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Astronomy's Heritage Data consist mostly of photographic plates, which can be of two sorts: sky images ("direct plates") or spectra. Some direct plates show spectra of each star ("objective prism plates") rather than actual images of stars, primarily for purposes of spectral-type classification. Exposures at all observatories, except perhaps some small, private ones, will have been logged in a formal or semi-formal log-book or log-sheet; those logs are routinely kept with or near the plates. When photographic observing was the chief modus operandi, Astronomy Departments retained a Plate Librarian whose tasks were (inter alia) to respond to queries to borrow plates, to locate and retrieve missing ones, and - in the case of stellar spectra - to update a card catalogue for each object observed. Writing the necessary metadata on the plate envelope was usually left to the observer, and when it was known that full log-book entries were in any case available many spectroscopic observers did not write much on the plate envelope beyond the plate number and the identity of the star.

Conventions regarding time-recording varied from one observatory to another. Most gave the time and date in UT (Universal Time), but the times given could sometimes be local, and even the latter could be unique (e.g., during the early decades at Mount Wilson the timings were in "solar time" so that the date did not change during the night; however, unlike JD (Julian Date), which begins its day at noon the *previous* day, the MW (Mount Wilson) date changed at noon the *following* day).

Astronomy's heritage of historic observations also includes hand-written measurements. Most have been incorporated into publications and are therefore adequately preserved in that way; the great majority of astronomical publications have been scanned and are in the public domain. However, both raw data (actual observations) and processed observations (reduced data) were also stored on floppy disk and on all varieties of magnetic tape and cartridge. Many may not now be recoverable, either through medium corruption or through lack of appropriate device reader.