The west side of the Moon is covered first during a lunar eclipse.

Here's why:

* Earth's Shadow: A lunar eclipse occurs when the Earth passes between the Sun and the Moon, casting its shadow on the Moon.

* Direction of Earth's Motion: The Earth rotates from west to east. This means that the western edge of Earth's shadow will encounter the Moon first, causing the west side of the Moon to darken.

So, as the Earth's shadow gradually covers the Moon during a lunar eclipse, you'll see the western edge of the Moon darken first.