The motto, CARPE DIEM, is one of the best known Latin phrases today, and it is often found on sundials. The words come from a poem by Horace: carpe diem, quam minimum credula postero; seize the day, trusting as little as possible in what will come next. You can read the entire poem in Latin and English at this Wikipedia article.
There's a bit of irony in the fact that earlier in the poem Horace rejects Babylonian astronomical science, the very science of celestial motion that provided the foundation for Greek and Roman astronomical science, including the Roman use of sundials.
The photo is by fdecomite at Wikimedia Commons: