Panel 11: Sunset of the Millennium (1995 to 2000 AD) © Walter Álvarez, 2019

Panel 10 was the last one for which there is historical sequence from one day to the entire detail, the available text space is used to consider major historical content to portray graphically and duration of the Cosmos — a zoom factor of 5 x 1012 broader issues of Big — Dating and describe. Two more panels, 11 and 12, have been (5 trillion). Blank space again is left for users, and a periodizing history (Panel 11), and the Character of added in order to continue to the last day of 1999, so few natural events have been selected to populate history (Panel 12). that ChronoZoom can display a continuous, linear the two panels, but rather than describing them in

Dating and periodizing history

Human beings are not born with the ability to radiometric dating in during the 20th century. grains in an ash bed will have had their age set to date events in the past, or even to remember and Two things make radiometric dating possible. First, zero when hot magma cooled during the eruption, pass on their memories of bygone events for more radioactive in a grain decay at a and if the grains then fell in an ash bed, their than a few generations. Before writing and written statistically constant rate given by the half-life, so radiometric age also dates the bed. Much work is documents became available, the more distant past that the ratio of daughter atoms to parent atoms currently being done on establishing evolutionary would become part of legend. Under those gives the age of the mineral grain (although this is trees based on the DNA of living organisms, and conditions, serious history was not possible. more complicated in practice!). Second, radioactive attempts have been made to date nodes in these Writing does not guarantee dated history, decay takes place in the tiny nucleus of the , family trees using the assumption that rates of however. First a is needed, and the shielded by the much larger surrounding mutation are constant through .3 This recognizable changes of the four seasons make years cloud, so the decay rate is protected from the assumption seem unlikely to be correct, since it the obvious calendrical basis. Modern and conditions that make involves chemical changes in DNA, unlike the count years in continuous sequence, but students of rates of chemical change vary with time. Hundreds protected nuclear changes that make radiometric early civilizations must deal with dates like “in the of thousands of radiometric dates now constrain the dating so successful. fourteenth of the reign of King So-and-So.” of history. In the later 20th Astronomers have come most recently to dating Much effort has gone into translating phrases like century, additional ways of dating rocks were history. With cosmic expansion understood, redshift this into years BC, but questions remain — for developed, including fingerprinting with allows the dating of light emitted from distant example, there are several decades difference geomagnetic polarity reversals, various kinds of galaxies. Globular clusters, huge gravitationally- between the “high” and “low” Egyptian Bronze Age stable , cyclostratigraphy and, bound collections of stars in the Milky Way can be . The Mayan calendar was still more for other solar-system bodies, crater-count been dated by the mass of the stars that turn off the complicated and challenging to decipher.1 Even for statistics.2 main sequence, for larger, brighter stars burn out and places with well-established chronologies, Archaeologists date their findings in ways more quickly.4 It has long been impossible to date many documents are not dated. However, once the similarly to those used in , although with individual stars, but this may be solved by a new modern times of Panel 9 are reached, there are few being of great importance, technique called gyrochronology, for it has been problems with dating the written historical record. because 14C, produced by cosmic-ray bombardment shown, perhaps surprisingly, that the rotation rate This has led generations of scholars to consider that of nitrogen in the atmosphere, has a half life of only of a star is a function of its age.5 The ages of events the only “history” is written history. Historical ~5,730 years, so it is suitable for obtaining dates during the are calculated theoretically. scientists and Big Historians consider this view back to about 40,000 years. In , artifacts, Dividing history into periods is essential for outmoded, and it is gradually passing away. especially pottery shards and coins, play the same understanding it. Historians and archaeologists use During the 20th century, historical scientists role as do in geology. informal (e.g., Roman Empire, Late learned to date many kinds of events that occurred Paleontologists cannot get dates in years Bronze Age). Only in geology and are before the invention of writing. Geology was the directly from fossils, except for very young ones there formal periodizations (e.g., Cretaceous), first, starting with fossils in the 19th century, which where radiocarbon dating can be used. Dating of established by standard procedures and accepted allowed rocks to be placed in chronological fossils is done indirectly, often by dating volcanic internationally, leading to the constantly improving sequence, followed by several systems of mineral grains in associated volcanic ash beds. The .6