THE STONEHENGE EFFECT IN NEW YORK CITY
  ====================================
 John Pazmino 
 NYSkies Astronomy Inc
 nyskies@nyskies.org
 www.nyskies.org
 2003 May 24 initial
 2013 July 16 current
Introduction
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    On the Salisbury Plains in the southern part of England is a
stone-age structure unique so far on Earth. It is Stonehenge, a
multiring of stones and holes which supposedly served as a calendar
and calculator of celestial events.
    What makes this structure unique is that it has several post-&-
lintel bays, which were once complete all around the ring, about 30
meters in diameter. There are many other circles of stones thruout
England, but they never had lintels, only posts.
    Regardless of the actual use of Stonehenge, which is still under
debate, it is true that on the first day of summer, near June 20th of
each year, the Sun does rise along the axis of the stone ring. Beyond
the ring were two stones, placed so the Sun rose between them. Today
only one remains, called the Heel Stone. The Sun rises alongside of
it, throwing off earlier attempts to explain the purpose of the Heel
tone and leading to speculation that the ecliptic suffered major
shifts of inclination.
    The sunrise on the first day of summer, erroneously called
'midsummer's day' is today a festival occasion at Stonehenge. It is
thronged by tourists and presided by local members of the druid
group. With the dismal weather of England, we can wonder just how
useful Stonehenge was for monitoring any sunrise!
New York's Stonehenge
 -------------------
    Stonehenge was reproduced all over the world. Sometimes the
alignment matches the original, with due consideration for differences
in latitude. Other models were haphazardly set and are not at all
aligned with anything celestial.
    New York does not have a specific Stonehenge model but in fact it
does after all by fortuitous accident. The axis is the east-west
streets on Manhattan and the stones are the very towers that so
dramaticly characterize the City.
Geography lesson
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    Manhattan is an island about 20 kilometers long and barely 2-1/2
kilometers at the widest. The gridiron of streets etched onto
Manhattan aligns with the island, a long skinny spar of bedrock. The
east-west streets, the 'streets', run along the short dimension of the
island. The longest continuous east-west street is 14th Street. The
north-south streets, the 'avenues', are orthogonal to the streets and
run along the long dimension of the island.
    The direction of movement along the avenues to the north is called
'uptown'. That heading south in the avenues is 'downtown'. These words
are directions and not placenames. There is no such a place on
Manhattan called 'downtown'.
    Now Manhattan is inclined from the lat-lon grid such that its
streets run 29-1/2 degrees right of the due east-west points on the
horizon. That is, their bearing is 119-1/2 degree to 299-1/2 degree of
celestial azimuth.
A little history
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    The City began as a Dutch settlement at the southern end of
Manhattan, near present South Ferry. The public commons was at the
foot of Broadway and was a field for playing the game of bowls. This
game in time developed into our modern bowling. This is now Bowling
Green, an urban greenspot.
    The main arterial road out of the City was Broadway. It ran on a
ridge along the central axis of Manhattan to the northern barrier
wall. The wall was merely a dirt berm cribbed with logs. It deterred
indigenous natives from intruding but would be easily overrunned by a
European attacker. This line is now Wall Street.
    As the City grew, Broadway was the main road leading north. It
exited the City thru a gate in the wall, and continued on the median
spine of the island. Today, in numerous places, despite land shaping
over the centuries, you can see the terrain sloping downward away from
Broadway.
    Broadway was linearly extended northward and most other new
streets in the growing City were laid out parallel and perpendicular
to it. By the 1810s, it was obvious that eventually Manhattan would be
completely covered by New York City and a formal plan of new streets
was enacted. The existing streets, mainly those in the older Dutch
part of the City and in Greenwich Village -- a separate town back then
-- were left alone. The new streets would parallel Broadway or be
orthogonal to it right up to the Harlem River on the northern flank of
the island.
    It just so happens that this pattern is skewed on the lat-lon grid
by that 29-1/2 degrees.
Some astronomy
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    The Sun cycles thru declination -23.5 degree to +23.5 degree over
the year. The Sun's declination is zero as he crosses the celestial
equator on the vernal and autumnal equinoxes. His declination is +23.5
degrees on the summer solstice; -23.5 degree, winter solstice.
    In consequence, the sunset point wanders from due west over the
year. From the vernal equinox, thru the summer solstice, to the
autumnal equinox he sets to the right (toward the north) of due west.
His maximum departure from due west is at the summer solstice, when he
also has the maximum northern declination.
    The swing of the sunset point is the Sun's amplitude. Astronomers
differ whether it is the full spread from farthest left to farthest
right or just the span from due west to either farthest right or left.
In any case, this amplitude is a function of latitude, being least for
the Earth's equator and most for the arctic and antarctic circles.
    In New York at latitude 40.7 degrees north, the amplitude,
counting from due west to the solstice sunset point, is 31.7 degrees.
This is surprisingly close to the 29.5 degree skew of the streets!
A compromise
 ----------
    If the streets were aligned to 31.7 degrees the Sun would set
along them on the summer solstice. I neglect the effect of the
atmosphere, which would raise the Sun as much as a half degree when he
is geometricly on the horizon. The Sun would appear to set later and
farther to the right than without an atmosphere. However, the extent
of this lifting is highly variable and can not be predicted in
advance.
    In the absence of the atmosphere, I note that the Sun would set
along Manhattan's streets when his declination is not +23.5 degrees at
the solstice, but at 21.9 degrees. This occurs on the 22nd day before
and after the actual summer solstice, on May 30 and July 12 in 2002.
The solstice was on June 21st.
    OK, New York does not have a perfect Stonehenge effect, but one
that's amazingly close given the purely terrestrial accident of its
street grid! I would allow a couple days of window to hedge against
clouds and summer shmutz. Also, the solstice floats on the calendar.
Let the window be May 29-31 and July 11-13.
    Note that the lifting of the Sun by atmospheric refraction
actually enhances the effect. When the Sun should be just disappearing
below the horizon on, say May 30th, it is in fact sitting on the
horizon centered along the street!
A little leeway
 -------------
    The geometric dates for Manhattan's Stonehenge are 30 May and 12 
July. Because the Sun is a disc, not a point, a street has some 
wideth, and the refraction of the atmosphere is so erratic, there is 
some leeway around these dates.  You can experience a good Stonehenge 
effect between 28 May and 1 June and between 10 and 14 July.
    The range of dates allows for adverse weather, schedule conflicts, 
remoteness from the viewing site, flubbed viewing attempts, and other 
contingencies. Yes, you can avail of the range of dates togo out and 
see the Stonehenge sunset more than once, 
Manhattanhenge 
 ------------
    In the first couple years that the Stonehenge effect was seriously 
anticipated there was no special name for it. It was the Manhattan 
Stonehenge sunset. The routine association of 'Manhattan' with 
'Stonehenge' soon merged the words into 'Manhattanhenge', It was a 
madeup word that had good currency among New Yorkers starting with the 
2006 pair of sunset events. 
    Manhattanhenge took off since 2006 to become the definitive term 
for the peculiar sunset show on Manhattan. It was adopted thruout the 
world and appeared in both astronomy and cultural litterature. 
    It is capitalized, after the name of the island and as the proper 
name for this specific astronomy phaenomenon. On 2013 June 1 the 
Oxford English Dictionary 'officiated' Manhattanhenge as a new valid 
word! Its definition was very brief: 
    'Q phenomenon occurring just before and after the summer solstice, 
in which the sun sets in alignment with the streets that run east to 
west on Manhattan's street grid; this year's first opportunity for 
seeing Manhattanhenge's glory came Tuesday night.' 
    This a bit TOO brief! It makes no mention of the Stonehenge effect 
of the flanking towers! This definition could apply for any linear 
sightline into the sunset. 
    It also is awfully fuzzy about the date of the event in 'this 
year'. I'm guessing, since the word entered the OED in 2013, taht it 
refers to May 28, which is a Tuesday in 2013. This is the start of the 
season, which runs thru May 31. 
    Let's see how Manhattanhenge is treated next year on the OED 
website or in a paper edition of the book. 
    Manhattanhenge was a joyful highlight of the 8th conference 
'Inspiration of astronomical phenomena' on 2013 July 7-12 at the 
American Museum of Natural History. Altho the weather was hazy and 
humid, the delegates enjoyed satisfying views on July 11 by standing 
in 79th Street near the Museum. The street by prior arrangement was 
closed from road traffic for safety sake. 
    Each season the hayden Planetarium offers a public show, as was 
done during the conference, to explain Manhattanhenge and prepare for 
clearsky viewing of it from a nearby street corner. 
    Other towns with an interesting sunset, or sunrise, alignment try 
to embed 'henge' in a name related to the town, like 'Chicagohenge'. 
None so far has the distinctive ring of Manhattanhenge and none has 
the Stonehenge effect any way close to that on Manhattan. 
Practicalities
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    Ideally, you would stand in the middle of a street, like 42nd
Street, and wait for the Sun to sink to the horizon. If you try,
you'll be rudely shoved off by vehicular and pedestrian traffic. You
can make do by standing on the sidewalk at curbside and looking down
the street. In this case, you'll see the Sun set on your same side of
the street rather than dead centered on it. 
    Or you can let the Sun set a bit off center, but brought back to 
center by your vantage point at curbside.  
    Pick a street with no obstructions. 27th Street is blocked by 
Fashion Institute and 64th Street is blocked by Lincoln Center. If 
you are on the Upper East Side, you better get to the west side to 
avoid the trees of Central Park. Check out an addiurnate street map 
of Manhattan.
    Standing on Broadway to get on the central spine, gives a good 
sightline along the street. On your peuliar block, the summit may be a 
a hundred or so meters to either side of Braodway. 
    Elevation helps. Viewing from a terrace or roof fronting an east 
west street throws you far off the line but gets you above trees and 
traffic obstructions. The ideal case would be a skywalk spanning an 
east-west street with a clear prospect of the City. One popular site 
is the Tudoe City overpass on 42nd St. 
    The stayed-cable bridge on 63rd Street, joining two parts of 
Rockefeller University, would be a possible viewing site.  So would 
the skywalk within Hunter College. You must have prior admittance to 
these facilities.. 
Public spectacle 
 --------------
    As word of the Stonehenge sunset spread, thru dialog with the 
Museum and NYSkies, ever more people watched it. From small gaggles of 
curious folk until 2005 the event blossomed into a massive public 
outpouring of celestial awareness. 
    With perhaps a hundred good accessible locations for viewing, 
there was no need to congregate in one special location. Dozens to 
hundreds assemble at each viewing site. 
    According as the interest and enthusiasm in each district, a 
certain street may be closed from traffic to let people stand safely 
on its centerline. This is arranged thru civic associations or the 
Community Board. They have the political liaison to get such favors 
done for at least one of the several sunsets in each season. 
    Without a street closing, there are instances of people filling a 
street, as if for a rally or demonstration, and clogging traffic from 
it. I do not encourage such behavior for safety and legal reasons! 
    So far there seems to be no organized public parties, like for the 
Venus transit or an eclipse, to watch the Stonehenge sunset. The shows 
at Hayden Planetarium seem to be the only ones. I suppose there are 
socials for friends and family at apartments with sightline over 
ground obstructions. 
    I myself recall the many occasions in the 1980s and 1990s when I 
had local astronomers stop at my housesitting crib on the Upper East 
SIde. It had a terrace with a west view along a street over the trees 
of Central Park. The effect in this situation was muted because there 
was no palisade of tall towers along that particular street. 
Weather 
 -----
    The two windows for viewing the Stonehenge effect are in the New 
York summer hot & humid season. You could be waiting for the sunset 
under oppressively uncomfortable conditions. In such combination of 
temperature and humidity, the body can not transpire properly, leading 
to sweatiness, drowsiness, internal loss of water. 
    Wear light loose clothes, have a bottle of fresh water. Walk and 
climb stairs slowly, don't run or jog. If feasible, view at a site 
with nearby shade like trees or walls. Bring a folding stool or chair 
to rest on. A battery -- or solar! -- powered fan can be very 
refreshing. 
    One horrible threat against comfortable Stonehenge viewing is the 
late afternoon thunderstorm. In New York commonly on the summer days a 
tumultuous storm erupts in the mid to late afternoon, throwing many 
centimeters of rain with blinding lightning and crashing thunder. 
    You will, if at all possible stay out of such a storm. Most likely 
it'll occur while you are indoors at work or school, or when you can 
retreat into a store or eatery. 
    The typical storm lasts only a half hour, but hour-long deluges 
are annoyingly frequent. On the whole, regardless of the start and 
duration, the storm abates before sunset. In fact, the sky usually 
clears up rather nicely for the Stonehenge effect. 
    However, you could then be soaked thru and thru, taken on a nasty 
disheveled feeling. The ground and street furniture, parapets may 
still be wet. Grassy sites may still be soft and squishy under the 
foot. In the event of the afternoon thunderstorm, arm yourself with 
newspaper to lay on wet furniture, be willing to  take splashes and 
drips. Protect your photo gear from water by keeping it in a carry bag 
or under your shirt/jacket between shots. 
Rainbows? 
 -------
   While the Sun is setting in the northwest along a Manhattan street, 
the antisolar point is rising in the southeast along that same street. 
If there was a rain before ou arrive at your viewing site, there is 
the chance to spot a rainbow. 
    On the whole, storms sweep over the City from west to east, When 
the sky is clearing in the west for the sunset, rain could still be 
falling in the opposite direction. This makes a wall of raindrops 
catching the rays of the setting Sun to generate rainbows. 
    There's plenty of slack time before the actual sunset to turn 
around and look down the street, away from the Sun, and look for a 
rainbow. The shadow of your head is the center of the rainbow. 
    Because of a rainbow's large diameter, about 23 degrees, you 
should ideally be at a site with an open view to the east and 
southeast. With a street hemmed in by towers, you may see only the 
upper segment of the bow centered over the street. 
    Nothing is promised, even if there was a rain that by sunset is 
retreating into the east. The rain may have ended, moved out of 
alignment with the Sun, or shrunk too small to make rainbows. 
    You could get a double or multiple bow! The extra ones are outside 
of, and concentric with, the main bow. 
    Photographing a rainbow is a bit tricky. Metering off of the sky 
itself can produce an overexposed picture. The rainbow is diluted with 
weak color. Meter off of the foreground scenery, lock the setting, and 
shoot with it at the bows. 
Viewing the Sun
 -------------
    While it is not healthy to stare at the Sun without protection, it
is safe to take quick glances at the setting Sun of summer. The air is
almost guaranteed to be shmutzed over, making the Sun a red ball of
tempered brilliance. You can safely photograph this Sun without fear
of burning out the shutter. Just don't look steadily into the Sun or
gaze at it for long moments.
    If the Sun is at all uncomfortably brilliant, don't try to
overcome the natural impulse to turn away. Monitor the Stonehenge
effect by your shadow in the street or reflection in a clear window.
Traffic hazard
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    Viewing the Stonehenge sunset from the ground can be dangerous! 
Standing in the centerline of a street puts you in the line of 
traffic, with potential abrupt termination of life. I'm almost sure 
one or two east-west streets have a safety median, but I'm not sure 
which., nor if they offer the required sightline to the west. 
    For the quick and nimble viewer, there is a trick, but you REALLY 
MUST stay alert. Watch the traffic flow for a few cycles of traffic 
light changes. Between the flow in one direction and that in the 
other, there may be a sensible delay, when there is for the instant no 
traffic in the street. Once you get the timing just right, you can 
walk across the street in this dead instant, look at the sunset, then 
continue to the opposite curb. 
    The window of opportunity is brief, only mere seconds. You may 
have to keep trying on following cycles of the traffic lights to get 
in a good view. SInce it takes the Sun a couple minutes to touch, 
cross, and sink at the horizon, you can get in several attempts. 
    To take pictures of the sunset, prepare the camera while on the 
sidewalk,aim it at the Sun, then hold the camera parallel to itself 
while walking across the street. Snap the shutter, and move off to the 
opposite side. This takes a bit of practice! 
    These cautions are tough enough to mind for a typical summer 
sunset with a subdued Sun. On the occasional clear dry summer day the 
Sun can be blinding. It will then take substantial discipline to keep 
the Sun from distracting you from the traffic. 
    You may have to, after dilligent effort, be content with an offset 
view, where you stay on the sidewalk and watch the Sun set into that 
same sidewalk. 
Other towns
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    The Stonehenge effect can be experienced in other towns with a 
long straight road aiming within the Sun's amplitude. The road should 
be at least one kilometer long westward from your vantage point and 
have a clear horizon beyond. A little relief is a plus so you can 
stand on an elevated part of it.
    The azimuth of the street is obtained from county or town street
maps or land surveys of property along the street. Calculate the
amplitude of the Sun by
 cos(azimuth) = sin(23.5deg) * cos(latitude)
          = (0.3987) * cos(latitude)
The answer will be the azimuth from north round to east, or the
azimuth of the summer solstice SUNRISE. This comes from the behavior
of inverse trig functions and quadrant quandary.
    Forget all the crazy rules for finding the correct quadrant for
the sunset point. Use the symmetry of the sky. That is, count off the
azimuth round to WEST, backwards from 360 degrees. Then reflect this
to the left (south) side of west to get the azimuth of the winter
solstice sunset. Your street better aim between these two azimuths.
    If your road passes the above test, you next find the declination
of the Sun such that he does set along the road. Rotate the above
equation
 sin(declination) = cos(azimuth of road) / cos(latitude)
    From a solar ephemeris or astronomy calculation program, find the
dates when the Sun has this declination. These are the two dates the
Sun sets along your road. You should double check all this number work
with a computer planetarium program. It's up to you to include a
nominal atmospheric refraction or not, according as the program allows
you to switch this feature on or off.
    The dates may have no relation to the equinoxes or solstices, but
could be close to some local celebration, like a county fair or
founder's day. You can include a viewing of the special sunset as part
of that celebration.
Rectified street grid
 -------------------
    A very common street pattern aligns with the cardinal compass
points. Streets run due north-south and due east-west. Sunset along
the east-west streets occurs at the two equinoxes. The Sun culminates
at true local noon along the north-south streets.
    There is a bonus for such a favorable street pattern. The clock
hour of the Sun's meridian crossing, when he is shining exactly along
the north-south street, may be far off from 12:00. I leave out the
silly shift of the clock for daylight savings time.
    There are two main reasons for this deviation from clock noon.
    First is the equation of time, combining the variable speed of the 
Sun
in the ecliptic and the inclination of the ecliptic from the equator.
These conspire to put the Sun alternatively a little ahead or behind
his position based on a steady motion. It's the latter that governs
the setting of clocks, due to the mechanical simplicity of uniform
movement of the hands around the clock face.
    The other is the offset of your town from the central meridian of
your timezone. With the gerrymandered frontiers of timezones, the Sun
may be so far off as 3/4 hour from clock noon! This is specially true
if you are west of the central meridian because over the decades there
is a slow creep westward of the zone boundaries. This effectively
builds into 'standard' time a defacto allyear daylight savings time.
Sunrises
 ------
    All of the considerations for the sunset apply to the sunrise. The
dates of the sunrise along your road are found in exactly the same
manner. The reason sunrises are not so well observed is that they
occur in the hour when you're still asleep or fixing up for a day of
work or chores. However, by knowing that on that certain day the Sun
rises in the line of a certain road, you may shuffle your daily
routine to go and watch that sunrise.
The Islamic Center
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    Spinoffs of New York's Stonehenge effect are at least two major
collateral features of astronomy interest. These may be experienced
without regard to date, so please include them in your next visit to
the City. They are the Islamic Center and the McGraw-Hill Gnomon.
    In Islam, the congregation in prayer faces toward Mecca. Ideally,
the prayer house, the mosque, also faces Mecca, but in New York most
mosques are built into existing buildings with no such perfect facing.
In all mosques, old and new, there is placed on the wall a marker so
the people inside can correctly face Mecca.
    The largest mosque outside the Islamic belt was built on Manhattan
in the late 1980s. It's the Islamic Center on 96th Street and 3rd
Avenue, taking half of the block. Being a new facility, it faces Mecca
so that it is all twisted relative to the street grid.
    What is the direction toward Mecca?
    Early Islamic astronomers wrestled with this problem and 
developed some good geography and navigation. The direction to Mecca 
is the 'qibla' and is the fundamental point of reference in the 
surrounds of the Moslem in prayer. The term is used in English for a 
person who is disoriented, like in an unfamiliar shopping mall. He 
lost his qibla! In modern navigation, the qibla is the azimuth of the 
great circle joining the praying person to Mecca. It is now a trivial 
exercise in spherical geometry to calculate this azimuth; for New 
York it is 59.5 degrees.
    The Islamic Center, therefore, sits turned by 30 degrees against
3rd Avenue or 60 degrees against 96th Street, making it look, well,
twisted out of line. It is correct for its purpose based on astronomy.
The McGraw-Hill gnomon
 --------------------
    At 48th Street on 6th Avenue stands the headquarters tower for
McGraw-Hill publishers. In its sunken plaza stands a stainless steel
triangle on a tall stalk. It at first looks like some funky artpiece,
like so many others along this reach of 6th Avenue. For sure its
twisted on the street grid.
    This structure is in fact a real noon marker or gnomon. When the
Sun is culminating on the meridian, the shadow of this gnomon
collapses into a straight line. Moreover, the sides of the triangle
are inclined to aim at the Sun at the summer solstice (south upright
side), the two equinoxes (upper longest side), and the winter solstice
(lower side on the stalk). Alas, a tower in 6th Avenue at 47th Street
blocks the very noon Sun at the winter solstice. You'll see him about
ten minutes later when he moves out from behind this building.
    Why is this thing here at all? 
    In the 1960s McGraw-Hill figured to build in its tower the 
largest fanciest planetarium on Earth, being that it then owned the 
Spitz planetarium company. The plaza would be the grand entry to this 
monster facility and had already been built to have three astronomy 
exhibits.
    The gnomon is one. The second is the stone maps on the floor
showing the continent drift over the eons. This is now intact but
ignored by McGraw-Hill. The last was a model of the solar system, now
a jazz cafe. The diameter of the circular plot, once having a stone
parapet, represented the Sun's diameter. Mounted on short standoffs on
the parapet were polished steel globes proportioned to the planet
sizes. Their placement on the parapet was angularly proportional to
their linear distance from the Sun. I leave out the tedious
explanation but that's how they were placed.
    In a short time, like a few months, the globes succumbed to
liberations and eventually were removed by McGraw-Hill. Then McGraw-
Hill sold off the Spitz company and gave up on the planetarium
project. The vast belly of the tower was remade into the New York
Experience multimedia tourist show. It ran for some fifteen years
before closing for being outmoded and outdated. Today the space is
occupied by community rooms, a theater, and various stores.