In the morning, I love to take a quick glance at the sky when I can. Sometimes, I notice that I can still see the moon. Seeing the moon in the daytime sky is pretty cool. It looks a little like a negative image of itself.
Well, I’ve always wondered why we can see the moon during the daytime. I have always thought that it was because of the sun.
A very common misconception in astronomy is that the moon is directly opposite the sun in the sky. In fact, the moon is only in this position for a single instant in the whole lunar month: the exact time of full moon, when it is 180 degrees away from the sun. The rest of the month it can be anywhere from 0 to180 degrees away and, at least in theory, visible in the daytime sky.
At full moon, the moon is exactly opposite the sun. This means that the moon rises just as the sun is setting, and sets just as the sun is rising. This is also the only night in the month when a lunar eclipse can happen. Even so, eclipses normally happen only one full moon out of every six; the other times the Earth’s shadow is either too high or too low to touch the moon.
Two things contribute to the moon being visible in daylight. First, it is bright enough that its light penetrates the scattered blue light of the sky. If you’re looking at exactly the right spot with a telescope, you can also see the planets Mercury, Venus, and Jupiter in daylight, plus a few of the brightest stars (though few casual observers can actually pull this off). Secondly, the moon must be high enough in the sky to be visible.
Because of the Earth’s rotation, the moon is above the horizon roughly 12 hours out of every 24. Since those 12 hours almost never coincide with the roughly 12 hours of daylight in every 24 hours, the possible window for observing the moon in daylight averages about 6 hours a day.
The moon is visible in daylight nearly every day, the exceptions being close to new moon, when the moon is too close to the sun to be visible, and close to full moon when it is only visible at night. The best times in the month to see the moon in daylight are close to first and last quarter, when the moon is 90 degrees away from the sun in the sky.
Cool! I love science.