Dating

Dating, or to be slightly tautological chronological dating, is the process of assigning an absolute or relative date to old things. A wide variety of methods are used, and they differ according to what types of material they can date, how accurate they are, what point in an object's lifecycle they date (when they were grown or made vs when they died, were buried, or last used), how far back in time they can measure, whether they require calibration by other dating methods, etc. Methods include radiometric dating (from decay of radioactive isotopes, of which carbon dating is one example), dendrochronology (tree rings), archaeomagnetic dating (changes in the earth's magnetic field), amino acid dating (changes in organic molecules), palynology (pollen), stratigraphy (deeper is older), tephrochronology (layers of volcanic ash), lichenometry (lichen growth), ice core layers, type Ia supernovas, forensic entomology (which bugs are currently consuming a corpse), and methods based on written sources and other human products.

Dating is important in several disciplines including archaeology, paleoanthropology, paleontology and evolutionary biology, geology, history, astronomy, climate change, and criminology. It's also important because a huge amount of evidence for the antiquity of the Earth disproves young earth creationism and similar nonsense.

When understanding the results of dating it is important not only to understand the accuracy and limitations of the dating method, but also to understand how dates are baselined: it is usual in fields such as carbon dating to present dates as "before present", where the present is 1 January 1950.

Radiometric dating
This uses the ratio of stable to radioactive isotopes to date a sample, and carbon dating is one type widely known for its use on archaeological and anthropological samples.

Luminescence dating
This measures the last time a sample containing certain minerals was exposed to sunlight, by measuring their luminescence and comparing the luminescence with known rates of radioactive decay. Hence this dating method measures when a sample was buried.

Dendrochronology
The counting of tree rings provides an accurate measurement of the age of a piece of wood, as well as estimates of historical rainfall based on the size of each ring.

Archaeomagnetic dating
This uses changes in the local magnetic field deriving from the earth's magnetic field, based on the fact that heating and then cooling certain materials will affect their magnetization based on the magnetic field they are in at the time. If you know the current orientation of a sample at the time it is found, and compare with historical records or estimates of the earth's changing magnetic field, you can date the last time a hearth, kiln, or oven was used (i.e. when it last passed the ). The orientation of crystals deposited from solutions can also be used in a similar way to date sediments. This method only works if you know the precise position where the object was found, and assumes an object hasn't moved since.

Amino acid dating
This uses the rate of a process in which the structure of amino acids shifts and a chiral molecule changes into its mirror-image form. Because almost all amino acids in living cells have a particular chirality (orientation) when the organism is alive, and the rate of change is well-understood, the ratio of the two mirror image forms can be used as an approximate method of dating the organism's death.

Stratigraphy
is a method of relative dating based on the idea that in geology, paleontology, and archaeology, older samples are deposited below newer samples and alongside similarly-aged samples. Hence you can tell which is older and which is newer by simple comparison, and if you can date a specific sample in the stack, you can get a limit for other samples above and below. The operation of stratigraphy, particularly in geology, relies on the application of a few simple laws, but there are pitfalls: if the strata have been subsequently disturbed you can get out-of-place artifacts or out-of-place fossils, and there may be a considerable time interval between the formation of an object and its deposition in the strata.

Pollen
Pollen dating (palynology) relies on changes in the types and ratios of pollen, which may be preserved in sediments. Events such as deforestation, drought, ice ages, extinctions, etc, can affect the types of pollen deposited at different times. To convert your pollen samples into a date requires comparison with existing samples that have been dated by other means such as carbon dating. Certain factors complicate it: some times of pollen may be better preserved than others, and some locations such as lake beds may collect pollen from wide areas while others like peat bogs may contain only local pollen.

Some specific cases have proven highly controversial: see Roraima pollen paradox and Grand Canyon.

Volcanic eruptions
Tephrochronology works on the principle that different volcanic eruptions have individual signatures which can be used to identify them in deposited strata, and therefore if the date of the eruption is known, then you can find and date the corresponding layer in strata captured over a wide area. It assumes volcanic eruptions are short-lived events in comparison to the age of the strata. To date, it has typically been used for events 10,000-15,000 years ago. It has the obvious limitation that it only works in areas subject to volcanoes, as well as all the other limitations of more general stratigraphic dating techniques.

Lichen
Lichenometry assumes that lichen grows at a steady, and very slow rate over a rock surface from the time that the rock is exposed to the air. Based on those assumptions you can theoretically date the time that a rock was uncovered, assuming nobody scraped the lichen off in the meantime. Hence its use is specialised but it can be valuable in understanding changes in glaciers and other features, in particular for research on global warming. There are also various methods used to reconcile the growth rates of different patches of lichen on the same rock or rocks, because they don't always grow the same amount. It is typically used for timescales up to 500 years, although some lichens can live as long as 5000 years.

Written markers
Sites can be dated according to written evidence found at the site. This includes coins and inscriptions which bear a more or less specific date (such as a reference to a specific ruler), the analysis of handwriting, and the interpretation of texts to see what datable things they mention. Generally things like coins which may be hoarded for a long time provide a firm limit of the oldest possible dating (equal to the newest date on a coin), but leave open the possibility that a site is newer than expected. In some cases, dating is complicated either because we don't have accurate ways of dating rulers and events (battles, etc) referenced, or because it is ambiguous as to precisely which event or ruler they refer.

Morphology and typology
When dating samples left by human beings, it is still common to judge the historical period by analysing features including the shape, design, composition, and abundance of objects such as potsherds, arrowheads, and other tools and weapons. These can be compared to known samples which are already dated.