If you were an astronaut orbiting Earth, you’d see the sun rise and set every 45 minutes. You’d look down on a line of color above that part of Earth where twilight is taking place. That line of twilight would be curved to match the curve of the round Earth. You’d see it in contrast to the awesome blackness of outer space.
Or imagine an earthly twilight seen from the moon. The line of twilight is what separates Earth’s sunlit side from its night side. A belt of twilight wraps all the way around the globe.
Every world orbiting a star has its own zone of twilight — its own encircling belt where night meets day. In the east Tuesday morning, you’ll see a bright object near the waning crescent moon. It’s the planet Venus — now more than half lighted but less than full as seen through earthly telescopes. So a telescope would show a convex line of twilight on neighboring Venus now.
And even without a telescope, you can see twilight on the moon tomorrow — again, the moon is near Venus in the eastern predawn sky. The moon is now in a waning crescent phase — so the line of twilight on tomorrow’s crescent moon is that concave line separating light from dark — or day from night — on the surface of the lunar crescent.