All commenters correctly determined that this was a moth:
At the time I took the picture, I assumed this was a butterfly. It acted like a butterfly, being active during the day and the way it held its wings (not folded over the back like many moths).
It was only when reviewing the photo later that I noticed it looked a bit odd for a butterfly. Like many commenters, I noted the lack of clubbed antennae. I didn’t try to identify it, but I remembered it when I read an interesting short article in a recent issue of Natural History magazine. The article was all about day flying moths in the subfamily Dioptinae (family Notodontidae). I emailed the author, James S. Miller, asking if he thought this might be one. Here’s his response:
That moth is either a Geometrid in the subfamily Sterrhinae, or a Noctuid
in the subfamily Agaristinae. I wouldn’t be able to tell without looking
at its wing venations and tympanum. Sadly, not a dioptine. It looks
exactly like several species in the dioptine genus Erbessa however, so you
were not far off. Mimicry in these taxa is phenomenal.
Thanks to Mr. Miller for responding.
If it is a sterrhine geometrid, then it must be near Astryria velata. I couldn’t find any agaristines that looked anywhere near this close.
Cool, thanks Ted!