Sphinx moths can be darn tough to differentiate, especially without spreading the wings and getting a good look at the 'eyes' (usually some nice pinks and yellows) that are on the lower wings and used for scare defense against birds. Halton Hills' suggestion is a good one. Popular sphinx is a good suggestion but also do a quick image search for Walnut sphinx, Virginia Creeper sphinx, and Lettered Sphinx. Compare your original (larger) image to those ones but it is still going to be tough because (unless I'm mistaken) your photo seems to show patterning on the underside of the wings, which can be slightly different than on top.
Halton Hills, your photo is definitely a pair of Waved Sphinx Moths. That was a great idea with the measuring tape in the photo! They're yet another ash tree menacer, but not numerous enough to be considered a pest species. Actually, any lilacs near where you photographed them? I know they probably just came to whatever light might be near that photo, but at the same time they're lilac destroyers too.
