The court asks witnesses about what they saw, regarding their testimony about the moon - and the celestial bodies change position over the course of a year, so the answers really would be different. How the sky actually looks makes a difference for the sake of accurate testimony. Plus, the formal sanctification of the new moon: Mekudash! But how critical is that part. Also, using a pre-fab poster, as it were, to quiz the witnesses regarding what they saw in the sky. Which raises questions about making images of the moon - out of concern for idolatry - and leads to answers as well.