The Phases of the Moon (‘Not Just Another Pretty Phase’)
Following are some excerpts from an article by Mark J. Coco about the phases of the Moon. It appeared in Astronomy in July of 1994.
- Watching the Moon over a month, you will notice its appearance change. Sometimes it is a thin sliver hanging in the western sky after sunset. At other times it is a lopsided football visible in broad daylight. At other times it is a giant white marble suspended against a black sky. These many faces of the Moon are its phases.
- These phases result from the changing angle between the Moon, Earth, and the Sun. Even though half the Moon is always lit by the Sun, we see different parts of its illuminated hemisphere as our satellite moves in its orbit.
- To picture how the Moon’s motion creates the phases, imagine you are holding a ball in a room illuminated by a single light. No matter where you hold the ball, it is always half lit by the lamp. But you see different amounts of the ball’s lit surface depending on where the ball is. When the ball lies between you and the lamp, the illuminated side faces away and the ball appears dark. This corresponds to New Moon – the Moon lies between Earth and the Sun and its lit side faces away from us, making the Moon invisible in the sky.
- Moving a quarter turn clockwise away from the lamp, you can now see half of the ball’s lit face. Likewise, the face of the Moon in this position appears half lit. But this phase isn’t called Half Moon. Rather, astronomers call it First Quarter, referring to the quarter of its orbit the Moon has traveled. For this and other visible lunar phases, you will see the Moon with the same phase everywhere on Earth, although the position of the Moon above or below the horizon will vary for different locations.
- After another quarter turn, you face away from the lamp and the ball appears fully lit. In lunar phases, this corresponds to Full Moon. A third quarter turn brings the ball to a position where it again appears half lit. but now the illuminated half of the ball is opposite the half illuminated at First Quarter. For the Moon, this is Last Quarter. Because these phases correspond with certain points in the Moon’s orbit, the Moon will appear in predictable parts of the sky at different phases. For example, at New Moon the Moon lies in the same direction as the Sun. Even though you see – or don’t see – the dark side of the Moon, it is there near the Sun and rises and sets with the Sun.
- As the Moon moves in its orbit away from the Sun, the amount of illuminated surface you see increases. Because the Moon is moving east from the Sun, the first time you see the Moon’s face after New Moon is in the west after sunset. The Moon appears as a thin crescent or, more properly, a waxing crescent Moon, with waxing coming from an old English word that means “to increase”. Waxing phases include all those from New Moon to Full Moon. The waxing crescent phase encompasses the crescents from the thing crescent seen a night or two after New Moon to the fat crescent visible five or six days after New Moon.
Click here to return to the Astronomy index page