Martian matters

This is a letter I just wrote to the Naval Observatory . . . anybody out there 
have an opinion? 

I have a question about the old 26-inch refractor, for a book I'm writing for 
Ace/Putnam publishers. 
 
Various references confirm that Asaph Hall discovered the Martian moon Phobos 
at 16:06 local time, 18 August 1877. My query is how could he possibly see 
such a dim speck, 12th magnitude, against a daytime sky? Even with legendary 
Clark optics. 
 
I've actually looked through the 26-inch at dusk, about a half-century ago. 
 In the fifties and early sixties I was a teenager who belonged to the National 
Capital Astronomers, and we were allowed to use the excellent Clark five-inch 
refractor that was just down the hill from the 26-inch. On a few occasions, 
the astronomers using the 26-inch let us come up before the sky was dark and 
glimpse the moon and planets. I strongly recall Saturn at about 500X, shimmering 
in the pale blue twilight. 

I suppose I should just accept the date and time. But the memory of that sight, 
slightly after sunset, makes me wonder. Could the time that Google offers be 
a typographical error that, through obscurity, has propagated over decades? 
 
Not very likely, I'm sure. But the odd fact is that I will be describing that 
moment in the novel, Phobos Means Fear, which mostly takes place in the future, 
but does have a flashback to Asaph and Stickney Hall. It's rather important 
that I know whether the observation was made in the daytime. 

Thanks in advance for any help you can give me. 

Joe