Thank you, The9! That answers a lot of questions! So the first hour is from sunrise, the sixth hour is at midday, and the twelfth hour is the last before sunset.
It's interesting that the article doesn't mention Ante Meridiem and Post Meridiem, the one little bit of Latin that everybody knows. Instead,
To indicate that it is a day or night hour Romans used expressions such as for example prima diei hora, prima noctis hora, hora prima noctis.
It depends on whether you want to say one a.m., which would be "Prima noctis hora" or the first Roman hour before noon, which would be "Prima hora ante meridiem."
"The first hour is before noon." is accepted as a valid solution but it seems incorrect.
Actually, it is correct. If you read the Wikipedia page that The9 referenced above, you will understand. Unfortunately I wrote, "It is one a.m." and reported it as an acceptable alternate, but now I see it isn't. My apologies to the course contributors. The translation of my sentence would be, "Prima noctis hora est."