What is the reference point for the beginning of a planet's orbital period? For example, if Mercury has an orbit every 88 days, at what point is it determined to reset?
There are several reference points for the beginning of a planet's orbit.
The problem with deciding at what point an orbit starts is that all planets' orbits precess.
According to Johannes Kepler, the start of an orbit is the perihelion when the planet is closest to the Sun.
The problem with perihelion is that it precesses. In the case of the Earth, perihelion is getting progressively later each year. Also, perihelion varies from year to year die to the gravity of the other planets. Earth's perihelion currently falls between 2 and 4 January. In the case of Mercury these effects are much more pronounced.
Another means of defining a start point for an orbit is using the Vernal Equinox. The Vernal Equinox is the moment of time, around 20 March, when the Sun crosses the equator in a northerly direction. Astronomers use the direction from the centre of the Earth to the Vernal Equinox as the
Again there is a problem that the Vernal Equinox precesses. So, an epoch date is used. A commonly used epoch is J2000. So, the direction of the Vernal Equinox on 1 January 2000 at 1200 is the reference direction. All planets' orbit start, and "reset" when they cross the J2000 Vernal Equinox which is