When witnesses come to Jerusalem to testify that they have seen the new moon, the judges interview them in order to ascertain that they have, in fact, seen the beginning of a new lunar cycle and not just the end of the previous one.
Someone who does not pay close attention to the position of the moon may very well walk into court and describe a situation that is physically impossible. To assist the witnesses in their testimony, the Mishna on our daf tells of models that Rabban Gamliel had in his study, which he would show to the people coming to testify. In this way, situations that might be difficult to describe verbally could be discussed with the help of visual aids.
The Gemara asks: And is it permitted to create these types of forms Isn’t it written: “You shall not make with Me gods of silver, or gods of gold” (Shemot 20:19), which is interpreted as teaching: You shall not make images of my attendants, i.e., those celestial bodies that were created to serve God, including the sun and the moon?
The answer offered by the Gemara is a difficult one – that Rabban Gamliel did not make the models himself; they were made by others. Tosafot and other rishonim argue that it is forbidden for Jews to have non-Jews perform tasks for them that are Biblically forbidden, which would seem to be the case here. A number of explanations are offered: