During the perihelia (minimal distance from the Sun) is the maximal angle distance visible from the Earth 15 degrees and 55 minutes.
It seems as if the planet stood in one place for 4-12 days in the time of maximal elongations.
Its angle distance from the Sun changes between 1-2 degrees. This insignificant movement could not have been discovered by the Mayan observing methods.
For that reason their determination of Mercury elongations moves on average inside the borders of that “mistake”.
3.The solar eclipses and the fullmoons and newmoons.
4.The observation of the heliacal risings and settings of the planets. The heliacal rise sets in after the planets╣ conjunction with the Sun, when they are visible in the morning sky before the sunrise. During their heliacal setting the planets are visible in the evening sky after sunset. In the period of conjunction the planets are invisible for a few days. By observing the heliacal risings and settings dates we are able to determine the length of the synodic circulations of the planets.
5.The observation of the planetary conjunctions (when two planets observable from the Earth get in line and are nearly covering each other). The Dresden Codex is mostly describing only close approaches of the planets because some of the dates are calculated to the past and to the future.
6.The determination of equinoxes and solstices.