Concerning precession of earth, is it possible that in 13,000 years it will be summer in December in the northern hemisphere? Or do we artificially change the definition of the length of the year to ensure this never happens?
We choose the length of the year to keep the seasons at the same time of year.
There are several different ways in which a year can be measured. Each has a slightly different duration.
The tropical year is the time it takes for the Earth to go from Vernal Equinox to vernal Equinox. It is on average 365.24 days. We use leap years and leap seconds to ensure that the Vernal Equinox falls on 20 March (occasionally it falls on 21 March). This keeps the seasons in check. The tropical years is based on a rotating frame of reference, so the effect of the precession of the equinoxes and perihelion doesn't affect its duration.
The sidereal year is the time it takes to complete a fixed orbit with respect to the fixed stars. The sidereal year is affected by the precession of the equinoxes so it is slightly longer than the tropical year.
The anomalistic year is the time between perihelion and perihelion. It is affected by the precession of perihelion and it is slightly longer than a tropical year.