#strictlyastronomy

Diagram showing the ‘radiant’ of the Perseid shower (marked with the red cross). Graphic by the blogger.

Sunday night (12th August) marks the peak of the year’s best meteor shower, and this year the circumstances for viewing them are the best they have been in over a decade.

Each August, the Earth’s orbit carries our planet through streams of debris shed by Comet Swift-Tuttle, which last flew through the inner solar system in 1992. The orbit of this comet is arranged just right such that it intersects our orbit, although fortunately the Earth has never found itself in the same place as the comet at the same time (although that outcome is not impossible!)

Comet Swift-Tuttle, 1992. Credit: NASA

With an orbital period of 130 years, no one who saw its last apparition will likely be alive to see its next. But on each trip around the sun, Swift-Tuttle leaves a trail of dust in its wake. As the Earth plows through this part of its orbit, some of those dust grains are intercepted by our atmosphere, and we see them as swarms of bright shooting stars across our night skies.

From a dark location, up to 100 meteors per hour may be seen, with occasional outbursts to even higher hourly rates. The meteor show is known as the “Perseids” because the tracks of the meteors appear to converge toward the constellation of Perseus.

2018 is an especially good year for the Perseids because the maximum intensity of the shower occurs a little more than 24 hours after the new Moon, so that there is effectively no moonlight in the night sky for the meteors to compete against. That enables good visibility of fainter Perseids, which would otherwise be lost in the glare of the Moon in other years. That means that hourly rates reported by observers in 2018 are likely to be higher than in other years.

Like other meteors, Perseids begin as pieces of cosmic dust little bigger than sand grains. After leaving the surface of Comet Swift-Tuttle, they may orbit the sun for hundreds or thousands of years before approaching the Earth. Impacting the upper atmosphere at relative speeds approaching 60 kilometers per second (134,000 miles per hour), friction with air molecules heats the dust to temperatures of over 1,000 degrees. Elements in the disintegrating dust grain, such as iron, magnesium, and sodium, are energized by the heating and radiate away some of the energy as light of particular colors like green and yellow. Some 4.5 billion years after their creation, these grains end their existence in mere seconds.

The best views of the Perseids come after local midnight, when the direction of the Earth’s rotation takes us toward the forward direction of the Earth’s orbit around the Sun; whereas earlier in the evening, meteors must catch up to Earth’s motion to reach the atmosphere, after midnight the Earth is more directly moving through the stream of dust particles.

To see the Perseids, find an outdoor space with a relatively clear horizon toward the northeast. Activity will begin to pick up by about 10pm local time, and steadily increase until after midnight. Here are some tips to improve the experience:

Safety first! Carry a flashlight to help navigate in the dark, especially in unfamiliar places. Red or brown paper placed over the front lens of the flashlight will help preserve your eyes’ sensitivity to faint light.

Use a reclining lawn chair or chaise lounge to relieve fatigue while looking up at the sky for long periods of time.

Bring plenty of water, too, because it’s possible to dehydrate in warm temperatures on summer nights.

If you’re observing after midnight, remember to dress warmly as the night may become quite cool in the pre-dawn hours.

Brave the cold and head out tonight to see what may well turn out to be the year’s best display of bright meteors: the Geminids! Little bits of dusty debris from the oddball ‘rock comet’ 3200 Phaeton slam into the upper atmosphere of our planet at some 35 km/s (79,000 miles per hour), leaving glowing trails that appear to ‘radiate’ away from the constellation Gemini (the small red cross in the figure above). Like train tracks converging in the distance, these bright trails are actually more or less parallel to each other, and their apparent origin at a common ‘vanishing point’ is an illusion. As they encounter our atmosphere at high speed, they are heated by friction with air molecules until they briefly glow at a temperature of several thousand degrees, being almost completely vaporized while still dozens of kilometers above the ground. The meteor debris is ionized, emitting bright colors with the characteristic fingerprints of their constituent atoms.

(AccuWeather graphic)

Photo credit: Jim Hendrickson

Saturday, 28th October, is International Observe The Moon Night! This annual event encourages people to get outside in the early evening and take a gander at Earth’s nearest neighbor in space. Here’s how the Moon will look this evening, using the Scientific Visualization Studio's Dial-a-Moon online app: 

Here are a few things to notice about the Moon if you view it through binoculars or a telescope. First, the Moon is in the waxing gibbous phase tonight, between first quarter and full. This means it is a little more than one-fourth the way along its orbit, taking the start to be when the Moon is “new” (seen roughly in the direction of the Sun in the sky). The vertical line running down the middle of the Moon’s disk is the terminator, representing where the Sun is just coming up over the horizon on the Moon. Terrain is seen is sharpest relief near the line, since shadows cast by tall objects tend to be longest there. 

Look for the lunar “seas”: the dark spots that form the familiar “Man in the Moon” pattern are remnants of giant impact basins that were filled in by lava early in the Moon’s history. Some of these are ringed by tall mountains, forming the rims of craters hundreds of miles across. And of course, much of the Moon’s ancient “southern highlands” terrain is covered in countless impact craters. These preserve a record of billions of years of lunar history shaped by the rain of comets and asteroids from above. Also, look for subtle shades of color on the lunar surface -- they’re real!

Lunar Reconnaissance Orbiter true-color image of the Moon.

If you don’t have access to a telescope or a pair of binoculars, there is still plenty to see with just your own eyes. And don’t forget the Earth around you. How bright is moonlight? We still don’t quite know exactly. Look at how the illumination changes in the evening, as the glow of twilight fades in the west and the Moon becomes the dominant source of light in the night sky. Some animals are more active at night when the moon is brightest.

Lastly, think about the human element. How do people relate to the Moon? Are they more or less likely to notice it during some parts of the lunar cycle? Consider, too, that we may owe our very existence to the fact that the Earth has a large moon at all. 

How is our galaxy built? What does it look like? (And how do we know?)

These questions have persistence since before we properly understood what galaxies are. Once upon a time, it was thought that what we know now as the ‘Milky Way’ was everything that was. The glowing band of light across the night sky was revealed as a swarm of faint stars when Galileo Galilei first turned a telescope on the Milky Way in 1610. The Milky Way was seen as a great system of stars, which themselves must be pretty far away if a telescopes required to see them, but it was viewed as the only such system until the twentieth century.

In the eighteenth century, people began trying to systematically map those stars in order to deduce something about the structure of the system. The Anglo-German astronomer William Herschel published a map of the Milky Way in 1785, drawn from his years of meticulous observations of its stars: William Herschel’s 1785 map proposing an overall shape of the Milky Way, based on his observations of its constituent stars.

William Herschel’s 1785 map proposing an overall shape of the Milky Way.

He also correctly identified the Sun as occupying a place within the system (the prominent “star” just to the right of center on his map) rather than lying outside of or beyond it. His logic was simple. If the Sun and its attendant planets, including Earth, resided within the system,

“the heavens will not only be richly scattered over with brilliant constellations, but a shining zone or milky way will be perceived to surround the whole sphere of the heavens, owing to the combined light of those stars which are too small, that is, too remote to be seen. Our observer’s sight will be so confined, that he will imagine this single collection of stars, of which he does not even perceive the thousandth part, to be the whole contents of the heavens."

In the generation before Herschel, thinkers began to speculate on the existence of other systems like the Milky Way. Thomas Wright suggested in 1750 that the Milky Way was really a single object, a large, flattened (and spinning) disk consisting of countless stars. The German philosopher Immanuel Kant read Wright’s musings and further developed the notion into what he called the 'island universe':

"Their analogy with the stellar system in which we find ourselves, their shape, which is just what it ought to be according to our theory, the feebleness of their light which demands a presupposed infinite distance: all this is in perfect harmony with the view that these elliptical figures are just universes and, so to speak, Milky Ways, like those whose constitution we have just unfolded."

By the turn of the twentieth century, astronomers were slowly coming to understand the the ‘spiral nebulae’ seen in their telescopes were systems like the Milky Way seen at great distances. However, these objects stubbornly refused to resolve into individual stars, even in the world’s largest telescopes, and some scientists held fast to the notion that the spiral nebulae were little more than gas clouds, perhaps illuminated by unseen stars embedded deeply within. 

The disagreement culminated in 1920’s “Great Debate” between American astronomers Harlow Shapley and Heber Curtis. Shapley argued the conventional position that spiral nebulae were small, nearby interstellar clouds, while Curtis advanced the notion that they were large and far away, and thus their constituent members must be stars. Curtis based his argument in part on his 1917 observations of a kind of stellar cataclysm called a “nova” in the direction of what we now know as the Andromeda Galaxy. Novae are very intrinsically bright, like extra-bright light bulbs, meaning that they can be seen at large distances within the Milky Way. Curtis looked back over the historical record and found 11 more novae in Andromeda on old plate photographs taken through telescopes. Based on these 12 objects, he computed a distance to Andromeda of half a million light years, much further away than the most remote known star.

Curtis was ultimately proven right before the end of the decade. The debate was properly resolved in the spring of 1929 when Edwin Hubble (of telescope fame) published a paper with a radical conclusion: the ‘Andromeda Nebula’ was certifiably far away. By monitoring a kind of variable star seen in the outer reaches of Andromeda’s spiral arms, Hubble was able to infer its luminosity; comparing this against the star’s apparent brightness revealed its distance: nearly 2 million light years. The controversy was settled: Andromeda must be something like the Milky Way itself. 

Edwin Hubble’s famous “VAR!” photograph marking the position of the Cepheid variable star he discovered in the Andromeda Galaxy. Carnegie Observatories photo.

What about Andromeda’s apparent shape? From images of it, and other objects that came to be known as “galaxies”, we find a variety of shapes and sizes that tell us something about their origins and histories. Hubble tried classifying galaxies by shape, arranging them into what became known as the “tuning fork diagram”:

Hubble’s tuning fork doesn’t sound like much.

There are basically two kinds of galaxies: spirals and ellipticals. A subset of spiral galaxies are known as “barred spirals” because of how their arms unwind from the center: the arms of ‘ordinary’ spirals curve all the way into the galaxies’ centers, while the arms of barrier spirals project off the ends of a linear feature running through their centers. Clearly Andromeda was some kind of spiral, seen at a fairly high inclination (whereas all of the model galaxies shown in the Hubble diagram are conveniently viewed face-on).

If the Milky Way was more or less like Andromeda, was it, too, a spiral of some sort? That seemed likely, because of how the Milky Way appears in our night sky. Keep in mind that we’re embedded within the galaxy, not flying above it somewhere in space. Rather than resembling the oval blobs of elliptical galaxies, the Milky Way looks more like a flattened disk seen along its edge. In certain cases, when we look out toward other nearby spiral galaxies, we see them in the same orientation, as in this image of the galaxy NGC 891:

A color image of the galaxy NGC 891 taken by the author.

We even have ways of deducing the presence of a bar in the Milky Way, so we know it properly belongs on the arm of Hubble’s tuning fork with the other barred spiral galaxies.

So by the turn of the twenty-first century, humanity had a pretty good idea of the shape and extent of our home galaxy based on observations, inferences, and some crafty detective work. Here’s what we think it would look like, if we could fly high above the disk and look back: 

A model of the Milky Way galaxy, viewed from over one of its poles. Illustration by Robert Hurt, IPAC; Bill Saxton, NRAO/AUI/NSF.

But how can we be sure? What’s happening in the parts of the Milky Way we can’t see directly?

In the image of NGC 891 above, note the dark stripe running down the middle of the galaxy, appearing to split it into two halves. This is a ‘dust lane’, composed of many individual clouds of interstellar dust that tend to form and settle in the galaxy’s mid plane. Dust particles are really good at scattering and absorbing light, so they transmit very little of the starlight coming from “behind” them as seen in the direction of Earth.  The Milky Way has its own dust lane, just like this one:

The “Great Rift” in the Milky Way, seen during northern hemisphere summer on Earth. NASA image.

With all that dust blotting things out, it seems like we could never know what’s going in really distant parts of the Milky Way. But some kinds of light can pass through the dust relatively unscathed — long wavelength varieties of light, like infrared. Here’s a Spitzer Space Telescope view toward the center Milky Way in the infrared part of the spectrum:

Staring at the Milky Way’s not-so-dark heart. Image: NASA/JPL-Caltech/S. Stolovy (Spitzer Science Center/Caltech).

There’s still some dust, but much less than in the visible light image shown previously. To look more deeply into our galaxy, we have to resort to longer wavelengths of light: radio waves. And that brings us to this week’s news about structures seen all the way through on our galaxy’s far side.

As reported last week in Science, Alberto Sanna and co-workers used a group of radio telescopes called the Very Long Baseline Array (VLBA) to measure the distance to a star-forming region on the side of the Milky Way opposite the Sun. VLBA uses a principle called interferometry to achieve extremely high resolution of astronomical radio sources. By distributing ten radio antennas, each with a diameter of 25 meters, across the western hemisphere from Hawaii to the Virgin Islands, VLBA achieves the same resolution as a single radio dish with a radius of thousands of kilometers. VLBA does this by very careful timing the arrival of the radio signals received by each dish. Software then reconstructs the incoming signal at very high spatial resolution.

The 10 VLBA dishes and their locations. Image: SeaWiFS Project NASA/GSFC and ORBIMAGE.

The authors then used a geometric principle known as parallax to determine the distance to the source of the radio waves, finding they originated some 66,500 light years away from Earth. That’s most of the width of the galaxy itself, thought to be around 100,000 light years. And with that they found, at exactly the expected distance, the continuation of one of the Milky Way’s spiral arms known as the ‘Scutum-Centaurus Arm’.

So why is this work important? Doesn’t it just confirm what we already thought was the case?

For one thing, it’s a demonstration of a very powerful technique for teasing out details of the structure of the Milky Way. The group’s parallax estimate blows the old record of about 36,000 light years right out of the water. As Sanna commented, "Most of the stars and gas in our Galaxy are within this newly-measured distance from the Sun. With the VLBA, we now have the capability to measure enough distances to accurately trace the Galaxy’s spiral arms and learn their true shapes.” 

"This means that, using the VLBA, we now can accurately map the whole extent of our Galaxy,” he added.

One of the most head-scratching stars in the night sky (at the moment) may be starting to finally give up some of its apparent mystery. The unceremonious-sounding KIC 8462852 is better known these days as “Tabby's Star” or “Boyajian's Star” after Tabetha S. Boyajian, professor of astronomy at Louisiana State University, who lead the team that first described the star in a 2015 paper. The paper is provocatively subtitled “Where’s The Flux?”, leading to the star’s other occasional nickname: the “WTF Star”. 

KIC 8462852 is an otherwise unremarkable 12th-magnitude star in the constellation Cygnus. What makes it remarkable is the recent history of its light output, which has baffled nearly every stellar astronomer who has examined its light curve. A light curve is a graphical representation of the light output of some astronomical thing over time. Most stars’ light curves are flat and boring: they emit essentially the same amount of light at all times. Other stars are known as “variables”. Their light curves show all manner of variation, from the highly regular to the highly irregular. In all those cases, however, we understand at least something about the underlying physics that causes these stars to vary.

Tabby’s Star, however, is in a league of its own. The first indications that something is up with this star came from the Kepler spacecraft mission in 2011. Kepler’s M.O. is pretty simple: in its primary mission, lasting from 2009-2013, it stared at a region of sky containing Tabby’s Star continuously, recording the brightness of every object in the field. Unlike previous instruments used in this manner, it achieved an unprecedented level of measurement precision, and was used to discover thousands of potential exoplanets. It also provided new information on the inner workings of stars themselves.

And it showed us that something about one particular star in its field of view is very, very weird.

Kepler light curves of Tabby’s Star (above) give some idea about its odd behavior. Long periods of stability (i.e., when the light curve is very flat) give way to inconsistent drops in the light received from the star. The star was seen to dim for many days at a time, losing up to 20% of its brightness, before stabilizing again at apparently full power. It didn’t at all resemble the light curve of a star hosting one or more exoplanets. Nor did it look like any other stellar light curve seen before.

Boyajian’s team wrestled with the cause of the star’s behavior. It considered explanations like a very young star prone to erratic light output as it settled onto the Main Sequence (nope: too old and mainstream for this kind of display). Or maybe it had a dusty disk around it that periodically blocked some of the light (nope: such a dust disk would re-radiate starlight in the infrared, leading to an obvious signal). After discarding some ideas like this for lack of evidence, the team proposed a radical idea: a chunky dust cloud made up of many dissolved comets (artist impression, below). 

The comets would probably tend to cluster, it was argued, if they originated as some larger object around the star that broke apart under the influence of the star’s gravity.Lots of other explanations have been proposed, and mostly dismissed, for want of supporting evidence, including:     

• Burps from a star that had actually eaten one of its planets;     

• A big planet with a set of rings tilted with respect to its orbit, periodically blocking some starlight from view;

• Chaotic stellar physics manifesting through something called “avalanche statistics”; and

• Some previously unobserved physical effect at or near the star’s surface.

The most exotic theory of all, and the one that got the most press, was that Tabby’s Star was being dimmed by a “Dyson swarm” (below): a ring or other structure around the star, composed of artificially designed and built objects, indicative of intelligent life.

Needless to say, it wasn’t the simplest explanation for the observations, and probably isn’t the right one.

New results seem to discount most of those theories and return to one of the original thoughts: the thing dimming Tabby’s Star is nothing more than garden-variety dust. New observations from NASA's Spitzer and Swift spacecraft, as well as the Belgian AstroLAB IRIS observatory, show that whatever is doing the light absorbing is better at it in the ultraviolet part of the spectrum rather than the longer-wavelength infrared.

That’s an indication that the absorbing particles are fairly tiny compared to the wavelengths of light illuminating them. But how do we know that? It turns out to be the same reason that the sky is blue: light interacts more strongly with particles when its wavelength is about the same size as the particles themselves. It’s for this same reason that the daytime sky on Earth is blue. So whatever is causing the dimming of Tabby’s Star can’t be made of particles much larger than the wavelength of light. Such particle sizes are consistent with the known properties of cosmic dust.

Lead study author Huan Meng (University of Arizona) says: “This pretty much rules out the alien megastructure theory, as that could not explain the wavelength-dependent dimming.” Instead, Meng says, the most likely reason for the observations is boring, old dust. “We suspect, instead, there is a cloud of dust orbiting the star with a roughly 700-day orbital period.”

However interesting Meng’s findings, they don’t completely explain what’s up with the “WTF Star”. There is no known mechanism to produce a dust ring or disk around a star that matches the observed properties of the KIC 8462852 system over the length of time we have seen its light varying. Other processes could be combining with the dust dimming to yield the observed variations. For instance, says AstroLAB volunteer Siegfried Vanaverbeke, “Tabby's Star could have something like a solar activity cycle.”

A Super-Duper Supermoon

Tonight (14 November 2016) sees a special occurrence of the “Supermoon”, a phenomenon much hyped by the media but not all that impressive to the casual observer. Yet it’s still a good chance to go out and see a big, bright full Moon — and to notice something important about its light. 

What is a ‘Supermoon’?

To understand how the Moon can be “super”, consider this NASA picture that shows the orbit of the Moon around the Earth: 

Although the shape of the orbit is greatly exaggerated for illustrative purposes, that shape really is not much like a circle and more like an ellipse, or an oval. Since the Earth isn’t at the center of the ellipse (according to Kepler’s 1st Law of Planetary Motion), that means sometimes the moon is a little closer to the Earth, and sometimes a little further away. The extremes of these two situations are marked on the figure as “perigee” (closest to Earth) and “apogee” (farthest from Earth). However, the orientation of the ellipse isn’t fixed in space. Instead it “precesses” over the course of about nine years, so the location in space of perigee and apogee keep changing.

Now add to this a simplified picture of the phases of the Moon, explained by orienting its orbit around us with respect to the (distant) Sun:

(Wikimedia Commons) 

Put all of this together, and two facts emerge: First, the phases of the Moon happen at different relative distances over the period of the precession of the Moon’s orbit around Earth. And second, at different intervals, any particular Moon phase will happen at some particular point in the orbit.

One possibility, therefore, is that the full Moon occurs at perigee, at which point it would be as apparently large in the night sky, and apparently as bright, as it can ever be. Full Moons that occur within a day or so of perigee have in recent years been commonly called “supermoons”. It’s also possible that the full Moon happens around apogee, making it apparently as small and relatively as dim as it can ever be. These full Moons, popularly known as “micromoons”, receive a lot less media attention. But the differences are noticeable: compare a supermoon (left, below) to a micromoon (right).

(Wikimedia Commons) 

A 12% difference in apparent size isn’t tiny, but it’s also not something that most casual lunar observers will notice. They may be more likely to note the different in apparent brightness, however, which is around 30%. Compared to a typical full Moon, that happens around the average Earth-Moon distance, the difference in brightness is more like about 15%. Still, these differences are small and only become readily noticeable when making the kind of comparison in the figure above. What is different about the November 2016 Supermoon? This month’s full Moon happens unusually close to the exact location of perigee, meaning that the Moon is both at its full phase and about as near to the Earth as it can possibly get. Perigee happened at around 11am GMT on Monday, just a couple of hours before the full Moon arrived at 1:52pm. At that time the distance to the Moon will be about 356,000 kilometers and its disc will span some 33.5 arcminutes across the night sky. The last time that full Moon occurred so near perigee in time was 26 January 1948, and it won’t be so close again until 25 November 2034. What will it look like?

The Moon may look a little bigger and a little brighter tonight, but odds are any such impressions are colored by the media hype. Since we’re a little more than a month out from the winter solstice in the northern hemisphere, tonight’s full Moon will be high in the sky around solar midnight, which is slightly after local midnight this time of year.  

If you go out tonight to see the Supermoon, look at the intensity of moonlight on the ground. That light provides about 0.3 lux of illumination, or about one-third of the brightness of twilight. By comparison, full sunlight is about 100,000 lux, and an overcast night away from sources of light pollution can be as dim as 0.0001 lux. Street lighting is around 20 lux, or roughly 60 times as bright as moonlight. Try reading a newspaper under full moonlight, however. You may be surprised at how easy it is.  

Notice, too, what colors look like under moonlight. The Moon is a nearly perfect reflector of sunlight in terms of its color, although it only reflects about 10% of the light it receives. The Moon’s surface is surprisingly dark, but it’s largely gray to white in color. Moonlight is a lot like dim sunlight, but you’ll notice that it doesn’t show some colors particularly well, such as red. As the human eye adapts to low light levels, its sensitivity to color shifts away from red and toward blue, something known as the Purkinje Effect. Why does this matter?

There’s some extra relevance to astronomy here that has to do with the ability to do astronomy from the ground. Not only is that hampered by the blurring effect of the Earth’s atmosphere, but also by the use of artificial light at night. Light pollution in the form of skyglow, caused by artificial light either emitted directly into or reflected upward into the night sky, forms a “fog” of light that makes it difficult to see the stars beyond. As the world converts its lighting from old technologies to new ones like white LED, with it comes a shift in the spectrum of that light toward shorter wavelengths that scatter efficiently in the Earth’s atmosphere and can make skyglow worse.

Some proponents of LED lighting make claims in the media that LED lights are “good for the night sky” or that they “reduce light pollution”, but that’s not strictly true. Even more confusing, sometimes they say that the color quality of white LED lighting is “just like moonlight”. Here’s why that’s not true at all:

This chart shows the spectrum of full moonlight (blue) with the spectrum of a white LED overplotted in black. Both sources have about the same color temperature, a metric that expresses something about how the intensity of a light source is distributed across the spectrum. Notice the huge spike of blue light in the LED spectrum that is not present in the moonlight spectrum; while the Moon reflects much of the blue light it receives from the Sun, much of that light is scattered away by our atmosphere. That, in part, explains why the night sky around full Moon is so bright.  

The result is that white LED light is considerably bluer than moonlight, if the two sources are matched for intensity. Unless cities that install these new lights also dim them, the night sky over those cities will get brighter. The failure to adjust light brightnesses to match the sensitivity of the eye and night has led to complaints that new LED lighting is glary and distracting. We’re still waiting for measurements that show how much the lighting actually affects night sky visibility, but we do know from extensive research that the additional blue light is potentially harmful to urban wildlife. The future of the nighttime environment in cities, and our ability to continue doing astronomy from the ground, depends on how these lighting conversions are managed, now and in the future.

And while you’re out tonight, have a look to the southwest where three bright planets (Saturn, Venus and Mars) are visible. The view below shows the positions of the planets just after dusk in the northern hemisphere.

A Celestial Pas De Deux

If you have been watching the western sky after sunset in the past few weeks, you may have noticed the curious dance of three bright planets: Mercury, Jupiter and Venus. Jupiter’s steady apparent slide to the west each night has brought it near the two inner planets, whose orbits interior to Earth’s keep them forever near the Sun in our skies. Mercury was more prominent in the night sky a couple of weekends ago, but has now begun its dive toward the Sun and inferior conjunction on 13 September, when it will pass nearest the Sun from our vantage point, reappearing in the pre-dawn hours a few days later. Meanwhile, Venus has just begun its long climb into the evening sky. It will be a regular feature of the hour or two after dusk throughout the fall of 2016, reaching its apex twelve days after the new year when the “Evening Star” will stand about 47 degrees east of the Sun in our skies.  

Tonight (27 August), Venus meets her chief rival in the night sky — at least in terms of apparent brightness. Most of the world will see the two planets pass a mere half-degree apart, or roughly the apparent width of the full Moon. By the time evening falls in the Americas, the two bright planets will be separated by only 0.1°, meaning they will fit neatly into the field of view of most telescopes. The image above shows the pairing in the west thirty minutes after local sunset, with Mercury more distantly off to the south and west. The inset shows the view expected in an 8-inh (20 cm) telescope at a magnification of 300 times. Venus will be so close to Jupiter tonight that it will nearly mingle among the latter’s bright moons. Such a close approach, called an appulse, only happens every few decades, and these two planets will not appear so close together in Earth’s skies again until the year 2065. 

In reality, of course, the two planets are very far from each other. Venus shines at apparent magnitude -3.3 and is roughly 1.5 times the average distance between the Earth and the Sun, known as an astronomical unit, or AU. Jupiter is much more distant at 6.4 AU from Earth. It’s a much larger planet than Venus, but the intensity of sunlight at Jupiter’s distance is only about 1/50 of its intensity at Venus. The faint light it reflects back to Earth wins over the large reflecting surface of the planet, meaning that Jupiter shines more dimly at magnitude -1.2. 

If you have a clear sky from the south to the west, tonight (6th August) is a great opportunity to see an unusual sight in the evening sky: all five planets bright enough to be easily visible to the unaided eye, plus a special guest appearance by the four-day-old crescent Moon. In order to see all five, begin looking shortly after sunset; Venus is the brightest of the lot at apparent magnitude -3.3, but it is also closest to the Sun and therefore will be the first to set. Saturn, the most distant of the planets visible, sets last and glows dimmest at magnitude +0.6.

Notice how the positions of the planets and the Moon trace out roughly a straight line across the sky. This apparent alignment does not result simply by chance, but tells us something fundamental about how the Solar System is constructed. The apparent path of the planets and the Moon as they move across Earth’s night sky delimit a line called the ecliptic. Related to the English word eclipse, it derives from its conceptual notion: formally, the ecliptic is the apparent path of the Sun on the celestial sphere. Eclipses of the Sun and Moon therefore take place only when the Moon is near the point in its orbit around the Earth where its apparent path crosses the ecliptic. 

(The ecliptic as traced out on the celestial sphere. Credit: Wikipedia) 

Since the planets all appear to nearly follow the ecliptic as they move across our sky, that means that their orbits must be more or less in line with Earth’s orbit around the Sun. The ecliptic therefore defines a plane of the Solar System in which the orbits of the major planets lie. Again, this isn’t chance, but an artifact of the way that the Solar System formed. According to current scientific thinking, the planets, asteroids, and other solar system bodies (except for long-period comets) formed from a rotating disk of material surrounding the forming Sun, some 4.6 billion years ago. Initially the cloud was more or less spherical, but as it began to collapse it started to rotate, forming a flattened disk, as in this artist’s concept: 

(A protoplanetary disk forming planets. Credit: Luís Calçada)