大气折射的计算

The Calculation of Mean Refraction

把普尔科沃大气折射表的第4、第5版进行完全计算机化，用多项式拟合大气折射表，直至天顶距83°为止，公式和表列值完全符合，同时用实际计算进行了精度比较。中丹水平子午环大气折射计算采用了计算机化的算法。大气折射是精密天体定位的重要研究课题之一，它除了影响光学天文定位外，还严重地影响射电天文、激光精密定位及GPS比对精度，随着天文定位精度的提高，大气折射的研究更显出其重要性。

The refraction affects the precise determinations of stellar positions attained withground-based instruments. The growth of knowledge on astronomical refraction is asiessed proceeding from the accuracy of the refraction tables. The accuracy of the refractionfables has been increased since Tycho's times. But the accuracy improvements of the reraction tables proceeded at a lower rate than that of the observational catalogues of stellarpositions. The best one among them is from Pulkovo Observatory. The refraction tablesof Pulkovo Observatory, the Fourth Edition and the Fifth Edition, have been published.But they are not convenient for our use because of the form of tables. For the observationof the DCMT, the zenith distance is not larger than 80°. It is easier to formulate it on acomputer for the zenith distance smaller than 83°. Therefore, there is no necessity to makelvailable formulate for all the zenith distances. The formulate for the refraction tables ofPulkovo Observatory, the Fourth and Fiffth Editions is given, with accuracy up to0. 00001' which is precise enough for the zenith distance smaller than 83°. There are nodiffrences between the results formulated and tablated up to the zenith distance of 83° withthe computation on a computer.A comparison between the Fourth and Fifth Editions has been carried out. Thenethod of calculation by formulate was emploied. The diffrences between them are smallerthan 0.05' up to zenith distance of 40°.