Note: This program first aired December 7, 2013.
If you have been doing your homework and keeping a moon journal, by this time you will have noticed many things about the moon, the first of which is that it doesn’t look the same every night.
The only reason we can see the moon at all is because it is getting hit with light from the sun, that light reflects off of it and that is what we see. Recognizing that relationship with the sun is the first key to understanding why the moon looks different on different nights. No matter where the moon is in the sky, fully half of its sphere is getting hit with sunlight. The same is true for Earth by the way, hold a tennis ball up to a lamp and you will see what I mean. Light illuminates half of the surface area of the sphere, and the other half of the sphere remains in shadow. The moon regularly changes its position in the sky, relative to the Earth and the sun, and what this means in practice is that sometimes we see the fully illuminated face of the moon, other times we see only part of that illuminated face, and still other times we only see the unlit face of the moon.
When the moon is oriented so that we see its entire illuminated face, we call this a full moon. This occurs when the moon is lined up with the Earth and the sun (in that order) in a straight line. The sun’s rays pass by Earth and hit the moon. The fully illuminated face of the moon is what is facing the Earth. If you are drawing this picture right now, you might say “Hey, isn’t the moon in the Earth’s shadow?” This does happen, but only very occasionally, and when it does, we call it a lunar eclipse. For the most part, the Earth is quite small relatively speaking, and the shadow it casts does not line up with the exact position of the moon.
When the moon is lined up with the Earth and the sun, but on the other side of the Earth, the one between the Earth and the sun, the only side of the moon that is facing us is the non illuminated side. We see precisely 0% of the lit side of the moon. This we all know as the new moon. To say we see it is really not accurate, we don’t see it at all, but there are a few reasons for that, we’ll get into in another program.
When the moon is not lined up with the sun and the Earth, we see something between 100% and 0% of the illuminated face of the moon. For example, when the moon is oriented exactly perpendicular to the line created by the sun and the Earth, we see half of the moon’s illuminated face, the half circle hanging in the sky. It is called a quarter moon, because technically, it is only a quarter of the moon’s face we are seeing, and because it is one quarter of the way through the sidereal cycle of the moon’s path around the Earth.
The moon changes position because it is a satellite, and it is orbit around the Earth. Each day during its orbital path, it is in a different position relative to the Earth and the sun, so each night, it looks a little different because a little more or less of its illuminated face is visible to us.
We will talk more about this pattern in coming weeks, but for now realize that it is indeed a pattern. How dull it would be if the moon rose at the same time in the same place and was the same shape every night. We would forget about it. The moon’s actual recurrent pattern keeps us on our toes, our very bodies are calibrated to it. When we have a week of clouds and I lose track of the moon, the first clear night brings with it surprise—Can it really have changed that much in just a few days?
The moon’s daily shape shifting waits for no clouds, no over scheduled lifestyles, no busy evenings. It carries on relentlessly, without us. It needs neither our attention nor our devotion to continue its trip through space and time. The responsibility is entirely on us, to pay attention to that celestial continuity and the tidal rhythms in our own bodies. We ignore these at our peril.
