Two reasons – first, the moon is bright enough to be seen above the blue/white hue of the sky. Second, the duration for which it is above the horizon of Earth coincides with the sun, making it visible during the daytime.
Everyone knows that we can’t see stars during the day. The most common explanation for this is that the faint glow of stars is washed out by the incredible illumination of the sun in the sky. Folks who are a bit nerdier will go a step further and tell you that the stars are actually washed out because of the Earth’s atmosphere, which scatters the sunlight reaching our planet.
Just like stars, we cannot (usually) see the moon during the daytime. However, there are days when you can clearly see the moon during daytime. Why is that?
The moon is bright enough to appear during the daytime
You almost certainly know that the moon does not give off its own light, but instead reflects the light of the sun to illuminate the night skies on Earth. It is interesting to note that the moon is actually quite dark, so only about 3% of the sunlight hitting the lunar surface is reflected. However, that scanty 3% of the reflected light is enough to illuminate our night skies back here on Earth.
All of this is to say that while the moon is nowhere nearly as bright as the sun, it’s still much brighter than even the brightest star in our night sky. Therefore, it can shine through the white hue of the daytime sky and be visible, even at high noon!
The role of Earth’s rotation in the appearance of the moon during the daytime
The visibility of the moon from Earth depends entirely on the former’s position in its orbit. The moon completes a revolution around Earth in a little less than 30 days, and it appears as different shapes in the sky over that period. These varied shapes are commonly referred to as the eight phases of the moon.
During a given portion of that time period (around the time of the full moon), the moon rises as the sun sets because the moon is opposite from the sun in the sky. Therefore, on every full moon, the sun, moon and Earth line up in such a manner that we can watch the sun setting and the moon rising (on the opposite side of the horizon) at about the same time.
However, with the passage of each day, the moon keeps getting nearer to the sun until finally it appears to be very close (around the time of a new moon), rising and setting at almost the same time as the sun. The upshot of this is that the moon becomes less visible at night and more visible during the daytime.
Since the Earth is constantly rotating, the moon appears above the horizon for around 12 hours out of the total 24. On some days, these 12 hours coincide with the sun’s 12 hours above the horizon, and lo and behold! We can see moon during the daytime!
At that point, the moon begins to move away from the sun until it gets back to the full moon phase and the cycle starts over.
It’s interesting to note that the moon is visible during the daytime almost every day (except on the days when it’s close to the new moon phase), but one needs to look at the sky very carefully to spot it. That’s the reason most casual observers cannot see the moon during the daytime every day.
For all you stargazers out there, the moon appears in the daytime sky after the full moon phase until a few days before the new moon (it’s not visible around the new moon phase, as the illuminated side of the moon is facing away from the Earth). If you keep track of the phases of the moon on a daily basis (here’s a useful link), you will know the exact days when you can spot that faintly luminous, whitish ball in the daytime sky!