On 4 July 2016, I offered a puzzle to readers.

I thought it was pretty easy. But, I’ve since concluded that I either missed my calling as the crafter of impossible puzzles or I should stick with straight-forward photographic tips.

I’m not sure which.

Ok, the whole post was the puzzle. In this  I offered a variety of clues, each of which—when deciphered—should have produced a hint toward the correct answer.

Perhaps to make this easier I should have written; ‘what common theme of this post can be found in various elements within the post.’

The first clue was the title: “Tracking the Light Reaches 1500!”

The second was in the time of the posting (not the normal time, but at 3 pm EST; 1500 hours).

(I put in an ‘FYI’ to hint that the time zone was important.)

Each photo also offered a clue (or clues):

Conrail 9578 was a model SW1500 switcher rated at 1,500 horsepower. (I guess you just need to know that sort of thing, or look it up in a book?)

I picked an SW1500 because the model number neatly matched the output (rather than say an F7, also rated at 1,500 hp).

Admittedly the final photo’s clue wasn’t especially obvious: this photo shows the overhead catenary on the Chicago, South Shore & South Bend (In the caption, I describe the railroad as the ‘South Shore’ along with the location  of the image). The clue was that the South Shore’s wires are energized at 1,500 volts DC. (Sorry, the signal is just an attractive distraction).

I’d mentioned that a one-word answer was all I was seeking; detailed explanations were optional.

The answer is an even four-digit number, evenly divisible by 1, 3, 5 and 100.

None of the answers from readers perfectly matched what I was aiming at, so  I’ve decided to award the free book to the first answerer (who guessed in the form of a comment, as required by the rules) that mentioned the correct number, which as it happens was also the first answer that I received.

Thanks to all the readers who participated, and especially to those readers who provided well-thought out responses that  produced answers that were more clever than I sought!

Tracking the Light doesn’t do Puzzles Everyday.

Incidentally, the puzzle behind the puzzle—my elusive allusive meaning—was that successful photography is like a puzzle: You have to put together a variety of pieces in the right order to make a picture, but kin to this puzzle, it is up to the photographer to pick which pieces to use.