I'm measuring the synodic period of Mercury using Stellarium. When measuring the synodic period one needs to choose a reference point to start the measurement, and I choose the point when Mercury is on top of the Sun. This is the start date:
And this is the end date:
Inputting these two dates into a calculator gives a synodic period of 104 days. However, as one can see from Stellarium's data, Mercury's synodic period is actually 115.88 days. This measurement is off by more than 10 days.
Why is there such a large difference? It's not large in the absolute sense, but it seems well above the error margins. For example if instead I shift the start date to 9 April 2021, Mercury is discernibly not between us and the Sun.