The May 10, 1909 NYT prints a letter to the editor concerning the hullabaloo over signalling Mars:

However large the signal, however ingeniously arranged, it would be absolutely invisible to any one upon Mars for the following reason:

When we look up into the heavens during the day we see only the sun, and at certain times the moon. All of the starry host are there, but they are blotted from our vision by the glare of sunlight. Not only are we blind to reflected light from the planets, but also to the direct glory of the fixed stars that shine with an intensity many million times greater than any device of man could possibly attain. Now precisely the same state of affairs exists on Mars. We are within its orbit, its illuminated side is always what we see, which means that whenever eye or telescope rests upon its ruddy disk it is broad daylight there. A Martian looking earthward would fail to see the earth itself, much less our puny signal. Just as with ourselves, he would find nothing save sunlight and blue sky above him.

All this upon the supposition that the signal be flashed from the earth in our night. If it be attempted during the day it needs no argument to prove that it would be lost in the blaze of sunlight reflected from the earth's surface.

I am open to conviction, and if the views here are erroneous would be glad to have some scientist explain in what particular.

I think this only works if the earth is flat, no?

[Update (5/11/2009 10:15EDT) Bob emails me some clarifying thoughts:

"If the earth was flat but still rotating (as it must so that we have a day and night), then I don't think it would help.

Flat or not, if a planet is in an inner orbit it is visible from the outer orbit planet. We can see Venus (and Mercury) for example. The difference is that planets on inner orbits will exhibit phases (like our moon does). Planets in outer orbits are always full. This is one reason why Venus varies so much in intensity to our eyes over the course of the year. It's much brighter when it's in its full phase. (Its closeness to the earth matters too.) Mars in contrast varies much less -- only varying in intensity because of variations in closeness to the earth, but this is less noticeable."]