Paleomagnetism: Earth’s magnetic polarity flip-flops about every 100,000 to 600,000 years.

The polarity is recorded by the orientation of magnetic crystals in specific kinds of rock, and researchers have established a timeline of normal and reversed periods of polarity.

Before more precise absolute dating tools were possible, researchers used a variety of comparative approaches called relative dating.

Both plants and animals exchange carbon with their environment until they die.

Afterward, the amount of the radioactive isotope carbon-14 in their remains decreases.

Paleontologists still commonly use biostratigraphy to date fossils, often in combination with paleomagnetism and tephrochronology.

A submethod within biostratigraphy is faunal association: Sometimes researchers can determine a rough age for a fossil based on established ages of other fauna from the same layer — especially microfauna, which evolve faster, creating shorter spans in the fossil record for each species.

Egyptologists, for example, created a relative chronology of pre-pharaonic Egypt based on increasing complexity in ceramics found at burial sites.

Whenever possible, researchers use one or more absolute dating methods, which provide an age for the actual fossil or artifact.

Researchers can measure the amount of these trapped electrons to establish an age.

But to use any trapped charge method, experts first need to calculate the rate at which the electrons were trapped.

Thermoluminescence: Silicate rocks, like quartz, are particularly good at trapping electrons.

Researchers who work with prehistoric tools made from flint — a hardened form of quartz — often use thermoluminescence (TL) to tell them not the age of the rock, but of the tool.

This includes factoring in many variables, such as the amount of radiation the object was exposed to each year.