# Which is the shape of the sky?

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I'm a software developer and I'm developing a planetarium and the first thing that I don't know is: What is the shape of the sky?

First, I though that it is a semi-sphere, but searching on Internet I have found a lot of youtube tutorials, and all of them draw a semi-sphere but flatten (you can search skydome on Google and find a lot of tutorials).

My question is, how can I represent sky on a virtual 3D world? How much do I need to flat a sphere to get a real sky representation?

For computer software, the easiest way to take a sphere (and/or hemisphere) and flatten it into a flat shape (usually a rectangle) is the equi-rectangular projection (also known as the plate carrée), because it has the simplest formula relating pixels and coordinates:

$$x = w imes fraclambda{360} + frac w2$$

$$y = -h imes fracphi{180} + frac h2$$

where $x$ and $y$ is the pixel point, $w$ and $h$ are the width and height (in pixels) of your map's rectangle, $lambda$ is the longitude in degrees and $phi$ is the latitude in degrees

This all assumes that your map puts the 0 longitude, 0 latitude point in the center. In other words, the prime meridian runs down the map's middle vertically and the equator runs down the map's middle horizontally. It also assumes that north and east are positive, while west and south are negative.

Notice that you don't even have to take the sine or cosine of anything. That's why the equi-rectangular projection is so simple. It basically just takes longitude and latitude and makes them $x$/$y$ coordinate with only an offset. The offset comes because, usually in computer programming, the origin is at the top left instead of the center of the screen.

Notice also that I negated the latitude in the $y$ formula, because most computer systems in 2D coordinates have the positive $f y$ direction pointing down. This is opposite of a standard graph in math, where positive $y$ goes up.

In your case, longitude and latitude will actually be right ascension and declination respectively. They are basically stellar coordinates for the celestial sphere as opposed to ground coordinates for the geode.

There are many, many other map projections possible, but equi-rectangular is the simplest.

However, your question is a little vague because you mentioned a "virtual 3D world", yet also asked how much you need to flatten it. If you already have a 3D model of a sphere in your computer graphics, you don't have to flatten it at all. You just draw the 3D model (sphere) with a texture on it. If you want to represent it as a map, then of course you have to flatten it, and equi-rectangular projection is the simplest way.

## The shape of galaxies: reconfiguring Hubble’s tuning fork

Using citizen science project Galaxy Zoo, citizen astronomers may have overturned a century-old galaxy classification system.

This competition is now closed

Published: October 8, 2020 at 8:56 am

In 1926 the American astronomer Edwin Hubble developed a classification system to describe the evolution of galaxies and their development from an elliptical to a spiral formation.

This was around the same time that he settled astronomy’s Great Debate by calculating that the Andromeda Galaxy must be well beyond our own Galaxy.

Hubble allocated numbers to the elliptical galaxies to characterise each one’s ellipticity, E0 being nearly round and E7 being very elliptical.

Spiral galaxies, according to Hubble’s system, were assigned letters from a to c according to how tightly wound they are, Sa being very tightly wound and Sc being more loosely wound.

The system was dubbed ‘Hubble’s tuning fork’ because of the way in which galactic evolution appears to generally go one of two ways once an elliptical galaxy begins evolving into a spiral (scroll further down for an illustration of this).

However, one of the key characteristics of the tuning fork system is a correlation between a spiral galaxy’s bright bulge and how tightly the arms are wound.

A study using the citizen science project Galaxy Zoo has found that this correlation might not be as accurate as Hubble predicted.

We spoke to Galaxy Zoo project scientist Prof Karen Masters to find out more.

### What is the Galaxy Zoo project?

It’s a website project that has been running for 12 years, where we invite anyone to look at galaxies and answer questions describing the different structures they see.

For example, they may see spiral arms, bars (straight formations across galaxies with a concentration of stars) or sometimes no structure at all.

We collect information from about 30-40 people per galaxy so we’re able to get a consensus crowd-sourced answer for what is visible in that image of that galaxy. We use this to investigate a galaxy’s evolution.

### What can you tell about a galaxy’s evolution from its shape?

The shape tells you quite a lot about how the galaxy is assembled over time and also about the motions of stars in the galaxy now.

We see spirals and discs where the stars are all moving in the same plane. Smoother galaxies, known as ellipticals, are less likely to have so much of that coherent motion, but have stars in a more random motion.

That is most likely a characteristic of having been assembled from lots of galaxies that merged together and came in at different angles.

### What did citizen scientists do in your spiral galaxy study?

The first thing they had to do was find all the spiral galaxies in the original list of 250,000 galaxies from the Sloan Digital Sky Survey (SDSS).

In the end, they had about 6,000 galaxies. Most people probably look at between 20 to 50 galaxies, and they compared their answers to classifications from other catalogues.

### What is Hubble’s tuning fork?

It is a galaxy classification scheme invented by Edwin Hubble in the 1920s.

The diagram used to represent it resembles a tuning fork because the spiral galaxies are split into two tines – one showing the spiral galaxy sequence with a bar and the other showing those without a bar.

Then the elliptical galaxy sequence is a single line that meets them, forming the handle of the tuning fork.

Hubble talked about the size of the bulge – the central concentration of light you find in some spiral galaxies – and its correlation to how the arms are wound. He claimed that most spiral galaxies had a big bulge and tightly wound arms, or a small bulge and loosely wound arms.

### What did your study show?

The surprising find was that the bulge size did not correlate with the arm winding.

The expectation from the Hubble tuning fork is that spiral galaxies with small central light concentrations would have loose arms and those with large central light concentrations would have tight arms, making almost a full circle.

When we plotted the observations for these two things in our study, we didn’t see that correlation.

### How does this help us understand galaxies better?

I think we are dealing with the questions of what spiral arms are, and what forms them. We have known about spiral arms for about 150 years and theories attempting to explain what they are have been well established since the 1960s.

So it was a surprise to me here to be thinking that maybe we don’t really have a good agreement over what is the dominant mechanism by which spiral arms form.

There are theories that predict Hubble’s correlation. People often talk of spiral arms as being density waves, much like with a traffic jam – where you get a build up of cars and then you suddenly pass through it.

The stars are doing that in the density wave model: they pile up and then pass through it. A static density wave is the model everyone said explained the spiral arm because it fitted the tuning fork, but we are saying it doesn’t have to be that.

We don’t see Hubble’s correlation.

Prof Karen Masters is an associate professor of physics and astronomy at Haverford College in Philadelphia, USA, and the Galaxy Zoo project scientist. This interview originally appeared in the October 2019 issue of BBC Sky at Night Magazine .

## Close-up on Cassiopeia the Queen

The constellation Cassiopeia the Queen can be found high in the northeast on October evenings, not far from Polaris, the North Star. At any time of year, you can use the Big Dipper to find Cassiopeia. These two star formations are like riders on opposite side of a Ferris wheel. They’re part of a great spinning wheel of stars seen moving counterclockwise around Polaris, the North Star, once each day. As Cassiopeia rises upward, the Big Dipper plunges downward, and vice versa.

Some of you know how to star-hop to Polaris, the North Star, by using the Big Dipper’s pointer stars, as displayed on the sky chart below. Because the Big Dipper’s handle and Cassiopeia shine on opposite sides of Polaris, an imaginary line from any star on the Big Dipper handle through Polaris reliably points to Cassiopeia.

You can find Cassiopeia somewhere in the north for much of the year, and much of the night. Here it is on an October evening, relative to Polaris, the North Star.

But you won’t need these details to find this constellation. That’s because Cassiopeia is very easy to pick out. It’s small and compact and looks like the letter M or W, depending on the time of night and time of year.

Like the Big Dipper, Cassiopeia can be seen even on moonlit nights.

Bonus for you if you live north of about 40 degrees north latitude, about the latitude of New York City! From that latitude and farther north, the Big Dipper and Cassiopeia are both circumpolar. That means they’re always above the horizon at any time of night, all year round.

Upside-down Cassiopeia on Mercator globe. Upside-down Cassiopeia via Johannes Hevelius.

Cassiopeia used to be known among astronomers and skywatchers alike as Cassiopeia’s Chair. In the 1930s, the International Astronomical Union gave this constellation the official name of Cassiopeia the Queen.

Cassiopeia was a queen in ancient Greek mythology. According to legend, she boasted she was more beautiful than the sea nymphs called the Nereids. Her boast angered Poseidon, god of the sea, who sent a sea monster, Cetus, to ravage the kingdom. To pacify the monster, Cassiopeia’s daughter, Princess Andromeda, was left tied to a rock by the sea. Cetus was about to devour her when Perseus the Hero looked down upon her from Pegasus, the Flying Horse. Perseus rescued the Princess, and all lived happily.

The gods were so pleased, that all of these characters were elevated to the heavens as stars. Only Cassiopeia suffered an indignity – her vanity caused her to be bound to a chair and placed in the heavens so that, as she revolves around the north celestial pole, she is sometimes in an upside-down position.

The official borders of the constellation Cassiopeia (and all 88 constellations) were drawn up by the International Astronomers Union in the 1930’s. Read more.

Bottom line: Spot the constellation Cassiopeia the Queen somewhere in the northern sky during much of the year, and throughout much of the night.

## How ‘The Land of the Stars’ Shaped Astronomy (and Me)

In the mountains and deserts of the Middle East, the region's role in shaping our modern view of the cosmos quickly comes into focus.

Land of the stars, I am here.
Gaze deeply…do you remember who I am?
I am that youth whose whole universe
Was right here.

— Illya Abu Madi, written upon returning to Lebanon

My father may have dedicated a lifetime to studying the heavens, but my mother comes from the land of the stars. In the Middle East, the night sky’s stories have been known for millennia, told by those who named its lights, aligned their tombs and temples with its shifting shapes, and divined the movements of the worlds wandering our solar system.

In some ways, the language of astronomy is derived from Arabic: Familiar stars such as Betelgeuse, Altair, Deneb, and Rigel retain their original names terms like “azimuth” and “nadir” have their roots in this exquisite and perplexing part of the world.

And as surely as I grew up surrounded by messages to the stars, Arabic was a language of my childhood.

This summer, after more than three decades on this planet, I finally visited the seaside village in Lebanon where my mama, Amahl, was born. Just north of Jbeil (also known as Byblos), Anfeh rests along the same coast the Phoenicians bid farewell to as they sailed into the unknown, using starlight to guide them in the dark.

It’s a place where salt is still laboriously harvested from the sea, where the moon shines down upon ancient churches, where the house my mother and grandfather were both born in still sits a few meters from the Mediterranean shore. In fact, all the houses of my mother’s family are still there, as are my cousins, and the people who knew her when she was a child.

Now, finally, they know me too.

I came to the Middle East for the first time not knowing what to expect, yet anticipating the awakening of a sleepy part of my myself. I’d grown up hearing Arabic and knowing the recipe for proper tabbouleh, but Lebanon was a place I’d only seen in my dreams. It was somewhere I’d never breathed in or dared hope to go, a land achingly out of reach.

As the mountains and buildings of Beirut emerged from the haze, I felt like I was coming home. I was that youth whose universe was right there among the salt and the cedars and the centuries of richness and strife, staring wide-eyed at a realm where the first twinklings of the stars were recorded.

I’ve always felt like a bit of an impostor Arab. My blondish hair and bluish eyes don’t fit the stereotypical image of my darker-eyed kin—but walking the streets of Beirut meant seeing that so many Lebanese women look just like me. It was an observation that offered solace, and I felt like I belonged—a nearly impossible privilege in a part of the world where wars are fought to make “belonging” even a smidge more possible. But there I was.

Part of traveling means finding the beauty in unfamiliar places and faces. Sometimes, it means acknowledging the pain, too. That’s there, in harsh landscapes and in the eyes of those who have seen too much. And it’s true that learning about others and acknowledging differences can flow both ways.

In rural Jordan, I was somewhat of a curiosity—a single woman, either brazen or crazy enough to hang out solo. “Where’s your husband?” so many men asked me. “He’s not here,” I would say, while proceeding to quiz my questioner about what it’s like to live in such a spectacularly beautiful place.

Tucked between Israel, Syria, Saudi Arabia, and Iraq, Jordan’s tourism industry is suffering from the distorted, myopic lens many people view the Middle East through. Yet I never once felt unsafe there or in Lebanon, where wartime scars still adorn buildings in the form of bullet holes and bombed out walls.

One evening, we camped in the Wadi Rum desert near Jordan’s southern border. Also known as the Valley of the Moon—and adopted home of Lawrence of Arabia—the Wadi Rum is moody and otherworldly in a harsh and commanding way. There, lumpy sandstone walls erupt from scorched, reddish sands crisscrossed by herds of camels and Bedouins shepherding tourists around in rickety Jeeps.

Too hot to sleep, I wandered outside and found myself completely distracted by one of the most dazzling skies I’d ever seen. A half moon bright enough to cast shadows obliterated the stars until it sank behind a ridge—and then, in the Valley of the Moon, those stars started screaming for attention.

I settled into the sand and lay there silently for hours, transfixed by the lights tracing paths across the sky. Antares, the red supergiant beating at the heart of Scorpius, soon emerged, and then the entire scorpion’s tail curled across the sky and dipped into the stream of stars that is the Milky Way. Cygnus glittered overhead, next to the starfield where NASA’s Kepler telescope has made such monumental planet discoveries. In the north, Cassiopeia clung to her throne, while in the south, the Sagittarius teapot pointed to the spot where a supermassive black hole churns away in the core of our galaxy.

I can only imagine how captivating this evening light show must have been during the ages when Earth’s skies were uncontaminated by artificial lights. It’s no wonder cultures all over the planet have their own version of its stories, their own solutions to its mysteries. What did the ancient Arabs think these shapes in the sky represented?

Twinkling overhead are the same exact stars humans all over the world have gazed at forever, beckoning to us and inviting us to solve their riddles.

Both sides of my family have brought the stars to my life. I’ve known my dad’s version of their stories for almost as long as I’ve been alive, but it wasn’t until I visited the Middle East that the thread connecting me with my mom’s history really began to breathe.

In a time when fear and hatred paint entire portions of the planet in a single shade of uninformed ugly, it’s more important than ever to live gently, to get to know those around you, and especially those who are different. There are greater goods and worthier goals than crushing those we disagree with, than giving in to hate and fear, than falling prey to tempestuous rhetoric and the manipulations of those who would do unspeakable harm in the name of their deities.

The Middle East is far from perfect, as is every place on Earth. Conflict disfigures the region in profoundly troubling ways. Yet if you ever doubt that we are all connected, just look up: Twinkling overhead are the same exact stars humans all over the world have gazed at forever, beckoning to us and inviting us to solve their riddles. And as long as those stars shine in the sky, we owe it to ourselves to help life on this planet sparkle as well.

## Which is the shape of the sky? - Astronomy

Figure 1: Southern Summer. As captured with a fish-eye lens aboard the Atlantis Space Shuttle on December 9, 1993, Earth hangs above the Hubble Space Telescope as it is repaired. The reddish continent is Australia, its size and shape distorted by the special lens. Because the seasons in the Southern Hemisphere are opposite those in the Northern Hemisphere, it is summer in Australia on this December day. (credit: modification of work by NASA)

If Earth’s orbit is nearly a perfect circle (as we saw in earlier chapters), why is it hotter in summer and colder in winter in many places around the globe? And why are the seasons in Australia or Peru the opposite of those in the United States or Europe?

The story is told that Galileo , as he left the Hall of the Inquisition following his retraction of the doctrine that Earth rotates and revolves about the Sun, said under his breath, “But nevertheless it moves.” Historians are not sure whether the story is true, but certainly Galileo knew that Earth was in motion, whatever church authorities said.

It is the motions of Earth that produce the seasons and give us our measures of time and date. The Moon’s motions around us provide the concept of the month and the cycle of lunar phases. In this chapter we examine some of the basic phenomena of our everyday world in their astronomical context.

## What's in this patch of sky

Modern astronomers use constellations to divide the celestial sphere into different areas (like state lines and borders on a map). Organizing the sky this way allows astronomers to easily find points of interest for their telescopes to spy on.

The patch of sky Boötes occupies faces away from the plane of our Milky Way galaxy and contains few astronomical objects. In fact, the constellation contains one of the most empty places in the known universe, the Boötes Void. This mysterious void is an area of the universe 250 to 330 million light-years across that is nearly empty, containing only a handful of galaxies, according to NASA.

Closer to home, there are three meteor showers associated with the constellation Boötes. The Quadrantid meteor shower is the first meteor shower of each year, typically occurring within the first week of January. The fiery display peaks for only a few hours and can be seen coming from the area between the constellation Boötes and the Big Dipper. The two other showers found in the constellation are known as the Boötids and occur in late January and June, although they're less brilliant than the Quadrantid showstopper that precedes them.

## Shape of the Sky

Shelagh Connor Shapiro’s novel, Shape of the Sky, drew me in from the first chapter. Not only because I appreciate the beauty of its Vermont setting, but because I quickly became immersed in each of the characters&apos stories. The novel moves among the various voices of the town of Resolute around a particular summer event, each perspective critical to the whole. At times in reading novels that change character voices every chapter, I feel jarred by the shifts (“No! No! I wasn’t done with THIS one Shelagh Connor Shapiro’s novel, Shape of the Sky, drew me in from the first chapter. Not only because I appreciate the beauty of its Vermont setting, but because I quickly became immersed in each of the characters' stories. The novel moves among the various voices of the town of Resolute around a particular summer event, each perspective critical to the whole. At times in reading novels that change character voices every chapter, I feel jarred by the shifts (“No! No! I wasn’t done with THIS one yet!"). In the hands of such a skillful author though, I felt confident that each of the characters was key to the evolving narrative. Far from feeling jumbled from one to the next and longing to go back to the familiar, Ms. Shapiro was seamlessly able to build the storyline through multiple perspectives. Each viewpoint was key to my understanding of the complete story and none felt superfluous.

The other reason I so enjoyed Ms. Shapiro's novel was her ability to create realistic dialogue through “showing” rather than “telling". Ms. Shapiro allowed her characters to act out their perspectives through interactions and conversations with the other characters. The success of this relies heavily though on the authenticity of the characters’ development. Whether through the voice of a teenage boy or a salt-of-the-earth farmer trying to make ends meet – I cared about each of the characters and felt that what they spoke was their truth. Each voice, however contradictory, made a whole lot of sense to me. I love reading books in which two or more characters clearly do not see the same situation eye-to-eye and by the end of the book, I feel like each of them are telling the truth! It all depends on the personal context. Through dialogue and just enough “telling" to fill in the holes, Ms. Shapiro gets us there with ease.

My only regret is that Ms. Shapiro has not yet written a sequel. Although the book feels complete in itself, I would love the opportunity to learn more about all of the characters. Perhaps I can suggest a reunion of the characters around some other plot line?? Though it’s just a small town in rural Vermont, as anyone from a small town knows, the possibility for new dramas are endless.
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## Bull’s face: The Hyades star cluster

The Hyades star cluster – Face of the Bull in Taurus – displaying its V-shaped pattern, which points to the right in this photograph. The brightest star here – an orange star – is Aldebaran, the Bull’s fiery Eye. Copyright Jerry Lodriguss/ AstroPix.com. Used with permission.

With the exception of the Ursa Major Moving Group, the Hyades cluster is the closest star cluster to Earth, at a distance of 150 light-years. This cluster is very easy to spot in the night sky, because it has a compact and distinctive shape of the letter V. The bright star Aldebaran is part of the V.

The V shape represents the Face of the Bull in the constellation Taurus. Aldebaran represents the Bull’s fiery red eye.

The bright waxing gibbous moon will shine in front of Taurus the Bull on December 26, 27 and 28, 2020. It’ll be difficult to see the Bull’s V-shaped Face on these moonlit nights, but you’ll likely see Taurus’ major signposts: the star Aldebaran and the Pleiades star cluster, otherwise known as the Seven Sisters. Read more. The Hyades star cluster can be found easily on January and February evenings, and is edging toward the western half of the sky by March and April evenings. It has the shape of the letter V. The brightest star in the V is Aldebaran. The small dipper-shaped Pleiades star cluster is nearby. Here is the relationship between Orion, the bright star Aldebaran in Taurus, and the Pleiades. Notice the 3 stars of Orion’s Belt, that is, 3 stars in a short row. These stars point to Aldebaran.

The Hyades cluster is easy to find by using Orion’s Belt, a compact and noticeable line of three blue-white stars in the constellation Orion the Hunter. Draw a line westward (generally toward your sunset direction) through the Belt stars, and you will come to the bright reddish star Aldebaran, the Bull’s fiery red eye.

Although Aldebaran isn’t a true member of the Hyades star cluster, this bright star is a great guide to this cluster. In fact, Aldebaran is only about 65 light-years distant. The Hyades lies about 2 1/2 times farther off. This is what we call a “line-of-sight coincidence.”

The V-shaped figure of stars (except Aldebaran) highlights the brightest of the Hyades’ few hundred stars. A dozen or more Hyades stars are visible to the unaided eye in a dark country sky, but several dozen of the cluster’s stars can be resolved through binoculars or low power in a telescope. From the Northern Hemisphere, the Hyades are best seen in the evening sky from around January to April.

The constellation Taurus the Bull is home to another bright star cluster, the Pleiades. The Pleiades cluster is more distant than the Hyades at some 430 light-years away. Both the Hyades and Pleiades are easily visible to the unaided eye. Also, both are enhanced by viewing with binoculars.

History and mythology of the Hyades. According to sky lore, the teary Hyades are the daughters of Atlas and Aethra, who are forever crying for their brother Hyas, who was killed by a lion or a boar. The Hyades are the half-sisters to the Pleiades, the daughters of Atlas and Pleione. The gods purposely kept Atlas’ daughters – the Hyades and the Pleaides – out of reach of Orion, giving them a safe haven from his lustful pursuits.

The gods transformed Hyas into the constellation Aquarius, and the lion that killed him into the constellation Leo. The gods placed Aquarius and Leo on opposite sides of the sky for Hyas’ protection. That’s why Aquarius and Leo do not appear in the same sky together. As one constellation sets in the west, the other rises in the east – and vice versa.

View larger. | David Rojas in Guatemala City, Guatemala, caught the waxing crescent moon, the Hyades star cluster with the bright star Aldebaran at one tip of the V, and the Pleiades star cluster on March 22, 2018. A telescope reveals over 100 stars in the Hyades cluster. The bright red star here is Aldebaran. Photo via astronomycafe.net.

Hyades science. Although the Hyades and Pleaides are half-sisters in mythology, science finds no close relationship in space between these two star clusters.

Astronomers find that the Pleiades are composed of hot blue-white suns in the heyday of youth, which puts the age of the cluster at about 100 million years. In contrast, the cooler red giant and white dwarf stars found in the Hyades indicate a vastly older cluster over 600 million years old.

Interestingly, astronomers suspect an actual kinship between the Hyades cluster and the Beehive star cluster in the constellation Cancer the Crab. Even though these two star clusters are separated from one another by hundreds of light-years, they are akin in age and travel in a similar direction in space. This leads astronomers to believe that these clusters might have originated from the same gaseous nebula some 700 to 800 million years ago.

The three bright stars in a row are Orion’s Belt. They point to the Hyades star cluster. Photo via Unishot/ Flickr. View a larger chart. | A detailed star map that shows the Hyades star cluster and its surroundings. Again, Orion’s Belt (seen at the lower left) points to the star cluster.

Bottom line: On January and February evenings, look for a V-shaped pattern of stars. The Hyades star cluster represents the face of Taurus the Bull. The cluster is easy to spot and beautiful through binoculars.

## Changes According to Location

In the Northern hemisphere, the Analemma curve has the broader loop at the bottom. This is opposite in the Southern hemisphere, where the broader loop is on the top of the curve.

Observers at the North Pole will see only the top loop of the Analemma, while those at the South Pole will observe only the bottom portion of their Analemma.

In addition, the direction of the Analemma also differs depending on the observers location on Earth,

The Analemma for the Sun has different shapes on each of the 8 planets. This is because the position of the Sun in the sky depends not only on the shape of the planet’s orbit around it, but also on the angle of the planet’s rotational axis.

The term Analemma comes from Greek and refers to the pedestal of a Sundial. Some globes have the Analemma curve printed on the surface.

## The Very Large Array: Astronomical Shapeshifter

When the Very Large Array was completed forty years ago, it was a different kind of radio telescope. Rather than having a single antenna dish, the VLA has 27. The data these antennas gather is combined in such a way that they act as a single radio telescope. As a radio array, the virtual dish of the VLA can cover an area roughly the size of Disney World. But the VLA can also do something ordinary telescopes can’t do: it can change shape.

The antennas of the VLA are arranged along three long arms, each with nine antennas. Each arm has a rail track, allowing the antennas to be moved to different locations along the arm by a 200-ton transporter. Thus, the antennas can be spaced widely apart, or clustered close together. Although each antenna can be moved individually, they are typically positional in standard arrangements or configurations. In many ways, each configuration is its own radio telescope. By moving antennas into these different configurations, the VLA can serve as many observatories rolled into one.

The power of a telescope largely depends upon two factors: the faintness of the light it can see, known as its sensitivity, and the sharpness of the images it can produce, known as its resolution. These two factors are often contradictory. To capture faint images a telescope needs to collect lots of light over a long time, but this can make images blurry. To capture a sharp image you often need a brighter source. It is similar to the effect of our own eyes, which adapt to brightness. It’s one of the reasons you can see clearly in bright daylight, while things can look more blurry in dim light. By arranging antennas into different configurations, the VLA can overcome this challenge, allowing it to capture both sharp images and faint objects depending on the needs of astronomers.

Hercules A as seen from each configuration of the VLA. Credit: NRAO/AUI/NSF

There are four primary configurations used by the VLA. They are each assigned a letter A – D, depending on the spread of the antennas. Configuration A, spanning more than 22 miles, is where the antennas are most widely spaced, and Configuration D is where they are closest together, with the antennas clustered into an area less than a mile wide. The VLA cycles through these configurations, staying in each one for several months.

The largest configuration gives the VLA its highest resolution. Radio astronomers often want to see fine details in a radio image, which is why Configuration A is the most requested. But smaller configurations have their own uses. Configuration D gives the VLA the greatest sensitivity. This makes it particularly useful in the study of diffuse hydrogen gas in nearby galaxies, and in capturing images of faint radio nebulae.

Configuration B is a workhorse configuration. It is a third the width of Configuration A and therefore strikes a balance between sensitivity and resolution. It is mostly used for the VLA Sky Survey (VLASS), which is a 7-year project to map 80% of the sky in radio light. When it is finished it will have a catalog of more than 10 million radio sources. VLASS also uses an additional hybrid configuration known as BnA. In this arrangement, the antennas in the north arm are arranged in A configuration, while antennas in the other two arms are kept in B Configuration. This gives the virtual dish of the VLA an oval shape.

Configuration BnA is used to see the southernmost region of the sky. Objects in the far south of the sky are near the horizon, and their light comes in at a low angle. By stretching the northern arm, the VLA can “circularize” the images gathered so they aren’t distorted by their low angle.

If in the future you happen to visit the VLA, you may find the antennas scattered near the horizon, or huddled close to the visitor center. If you visit at another time, you will likely see the antennas in a different configuration. All because VLA shapeshifts to see the universe in wondrous new ways.