Here's why:

* Lunar phases: The moon doesn't produce its own light; it reflects sunlight. The different phases we see are due to the changing angles between the Sun, Earth, and Moon.

* First quarter: At the first quarter, the Moon is roughly 90 degrees away from the Sun in the sky. This means we're seeing half the illuminated side of the Moon, with the right side being lit.

* Appearance: The shape resembles a half-circle or a "D" (for "decreasing" as the lit portion shrinks before becoming a new moon).

Let me know if you'd like to know more about the moon's phases!