Why can I sometimes see the moon during the day, and not just close to evening or morning, but in the afternoon?
The moon orbits the Earth in a near-circular orbit once a month; it will appear at various places on that circle during the month, sometimes close to the Sun, sometimes on the opposite side of the heavens. Some times it is above the horizon, sometimes it is below; if it is above the horizon the Moon is there whether it is day or night at the observer's position. It is easy to see the Moon at night, when the sky is black; but during the day the Moon will still be there, unless it is below the horizon. The blue colour of the daytime sky can be assigned a brightness which is similar to the magitude of the planet Jupiter; anything brighter than that can be seen during the day, including the Sun, the Moon, and sometimes Venus. However the sky brightness increases when you look in the direction of the Sun; so if the Moon and/or Venus are close to the Sun, they will not be visible. Sometimes the sky is dark enough toward sunset for Jupiter to be seen too; I have seen it long ago, but my eyes are not so keen these days.
Thank you so much. And I understand the brightness issue, but how does the moon get above or below the earth's horizen? Is it just a function of the orbits of both the moon and the earth?
The Moon orbits the Earth almost on the same plane as the Earth orbits the Sun; so the Moon is sometimes above the horizon and sometimes below, just like the Sun is. This plane is called the ecliptic; most of the planets orbit in ths plane; at night the stars that lie along this pathway in the sky are known as the Zodiac. Generally the Moon and all the planets, and of course the Sun, are somewhere in the twelve houses of the zodiac; half of which are below the horizon at any one time, and the other half are above the horizon. Sometimes in fact the orbital planes of the Sun and Moon are so close the Moon goes in front of the Sun (a solar eclipse). But that doesn't happen very often.
