From the inner breast pocket of the Doctor's jacket, he pulled out a straw, which he handed to Lydia. "Straws make drinking more fun," he instructed, and then brought one out for himself, and plopped it down the neck of the bottle before taking a sip. He then regarded Castiel with great curiosity. It wasn't his unusual behavior that interested the Doctor - goodness knows he had his own share of quirks. No, it was a combination of what he'd sensed earlier, and now the way he responded to Lydia's question. It was definitely an evasive tactic, one he'd pulled himself on many an occasion - Castiel definitely didn't want to discuss this fallen angel bit, for whatever reason, so the Doctor, taking into Castiel's awkward situation, decided to help a bloke out.
"The Earth is between the moon and the sun when that happens. Full or new moons, that is. In both cases, the gravitational pull of the sun is "added" to the gravitational pull of the moon on Earth, causing the oceans to bulge a bit more than usual. Funny though..." the Doctor looked up a the night sky. "It's all an illusion. It may look like the sky from Earth's viewpoint, but it's not." He then looked directly at Castiel. "But you already knew that."