The Cosmos in a Heartbeat 2: Coming of Age

by Shane L. Larson

Astronomy from your backyard is dominated by naked eye stargazing, and optical telescopes that gather exactly the same kind of light you see with your eyes. There are a few amateurs who ply the skies with radio telescopes, or build cosmic ray detectors in their kitchens, but for the most part it is traditional telescopes. When I first started my studies in astophysics, this was still largely true of professional astronomy, though there were a few advanced experiments that were building the technology needed to survey the Cosmos using astroparticles, and a robust effort to design what would become the world’s first successful gravitational wave detectors.

One of the things that is true of all telescopes, no matter how they are designed to see the Cosmos, you can always build a better instrument that can observe more. In the context of astronomy, “more” means detecting cosmic phenomena that are difficult to perceive, or probing deeper into the distant Universe.

My two current telescopes (both homebuilt). The one on the left is called Equinox (12.5″ f/4.8 Dobsonian Reflector) and the one on the right is called Cosmos Mariner (22″ f/5 Dobsonian Reflector).

For optical telescopes, bigger is better. The larger the mirror, the easier it is for the telescope to capture a few tiny bundles of precious light that arrive on Earth and gather them all together in one place (your eye, or a camera) so there are enough to be bright enough to see. When I first started in backyard astronomy, I had an 8-inch telescope, which is about 30 times larger than the pupil of my eye, and so can gather almost 850 times more light than my eye. That first telescope of mine has been passed down to my daughter as part of her growing interest in backyard astronomy. It has changed its look, but its heart is still the telescope that I spent many hundreds of hours out under the night sky with. I’ve built a new, bigger telescope to replace my old one. Shown above is my telescope called Cosmos Mariner, and has a 22-inch mirror in it. That mirror is about 80 times larger than my pupil, and can gather more than 6000 times the light my eye can!  Mariner can probe deep into the Cosmos, and I use it as often as I can to soak in the wonder of the night sky.

The Hubble Space Telescope. Arguably the most capable and successful telescope ever built by humankind. [Image: NASA]

Professional astronomers have also been building bigger and better telescopes. Shown above is the Hubble Space Telescope, which launched in 1990, and is arguably the most successful scientific instrument in history. It’s mirror is 2.4 meters in diameter, but it peers at the Universe from a vantage point high above the atmosphere of Earth. In its three decades of observing, it has seen farther and seen more than any telescope in history. All told, some 10 to 15 thousand papers have been written about what Hubble has seen in the Cosmos. When we build a machine like Hubble, we always have grand plans for it. One of the tasks we had in mind was to use Hubble’s size and vantage point high above the Earth to take a picture of the Cosmos unlike any other before it. We picked a non-descript region of the sky in the constellation Eridanus, and over the course of a decade, asked Hubble to go back and look at that point over and over and over again, until it had stared at that one spot for a total of 23 days. You take all those individual pictures, and you stack them together to make a single new picture.

The Hubble Extreme Deep Field (XDF). [Image: NASA/ESA]

What you find is that in a region of the sky where you thought there was nothing, there is very definitely something. We call this picture the Hubble Extreme Deep Field. This image covers an area on the sky roughly the size of the eye of needle, held at arms length. Within it, virtually every fleck of light and color you see, is another galaxy. All told there are some 5000 individual galaxies in this image, implying that across the entire sky there are several hundred billion galaxies. This is what we mean when we say the Cosmos is vast beyond our wildest imaginations.

The Hubble Extreme Deep Field is a flat, two-dimensional image not unlike other pictures you are used to looking at. But one of the things we can do in modern astronomy is measure the distances to galaxies, so we can make a movie of what it would be like if you could dive into this image, making the impossible journey from Earth to the most distant galaxy in the image. If we launch ourself on that voyage, the first thing we notice is that over most of the journey, we are travelling through nothing at all.

The Universe, on the grandest scales, is mostly empty of any ordinary matter like you and me or stars or galaxies.  As we plunge deeper, we do encounter groups of galaxies — they tend to cluster and grow together, making a vast cosmic web that fills the entire Cosmos. Many of the galaxies we pass look familiar, like we expect galaxies to look. But as we travel farther and farther into the Deep Field, we are looking back farther and farther in time, until we reach the most distant galaxies in the image. These are the youngest galaxies we’ve ever seen, born barely 450 million years after the birth of the Universe. Astronomy is a special kind of time machine — looking back across the Cosmos is looking back in time. We can see how galaxies were long ago, in an effort to understand how our own galaxy might have been long ago as well.

The surface facilities of IceCube at South Pole. [Image: IceCube Collaboration]

Our colleagues in neutrino astronomy have been building their own new generation of observatories as well. Today, the pre-eminent neutrino observatory in the world is located at South Pole Station, and is called IceCube. IceCube is a series of 86 deep cores drilled 1.4 to 2.4 kilometers down into the Antarctic ice. Long strings with camera modules are lowered into the bores on cables, and the ice refreezes around them; all told there are more than 5000 cameras (“Digital Optical Modules”) in the array.  Observing with IceCube is in principle the same as KamiokaNDE — neutrinos pass through the Antarctic ice, and sometimes interact with the atoms of the ice to produce a burst of light that is picked up by the cameras on the strings. 

The IceCube array is encased in the ice underneath the surface station. [L] The long ice cores contain strings with cameras, and when an event passes through the array the light is detected by some of them. [R] The most energetic neutrino event yet detected, on 22 September 2017. [Images: IceCube Collaboration]

In September 2017, IceCube picked up the most energetic neutrino ever observed. It blasted through the array at  20:54:30 UTC (around 2:54pm Central Standard Time), and was detected by a long sequence of camera modules that stretched from one side of the array to the other. Looking at the shape of the data in the array, and in particular which side of the array lit up first in the detection, allowed astronomers to point backward from IceCube to the point on the sky where the neutrino came from.

An artists impression of a blazar, a type of active galactic nucleus with a supermassive black hole at the center, and an energetic jet pointing at the observer (us, on Earth). This is the type of object the IceCube neutrino event was traced back to. [Image: DESY Collaboration]

Using this pointing information, astronomers searched that region of the sky and discovered a blazar called TXS 0506+056. A blazar is a kind of galaxy that has an active galactic nucleus (AGN). AGN are supermassive black holes that have a swirling maelstrom of gas and material around them that slowly feed the black hole. When gas gets close to the black hole, the gravitational attraction propels it onto high speed orbits around the black hole. All of the gas swirling around together interacts strongly with the rest of the gas and as a result gets very hot; hot gas glows brightly, especially in ultra-violet and x-rays, both very energetic forms of light. The bright light can be seen in telescopes from Earth indicating the presence of a supermassive black hole. As the gas swirls ever inward, some of it eventually falls into the black hole, vanishing forever. Some of it, however, is spining so fast it cannot fall onto the black hole, and gets squirted out into an energetic jet, propelled away from the nucleus of the galaxy at enormous speeds. This is characteristic of AGN; a blazar is simply an AGN where the view from Earth is staring directly down the jet.

All the material being ejected down the jet is moving at very high speeds, and still collides with other material making its own way out through the jet. This energetic environment is not unlike our own particle accelerators here on Earth, and at some point along the jet a collision between particles created the neutrino we detected with IceCube.

This is once again multi-messenger astronomy — observing the same astrophysical object, using both astro-particles and telescopes. What is different in this case is the discovery was led by neutrino astronomers, who guided telescopes toward the right place in the sky.

The twin 10-meter Keck Telescopes on Mauna Kea. [Image: Wikimedia Commons]

Now, most of us know what astronomers know: black holes are AWESOME. Active Galactic Nuclei are not the only big black holes in the Cosmos. There is, in fact, a massive black hole much closer to home, in the center of our own Milky Way. Astronomers on the ground have been building bigger and bigger telescopes, and today among the largest telescopes in the world are the twin 10-meter Keck Telescopes on Mauna Kea, in Hawaii. We’ve been using the Keck Telescopes, togther with others around the world, to peer at the exact center of the Milky Way for the past two and a half decades. The telescopes have been able to detect a small cluster of stars we call the “S-Cluster.” We’ve been watching them long enough now that we have not only seen the stars move on their orbits, but we’ve seen some of them complete their orbits.

Two decades of observations have shown the orbits around the 4 million solar mass black hole at the center of the Milky Way. [NCSA/UCLA/Keck]

If you remember back to your early classes in physics or astronomy, you may remember that someone like me once told you that if we can measure orbits, the Universal Law of Gravitation (published by Newton in 1687) can be used to discover the size of the mass that is driving the orbit. For the S-Cluster of stars, the size and timing of the orbits says there is a 4 million solar mass black hole at the center of the galaxy. How do we know it is a black hole? It is emitting no light, and it is so small it competely fits inside the orbits of the stars!

Among this cluster is a star we call SO-2. It is the star closest to the black hole. Every 16 years, its highly elongated orbit dips down to its closest point (what astronomers call the “periapsis”), and zips around the black hole in a quick slingshot. In just the course of a few months, this 15 solar mass star completely changes the direction it is moving through space. THAT is the power of a massive black hole! In May 2018, we watched SO-2 make the second periapsis pass we’ve seen since observations began (the last was in 2002), giving us the most precise measurements to date of the properties of the black hole. By all accounts, the black hole at the center of the Milky Way has all the properties and behaviours predicted by general relativity.

This story serves to introduce us to one of the emerging wonders of modern astronomy — that we are beginning to understand that we can use gravity itself to probe the Cosmos. In the case of SO-2 we are using the influence of gravity to tell us something about the black hole, an object which by definition emits no light. But a century ago, when general relativity was newly minted and first being pondered by Einstein, he had another notion: perhaps we could observe gravity itself — don’t use telescopes at all, but instead build a machine of some sort that plumbs the Universe with some other sense, a sense that we humans do not posses at all.

This illustrates the basic premise of gravitational wave detection using laser interferometers. [TOP] Imagine a ring of small masses. If a gravitational wave is coming straight out of the screen at you, it distorts and warps the ring, first making it long and skinny, then a bit later making it short and wide, and then back again. [BOTTOM] The idea of detection with an instrument like LIGO or LISA is to use mirrors for three of the masses on the rings. As the distance is warped between the masses, the time it takes a laser to travel between them changes. [Images: Shane L. Larson]

Einstein’s idea was simple. What you and I call “gravity” fills space and can change with time, and so the information about how it is changing must be able to be transmitted from one place in the Cosmos to another at the speed of light or less. That propagating message is what we today call a “gravitational wave.”  It is one thing to deduce that such gravitational signals must exist, and quite another to decide what that means and how to build an instrument to detect them. It took until 1957 for physicists to even come to agreement on what gravitational waves do to the world around us. After much arguing and debating and confusion and aggravation, it was realized that they warp spacetime — they change the proper distance between any two masses in a repeating pattern of stretching the distance out and compressing the distance down.

The effect is extremely tiny, so tiny as to be unnoticeable in everyday life, and so tiny as to be discouragingly small if you want to build an experiment to look for the effect. But physicists are a diligent and resilient bunch, and a few of them began to think about exactly how to build such an experiment. Fast forward to today, and six decades of thinking have culminated in one of the most exquisite astronomical observatories ever built: the Laser Interferometer Gravitational-wave Observatory — LIGO. In 2015, LIGO was the first gravitational-wave observatory that was able to successfully detect gravitational-waves, in that case from two merging stellar-mass black holes. Many more discoveries followed, all of them of black holes, until August of 2017.

Left to Right: LIGO-Hanford (Hanford, Washington), LIGO-Livingston (Livingston, Louisiana), and Virgo (Pisa, Italy).

In late August, people across North America were gearing up for a total solar eclipse that was going to race across the continent from Oregon to South Carolina on August 21. But just four days before, in the early morning hours in North America (7:41am Central Daylight Time), the Universe blasted us with gravitational waves. This particular event was unlike any previous gravitational wave event we had seen. It was accompanied by an almost simultaneous burst of gamma rays, detected by NASA’s Fermi Gamma Ray Telescope, in orbit high above the Earth. The two LIGO facilities, together with our European colleagues using their Virgo detector outside of Pisa, measured the masses of the event telling us we had just observed the merger of two neutron stars — a kind of dead stellar skeleton that can be left over when stars explode at the end of their lives. LIGO and Virgo were able to pinpoint the location of the event to a small region on the sky. By the end of the day, as the Sun set on professional telescopes around the world, the search was on and the fading light of the event was discovered in a small galaxy known by the name NGC 4993. The light gathered by observatories around the world over the next many months (and continues today) showed that this event was an explosive phenomena known as a kilonova. In less than 24 hours, this single event transformed modern astronomy — literally, in the blink of an eye.

The initial detection resolved a great mystery that had confounded astronomers since the 1970s — short gamma ray bursts (like the one detected by Fermi) were the signature of merging neutron stars (detected in gravitational waves). The continuing observations of the kilonova resolved another great mystery — kilonova are the expanding, cooling shattered remains of merging neutron stars. From that cooling and expanding morass of nuclear material that was once the neutron stars, the stuff of us is made in large quantities — the heavy elements on the periodic table, built from the death of dead stars!  All of these ideas are ones that astronomers have speculated about, imagined, and calculated for many years up to now, but the observation, the multi-messenger detection of gravitational waves and light has, for the first time, shown us that some of our thinking is along the right tracks.

It is remarkable to witness this evolution in our thinking about the Cosmos. When I started in astronomy (both in my backyard and in my professional endeavours), our thinking was dominated by telescopes because those were the instruments we had and the tools we had been successful at building and using. We were just starting to try to expand our toolbox, to develop a new repertoire of machines to unravel the story of the Universe and our place within in. Now, at the middle of my life and career, those idle daydreams, those grand ponderings of what might be possible, have come to fruition. 

Now we turn our eyes to the future, and ask “what next?”

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This post is the second of three based on a talk I have given many times over the last few years, updating it each time to reflect the latest coolest things. The complete set posts of the series are:

The Cosmos in a Heartbeat 1: A Love Affair with the Cosmos

The Cosmos in a Heartbeat 2: Coming of Age (this post)

The Cosmos in a Heartbeat 3: The End is Just the Beginning

This post was enabled by a new version of the talk done as a Kavli Fulldome Lecture at the Adler Planetarium in Chicago. The talk was captured in full 360, and you can watch it on YouTube here. If you have GoogleCardboard, click on the Cardboard Icon when the movie starts playing; if you watch it on your phone, moving your phone around will let you look at the entire dome!

I would like to thank all my colleagues at Adler who worked so hard to translate what was in my brain into a story told in the immersive cradle of the Grangier Sky Theater. The talk was given on 9 Nov and 10 Nov 2018.

The Saturn Moment

by Shane L. Larson

I just returned from the 33rd annual Winter Star Party, hosted by Miami’s venerable Southern Cross Astronomical Society. Every February, for a week during the new moon, 400 amateur astronomers and their families descend on Camp Wesumkee in the Florida Keys.  During the idyllic days, we sit in lawn chairs, enjoy the gentle sea breezes, watch sandpipers running along the tideline, or beachcomb on the key front looking for pretty shells or little fishies trapped in tidepools.

Sunset over Scout Key, Florida, the site of the Winter Star Party. [Image: S. Larson]

But the real reason we are there becomes apparent as the Sun sinks over the western sea, and the black velvet of night emerges, studded by brilliant diamonds of light. The vast majority of us live our lives under the glaring lights of modern cities, and all too often we forget that the Cosmos is there, hiding behind our artificial fluorescent glow, waiting for us to remember. At the first sunset of the Winter Star Party, it all comes roaring back and you remember what you’ve been missing.

The Milky Way rises over Scout Key around 3am in February. You can watch a timelapse movie of the whole night, including the rising of the Milky Way, on YouTube. [Image: S. Larson]

People often ask me, “are you religious?” My answer is that I am not in the sense of modern churches and institutions, but I do know that we are part of something larger — a Cosmos infinitely vast and wonderful and intricate beyond anything we can imagine or will ever know. The cathedral of night is my church.

In his poem “When I Heard the Learn’d Astronomer,” Walt Whitman espoused the idea that you don’t need sages to know a deep connection to the sky, only the solitude of the night.

When I Heard the Learn’d Astronomer 
by Walt Whitman 

When I heard the learn’d astronomer, 
When the proofs, the figures, were ranged in columns
     before me, 
When I was shown the charts and diagrams, to add,
     divide, and measure them, 
When I sitting heard the astronomer where he lectured
     with much applause in the lecture-room, 
How soon unaccountable I became tired and sick, 
Till rising and gliding out I wander’d off by myself, 
In the mystical moist night-air, and from time to time, 
Look’d up in perfect silence at the stars. 

(http://www.poetryfoundation.org/poem/174747)

But in today’s fast paced world, driven by small screens, instant communication, and more information than has ever been gathered by a civilization before, it is hard to slow down enough to realize those moments of solitude. Living beneath the glare of our cities, there are generations of people who have never truly seen a starry sky and thus never built a deep personal connection to the night.

My telescope (named EQUINOX), on the observing field at Scout Key. [Image: S. Larson]

While the Winter Star Party is dominated by amateur astronomers who, like me, do this as often as we can, there are also a lot of people who are experiencing the dark night sky and the Milky Way for the first time. They walk among the telescopes at night, peering at a nebula here or a star cluster there, all the while being regaled with tales and facts of all that we have learned from 400 years of telescopic study of the sky.

This year, at around 4 in the morning, the Milky Way climbed up above the horizon, it’s center studded by a pale yellow “star.”  A young couple, at their very first star party, had stopped by my telescope for some quiet conversation and some views of the sky.

“Do you want to see something cool?” I swung my telescope over to the pale yellow “star” and let them peer through the eyepiece. The view elicited startled gasps, and loud exclamations of joy.

View of Saturn through the telescope, taken with an iPhone [Image by Andrew Symes; visit his blog here]

“There is no way that is real!”  The pale yellow “star” was in fact not a star at all — it was the planet Saturn, a cream colored orb bejeweled by its famous ring, the ring itself narrowly divided by a thin black gap known as the “Cassini Division.”

Delivering a personal experience with the night sky is part of the promise of amateur astronomy. We show people the Moon, stars, clusters, perhaps an occasional galaxy. But nothing moves people like their first view of Saturn through a telescope. Most people who take the time to look walk away remembering that moment for the rest of their lives.

We call this “the Saturn Moment.”

More than any other far away object in the sky, Saturn looks like what people expect. They often respond to their view with incredulity, joking that it looks almost painted, or like a picture that has been taped over the end of the telescope.

The Moon often engenders similar responses, but people expect the Moon to look that way. They can see it with their eyes, and imagine craters and mountains, so they aren’t necessarily surprised by the telescopic view.

By contrast, most people have never seen Saturn, except through the eyes of space probes. The telescope somehow takes the NASA pictures we see on our computer screens, and makes it real and visceral.

At the Adler Planetarium in Chicago, you can see a “20 foot Refractor” similar to the kind used in Huygens time (left). We have it set up so you can look through it, and see the same kind of fuzzy image of Saturn he may have seen (right). [Images: S. Larson]

The first person to have a Saturn Moment was Galileo, who turned his telescope on the skies in 1609. His views of Saturn were not the greatest, as his sketches published a year later in Sidereus Nuncius show. It was clear Saturn wasn’t normal because he could make out blobs on either side. He wrote in a letter to his student Benedetto Castelli that Saturn had “ears.” It wasn’t until 1655 that Dutch astronomer Chrstiaan Huygens, using a much better telescope (though still fuzzy) was able to discern that Saturn was surrounded by a thin, flat ring.

A 57 mm diameter lens, all that remains of the telescope Huygens used to observe Saturn. Around the edge is carved a verse from the Roman poet Ovid: “Admovere Oculis Distantia Sidera Nostris” (They brought the distant stars closer to our eyes). It is an anagram, establishing the details of Huygens’ discovery of Saturn’s moon, Titan. When translated, it reads “A moon revolves around Saturn in 16 days and 4 hours.”[Image: Utrecht Univ. Museum, from APOD]

Today, ordinary people like you and me can own telescopes that would have made Galileo and Huygens swoon with envy. Technology is better, and available to everyone.

My Saturn Moment happened long ago, at a sidewalk astronomy event. An amateur astronomer invited me over to look through her “telescope” — it wasn’t an ordinary telescope, it was a spotting scope for birding that she had pointed at the sky. But what I saw blew my socks off. I was seeing Saturn, with my own eyes, and I could see the rings! Though I don’t remember it, I’m sure the rusty dot of Titan, Saturn’s largest moon, was also lurking nearby.

The ultimate result of that encounter is that today my wife and I are both amateur astronomers ourselves, and we guide people through their own Saturn Moments every year. Each moment is unique, exhilarating, and moving in their own way. Among the most memorable was several years ago, my wife had guided a young boy to our telescope to have a peek at Saturn. The view elicited a loud gasp, and the exclamation, “It looks just like a Chevy symbol!” Yep, it kind of does!

If you’ve never seen Saturn before, go to your local planetarium or astronomy club. They would love to show you Saturn for the first time. And when you’re done, tell everyone what you’ve seen, and encourage them to have their own, first #SaturnMoment, a moment of perfect beauty between us and the Cosmos.

Humanhenge — Marking Time in the Modern World

by Shane L. Larson

The other morning, when she was trying to decide what to wear for the day, my daughter said to me, “Is it cold outside?” Then, without missing a beat, she says “Never mind,” and picked up an iPhone and checked the current weather conditions. This brought to mind a vivid memory from my childhood of me asking the same question of my mother, and her saying “Go out and check!”  She had a rock on her porch; if it was wet it was raining, if it was white it was snowing, if it was hard to see it was foggy, and so on.

Far be it from me to eschew modern technology; it can do awesome things!  But it strikes me that if there is any truth about modern society, it is that it is entirely possible to live your life looking through the electronic portal of digital technology, rather than by looking out the window.

My schedule is kept by a modern calendar application, accessible from my phone. This is modern progress. Sigh.

This was not always the case, of course.  Our forebears had to pay attention to the world, literally because it was a matter of life and death. Today, if strawberries are out of season in the northern US, no worries — your local store is shipping you some from far away, the cogs of the modern economy meeting your every whim.  But hundreds and thousands of years ago, crops had to be planted in time for the life giving rains, and harvest had to be brought in before fall frosts destroyed the yields that had to last through the winter.  Before quartz watches, before Greenwich Mean Time, and before Google Calendar, people still had to know what time of year it was.

So what did people do before the world worked on linked times and calendars? They watched the sky! The sky is filled with a regular clockwork of motions that ticks off the seconds, days, and aeons as precisely and regularly as the finest timepiece humans have ever made. The motions in the sky are a combination of the Earth’s orbital motion around the Sun, the spin of the Earth on its axis, and the fact that the North Pole of the Earth is not pointing straight up from the orbit.

One observational consequence of these three facts is that the Sun’s position in the sky changes over the course of a year. Each day it rises and sets at a different point on the horizon, and it tracks across the sky along a different pathway.  The equinoxes are the days during the year when the Sun rises and sets directly in the East and West. The solstices are the days during the year when the Sun rises and sets at the point furthest North and South on the horizon.

The ever-changing rise and set position of the Sun along the horizon can be used as a calendar. One of the most famous examples is the Shungopavi horizon calendar of the Hopi in the American southwest, shown below.  As the sunrise marches north and south along the horizon, its passage by certain landmarks indicates times of planting, harvest, and cultural ceremonies.  While you likely have no need for a horizon calendar, it can be fun to construct one via sunrise observations at your own home, like mine in Paradise, Utah, also shown below.

Example horizon calendars. The Hopi Shungopavi horizon calendar (top), and my personal horizon calendar for Paradise, Utah (bottom).

There are many ancient sites around the world where the rising and setting of the stars and Sun are aligned over carefully placed stones and architectural structures.  Stonehenge on the verdant Salisbury Plain, and the great Anasazi kiva Casa Rinconada in Chaco Canyon, and the Great Medicine Wheel on top of Wyoming’s Bighorn Range.  All of these sites, and many more besides, bespeak of the intimate knowledge our ancient ancestors had about the sky.

Ancient astronomically aligned sites. (L) Stonehenge, (C) Casa Rinconada, (R) Bighorn Medicine Wheel.

Manhattanhenge, looking down 42nd Street.

Sometimes modern landscapes align with the celestial clockwork, possibly by design but often by accident.  Consider the island of Manhattan. The island sits in the mouth of the Hudson River, canted diagonally to the north-south direction.  When it was first settled, the streets were laid out willy-nilly, in the usual haphazard organic way of ancient cities.  The expanding maze of twisty little streets that all look the same was abruptly ended by the Commissioners’ Plan of 1811, which established a grid plan for the growth of the city.  The grid was laid out with the long northish-southish thoroughfares running parallel to the coastline of the island.  As a consequence, the cross-streets running eastish-westish are all canted roughly 25 to 30 degrees to the east-west direction.  As it turns out, that is just the right tip for all the streets to be close to lining up with the sunrise and sunset directions on the Summer Solstice!  This was famously noted about ten years ago by Neil deGrasse Tyson, who dubbed the phenomenon “Manhattanhenge.”

The street grids for Manhattan (L) and for Chicago (R). The geometry of the streets determines when you will have a well placed sunrise or sunset to make a cityhenge.

But you don’t have to travel all the way to Manhattan to enjoy modern astronomical alignments.  Many towns and cities are laid out on a grid system.  For grids lined up on a north-south and east-west system, magic happens every March and every September, on the equinoxes.  For instance, in the city of Chicago, the streets are laid out (more or less) on a rectangular grid aligned to the compass points, and a “Chicagohenge” can be photographed on the Spring Equinox in March, and on the Fall Equinox in September!

Examples of Chicagohenge [Images from Ken Ilio’s Uncommon Photographers].

No matter where you are, there is probably an opportunity to use your city as a henge for photographing the ever-changing motion of the Sun.  Even small towns, like Vankleek Hill, Ontario.  The town is about half-way between Ottawa and Montreal, and has a population of only about 2000 people. The grid is laid out canted to the compass points, roughly parallel to the drainage from Lake Ontario into the Gulf of Saint Lawrence.  It’s just enough to line up with the Sun on the solstices, captured in this image by Gabriel Landriault.

(L) The street grid in Vankleek Hill, Ontario is canted about 20 degrees to an east-west line. (R) A summer solstice along Vankleek Hill’s streets [image by Gabriel Landriault].

The Vankleek Hill picture hints at another point in this whole game.  The streets in Vankleek Hill are only canted 20 degrees or so from east-west, whereas the Sun is a bit farther north on the solstice, so is not perfectly aligned in this case.  But there is some day during the year when it is aligned!  This is a subtlety that was hinted at by our horizon calendars — the Sun is always on the move, marching up and down the horizon.  On the soltices and the equinoxes it is in a definite, predictable location on the horizon.  But with some careful planning, observations, and simulation (such as with a planetarium simulator like Stellarium), you can figure out when the sunrise and sunset will line up with the streets in your own town.

An example simulation of sunrise and sunset from the Adler Planetarium on the Spring Equniox, using StarryNight desktop planetarium software.

The truth is, it is not a life and death matter if I don’t pay attention to the daily motion of the Sun and use that information to lead a subsistence, agrarian lifestyle.  I have a calendar, the Farmer’s Almanac, and Google to tell me when to plant my snap peas.  I can really just do astronomy for fun — because it is cool, and it lets me capture some beautiful pictures that I can use to impress my friends and woo members of the opposite sex.

The Spring Equinox is just around the corner!  So go out with your cell phone and catch a sunrise or a sunset, directly down your nearest East-West street. Make sure you tweet the picture to the rest of us, so we know it is spring and can start to think about planting our gardens; strawberry season is on its way!