Lesson 1: Pre-Unit Assessment

Key Idea

When observing the Moon, we see a sharp, curved boundary separating the bright, lit portion from the dark portion. This border is technically known as the terminator.
>
Over time, the shape of the lit portion changes drastically. Sometimes the Moon appears fully round, sometimes it is a thin crescent, and other times it seems to disappear completely.

The central challenge of this unit is to explain these changing patterns. Since the Moon is a solid rock sphere that doesn't change shape, the changing light patterns must be caused by the way light interacts with it in space.

Key Idea

To solve the mystery, we must first establish a basic physical principle: Light travels in straight lines.

The Sun acts as a powerful light source, sending energy outward in all directions. Because light rays travel in perfectly straight paths, they cannot curve around objects.