Photographic Determinations of Stellar Parallaxes

Abstract

In many astronomical problems that depend for their solution upon precise measurements, the chief difficulty to be overcome is not in attaining the requisite accuracy in the measurements themselves, but in planning the observations in such a way as to disentangle phenomena that run nearly or quite the same course. This is particularly true of phenomena that are annually periodic, for then the difficulty may become quite insuperable because of the great number of effects having this period. To cite one example out of many, it is almost impossible to determine a system of absolute positions of stars that shall be free from periodic errors; the size, the shape and position of the instrument, refraction, aberration, the quality of the seeing and therefore the amount of personal equation, and many other effects directly concerned, all vary with the season and all must be accurately evaluated or eliminated before we can hope to attain thoroughly satisfactory right ascensions and declinations.