Seeing the daytime
Q: Why do we see the moon during the day?
A: What a good question, I thought, when I read your email. Why do we see the Moon in the day? Of course, it's there but many
things are there that we don't see. We don't see the bright star Sirius in the day. We do see the Moon. Often it's like a beacon in
the blue sky we follow through summer woods.
[Johnny Horne- Fayetteville Observer- Backyard Universe] A beacon in the blue sky
I dig up an astronomy book and find the Moon's relative brightness. The Full Moon has about 1/400,000th the brightness of the
Sun. Yet the Full Moon sheds enough light that we can read a newspaper at night. It's over 33,000 times brighter than Sirius, which helps explain why, in the
daytime, we don't see Sirius but do see the Moon.
I read that the Moon has one of the lowest reflectivities of all the objects in the Solar System. Curious. It looks bright and white even with an average
reflectivity of an asphalt parking lot.
Almost every day in a lunar month, we can see the daytime Moon. The days we cannot are when the Moon is Full, when it's New, and a few days before and
after the New Moon.
An exactly Full Moon is invisible (or at least not easily seen) during the day because then the Moon is opposite the Sun with the Earth in between. The Full
Moon sets when the Sun rises (except near the poles) so we can't see the Moon during the day. It's below the horizon, shining brilliantly on the other side--the
night side--of Earth.
When the Moon is New it will be dark and unseen and, for at least two days either side of the New Moon, the Crescent Moon will be "too faint and close to the
Sun to be seen with the naked eye--although it might be glimpsed at sunset," says Robert Massey, astronomer at the Royal Observatory Greenwich.
The Moon is close to Earth and that's why we see bright sunlight reflected from its asphalt-parking-lot surface even in the daytime and even when part of the
Moon is in shadow. Step outside on or about the 14th of June, and find the daytime Moon. It's there--a shiny half moon, high and West.
(Answered by April Holladay, science correspondent, June 13, 2001)
UCLA Physics: Daymoon
Royal Observatory Greenwich: The Moon