The Zodiac Killer and Celestial Navigation

[Note: This article was originally published in June of 2006.]


One particular piece of evidence in the yet-unsolved case of the serial killer known as “The Zodiac” suggests that he was inspired not by astrology, as commonly assumed, but rather by celestial navigation. The definitive reference book for the subject of nautical navigation, first published over 200 years ago, appears to tie together some of the concepts found within the persona of “The Zodiac”. Furthermore, the book also suggests there indeed may be something to the idea that the Zodiac killer was incorporating the dates of the summer/winter solstices and spring/autumn equinoxes into his killing methodology.

The Compass Rose, Bowditch, and “The Zodiac”

On June 26, 1970, the Zodiac killer mailed a letter accompanied by a Phillips 66 map of the San Francisco Bay Area. On that map, the killer drew the following symbol centered on the peak of Mt. Diablo.

Anotated Symbol from the 1969 Phillips 66 Map of California

Annotated Symbol from the 1969 Phillips 66 Map of California [1]

Over the years there has been much speculation as to the meaning of this annotated symbol. However, if we consider the following points of commonality, it is clear that the symbol is intended to be a derivative of a compass rose, an analytical figure used in the practice of nautical navigation found primarily on nautical charts.

  • Both appear on maps
  • Both are constructed from graduated circles
  • Both graduated circles are numbered from 0
  • Both give primary emphasis to the lines associated with the four cardinal directions
  • Both have an arrow on the ray associated with 0
  • The instruction on the Phillips 66 map reads “0 is to be set to [magnetic north]” whereas the standard compass rose has an internal “magnetic compass rose” aligned with magnetic north.

Figure 2 shows a standard compass rose taken from a nautical chart published in 1956.

A standard compass rose found on an example nautical chart in 1966 Bowditch

A standard compass rose found on an example nautical chart in 1966 Bowditch[2]

In relatively short order, my investigation into the usage and history of the compass rose lead me to a book entitled American Practical Navigator, often referred to simply as Bowditch in tribute to its original author Nathanial Bowditch. This definitive reference for the subject of nautical navigation has been published continuously since 1802. In order to ascertain the potential for this book and/or its subject matter to have served as inspiration for the Zodiac killer, I obtained a copy of the 1966 version of Bowditch.

Interestingly, one of the book’s eight parts, some 225 pages, is devoted to the topic of Celestial Navigation.  Within that part, a chapter entitled Navigational Astronomy defines “the zodiac” with the following two paragraphs:

1420. The zodiac is a circular band of the sky extending 8° on each side of the elliptic. The navigational planets and the moon are within these limits. The zodiac is divided into 12 sections of 30° each, each section being given the name and symbol (“sign”) of the constellation within it. These are shown in figure 1420. The complete list of signs and names is given in appendix A.

The sun remains in each part for approximately one month. When the names were assigned, more than 2,000 years ago, the sun entered Aries (<symbol>) at the vernal equinox, Cancer (<symbol>) at the summer solstice, Libra (<symbol>) at the autumnal equinox, and Capricornus (<symbol>) at the winter solstice. Even though this is no longer true because of precession of the equinoxes, The American Ephemeris and Nautical Almanac still lists the sun as entering these constellations at the times of the equinoxes and solstices, for this has come to be their principal astronomical significance. The pseudo science of astrology assigns additional significance, not recognized by scientists, to the positions of the sun and planets among the signs of the zodiac. [3]

This last point has significant implications with respect to the Zodiac killer. Astrology and “the zodiac” are so intertwined in contemporary popular culture that many people never realize “the zodiac” is a well-defined concept apart from its usage within the practice of astrology. Relatedly, many people have searched the evidence in the case of the Zodiac killer hoping to find some meaningful connection to astrology. In my opinion, none have succeeded, at least not convincingly. Given the information in Bowditch, I suspect this lack of success is due to the connection not existing. The killer adopted the moniker “The Zodiac” because of its meaning associated with celestial navigation, not astrology.

Bowditch figure accompanying the description of "the zodiac"

Bowditch figure accompanying the description of “the zodiac” [4]

The integral part that the solstices and equinoxes play in Bowditch‘s definition of “the zodiac” suggests these astronomically-significant events warrant further consideration. Just three pages prior, Bowditch describes the equinoxes and solstices as follows.

…. The earth is at perihelion early in January and at aphelion six months later. On or about June 21, about ten or eleven days before reaching aphelion, the northern part of the earth’s axis is tilted toward the sun. The north polar regions are having continuous sunlight; the northern hemisphere is having its summer with long, warm days and short nights; the southern hemisphere is having winter with short days and long, cold nights; and the south polar region is in continuous darkness. This is the summer solstice. Three months later, about September 23, the earth has moved a quarter of the way around the sun, but its axis of rotation still points in about the same direction in space. The sun shines equally on both hemispheres, and days and nights are the same length over the entire world. The sun is setting at the north pole, and rising at the south pole. The northern hemisphere is having its autumn, and the southern hemisphere its spring. This is the autumnal equinox. In another three months, on or about December 22, the southern hemisphere is tilted toward the sun and conditions are the reverse of those six months earlier, the northern hemisphere having its winter, and the southern hemisphere its summer. This is the winter solstice. Three months later, when both hemispheres again receive equal amounts of sunshine, the northern hemisphere is having spring and the southern hemisphere autumn, the reverse of conditions six months before. This is the vernal equinox.

The word “equinox,” meaning “equal nights,” is applied because it occurs at the time when days and nights are of approximately equal length all over the earth. The word “solstice,” meaning “sun stands still,” is applied because the sun stops its apparent northward or southward motion and momentarily “stands still” before it starts in the opposite direction. [5]

Perhaps even more interesting than the description itself is the accompanying explanatory figure, shown below.

Bowditch figure illustrating the solstices and equinoxes.

Bowditch figure illustrating the solstices and equinoxes.[6]

Readers familiar with Robert Graysmith’s Zodiac [7] will recall Graysmith theorized that the Zodiac killer was planning some of his actions around the solstices and equinoxes. Unfortunately, Graysmith was not able to draw a compelling connection between “the zodiac” (the concept) and the four astronomically-significant times of the year. Furthermore, he confused the issue by attempting to incorporate holidays, astrology, horoscopes, and phases of the moon into his theory. By the time it was complete, the theory was an unconvincing hodgepodge that could seemingly account for a large subset of dates throughout the year.

However, the strong relationship between “the zodiac” of navigational astronomy and the solstices/equinoxes as described by Bowditch suggests there is something to the idea that the Zodiac killer acted in accordance with the timing of these events. Below is a table showing the relevant solstices/equinoxes and nearby Zodiac-killer-related events.

Celestial Event Date Zodiac-Killer Event Date Diff (days)
Winter Solstice 12/21/1968 Lake Herman Road 12/20/1968 -1
Spring Equinox 3/20/1969 <No Known Event> n/a n/a
Summer Solstice 6/21/1969 Blue Rock Springs 7/4/1969 +13
Autumn Equinox 9/23/1969 Lake Berryessa 9/27/1969 +4
Winter Solstice 12/22/1968 Belli Letter 12/20/1969 +2
Spring Equinox 3/21/1970 Kathleen Johns 3/22/1970 +1
Summer Solstice 6/21/1970 Richard Radetich [8] 6/19/1970 -2
Autumn Equinox 9/23/1970 Donna Lass? 9/6/1970 -17
Winter Solstice 12/22/1970 <No Known Event> n/a n/a
Spring Equinox 3/21/1971 Pines Card 3/22/1971 +1


Clearly, this correlation is not perfect. However, perfection is an unrealistic expectation given the numerous other constraints with which the killer likely had to contend.

As a final observation, it’s interesting to note that a copy of Bowditch is maintained on every ship in the United States Navy. Furthermore, at times in the past the book has been required reading at the United States Naval Academy. These facts further suggest that some type of association exists between the Zodiac killer and the Navy, albeit the nature of such an association remains unknown.


By drawing a compass-rose inspired Zodiac symbol on the Phillips 66 map that accompanied his June 26, 1970 letter, the Zodiac killer created a clear and compelling connection between his persona and the subject of nautical navigation. As I investigated this connection, material in the book American Practical Navigator (also known as Bowditch) strongly suggested the self-assigned moniker, “The Zodiac,” is derived not from the subject of astrology, but rather that of celestial navigation. Furthermore, the description of “the zodiac” in Bowditch makes clear that the solstices and equinoxes are intimately tied to the concept of the zodiac. An analysis of the dates associated with the Zodiac killer’s activities does strongly suggest a correlation between some of the events and the solstices/equinoxes. Finally, the fact that a copy of Bowditch is maintained on every ship in the United States Navy strengthens the previously suggested association of the Zodiac killer with the Navy.


[1] Image used with permission of Tom Voigt,
[2] Bowditch, Nathaniel, original author. American Practical Navigator. 1962 Ed. Washington D.C.: U.S. Naval Oceanographic Office, 1966. 106-107.
[3] Bowditch 374.
[4] Bowditch 374.
[5] Bowditch 371.
[6] Bowditch 371.
[7] Graysmith, Robert. Zodiac. New York: Berkley Books, 1987.
[8] The Zodiac killer appears to have implied that he was responsible for the murder of San Francisco Police Officer Richard Radetich when he wrote the following in his letter of June 26, 1970: “I shot a man sitting in a parked car with a .38.” Officer Radetich was shot in the early morning hours of June 19, 1970, not June 26 as implied in Graysmith’s Zodiac. SFPD has maintained consistently that the Zodiac killer was not responsible for the death of Officer Radetich.

Revision History

Version Date
1.0 June 19-26, 2006


All content, unless otherwise noted, is © 2006 Michael F. Cole. All rights reserved.