And welcome as well to the new home of the blog-formerly-known-as AstroWoW. Make yourself comfortable. While I figure out what to do with the place, hang out here and enjoy the carnival for a spell.

Starting with our nearest celestial neighbor: Moon Mappers needs your help! Stuart Robbins is presenting research on the craters around the Apollo landing site, and we need YOUR help to get it done. Here is how you can learn about the Moon and do science along the way.

Moving out a little farther, lots of news from the Red Planet this week. Over at Discovery News, Ian O’Neill asks if, instead of waiting for Martian life to walk up and introduce itself, could we try sniffing them out?

Looking for a Mars Curiosity landing cheat sheet? The rover makes its daring landing attempt in August. If you’re hoping to attend one of the many social events taking place to watch and celebrate the event—or even if you’re just following along at home—this one-page guide will make your planning easier.

Happy Canada Day for all the Canadians in the world and beyond! The Canadian Space Agency is eager to see the rover Curiosity land on Mars. Find out why!

Finally, Bans Lansdorp is a European entrepreneur who wants to send humans on a one-way trip to Mars, and to pay for it as a media event. If Mr. Lansdorp meets his goals, the first four settlers will arrive on Mars in 2023, and four more settlers will join them every two years. Nextbigfuture has an interview with Mr. Lansdorp in which he and Sander Olson discuss the Mars One project, how he believes that the project could be done for $6 billion, and could eventually lead to a permanent manned presence on Mars.

Expanding our scope to the rest of the solar system, Riding With Robots talks about how a set of beautiful old planetary maps changed the way he sees space exploration. Thera also some fun pictures of toy space probes!

You know those videos and pictures—pointing at or very close to the Sun—that purport to show unknown planets? Astroblog asks: if these objects really were planets—rather than lens flares, internal reflections, clouds or aircraft—why don’t we see them after sunset?

A second entry from Nextbigfuture: the B612 Foundation has unveiled its plans to build, launch, and operate the first privately funded deep space mission. SENTINEL is an infrared space telescope to be placed in solar orbit, ranging up to 170 million miles from Earth. The telescope’s mission is to protect humanity, map the inner solar system, and enable exploration.

Moving out to the star forming regions of our own Galaxy, Silver Rockets is drawn to the vivid color of an archive Hubble Space Telescope image of NGC 2467.

Speaking of galaxies, why stop at the Milky Way’s edge? StarryCritters explores irregular dwarf galaxy DDO 82 in a new image from the NASA/ESA Hubble Space Telescope. What shapes do you see in this salt and pepper cloud of stars?

Want to see all these celestial wonders for yourself? Maybe you should stop by the Virtual Star Party! Google made a lovely video about CosmoQuest’s weekly online star-gazing that brings people together from around the world. Won’t you join them?

If we’re looking out into the heavens, it’s not unreasonable to think about other civilizations looking back at us! Would it be possible to spot the light from Earth’s cities from light years away? Supernova Condensate ponders detecting extraterrestrial city lights, and the flipside of light pollution.

From the realm of technology developments: a blog post double feature! The arrival of a new pulsed energy device for testing in Huntsville has Centauri Dreams pondering fusion, its past and its future, in space propulsion systems.

Completing its Carnival trifecta, Nextbigfuture reports on exciting developments at SpaceX and Orbital, both of whom have recently fired their new engines! SpaceX’s Merlin 1D rumbled for a full mission duration firing, while Orbital’s AJ-26 continued its testing ahead of its debut on their Antares launch vehicle.

And finally, we’ll close the Carnival with some poetic inspiration and musings on the heavens. I’ve long maintained that art and science are intimately intertwined. Apparently, I’m not the only one. The good folks from the Chandra X-ray Observatory present us with the final two winning entries from their second successful AstroPoetry Competition.

Late addition special! It’s never too early to get a head start on the inevitable return of the annual Mars Hoax.  The good folks at The Venus Transit have had just about enough of this silliness and suggest how you can fight back!

So come on over and make yourself comfortable. You’ll learn how astronomers use subtle shifts in the color of starlight to find planets and discover the expanding Universe.

The astronomy word of the week is “redshift”!

In the 1950s, radio astronomy was maturing. Astronomers at the University of Cambridge published the first detailed maps of the entire sky at radio wavelengths. These new observations revealed, among other things, a zoo of mysterious objects in the sky: point-like sources of radio emission which did not, apparently, emit any visible light. Up until then, visible light was all astronomers had to go on; radio astronomy was revealing a previously invisible Universe!

The mystery deepened in 1960 when astronomers Allen Sandage and Thomas Matthews identified what appeared to be a faint blue star sitting on top of one of the radio source “3C 48″. Spectroscopic chemical analysis – the breaking up of light into its component colors or wavelengths – baffled investigators. The chemical signature in no way matched any known objects. The experience was repeated in 1962 when astronomer Maarten Schmidt, using the 200-inch Hale Telescope at Palomar Obseratory, found nearly the same thing with another radio source.

Schmidt figured out why the chemical fingerprint seemed so odd. The object was not made up of exotic elements but rather very mundane elements – hydrogen, mostly – traveling at an enormous speed. With the realization that they were seeing plain old hydrogen, they were able to clock 3C 48′s velocity at 37% the speed of light. This “star” was receding from Earth at nearly 370 million km/hr! And each object that was measured told a similar story: all of these radio sources were caught up in the expansion of the Universe.

These “quasi-stellar radio sources” – or quasars, for short – were not nearby objects at all, but rather extremely luminous entities far out in intergalactic space. Today, astronomers know of rougly 200,000 quasars, the furthest of which sits over 28 billion light-years from Earth. At that distance, the light has been traveling through space for 13 billion years – nearly three times the age of our planet! And this is one of the peculiar things about quasars: none of them are “local”. Most are at least three billion light years from Earth. The Universe appears to have gone through a phase early in its development when it produced many quasars…and then stopped.

With the aid of deep imaging from the largest telescopes, astronomers have learned that quasars are actually the hyper-luminous cores of very distant galaxies. Some of the most luminous shine with the light of two trillions suns — that’s about 100 times the light output of our entire galaxy! All of this energy is coming from a region of space no larger than our Solar System. This kind of energy can not be produced by star light alone. Quasars, therefore, require a phenomenonly powerful engine.

The driver at the heart of every quasar is a supermassive black hole – an exotic creature that contains the combined mass of several million suns within a region so compact that the intense gravity can not allow even light to escape. It’s gravitational tentacles reach out many light-years, drawing in stars, gas, and dust that venture a little too close. But, limited by the dynamics of orbital motion, this cosmic detritus can not simply plop down to the black hole’s surface. Rather, it spirals in, like water running down a drain. Every bit of matter that is funneled down the black hole’s throat must pass through this vortex. In the case where material is collected faster than it can spiral in, a celestial traffic jam ensues and the black hole builds what astronomers call an accretion disk. The high velocities of material running into each other in this disk generates friction that heats the disk to many millions of degrees. And, as it does so, it begins to pump out prodigious amounts of energy at all wavelengths of light: radio waves, infrared, visible light, and x-rays. The disk lights up and a quasar is born.

The observation that quasars proliferated in the early Universe and then slowly died out tells us something about the evolution of galaxies. A quasar needs two things: a supermassive black hole in the galactic center and enough material to continuously feed it. The largest quasars consume material at the rate of 600 Earths a minute! Almost every galaxy we observe – our own included – has a supermassive black hole in its core. What appears to have gone missing is the steady diet of interstellar gas and dust that creates the accretion disk. In the early Universe, galaxy collisions were far more common then they are today. This game of galactic bumper cars provided the cores of these galaxies with fresh material to feed the black holes. As the Universe aged, the galaxies drifted apart, the material got used up, and the quasars turned off. Astronomers hypothesize that the Milky Way may once have housed a quasar in its center that has long since shut down. But, it may reactivate in a few billion years when the Andromeda Galaxy collides with our own and the new combined super-galaxy flares up one more time.

The powerful light from quasars provides astronomers with a unique way to measure changes in the Universe. As the light from a distant quasar crosses through space en route to Earth, it frequently passes through intervening clouds of extragalactic gas. When it does so, the chemical signature of each cloud is imprinted on the spectrum of the quasar’s light. Each cloud leaves its own unique stamp. For the most distant quasars, the light travels for nearly the age of the Universe and every time it passes through one of the clouds, it takes a snapshot of the cosmic stew at that time. This has proven to be a fantastic test of theories describing the origin and evolution of the Universe. By tracing these snapshots back across space and time, we get an unprecedented peek into the chemical makeup of the cosmos from its first billion years of existance.

Hop on over to Amy Teitel’s Vintage Space blog to see what the buzz is this week.  In the 250th edition, we fly to the spiral galaxy NGC 891, examine the details of a proposed space drive that could accelerate a spacecraft up to 69% of the speed of light, and check out highlights from the Cassini mission orbiting Saturn.

Those articles, and many others, await at the 250th volume of the Carnival of Space.  Go explore!

How do we know that the nearest star to our Sun is 4.2 light-years away?  It’s not as though we can stretch out a long tape measure between here and Proxima Centauri.  Barring drastic advances in space travel, if we’re going to figure out the distances to the stars, we’re limited to measuring them while not leaving the Solar System.

Let’s try an experiment.  Hold up a finger several inches in front of your face and focus on something across the room.  Now close one eye and notice where your finger appears to be.  Without moving your head or your hand, close that eye and open the other one.  Notice anything different about your finger?  It appeared to move!  If you rapidly alternate which eye is open you’ll see your finger appear to dance back and forth in front of your face.  Every time you switch eyes, you’re seeing your finger from a different angle.  To your left eye, the finger is off to its right while the opposite is true for your right eye.  Now, continue to alternate eyes while moving your finger away from your face.  You’ll notice that your finger won’t bounce back and forth as much when it’s further away.  Now try it with an object on the other side of the room.  The more distant the object, the less it moves as you alternate eyes.

This effect is known as parallax and it’s one of several ways our brains interpret distance.  Having two eyes lets us view everything from two different angles.  By comparing how objects appear in both our eyes, our brain can figure out whether an object is close or far.  Successfully picking up a glass without missing would be a lot harder if you only had one eye.

But we can do a lot better than just saying whether an object looks close or far.  We can actually calculate precisely how far it is.  If you know how far apart your eyes are and you measure the angle over which your finger appears to move, you can construct a triangle.  The base of the triangle is the distance between your eyes and the opposite angle is the one you just measured.

Remember in high school trigonometry when you wondered what use all of it was?  Well, here you go: you can use those two numbers, and a simple equation, to calculate the height of the triangle which, in this case, is the distance to the object.

What ultimately limits us is how far apart our two viewing angles are.  Try the alternating eye trick with a far away flag pole or even a cloud and you’ll note that your eyes don’t see those things much differently.  You need to move your eyes farther apart for that to work.  Since that sounds like an unpleasant solution, what if we just stand at two different locations?  This is the whole basis of triangulation and is something surveyors have been using for a very, very long time.

If you pick two locations that are sufficiently far apart, you can start to measure some seriously large distances.  By standing on opposite sides of the Earth, you can measure the distances to the Moon, Sun, and planets.  But even that’s not enough to measure the distances to the stars.  Instead, we let the Earth do our work for us by taking us around the Sun!  As the Earth travels on its orbit, we constantly see the stars at slightly different angles.  If we take a picture of a field of stars and then wait six months to take another, we see those stars from two locations which are as far apart as the Earth’s orbit is wide – roughly 200 million miles!

Interestingly enough, this was for a long time used as an argument against the idea that the Earth went around the Sun.  Ancient Greek scholars knew that if the Earth moved, they should see the stars shifting back and forth.  They didn’t, so clearly the Earth was stationary.  It didn’t occur to them just how far away the stars actually were!  It wasn’t until 1838 that German astronomer Friedrich Bessel successfully made the first stellar parallax measurement and deduced the distance to the star 61 Cygni as roughly 10 light-years.  This is no easy task.  Even for the closest star to our Sun, measuring the parallax angle is equivalent to measuring the width of a human hair from roughly 70 feet away!

By measuring stellar parallaxes, astronomers have mapped out the distances to hundreds of thousands of the closest stars.  To get really accurate measurements, you need to get above the turbulence of the Earth’s atmosphere.  This led the European Space Agency (ESA) in 1989 to launch the Hipparcos satellite and map stars out to 1600 light-years away.  But even that is a small percentage of our entire Galaxy.  To probe deeper, ESA will launch the Gaia satellite in late 2012.  Gaia will provide an unprecedented galactic census of some one billion stars and let astronomers map their positions right to the core of the Milky Way Galaxy.

The power of parallax measurements, however, ends pretty much at our Galaxy’s edge.  Beyond that, the angles become much too small for even the most sophisticated instruments to measure.  To determine galactic distances, and beyond, astronomers have a whole host of other tricks up their sleeves.  But they all depend upon the parallax measurements of the closest stars for calibration.

Amazingly, the journey to the edge of the Universe starts by measuring the tiniest of angles right here at home.

She bobs and wobbles, ducks and weaves, as she makes her annual trek around the Sun.  Our home constantly struggles with forces both external and internal; the Sun, the Moon, and the planets tug her back and forth while shifting masses within force her to constantly find new balance.

The tilt of our planet’s axis – what astronomers call its obliquity – is one measure of this interplanetary tug-of-war.  Obliquity measures how far over a planet or moon is tipped relative to its orbit; in the Earth’s case, it’s about 23 degrees.

We feel the obliquity in the changing seasons.  In June, the north pole is tipped towards the Sun.  The northern hemisphere experiences longer days and more direct sunlight giving us warm summer days.  In December, the north pole is tipped away from the Sun and everyone above the equator experiences the bitter chill of winter as the days get shorter and the sunlight reaches us more obliquely.

The other planets in our solar system exhibit a wide range of obliquities.  Mars, Saturn, and Neptune are all tipped by roughly the same amount as Earth.  Mercury and Jupiter have hardly any obliquity at all – which means no seasons for them.

Venus’ obliquity is 177 degrees; it is almost completely upside down.  Put another way, it rotates in the opposite direction from Earth.  Were there ever to be a break in Venus’ stifling cloud layer, native Venusians would see the Sun rise in the west and set in the east!

Uranus is another strange character; his obliquity is 97 degrees.  Uranus is lying on his side!  This leads to some very strange days and seasons.  Each pole of Uranus alternates between being in constant sunlight for 42 years, then perpetual darkness for another 42 years.  Only a very thin strip along the equator experiences anything like day/night cycles that we experience – and even then the Sun is frequently sitting very low on the horizon.

Astronomers aren’t entirely certain how Venus and Uranus came to have such extreme  obliquities.  A leading hypothesis is that both experienced massive collisions at some point in their distant past, essentially knocking them over.

A glancing blow from a minor planet isn’t the only thing that can change an obliquity.  The tilt of our planet’s axis is drifting – ever so slightly – all the time.  The poles actually trace out a rather complex dance over time scales ranging from minutes to thousands of centuries.

Like a spinning top, our planet also wobbles – though it does so very slowly.  Over a period of 26,000 years, the poles trace out a giant circle in the sky.  Right now, the north pole points pretty close to the star Polaris.  But over time that will change, and in some epochs it will be pointing at nothing at all.  This wobble – or precession – doesn’t change the amount of obliquity; it simply changes where in space the poles are pointing.

The Moon, however, does tug on our planet’s axis.  In addition to rotation and precession, the Earth experiences something called nutation.  This is a slight rocking of the Earth’s axis caused by the changing positions of the Moon, the Sun, and all the other bodies in the solar system.  With many players involved, the motion from nutation is fairly complex.  The main component nods the planet by a couple hundred meters over a nearly 19 year cycle that is tied to how the Moon’s orbit drifts around the Earth.

Our planet also has to contend with forces much closer to home.  Internal motions of magma, deep sea currents, changing ocean salinity, winds, melting ice sheets, and even earthquakes all conspire to constantly change the distribution of mass within the Earth.  The planet responds by continuously shifting its spin axis – an effect known as polar motion.  This is a bit different than nutation; it actually changes where on Earth the north and south poles are located.  Hard as it may be to fathom, these are not fixed locations on the globe. The drift is very small – centimeters per year – but it does add a complex twist to understanding our planets motion in space.

The presence of a large moon is believed to help stabilize our obliquity.  The tilt does change by a few degrees over many millions of years, but manages to hold steady at around 20-25 degrees.  Mars, conversely, appears to be going through a chaotic evolution in its obliquity.  Computer simulations of its stability suggest that, over the past few million years, the Red Planet’s tilt has oscillated wildly between zero and sixty degrees which may lead to drastic swings in the environment.  Many astrobiologists – scientists who study the evolution and origin of life in a cosmic context – argue that our moon may be an essential ingredient for providing a stable environment within which life could thrive.  Does this mean that the existence of complex life and advanced civilizations is dependent on the presence of a large satellite?  That’s hard to say.  It is certainly plausible that our closest neighbor in space has been an indispensable partner in the evolution of our species.

I’m continually amazed at how a simple concept – like the tilt of a planet – can actually lead down a labyrinthian path of subtleties.  Studying Earth’s obliquity, and the obliquity of all the other planets and moons, reveals much about the dynamics of our planet, the interplay of our home and neighboring worlds, and even the often times cataclysmic evolution of the solar system.

While the subtleties are far removed from our day-to-day lives, our Earth’s position in space actually is inseparable from the human experience.  The June solstice is just over a month away. Whether you’re bundling up for winter, or heading to the beach for summer, remember: the Earth’s obliquity is the reason for the seasons!

Cool stuff this week on how astronomer’s know the age of the Universe, a nova caught by NASA’s spaced-based solar telescope, STEREO, and even a look at the energy demands for setting up your very own Galactic Empire.

Check it out: Carnival of Space #248!

The celestial intruder came to be known as a “nova” – after Tycho Brahe’s extensive manuscript on the phenomenon – where he referred to the object simply by the latin term for a new star: “stella nova”. While no one could have known it at the time, the new star was actually not new at all but rather a very old star experiencing a cataclysmic event.

Brahe’s illustration of a “new star” in Cassiopeia (labeled “I”)

Most of the stars in the sky are not singular, like our Sun, but rather members of binary star systems – two stars locked in orbit around one another. Of this pair, one star is often more massive than its companion and therefore burns through its nuclear fuel at a faster rate. The heavier star reaches the end of its life before its sibling. When it does, the star inflates to become a red giant and sheds its outer layers into space leaving behind the hot, dense stellar core. The naked core, known as a “white dwarf“, continues to orbit its stellar neighbor while taking the next few billion years to passively cool from a white-hot 100,000 degrees.

In the case of close binary pairs, the white dwarf can actually siphon matter off of its companion. The intermingled gravitational fields of the two stars builds a bridge of predominately hydrogen gas spanning the distance between them. Gas from the still active star flows through this gravitational funnel and spills on to the surface of the white dwarf. The mass of the white dwarf controls the rate of mass accretion; if the rate is high enough, then something quite spectacular can happen.

Artist’s rendition of a white dwarf orbiting a blue giant star.

White dwarfs are an example of a rather exotic type of matter that physicists call “degenerate“. Most gasses expand when you heat them up: the increasing temperature causes the molecules to zip around faster which in turn increases the gas pressure. At extreme densities – like those seen in the cores of stars – the gas behaves rather peculiarly. The pressure is no longer a slave to temperature: turn up the heat and the gas does not expand.

This counterintuitive behavior is crucial to what happens next on the white dwarf’s surface. As hydrogen from the donor star comes crashing down, the crushing force of gravity compresses it into a degenerate state and the intense radiation heats the gas to many millions of degrees. But having now obtained the properties of a degenerate gas, the newly acquired hydrogen shell doesn’t expand in response to the rapid heating, but holds its pressure steady. And now things get interesting.

At temperatures exceeding 16 million degrees Celsius, conditions on the surface mimic those deep in the cores of stars and a thermonuclear explosion is the result. Hydrogen is rapidly fused into helium and the resulting release of energy blows the outer hydrogen shell off the surface of the white dwarf at speeds exceeding ten million kilometers per hour. In mere days, the white dwarf can increase its brightness by 100,000 times. It can then take months – or in some cases years – for the star to slowly fade from view.

Our Milky Way Galaxy experiences several dozen novae each year, only about ten of which are visible from Earth. A few are even visible to the naked eye. The most recent nova to be seen without the aid of binoculars or telescopes did so in the constellation Scorpius, reaching its peak brightness on February 17, 2007. Even more rare are what astronomers call “recurrent nova”. Once the white dwarf has blown the hydrogen shell into space, the gas can slowly start building up again. These are novae that flare up repeatedly, sometimes once a year, sometimes once every couple of decades. In our whole galaxy, only ten novae are known to be recurrent.

In the centuries since the new star of 1572, astronomers have come to realize that the event which gave novae their name was not what we know now as a nova. Up until early in the 20th century, a nova referred to any rapid brightening of a star. But there are many reasons stars suddenly flare into view. What Tycho saw was actually something far more powerful – a type of, aptly named, supernova. The supernova of 1572 was not the result of a flash of hydrogen burning on a white dwarf shell, but rather was caused by the complete detonation of a white dwarf. By stealing gas from a nearby companion slowly enough, the progenitor of the supernova delayed a nova flash while gradually increasing its temperature and pressure and eventually igniting hydrogen fusion throughout the interior of the once dead stellar core. With out the counterbalancing force of the rest of the star to throttle these reactions, the now violently active core obliterated itself in one of the most powerful explosions the Universe can produce.

Composite X-ray and infrared image of the remnant of the 1572 supernova.

The event of 1572 ushered in a new era of astronomy, one in which the constancy of the celestial sphere could no longer be assumed. Novae – and their more powerful supernovae cousins – are constant reminders that we, in fact, live in a highly dynamic and energetic Universe.

To trace out the meridian yourself, point your arm north.  Now swing your arm up directly over your head and then down again to the south.  You’ve just drawn a line dividing the sky neatly into eastern and western halves.  Facing south, everything to the left of the line is rising; everything to the right is setting.  The meridian is different from celestial markers like the ecliptic because it is completely local.  One observer’s meridian is different from someone else’s.  Only observers standing at exactly the same longitude will mark out the same meridian.

As a star crosses the meridian, it reaches its highest point on its nightly journey – a moment known as “upper culmination“. Knowing a star’s time of upper culmination is essential for an astronomer trying to get the most out of her time at a telescope.

Light from a distant star or galaxy can travel for thousands or even millions of light-years with out being perturbed…and then in the last nanoseconds of its journey become irreversibly entangled in our atmosphere.  Much like looking at oncoming headlights through a thick fog, the air we breath dims starlight.  Atmospheric turbulence also causes the stars to twinkle.  This effect can be lovely to look at but can be rather frustrating for the astronomer trying to make a detailed measurement of some celestial object.

The more air one looks through, the greater both of these effects become.  You can see this effect for yourself by looking at a bright star near the horizon and comparing it to one high in the sky.  At this time of year, the stars Sirius and Regulus are good stars for comparison.  For those living in mid-northern latitudes, bright blue Sirius sits low in the western sky shortly after sunset while Regulus is almost directly overhead, sitting close to the red speck of light that is Mars.  Sirius dances and twinkles with all the colors of the rainbow while Regulus burns steady and bright, only a slight shimmer revealing the sixty miles of air sitting on top of your head.  Starlight coming from low in the sky must pass through more air than light coming straight down.  The twinkling, therefore, becomes more apparent when stars are close to rising or setting.  And while it’s hard to discern with unaided eyes, the light is considerably dimmed as well.

For this reason, astronomers try to time their observations for when the star, planet, or galaxy of interest is crossing the local meridian.  This ensures that their target is as high in the sky as it can be and therefore maximizes the amount of light reaching the telescope.

But the meridian has an impact well beyond the needs of professional astronomers.  It is also central to how we mark time.   The duration between successive crossings of the meridian by the Sun is how we define one 24 hour day.  Using the Sun’s meridian crossing greatly simplifies time keeping – when the Sun is sitting at its highest point in the sky, shadows on the ground are shortest.  Marking noon therefore becomes a simple matter of watching shadow lengths.

One downside of this is that any two towns lying on different lines of longitude will mark noon at different times.  This may seem obvious when comparing widely separated cities like New York and London.  But what about the difference between London and Oxford?  Or New York and Trenton?  These are cities separated by only about sixty miles and yet their noons are different.

For most of history, this is exactly what happened.  To people that rarely left their home town, the fact that every town and city had their own local time was irrelevant.  It only became a problem with the advent of rail travel.  Suddenly, Boston and New York were reachable after mere hours in a train and the problem of synchronizing clocks for the purpose of scheduling quickly became a nightmare.  Different rail companies took to using their own time – which made connecting from one rail line to another in a distant city an adventure in time table translating.

The terrible burden and confusion this introduced directly led to the creation of time zones.  Now, towns within agreed upon boundaries of – nominally – constant longitude would all use a common time known as the “zone time”.  Approximately, this fell to synchronizing the entire zone to local noon measured somewhere in the middle of the time zone.  Noon in your town may therefore no longer happen at exactly  the moment of the Sun’s upper culmination.  Depending on your location within the zone, it could be as much thirty minutes before or after the local solar meridian crossing.

Keeping time with the Sun actually becomes even more complicated than this.  Because the Earth’s speed around the Sun is not constant over the course of the year, the duration between successive solar culminations varies.  A day tied strictly to solar noon is longer in July, when the Earth is farthest from the Sun and therefore traveling slowest, than it is in January when the Earth speeds through its point of closest approach.  To keep a constant day,  zone time is therefore not tied to apparent solar noon but what is called “mean solar noon” – or where the Sun would be if the Earth orbited the Sun at a constant speed.  The difference is not insignificant – apparent solar noon can be ahead of mean noon by 16 minutes (in November) or behind by 14 minutes (in February).

The “equation of time” plots the difference in noon as measured by a sundial and “mean solar noon”.

Something as esoteric as maximizing telescope efficiency and something as basic as defining a day – probably not two things you’d put together.  But they share the common thread of determining the moment that a celestial object – be it the Sun or a distant galaxy – reaches its highest point in its daily trek across the celestial sphere.  Some concepts in astronomy are pretty far removed from our daily lives.  But an idea as fundamental as noon starts with a simple line bisecting the sky.

Let’s start with clearing up one popular misconception: it has nothing (or very little) to do with time. Ignore the word year. It’s a distraction. When we’re talking about light-years, we’re actually talking about a way to measure distance. A light-year is just the bigger, badder cousin of the inch, the mile, the kilometer, and the furlong. In short, it’s simply the distance a beam of light will travel in one year.

What we’re doing is using time as a proxy for distance. It’s not such a crazy idea. Have you ever been on your way to meet friends at a restaurant or movie theater and, while en route, called to tell them you’re running late? What do you usually say? If you’re like me, you might tell them “I’m about fifteen minutes away”. You’re using how long it will take you to get there as a substitute for how far away you are. Astronomers do the same, but use a beam of light as their reference. If we can figure out how long it will take light from a star to get here, we know something about how far away that star is.

Why do things this way? Partly because the distances we deal with in space are immense. They are mind-boggling, ginormous, colossal, stupendous…..astronomical. You really can’t go overboard with the hyperbole on this one. If we were to stick to miles or kilometers we quickly run into very unwieldy numbers just measuring the distance to the closest star to our Sun (a distance of roughly 24,000,000,000,000 miles!). So using a longer yard stick, so to speak, helps keep the numbers at least manageable.

Light is also convenient because, throughout the Universe, all light travels at exactly the same speed: about 670 million miles per hour. We don’t usually think about light “traveling” anywhere because when we turn on a light switch…there it is! We don’t have to wait for the room to light up, it just happens instantaneously. Except that it’s not instantaneous, just insanely fast. In fact, let’s pause for a bit and ponder how ludicrously quick the speed of light is.

Traveling at that speed, you would encircle the globe almost eight times in one second!

Eight times! Around the world! In one second! That’s quick. So if you were to travel off the Earth in a straight line at that speed, you’d get pretty far in the same amount of time, right? Actually, you wouldn’t have even made it to the Moon yet. For that, you’d have to wait half a second more. To get to the Sun at that speed would take you about eight minutes. Can you even imagine it? Traveling at a speed where you cover 200,000 miles every second for eight full minutes would only get you to the center of our Solar System.

If your brain hasn’t imploded yet, here’s another way to think about it: the light from the Sun takes eight minutes to get here. A side effect of that is, when you look at the Sun (which you really should never do!), you’re seeing the Sun as it was eight minutes ago. You’re looking into the past! While this sounds a little crazy, it’s actually something with which you’re already familiar. If you’ve ever seen fireworks, for example, you know that you see the explosion and then a few seconds later you hear it. If you close your eyes during the fireworks show, you’d only have your hearing to know when things were happening. Since it takes some time for the sound to get to you, you’d always be hearing things a few seconds after they happened. You’d be hearing from the past. The same happens with light: we only see something once the light from that event actually gets to our eyes and it takes time to do that. When we’re looking across a room, the time delay is only a few billionths of a second so in practical terms it doesn’t really matter. But when you start looking across large enough distances, the light from those objects is delayed like the sounds are from exploding fireworks.

So, going back to how far we can travel on a beam of light, where would we be after one year traveling at that speed? Surely we would have made some progress across the Galaxy by now! Not even close. We’re not even a quarter of the way to the nearest star! To get there, a dim red star called Proxima Centauri, we would have to travel at 670 million miles per hour for four years! That’s not counting bathroom breaks. So, because of that, we say that Proxima Centauri is four light-years away. Once again, when we look at that star, since it took the light four years to get here, we’re seeing the star as it was in 2008!

The red star in the middle is Proxima Centauri – the closest star to the Sun. It takes the light from this star four years to reach Earth.

So exactly how big is one light year? It’s about six trillion miles. That’s a six followed by twelve zeros. And that’s just one light-year! The Milky Way galaxy in which our Sun and all the stars we see at night reside is 100,000 light-years from one end to the other. To put that into perspective, the span of recorded human history is roughly 5000 years. So light from a star at one end of the Galaxy takes twenty times longer than all of recorded history to get to the other end. If we start moving beyond our Galaxy, it’s just over two million light-years to our nearest galactic neighbor, the Andromeda Galaxy. The light we currently see from that galaxy left there about the same time the ancestors of modern humans were first discovering stone tools.

At a distance of over two million light years from Earth, the Andromeda Galaxy is the closest galaxy to our own.

Our last stop takes us to the edge of the visible Universe. It’s also here where the trickiness of measuring distance in an ever-expanding Universe becomes apparent. The light we see coming from the farthest depths of the Universe has been traveling across the cosmos for almost three times longer than our planet has existed: nearly 14 billion years! But here’s a catch: we can not say that the edge of the visible Universe is 14 billion light-years away. Why? Because the Universe has grown larger in that time! A galaxy whose light took 14 billion years to cross the cosmos to our little planet has, in the intervening aeons, moved even further away. The current physical distance to that remote beacon, if we stopped the Universe from expanding and stretched out a really long tape measure, is just over 46 billion light years! Even in light years, measuring distances across the Universe becomes unwieldy. But measuring in something familiar, like miles, is truly humbling. From here to the edge of our vision spans a distance of approximately 276,000,000,000,000,000,000,000 miles.

And it’s getting bigger every day.