This is a follow-up to @usernumber's answer to Has a near earth object in heliocentric orbit ever been bright enough to be visible to the unaided eye? I'm not afraid it will hit me, so if I'm still alive in 2029 I'd like to go take a look - perhaps it will even do a little something interesting d...

In a letter to Robert Hooke in 1676, Newton wrote: If I have seen further it is by standing on the shoulders of giants. Do we know which giants Newton was referring to? And was he referring to a particular piece of work or in general to all his accomplishments thus far?