Factual error: Celestial navigation can determine one's position on the earth's surface to within about one mile, not precise enough to give the location of a buried treasure. It also requires more than a sextant. It requires tables published in a nautical almanac (which is updated each year), charts on which to plot lines of position, and an accurate chronometer that is set to the precise time at the Prime Meridian (Greenwich Mean Time).
Factual error: When Buck's body is found, his blood is red when it should be brown due to oxidation after how much time has passed.
Answer: The storyline was that Jessica was teaching classes in New York. However, after so many seasons and as ratings declined, it was probably to freshen up the series with a different locale, introduce new characters, and have a greater variety of situations in which Jessica could solve murders. It was already unbelievable that so many people could be murdered in the small town of Cabot Cove. Being in a large city like New York makes it a tiny bit more plausible.
raywest ★