It looks like it might be an emerald moth but its hard to tell from the discoloration in the wings and it is too high up for me to get a better look.
When: July 23rd 2018
Where: Central Illinois
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Sign up to join this communityIt looks like an Imperial Moth (Eacles imperialis):
Source: https://www.butterfliesandmoths.org/species/Eacles-imperialis
As shown on the distribution map at this link, they can be found in central Illinois.