Astronomical Time Scale

This section on Astronomical Time Scale was written and illustrated by Linda A. Hinnov, Associate Research Professor, Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218

An important innovation in modern geochronology is the development of the Astronomical Time Scale ('ATS'). Periodic variations in the Earth's orbit and tilt relative to the Sun influence climate, and in turn, climate-sensitive sedimentation. High-quality records of these variations have been found in cyclic sedimentary sequences ('cyclostratigraphy') across the globe, stretching back in time for hundreds of millions of years.

Metronomic attributes of the recorded variations allows cyclostratigraphy to be exploited as a geologic 'clock' at an unprecedented resolution of 0.02 to 0.40 million years. For example, in the above photo, each of the light-dark layer pairs represents a 0.02 million year time increment that has been tied to a high-precision astronomical solution of the Earth's precession. This resolution is comparable to the uncertainty of modern radioisotope dating. The ATS also provides continuous time information between radioisotopically dated rocks, which are often separated by substantial stratigraphic thicknesses with no other time information. Thus, recovery of the continuous, high-resolution ATS throughout the geologic record is an important goal in modern earth science.

Today, A Geologic Time Scale 2004 (Gradstein et al., 2004) uses 'absolute' astronomical calibrations for most of the Cenozoic Era, in which time is tied to 12h Terrestrial Time on 1 January 2000. 'Floating' astronomical calibrations tied to radioisotopically dated stratigraphic points are given for entire epochs or stages in all three Mesozoic periods. By 2008, the GTS will include absolute astronomical calibrations for the entire Cenozoic Era and Cretaceous Period; floating astronomical calibrations will make up 85% of the Jurassic and 75% of the Triassic time scales.

Want to learn more? Download a 4.7 Mb pdf file of a recent overview by Linda Hinnov and James Ogg entitled Cyclostratigraphy and the astronomical time scale

EARTHTIME is supported by the National Science Foundation.