Moon Phases

Watching the moon over successive nights will show how the illuminated portion of its face changes from night to night. This changing fraction is known as the moon's phase.

The moon, like the Earth, is a sphere and so, from anywhere on Earth, the Moon appears to be a circular disk which, at different times, is to a greater or lesser degree illuminated by direct sunlight.

Like the Earth, the Moon is always half illuminated by the Sun, but as the Moon orbits the Earth we get to see sometimes more, sometimes less, of the illuminated half. During each lunar orbit, the Moon's appearance changes from not being visible at all, to being partially illuminated, fully illuminated, then back through partially illuminated to not illuminated again. Although this cycle is a continuous process, there are four distinct, traditionally recognized stages, called major phases (New, First Quarter, Full and Last Quarter).

Here's a question for you:

How much of the Moon's surface can we see from Earth?

You might answer: "That's obvious! 50%"

But you'd be wrong.

Over the course of a lunar cycle we can see more than 50% of the Moon's surface from Earth. This is because of a combination of effects which are known as "librations" of the Moon.

If we view the face of the Moon over the course of its orbit in fast motion, it is as if the Moon is both nodding its head "yes" and shaking its head "no" at the same time.

The lunar libration in latitude is due to the Moon's axis being slightly inclined relative to the Earth's axis (by about 5 degrees). From our angle we can at one time peek over the north pole of the Moon, and then later in the lunar month we peek over the southern pole.

Over the entire four week cycle it gives the the effect of the Moon slowly "nodding its head yes."

The diurnal (daily) libration of the Moon is due to the observer first viewing from the western edge of the Earth as the Moon is rising, and then later from up to 4,000 away to the east as the Moon is setting.

This is due to the rotation of the Earth. The difference in perspective between the rising and setting of the Moon appears as a slight turning of the Moon first to west and then to east, as though "shaking its head no."

Libration of longitude is an effect of the Moon's varying rate of travel along its slightly elliptical orbit around the Earth. The Moon travels faster when closer to Earth, and at its slowest when farthest away. Its rotation on its own axis is more regular, the difference appearing again as a slight east-west "no" oscillation.

Although the Moon always presents us with the same face towards the Earth, due to its rotation and revolution being tidally locked to the same period (think swinging a bucket on a piece of string around your head), the combined effect of all these different librations allows us over time to see some 59% of the Moon's surface.

Here's this month's libration diagram. Each light blue dot is a date in the month (each 5th one is numbered).

Here's a time-lapse sequence demonstrating lunar libration:

All diagrams on this page were created by LunarPhase Pro.

The software predicts a wide range of Moon phenomena for whereever you live. And it also includes an interactive Moon Atlas, so you can get familiar with the Moon. Maps can be zoomed, panned and printed out and features on the Moon can be labelled for easy identification.

Check out the above link to see just what the software is able to do.

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