Why can we see the Moon during the day?
Our natural satellite the Moon takes 27.3 days to orbit the Earth, and we only ever see one side of it because it’s tidally locked, meaning that for every orbit, it spins on its own axis exactly once. But the Moon is not luminous in its own right—we can only see it due to sunlight reflecting off its dusty surface: the side of the Moon facing the sun is fully illuminated, while the other half is in shadow. Because the Moon is constantly moving, sunlight hits it at different angles each day, and these changing angles create the lunar phases we see on Earth. A new moon occurs when the Moon slides between the Earth and sun, so the sun illuminates the side facing away from us and casts our side into shadow. A full moon is the opposite—the Moon, sun and Earth are in approximate alignment again except this time Earth is between the other two, so sunlight hits the side of the Moon facing us. Since the Moon’s orbit is about 5 degrees off the Earth-sun orbital plane, the Earth doesn’t block the sunlight. All the other phases are gradual transitions between, since from our perspective on Earth we can see both the part of the Moon in sunlight and the part in shadow. But the Moon is not exclusively a creature of the night—in the weeks before and after the new moon, the Moon is on the sunward side of Earth. This means it can be seen in the sky at the same time as the sun, i.e., in daylight hours. After all, it’s just another object in the sky lit by the sun.
(Image Credit: Southern Skies, Dr Doug Welch, 2008)