When looking into the night sky, nothing compares to the brightness of the moon. Unless, of course, you are looking up during a new moon, in which case you can barely see it at all.

As the moon travels around the Earth, it is illuminated in different ways, ranging from a new moon that has nothing illuminated all the way to the full moon, where the entire surface is bright.

Humans have known about this cycle, which takes 29.5 days to repeat, for a very long time. In fact, most of the earliest calendars used the lunar cycle to create the months of the year.

When you look up into the sky, especially if you have a good telescope, you can observe other celestial objects, such as the planets.

At first thought, many people would assume that the planets should also go through these types of phases, and indeed, some of them do go through something similar, but not all.

The planets that are further from the sun than us don’t have any noticeable phases at all. If you look at Jupiter, for example, it will always appear completely illuminated.

If you look toward the inner planets such as Venus, however, you will see phases, but not exactly like the moon.

The reason is simple, and it is all based on what parts of the planets are truly exposed to sunlight at a given time, and what parts we are able to observe from Earth.

All planets, and the moon, are about 50% illuminated by the sun at any given time (as long as there is no eclipse happening). If the moon is positioned between the sun and the Earth, we can only see the dark side, so it appears like none of it is lit up at all. This is known as a new moon.

When the moon is somewhere to the sides of the Earth in relation to the sun, then we can see some of the illuminated half and some of the dark half, giving us the other phases. Finally, when the moon is on the opposite side of the Earth (but not directly lined up with the sun), we see the entire illuminated half.

Planets like Jupiter, which are further away from the sun than we are, will always appear almost entirely illuminated from our perspective. Technically speaking, we can see a very slight phase change based on its position, but it is always over 99% illuminated, so without powerful instruments, you can’t even notice the change.

Inner planets like Venus, on the other hand, do have phases, but they are not as complete as what we see with the moon. When Venus is directly between the Earth and the sun, it could be considered a ‘new Venus’ in that we only observe the dark half.

As it travels to either side of us in its path around the sun, we will see more or less of the illuminated half. We will never, however, see a ‘full Venus’ where we can observe the entire illuminated side of the planet. This would only occur when Venus is on the opposite side of the sun from the Earth. When that is the case, the sun is obviously in our way, and we can’t see it.

All of the planets, moons, and other objects in the solar system are illuminated by the sun. What we can see of them all depends on our position in relation to them and the sun at any given time.

If you thought that was interesting, you might like to read about a second giant hole has opened up on the sun’s surface. Here’s what it means.