Further, furthest - English editing.

In some contexts further and farther are completely interchangeable: she moved further/farther down the train. The two words share the same roots and are equally correct when the meaning is ‘at, to, or by a greater distance’. Further is a much commoner word, though, and is used in various abstract and metaphorical context, for example referring to time, where it would be unusual to use farther, e.g. without further delay; have you anything further to say?; we intend to stay a further two weeks. The same distinction is made between farthest and furthest: the farthest point from the sun, but: this first team has gone furthest in its analysis.
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