Nychthemeron. The word of the day was Nychthemeron.

Nychthemeron: noun. A complete cycle of day and night. A word that seems so well defined, but whose solidity is an illusion. On what planet is this day and night taking place? How long is that day and night? Some planets are so snared by the gravity of their star that one face is always in the sun while the other is always in shadow. There is no cycle of day and night there, no rising and setting of the solar disk to herald the next great round. There you can touch eternity.

But that is not to say that no days pass. One day on a planet is measured by one complete rotation on its axis. Even a "tidally locked" planet rotates—at the same rate as it revolves around its star. So, on these orbs of eternity, one day is equal to one year, and by the movement of the stars some time can be reckoned. However, in this cycle there is no alternation of sunlight and starlight, so even in this day there is no nychthemeron.

However, day does change to night as you move across the planet's surface. As you move east the sun sets in the west, twilight comes and the sky opens up to the universe, then the sun rises again. When you return to the beginning, you will have travled through one night and one day. You will have completed a nychthemeron—no longer a measure of time, but of space.