Go here and look up both these words:
Further
Farther
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"The lighthouse is further away than it first appeared."
Further
Farther
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http://www.quickanddirtytips.com/education/grammar/further-versus-farther?page=all
The word "further" is often used as a comparative form of "far" to indicate a greater degree or distance. However, in this context, it seems to be comparing the lighthouse's distance from two specific points: its initial appearance and its current location.
To convey the intended meaning more accurately, it would be more appropriate to use the word "farther" instead of "further." "Farther" is specifically used when comparing physical distances. Therefore, a more appropriate revision would be:
"The lighthouse is farther away than it first appeared."
By making this correction, the sentence becomes more clear and correctly conveys that the lighthouse is currently at a greater physical distance than it initially appeared to be.