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Sebastianus sat down on one of the stools without looking at me. “I can’t be sure, but it doesn’t really matter. My presence here isn’t a secret. If he doesn’t know already, he’ll find out soon enough.”

“And send word to Jovinian as quickly as he can find a quill,” Don said.

Sebastianus unwrapped the bundle, revealing a fragile bound volume. He opened it gently and laid it on the table. He cleared his throat. “Anna, after Don told me that you had arrived in Corlivo, I thought we should continue with our plan. I visited the little library you told me about. I’ve wondered for some time whether we might try to research the dates of Straba’s visit to see if they line up with some of the other clues we have, but when— Well, suffice it to say that I didn’t look until today.”

“So that’s it?” I asked. “Just, I see you’ve arrived after all; let’s get back to our research?”

He kept his eyes on the yellowed page. “What would you have me say?”

“Oh, I don’t know. How are you, maybe?” I looked again at the worn, aged book he had carefully unwrapped. It was a ledger. “Is that— Did you take that from the records room?”

“Why? Are we not supposed to take items out of that room?” He turned his head slightly to take in my reaction.

I pretended I hadn’t heard him and bent to examine it more closely. “You found something?”

He shrugged. “It took me several hours to find the right volume, but yes, I think I did.”

The lines were written in the thin, spidery hand of a long-ago matron. There he was:

5th day of Iunius. Straba. For Aemilia. 2.

6th day of Iunius. Straba. For Aemilia. 2.

7th day of Iunius. Straba. For Aemilia. 2.

Straba. His visit to Casa Caligine, an unknown piece of history, hiding right there in plain sight. I was so excited to see it that it took me a few moments to absorb the details of the entry.

“He really was here,” I said. “In Iunius. The beginning of the high season. And so was she. Aemilia.”

Sebastianus nodded. “The question is, however, was that the right time to coincide with his discovery of the caves? ‘Guided by the Mother and the full moon, I have found it.’ Were the Mother and the full moon in the sky on the fifth, sixth, and seventh of Iunius?”

“What is the significance of this?” Pollux stretched a wiry arm over his head.

I gestured toward the book. “Straba, an ancient geographer, claims that he found Arcadius’s lost caves. From what we know, a full moon was in the sky at that time, and a certain constellation, the Mother, was visible.”

Pollux looked at Don in confusion. “This isn’t a time for solving riddles. You’ve said that this man, Clemens Fabius, will notify the Imperator. We must prepare to flee.”

Don sighed. “Pol, you’ll never convince the two of them to stop trying to solve this particular riddle together.”

“This is what we came here for, isn’t it?” I waved a hand at the parchment. “This is why I walked miles through the forest alone.”

A pained look crossed Sebastianus’s face. If I wasn’t feeling so hurt myself, I’d have felt sorry for him.

“Is there a way to know about the movement of constellations in the distant past?” I wondered aloud. “Are there records anywhere?”

“Perhaps at the university at Arcadium, but certainly not here,” Sebastianus said. “It’s a shame we aren’t trying map the future positions of constellations. That device you and Cassius built would be very helpful.”

I sat down on the couch. “Yes. Strangely enough, the future is easy to predict with the orrery. Mathematical laws are unchanging. It’s the past that we can’t—” I got to my feet again.

Couldn’t the same mechanics, operating in reverse, help us to know where these heavenly objects once were?

“What idea just occurred to you?” Sebastianus asked. “Whatever it is, it’s written all over your face.”

“I’m thinking that I might be able to reconfigure the orrery so that it can calculate past movements, rather than future. The mathematics would be the same.”

He leaned forward, and I had the sense that he was trying to moderate his excitement. “What would you need to figure this out?”

I considered for a moment. “Your best guess at the year Straba found the caves, and then many hours and a quiet place to work.”