The observational archive of the Minor Planet Center does not contain every single observation of a minor body (comet or minor planet/asteroid) ever made. Not all published observations are good, there are many duff observations in the literature. Some refer to flaws on plates, or stars or galaxies. In some cases, it is possible to fix these bad observtions by applying a time (an integer number of hours of time) or date correction. Other valid corrections that might be made include a sign change in the declination, or correction of the hours of R.A. or degrees of declination. If the observations is a micrometric observation and the offsets and comparison star positions are given, it may be possible to change the signs of the offsets or correct the identification (and hence the position) of the comparison star. Any corrections are applied during the reduction process, not the data entry process.
The very earliest observations of planets and comets were made by measuring the angular distance from comparison stars. The measurements were rather crude, and mesurements of the same object taken over a short interval on the same night might show discreancies of 10 or more arcminutes.
In the mid-17th century, the micrometer was invented. Early implementations used...
The filar micrometer was invented around 1880... This was a mechanical-screw device that allowed small separations to be measured accurately...
A discussion of the various forms of micrometer and the procedures used by observers to reduce their observations is too involoved to be included here. Interested readers can find more details in volume 2 of William Chauvenet's A Manual of Spherical and Practical Astronomy.
Although the first photographic observations of minor planets were made in 1892, it was not until the 1920s that photography replaced the micrometer.
Initially, it was difficult to obtain accurate positions from photographic plates, partly due to the poor plate scale.