Friday, December 04, 2020

Manhattanhenge, Milankovitch, and Mystical Miscalculations

 Last weekend was the reverse Manhattanhenge in New York City. In order to explain what this is, I must first explain the Manhattanhenge, and in order to explain that, we need to start with the Stonehenge, in England.

Stonehenge is a Neolithic-to-Bronze age stone construction in Southwest England, It’s key feature, for the purposes of this article, is that part of it, the heel stone, is astronomically aligned such that at the summer solstice, the sun rises over it (as seen from the middle of Stonehenge.)

Manhattanhenge, then, is when the sun sets in alignment with the cross streets of Manhattan. This does not happen on the solstice, however; it generally happens about a month before and after the summer solstice. A reverse Manhattanhenge, then, is when the sunRISE is aligned with the cross streets in Manhattan. This generally happens a month before and after the winter solstice. For example, last Monday.

The reason for this is that the streets of Manhattan are aligned 29 degrees north of west, and the sun doesn’t set that far north at New York’s latitude until it reaches a declination of 22 degrees. That is, the sun is straight overhead at noon at a latitude of 22 degrees north. The maximum declination reached by the sun is about 23.2 degrees, which is the axial tilt of the Earth at the present time

Of course, as geologists, we are not bound to the present time. We can, and do explore deep geologic time routinely. And one of the things that changes over geologic time is the magnitude of the Earth’s axial tilt.

Milankovitch cycles are regular changes in the Earth’s orbit that are caused by the gravitational pull of the moon and other planets. One of the parameters which changes is the Earth’s axial tilt. When the tilt is larger, the seasons are more extreme, and when it is smaller, they are less extreme. For most of the Quaternary, from about 2.6 million years ago to about 1 million years ago, this cycle was the dominant influence on the Earth’s ice ages.

In terms of its influence on Manhattanhenge, that is easier to explain. A larger axial tilt would move both the normal and reverse events away from each other, so that the May and July events would both be farther from the Solstice. A smaller axial tile would move these events closer together, and nearer to the solstice. In fact, when the Earth’s axial tilt drops to 22 degrees, then, just like at Stonehenge today, the sunrise and sunset alignment will coincide with the winter and summer solstices.

A common theme in 20th century speculative fiction, which is still sometimes repeated today, is that Earth’s Neolithic to early bronze age edifice-building civilizations worshipped the sky because they had alien patrons which would come to Earth and assist them with the otherwise implausibly large edifices which they built. While this theory has previously been used in science fiction to explain the Egyptian Pyramids or the Stonehenge, I do not know that it has been applied to Manhattan. However, as geologists, we have the tools to do seen if and when this happened.

First, we need a Milankovitch calculator. These can be found on the web, via either NASA or Colorado State University. Then, we can use the Milankovitch cycles to date *when* the aliens came to set up the urban street grid, based on when the Milankovitch cycles last dropped the Earth’s tilt to 22.0 degrees.

Unfortunately, like many science fiction ideas, this one gets less brilliant the longer we look at the numbers.  Any date for the alien founding of Manhattan needs to be less than about 17000 years ago, since before that the island was buried under a glacial ice sheet. This is a problem, because Axial tilt is currently declining; it reached a maximum about 9000 years ago, and hasn’t been lower than the current level for 19,000 years, a time when Manhattan was in the deep freeze. So it seems unlikely that the streets were laid out to align with the solstice. But there’s a more fundamental problem.

The last tilt minimum was 29,000 years ago. But the tilt at that time was 22.2 degrees- in otherwords, the tilt never got low enough to put Manhattanhenge on the Solstice. Now, as it happens, the magnitude of the tilt cycle varies a fair bit from cycle to cycle. The previous minimum, 70,000 years ago, had a tilt of 22.3 degrees (according to the Colorado State University model linked above). In fact, at no time on the past 5 million years does the tilt drop to 22.0 degrees to put Manhattanhenge on the Solstice.

A responsible researcher would give up at this point and conclude that the streets were laid out on the angle decided in 1811, just like all supporting documents state. But because I’m blogging after midnight, I’m going to double down on the crazy:

What if TIME TRAVELLING aliens sent a message back in time to set New York up on this angle to align with a future solstice? After all, it turns out that in 4.77 million years, if New York is still around, the Earth’s tilt will drop to 22.0 degrees, and whoever is still left in the Tri State Area will be able to head into The City and watch the sunset between the skyscrapers on the longest day of the year. I hope they aren’t still in lockdown when that happens.