It’s All About Geometry Or – It Takes 3 To Create Phases
Let's imagine for a moment that we have a space ship that allows us to launch from Earth and zoom up above the ecliptic (the plane as defined by the Sun's equator) just far enough to get a good view of the Moon orbiting about the Earth. The image below shows the Moon, Earth and Sun system from this perspective. From this view it can easily be seen that half of the Moon (and Earth) is constantly being illuminated by the Sun. A rather boring view of the Moon from day to day – no phases from this vantage point!
Now, if we return to the surface of Earth, over the course of a month we would see the familiar phases of our Moon as illustrated in the following image.
What has happened here? We've gone from being outside the plane, or ecliptic of the Sun to moving back into it by landing on Earth. We've placed ourselves in a position such that the Moon can now move between the Earth and Sun and it's illumination from our perspective on Earth will vary with time as it moves about the Earth. We haven't changed anything in the Solar System except our location.
Planets Have Phases Too
The fact that the Moon can come between the Earth and the source of illumination, the Sun, which partially illuminates the Moon's surface from our point of view, is the same situation that occurs with the inner planets, Venus and Mercury. Both of these planets exhibit similar phases as the Moon, one just needs a telescope to view them. (A note of caution: Mercury's location near the Sun makes it only visible just after sunset or before sunrise. Extreme caution needs to be exercised when looking through a telescope at the planet. Make sure that the Sun is below the horizon before trying to view the planet with a telescope.) The distant planets can also show partial illumination – gibbous phases during different times of the year The effect is smaller the further away the planet is from Earth.
Other Moons In Our Solar System
All the planets, from Earth outward in our Solar System have moons, some many more than others. If we again take our rocket and zoom up above the ecliptic we would see the same thing for these planets that we see in first figure above for our moon, half of the surface illuminated by the Sun.
Now if we maneuver our rocket so we can hover just above the clouds of Saturn for instance, (since it is a gas giant there is no solid surface to land on) we would effectively be in a position represented by the second figure above. The moons of Saturn—all 62 of them (as of last count in May 2010) would exhibit phases. But, the amazing thing is, they would all be showing different phases, because they would be in different positions about Saturn! Some would be full, others would be crescents or gibbous and to top it all would be the glorious rings painting a path in the night sky. All of this because you have placed yourself in the within the system where the moons are close to you, and can come between you and the Sun.
Below are images of some of Saturn's moons as taken by Cassini as it winds its way through the Saturnian system. Note that although Cassini is not on the surface of Saturn, it is still in a position that places it within the system and therefore it sees different phases of the moons.
Do Moons Of Other Planets Have Phases Like Earth’s Moon?
Moons and other objects in any solar system will show different phases of illumination, it just depends on where your location is relative to the object and the source of illumination.
All images are courtesy of NASA