Here's why:
* Lunar Phases: The Moon's phases are caused by the changing angles of sunlight reflecting off its surface as it orbits the Earth.
* 90-degree Angle: When the Sun, Earth, and Moon form a right angle, the Moon appears half-illuminated. We see either the right half (first quarter) or the left half (last quarter) of the Moon illuminated, depending on its position in its orbit.
To visualize this:
1. Imagine the Sun as a light source.
2. The Earth orbits the Sun.
3. The Moon orbits the Earth.
4. When the Moon is positioned at a 90-degree angle to the Sun, with the Earth in the middle, only half of the Moon's surface facing the Earth is illuminated by the Sun.
This configuration results in either a first quarter moon (when the illuminated half is on the right side) or a last quarter moon (when the illuminated half is on the left side).
