocks. All the data from this planet and beyond has led to the estimated age for the Earth of 4.5 billion years.