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.