The ancient Greeks were a people of many fathers, at least according to their modern academic fan clubs. Hippocrates is considered the Father of Medicine, to the point that the “First do no harm” mantra recited by all physicians is called the Hippocratic Oath. Euclid is known as the Father of Geometry, or at least patron of the huge swath of the field known as Euclidean geometry. Homer is called alternately the Father of Epic Poetry and the Father of the Novel, such that any sufficiently poetic and epic work is described as Homeric. And, of course, there’s Hippodamus, who gave the world the Hippodamian Plan.

Yeah, we probably lost you on that last one. Nonetheless, the modern world owes nearly as great a debt to Hippodamus of Miletus as we do the likes of Aeschylus, Father of Greek Tragedy; Herodotus, Father of History, or Aristotle, Father of… well… Modern Thinking.

Hippodamus, you see, is the Father of Urban Planning, and arguably the first great scholar of the notion that cities (and even societies) could be ordered, structured, rational objects. During his lifetime (498 BC to 408 BC), Hippodamus quarreled with Aristotle over the philosophical antecedents of patent law and counted among his clients Pericles, for whom he designed the harbor district of ancient Athens.

Hippodamus’ namesake, the Hippodamian Plan, pioneered the grid layout of planned cities, as well as the distinctions between public, private, and sacred land. While grid-like city plans date back two millennia before Hippodamus, his writings on the subject define the features and rationale behind urban planning. It is this legacy that impacts the modern world in ways that few suspect. For an example, one needs look no further than contemporary New York City.

In 1811, the legislature of the state of New York adopted the so-called Commissioner’s Plan, which laid out the bulk of land on Manhattan Island into its modern — and largely still intact — grid street layout. The vast majority of the city layout between Washington Heights and 14th Street is defined by the 1811 Commissioner’s Plan, which is in turn directly derived from a Hippodamian Plan. Streets meet at regular intervals in right angles, and intersections near the shoreline are more common, as land there is deemed more valuable, thus requiring more traffic allowance and subdivision.

Yet, for all its Hippodamian origins, the Commissioner’s Plan has a notable astronomic side effect, one that evokes an ancient monument whose earliest construction predates every planned city on Earth.

WHAT ANCIENT ASTRONOMIC MONUMENT IS ACCIDENTALLY MIMICKED BY THE COMMISSIONER’S PLAN THAT DEFINES THE LAYOUT OF NEW YORK CITY?

Get the answer.

To quote comedian Eddie Izzard, we’re talking about “one of the most famous henges in the world,” Stonehenge. While many debate the purpose and function of Stonehenge, one unassailable fact of this ancient megalithic structure is that, during the summer solstice, the first rays of morning sun fall upon the center of the monument through its horseshoe opening. This event occurs generally along the axis of its Heel Stone — a particular megalith near the “mouth” of Stonehenge. Whether this alignment was intentional on the part of the builders of Stonehenge — and if so, what ritual or scientific significance that alignment held — no one can definitively say. Nevertheless, it is a striking phenomenon, and one that is spectacularly imitated by the streets of Manhattan.

Thus, we have the phenomenon known as Manhattanhenge. On or around January 8, May 28, July 12, and December 5 each year, the east-west Streets (as opposed to north-south Avenues) defined by the Commissioner’s Plan of 1811 align with the setting sun. The window glass and reflective surfaces lining these avenues are dramatically illuminated on these dates (weather permitting), which is what led celebrity astrophysicist Neil deGrasse Tyson to dub the event Manhattanhenge.

The Commissioner’s Plan grid is offset by 28.9 degrees from true east-west alignment, which is how the “north-south” avenues can actually face the westerly setting sun. The exact dates of Manhattanhenge vary slightly year to year, but they are usually evenly spaced before and after the winter and summer solstices — much like the alignment events at Stonehenge.

Manhattan doesn’t have a monopoly on this faux-henge feat. Presumably, most any Hippodamian-style city layout could create the effect at some point in the year, so long as the horizon is visible at sunset. However, no planned city’s solar alignment has the equivalent notoriety — and/or equally catchy nickname — of Manhattanhenge. That’s not just particularly ponderous public relations, it’s some cartographically commemorative Geek Trivia.

The Quibble of the Week

If you uncover a questionable fact or debatable aspect of this week’s Geek Trivia, just post it in the discussion area of the article. Every week, yours truly will choose the best quibble from our assembled masses and discuss it in a future edition of Geek Trivia.

Check out this week’s quibble.

Falling behind on your weekly Geek fix?

Check out the Geek Trivia Archive, and catch up on the most recent editions of Geek Trivia.

Test your command of useless knowledge by subscribing to TechRepublic’s Geek Trivia newsletter. Automatically sign up today!