Expert: Courtney Seligman - 10/5/2008

Question
You can't see the Dark Moon (commonly often referred to as the New Moon) in the daytime sky using the naked eye. (or so I've heard) But it is there. Isn't there some way to view it? Using a polarizing filter? Or something? (preferably that a layperson could do, or using something that wouldn't cost too much to purchase or DIY make at home)

Answer
You can't see a new moon because it is so close to the Sun that the scattered sunlight that lights up the sky around the Moon would be far brighter than the Moon. If the daylit sky only involved a few "emission lines", you could use a filter to remove those; but it includes all wavelengths of light, just like the light reflected off the Moon, so there is no way to filter it without making the Moon equally fainter.

To see the difficulty involved, take a look at the Moon when it is only a day or two past new moon (it's too late for that this month, but you could try it next month), during early evening twilight. Such a thin crescent is difficult to see even against the twilit sky, and in full (or half-full polarized) daylight, it couldn't possibly be seen. At new moon, what little moonlight there is would be far fainter, and even harder to observe.

That doesn't mean the Moon couldn't be observed at all. Where the Moon isn't, the sky is lit by a certain amount. Where it is, the sky is lit by about the same amount, plus the faint light of the Moon. A photograph taken with just the right exposure, run through photo-editing software to greatly increase the contrast, might barely reveal the presence of the Moon. (An important consideration in that case is that you'd have to be very careful not to accidentally point the observing instrument at the Sun, as that could destroy your eyesight and/or your equipment.)

Courtney Seligman
Professor of Astronomy
Long Beach City College