The Radian

In his letter of 26 June 1970, the Zodiac wrote that he had buried a bomb somewhere in the San Francisco Bay Area, and that an enclosed code and map, annotated with a crossed-circle on Mt. Diablo, could be used to determine its location. The code was never solved, and exactly one month later he wrote in a postscript that "The Mount Diablo Code concerns Radians + # inches along the radians." A radian is a specific angular measurement based on the transcendental number pi. It is equal to a circle (or 360 degrees) divided by 2pi (or 6.23818...). Rounded off to the nearest minute and second, this is expressed in degrees as 57° 17' 45" and decimalized as 57.29578... degrees.

In late 1980, freelance writer Gareth Penn heard about the radian hint from a relative working in the California Attorney General's office. They both thought it odd that a homicidal maniac would make reference to an obscure geometric term used primarily by engineers and mathematicians. He describes his discovery in TIMES 17:

"Using a protractor and straight-edge, I drew an angle of between 57 and 58 degrees on the acetate and then laid the acetate over a map of the Bay Area. I placed the apex of the angle on Mount Diablo, then rotated the angle until one leg passed through the scene of the murder at Blue Rock Springs. Then I felt as if a ton of bricks had fallen on me.

"The other leg of the angle went straight through Presidio Heights in San Francisco where the Zodiac had murdered the cabbie. " [Footnote 1]

Penn goes on to suggest that the Zodiac logically chose a cab driver as his final victim because his plan required a murder in a particular location, and a cabbie could easily be directed to a predetermined site without arousing suspicion.

An exact measurement of the angle formed by Mount Diablo and the murder sites in Vallejo and San Francisco is difficult to make given the curvature of the earth and the distortion of any map covering approximately 400 square miles, but it has been gauged at 58-59 degrees. Some skeptics have argued that a variance of even one degree renders Penn's hypothesis unfounded, but it should be noted that the execution of a true radian would be impossible given its irrational mathematical nature. It has also been pointed out that, according to the Zodiac himself, "radians" -- plural -- were for use in finding a buried bomb, not marking murder sites. No bomb, though, was ever located through any means, and it goes without saying that all of the Zodiac's previous bomb threats were phony, and that there is no reason to believe that this one was any different. In any case, the fact that the Zodiac used the word correctly in conjunction with a map that prominently flagged Mt. Diablo as its most important feature strongly suggests that the mountain was intended as the angle's apex. Since no other application on this central point yields any meaningful result, Penn's solution stands out as the most reasonable explanation of the Zodiac's reference.

While it remains unknown whether the Zodiac intentionally killed at his chosen locations in order to execute the radian or only realized its significance after the murders were committed, the radian clue is a valuable key to understanding the Zodiac crimes, and no analysis of the case that overlooks it can be complete.