The phase of the moon that we see depends on the angle between the moon, the sun, and the earth. The visible moon is fully lit, known as full moon, when the earth is directly between the moon and the sun. The moon is not visible at all, known as new moon, when the moon is directly between the earth and the sun.
When we see half of the moon, at first or last quarter, the moon is at the same distance from the sun as the earth. When we see less than half of the moon, a crescent moon, the moon is closer to the sun than the earth. When we see more than half of the moon, such as a full moon, the moon is further from the sun than the earth.
The same side of the moon always faces the earth. The far side is sometimes called "the dark side of the moon", but this is incorrect. The far side actually gets more sunlight than the side that faces us because it faces the sun when the moon is closest to the sun, at new moon.
But at all other times, when the moon is partially lit, the lit portion must be either to the left or the right. Suppose that there is a crescent moon. The crescent can be either to the left, as in the first of the following illustrations, or to the right, as in the second illustration.
At the beginning of the lunar cycle, just after new moon, a crescent moon will appear in the western sky in the evening with the crescent to the right. Near the end of the lunar cycle, just before new moon, a crescent moon will appear in the eastern sky in the early morning with the crescent to the left. The lit side of the moon always faces toward the sun.
Most countries having the crescent moon on their flag have the crescent to the left. It is easier for a right-handed person to draw a crescent moon with the crescent to the left. But the moon with the crescent to the left is near the end of the lunar cycle and is seen only early in the morning. The crescent moon that most people will see is the new moon, visible in the western sky in the evening with the crescent to the right.
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