Let's Explore this Further (or is it Farther?)

Here's another one of those pesky word pairs that tends to get mixed up by a lot of well-meaning folks.  When do you use "further" and when is "farther" the correct word to use?  (And, no, it's never "Happy Farther's Day")

This jockey is riding farther (and faster)
 than anticipated!

So it's pretty easy, actually.  "Farther" is correct when you are talking about an physical, measurable distance (as in, Her car is parked farther away than mine).

"Further" should be used when you are talking about additional time, quantities or degrees (as in, We need to discuss this word pair further to be totally clear on it).

I keep the two straight by using the "far" in farther to remind me that that one relates to distance.  We ran farther than she did.  The sun is farther from the Earth than is the moon.

Have any further comments or questions, please share them below!