Why Can We See the Moon During the Day?
That's The Moon?
Throughout the month, we frequently see the moon late at night, well past when the sun sets for the day and darkness reigns supreme. Each night, the moon's shape changes and as the month progresses it goes through crescent phases, quarter phases, gibbous phases, a full phase and a new phase. Yet many do not understand how those same phases of the moon that can be seen at night can also be seen during the day, yet no stars are visible in the blue sky. Despite all the factors that led the moon to be in an optimal position for viewing, the only remaining factors that contributes to seeing the moon during the day depends on is what time of day it is and if the sky is clear. So how can we see the moon and not the stars during the day?
The Moon And The Sun
It is important to make sure we understand how we see the moon at all, regardless of the time of day. The moon is only viewable to us because of the light that reflects off of it (and this is true for any object you see). The regolith, or the surface material of the moon made of dust, has a high albedo, or reflectivity. All that sunlight that hits the surface of the moon gets bounced off and makes its way to Earth. So when we look at the moon we are not seeing some light that it generated but in fact what it reflects to us from the sun. This is a subtle but important concept to understand. No moons or planets generate enough light to have their disc be viewable to anyone. Any such shape is entirely reliant on having enough light to bounce off the surface and be received by someone to see.
Light and the Atmosphere
Another important point to make is that the sky is blue only because that is what reflects off the atmosphere to our eyes. Light that enters the sky from space has to go through many layers of our atmosphere and it is during this process that certain wavelengths of light get reflected into space. Red light is a longer wavelength than is blue light, and as sunlight enters our atmosphere the sunlight will encounter particles in our air. These particles will cause the blue light to be scattered into the air and will force the red light to be scattered into space, for they lack sufficient energy to penetrate into the lower atmosphere. That being said, if the sun is low enough in the sky then the angle in which red light enters the atmosphere is more direct and can be scattered into the lower atmosphere. This is why sunrises and sunsets have red tones to them.
It's All There!
Now, all those stars you see at night do not simply vanish into nothing once the Sun rises. They are all still present in the sky, but the blue light that enters our atmosphere obstructs us from seeing them during the day. That blue light is scattered to our eyes and with the many layers to the atmosphere that blue color builds up so that during the day we are denied the chance to view the stars. They are simply not bright enough to overcome the magnitude of a blue sky. But some objects, such as Venus, can be seen during the day so long as the amount of sunlight striking Venus and reflecting back to us can overcome the brightness of the sky. It is a complicated relation and thus a rare event.
But the moon has such a high albedo that it can overcome the brightness of the blue sky, thus why it is so frequently a visitor in the sky. Proximity is key here. Even though Venus is still far away from us, we can still see it depending on the conditions. But all the stars that are so far away have no way to overpower the sunlight. If the moon were further away from us, it would be more difficult to see.
The light that reflects off the moon and into our atmosphere does go though the same scattering of light as does the sunlight, so when you see the moon during the day it does have a blue tint to it. It is ironic that the Sun is the same source of the blue sky and the phases of the moon. We are ultimately reliant on the sunlight in more ways than we could imagine.
© 2014 Leonard Kelley