Prima walked out to the observatory's main deck. There, as she had expected, Secunda was sitting in the grass, looking up at the stars.
"Pretty, isn't it?" said Secunda, as she craned her neck backwards to look at her sister. "You can see everything tonight."
"Everything?" replied Prima. Slowly wading through the grass, she glanced up and was instantly spellbound by the celestial garden of steady pinpricks of light, white and yellow and blue and red, smeared into blooming radiant petals by the thick observatory dome.
"Everything," whispered Secunda. "Well," she added, "except for Terra, I suppose." She stood, stretched, sighed, and turned to her starry-eyed sister. "Hey," she said, "I want you to follow me. There's a telescope over there that has a lens outside the dome. I bet we can see Mars from here."
Prima frowned slightly. "Really? Where is Mars from here, anyway?"
Suppose that Prima and Secunda are from our Earth, only a few centuries in the future. Could Mars be visible from Luna's night sky? What else would be visible? Would it be normal for Terra to not appear at night?
I know that, at a minimum, Luna's lack of atmosphere would mean that stars don't twinkle. I suspect that the Milky Way is apparent sometimes, and that the sky's view changes as Terra orbits Sol, so that the seasonal constellations are either similar or analogous in scheduling.
Edit: Thank you to all contributors. I've tried to upvote every helpful response, and will be thinking of tidal locking and thin atmosphere quite a bit now. I need the practice, so have some more:
"Got it! Aw, Jupiter's not as big as I had hoped," whined Secunda, pulling her face away from the goggles. The optical tube trailing the goggles, large and padded, made a sympathetic groan as it sagged slightly.
"What‽ No way, let me see already," Prima said as she groped for the goggles in her sister's hands.
"Okay," said Secunda. Prima's hands met hers on the goggles. She stepped towards Prima, and helped orient the optical tube to a position approaching stability. Prima pressed her face to the goggles, while Secunda giggled quietly.
Prima frowned, then jerked up from the goggles and glared at her sister. "You cheat! That's a photograph, isn't it? That's not real!" she protested, before breaking out into giggles herself. "You're such a joker sometimes," she snorted between laughs.
Secunda frowned. "No, I didn't," she started, then gripped the goggles harder and pushed them back into her sister's face. "Look carefully. Do you see it?"
Prima stared closely at the beautiful storm-ridden gaseous orb for a few moments, then gasped, "It's…moving? Crawling? It's alive! I can only barely see it, but it's moving somehow. Rotating?"
Both sisters were quiet for a moment. Then Secunda poked Prima. "C'mon, it's my turn again!" she said.