Climate and Weather Conditions

What do we know about the climate in various regions of the world over the past 6000 years? There is ample evidence of changing conditions in most of the world over this period of time. The evidence is in several forms:

at the same time. One such device is a two-star photometer, in which automatic and precise measurements are taken of the two objects at the same or nearly the same instant. The Rapid Alternate Detection System (Milone et al. 1982) in use since the early 1980s at the Rothney Astro-physical Observatory of the University of Calgary measures consecutively the light of two stars and samples the sky near them as well, permitting the measurement of relative brightness even through light cloud, and sky measurements to correct the results for sky brightness.

(1) Biological data in the form of pollen and other plant spores in living sites

(2) Changes in the distribution of fauna known from historical and archaeological sources

(3) Forest and other vegetation limits

(4) Stable isotope studies suggesting changes in precipitation patterns, ice coverage, and ocean salinity

(5) Geophysical data such as widths of river beds and sea level heights

(6) Geographical and historical data such as the absence of ice on sailing routes

Results of climatic investigations by Lamb (1974) for the Northern Hemisphere indicate a pattern of fluctuating wet and dry periods that lasted many years but which accompanied global retreat from a glacial period and subsequent stabilization about 5000 years ago. Although wet intervals might have been more conducive to better agricultural yields and increased populations, dry intervals would probably have permitted more systematic investigations of the sky. The forest limits of 2000 b.c. compared with those of today show that conditions appear to be slightly cooler at present with both cooler and warmer intervals between (Lamb 1974). As ingenious as such reconstructions are, we do not have astronomical records in the normal sense of the term (see §6.2). From Mesopotamia (§7.1), at a somewhat later time, however, we have a rich store of such information. In Babylonian "diaries," cuneiform records on baked mud bricks, we have detailed information about precisely what objects were "observed," although clouds were no impediment because, in general, planetary positions were computed using prescribed methods (see §7.1.4 for the flavor of that work). The diaries functioned as a kind of daily news report, including commodity prices, meteorological phenomena, and historical occurences, as well as astronomical phenomena. Coe (1962/1972) summarizes some of the broader changes in pre-Columbian Mexico. It can be said that sufficiently clear viewing opportunities, when extensive series of astronomical observations could have been made, did exist sometimes at sites in these areas. We next describe the empirical properties of light from astronomical objects before discussing the atmospheric effects.

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