It is a difficult task - to understand the main point of the historical source correctly. There are dozens of facts that would make the reading awkward. First of all, it is a time barrier. Almost five hundred years ago people spoke and thought differently. So, for our contemporaries understanding of our ancestors is not an easy mission by itself.
Besides that, a very important fact is who the author was. A lot of things depend on it. For example, does the author belong to an upper class of a society, or is he a commoner? Does he believe in God, or not? If yes, than what kind of believer is he? Is he Catholic, Protestant, or Muslim? Another important question in analyzing the source is what was the reason of creating the document? Is the document neutral, or it reflects the point of view, lets say of a church, or a monarchy?.
It might be helpful to find out if the author has other works. It would be a great outcome if we could compare other works written by different authors of the same epoch. If the authors have different interests, but their works are about the same problem and they are similar to each other, that mean we can believe to the source, and the source is reliable.
Another important question is was the work written by the author because of his will, or was it the order of somebody else and the author just did his job? To whom this work was intended?.
If we are reading a historical source document written by a scientist in 1550, in England, it would probably reflect the "new" concepts of creation of the Universe. It was the time of the beginning of the Scientific Revolution in the Europe. At the beginning of the sixteenth century the old believes of creation of the Universe still were strong, but at that time the point of view started to change thanks to such scientists as Copernicus and his followers: Brahe, Kepler, and many others. They started to ask questions and to look for the answers not in the religious texts, but in their own observations, and scientific experiments.