FARTHER and FURTHER both mean “at a greater distance, ” and they often are used interchangeably, though they aren’t quite the same.
FARTHER typically refers to physical length or distance. It is the comparative form of the word “far” when referring to distance.
FURTHER typically refers to abstract distance. It can also mean “to a greater degree,” or “additional.” It refers to time or amount.
Correct: New York is farther north than Washington, DC. (Refers to physical distance)
Correct: The proper solution requires further study. (Meaning “additional,” refers to amount)
Correct: According to the project timetable, we should be further along. (Refers to time)
TIP: Use FARTHER for physical distance and FURTHER for metaphorical/abstract distance. FARther has FAR in it, and “far” obviously relates to physical distance.