DAY INTO NIGHT

By Jim Kaler

First published in Ninth Letter, Fall-Winter 2005

(Return to Skylights or STARS.)

Nightfall. When day turns softly to evening. But night does not fall. It rises.

Wait for a brilliant clear day and find a place of contemplation, one with a clean horizon that allows you to see the changing colors of sunset and twilight. Like everything else in sunlight, the Earth casts a shadow into space. When our Moon passes through the shadow at full phase (opposite the Sun), we see a lunar eclipse. But the shadow starts at the surface of the Earth itself and must first cut through our own atmosphere. So as the Sun dips below the western horizon, a gray band rises in the air to the east, the earthshadow. The Sun is far and the air near, which produces a lever action that causes the shadow to climb faster and faster until it sweeps across the darkening sky, allowing the stars to shine. Night thus rises. It is in morning twilight, just before sunrise, when the gray band descends to the western horizon, that night falls, the Earth's shadow finally disappearing as the Sun pops up in the east and a new day begins. Welcome to the sky -- half your world. Look upward and you won't want to look back.

If the stars come out at night, where are they during the day? Distant suns, they can hardly stop shining only to appear when they think night will blanket the heavens. They are just not bright enough to be seen against the cover of the blue sky. Have you ever really looked at it?

Of blues, that of the sky is the most remarkable. So few notice it: we ignore the familiar. Always there, and not just blue, but blues that take on an infinite variety of shades, changing with elevation and the time of day. High up, away from the Sun, a dark blue, from the mountain top almost blue-violet, near the horizon the light blue of pretty eyes. Of all the blues in the celestial dome, which is my favorite? I tell myself yes, about 20 degrees up perhaps in a perfectly clear sky, and then I look overhead and fall in love with another shade. You can lie on a hillside and fill your eyes with nothing but blue, you can lose yourself in it. Near sundown, the blue near the horizon becomes blue-green, an ocean of color matching a sea-washed shore, but deeper, cleaner. Blue, a symbol of purity, of the spirit, of heaven itself. Enhanced by the whiteness of clouds, blue through the broken cover becomes intensified. Let me fly through the blue, let me live within its confines, and I do as I walk through life looking overhead, the blue of the sky symbolizing all that is true and beautiful in the world.

The origin is the Sun. Sunlight -- so showed Mr. Newton -- can be broken into a spectrum made of the colors of the rainbow. All six of them, from long to short wavelength: red, orange, yellow, green, blue, violet. What happened to indigo? It's there along with hundreds of other spectral shades. There are seven ancient moving bodies of the sky: the Sun, the Moon, and the five classical planets, all ancient gods. There must be seven colors, just as there are seven days of the week, so we stick one in whether we need it or not.

A breath of air is made mostly of molecules of nitrogen and oxygen. Light waves from the Sun hit random clumps of those molecules and scatter -- reflect in a way -- from them. Because short blue and violet waves are closer in size to the tiny particles, they scatter more than a dozen times better than longer red waves. After bouncing around a few times, blue and violet waves of sunlight can come at you from anywhere in the sky, while the longer red rays get through more or less unhindered. Violet is a lesser part of sunlight than blue; moreover, the eye is not very sensitive to violet. The sky therefore turns blue.

Four matters complete the explanation. The Sun is not set into the near sky where the blue is formed, but is nearly 100 million miles away. Scattering works best in the forward direction with no angular deflection, and worst at right angles from the scattering particles. Water vapor and dust also preferentially scatter and absorb bluer sunlight. And the air is a thin sheet that rides parallel to the ground, so the thickness through which you look depends on angle above the horizon; toward the horizon, you look into space through thirty-eight times more air than when you look overhead. The result is that the sky's color depends on direction. It's bluest at 90 degrees facing away from the Sun, where only the very shortest waves can be scattered, and gets lighter toward the horizon because of absorption and scattering through the greater thickness of air.

Since blue and violet are partially removed from sunlight, the Sun itself must be reddened. When the Sun is high in the sky, it has a slightly yellowish color. Although the Sun is generally far too bright (and dangerous) to look at, you can sense the color indirectly by looking at shafts of sunlight reflecting from dust particles floating in a darkened room. As it sets and the atmospheric path through which its light shines lengthens, the Sun takes on a deep golden hue that can be so dramatically enhanced by water vapor and dust that it can turn orange, even deep red, all blue light taken out, so much light absorbed that one may look at it directly (so long as it is comfortable).

As the Sun drops below the horizon of the turning Earth, the sky does not become instantly dark. We instead enter a period of twilight in which sunlight is still being scattered from the atmosphere above our heads. Gradually, as the Sun drops farther below the horizon and the sky darkens, the brighter stars and planets come out to greet us-"Starlight, star bright..." until the Earth's shadow finally sweeps over our heads. Perfect darkness is achieved when the Sun sinks to 18 degrees below the horizon, at which time no light reaches even the upper air, and twilight is over. The stars now have no competition and shine down upon us in full glory. In the morning, at dawn, all effects appear in reverse.

Clouds add striking effects. As the Sun sinks below the horizon, they reflect its reddened light to produce multicolored sunsets that bring us outdoors to take pictures, no two of them alike. While pure air is clear and invisible, in reality it glows with dust and water vapor that is illuminated by the Sun. Shining through holes in a cloud deck, a lowering Sun appears to send shafts of golden light to Earth. While the rays are really parallel to each other, perspective makes them seem to converge toward the Sun, giving rise to the countryside myth that the Sun is "drawing water." When the Sun hides behind a cloud, the shadows and resulting sunrays can create brilliant fingers of light and a stunning sunburst effect. Place both the Sun and clouds below the horizon, either in evening twilight or at dawn, and the shining fingers radiate mysteriously upward from the distant horizon as from the hand of God quietly signaling us to put away our work or to announce the coming of the new day.

An afternoon thunderstorm rolls across the prairie. As it passes from west to east and the clouds clear the Sun, sunlight shines upon still-falling raindrops and a glorious rainbow arcs across the sky, not just one of them, but two, one bigger and fainter than the other, both centered exactly on the point opposite the Sun. The effect is caused by a combination of refraction and reflection. Light sets the speed limit of the Universe, 299,792 kilometers or 186,282 miles per second. Nothing can go faster. Moreover, in a vacuum, it is constant no matter what the motion of the source.

Pass light through a substance, however, and it slows down. If it enters glass or water at an angle, the light will change direction, or refract, toward the perpendicular to the surface, the degree depending on the nature of the substance. The speed in matter also depends on the wavelength or color, long waves (red light) refracting less than short (violet). The result is a dispersion of light, different colors going off at different angles. Newton used a glass prism to disperse sunlight into its colors. Nature uses raindrops.

Imagine a drop falling toward the eastern horizon. As the afternoon sunlight enters the droplet, it refracts and disperses, bounces off the drop's backside, and then refracts and disperses even more upon going out of the drop into the air and then into the observer's eye. The combination of countless drops give a rainbow with a radius of 42 degrees around the anti-solar point with red on the outside because it refracts less and blue and violet on the inside, all of which agrees nicely with the results expected from the refractive properties of water. A double reflection in the raindrop gives us an outer bow with a radius of 54 degrees with the colors reversed.

Morning sunlight falling on drops from a westerly approaching storm works just as well. But the Sun must be low in the sky, below 42 degrees elevation, to bring the anti-solar point upward enough so that the primary inner bow clears the horizon. Unless you wash your car with a spray hose, or visit a waterfall, you will never see a rainbow with the Sun high, near noon. The intensities of colors depend on those inherent in the sunlight itself. If the Sun is near setting or rising and highly reddened, the short-wave colors simply are not there to be dispersed, leaving us with a rare red rainbow. More rarely still, the Sun below the horizon can still hit high raindrops to make a twilight red bow. Rainbows do not even need a storm. Enough high mist over the ocean creates the bows common to Hawaii.

High cirrus clouds are made of ice, which refracts and disperses every bit as well as droplets of liquid water. A snowflake on a mitten is six-sided. Ice crystals in clouds similarly can take on rough shapes of hexagonal prisms with sides that meet at 60-degree angles. Sunlight passing through the 60-degree sides is refracted by 22 degrees and, as in the rainbow, dispersed into colors. The lovely result is a halo around the Sun with a 22-degree radius that is colored red on the inside and, since it refracts more, blue on the outside. Light as they are, the ice crystals can still fall through the air. If shaped like six-sided dinner plates, they will align themselves parallel to the ground. The halo then brightens dramatically at points on a line through the Sun that are parallel to the horizon, resulting in brilliant colored "mock suns" or "sundogs" that are common during colder months and often appear without the visible halos.

The more you are willing to look upward, the more you will see, including a variety of arcs that lie tangent to the 22-degree halo. Refraction through the 90-degree sides of the crystals can yield a rare 46-degree halo. Crystal alignments can then create sundog analogues in the form of an arc around the point overhead and a gently curving arc that lies far below the Sun parallel to the horizon, one that is so brightly colored that it is commonly confused with the rainbow.

Ice crystals also reflect sunlight from their flat surfaces. One outcome is a white ring through the Sun that extends through the sundogs and that can very rarely be seen to go all the way around the sky. Much more common, and often spectacular, are sun pillars. Go to a romantic movie scene: a couple on a ship look off into the sunset, the sunlight reflecting from a rippled oceanic surface that stretches out in a long line all the way to the horizon. Tip it upside down. For the ocean, substitute a flat layer of high, ice-crystal clouds. The rising or setting Sun now lies below the clouds and reflects off the bottom surface, forming a long line of sunlight that stretches toward the observer. Because of the great distance, we lose any sense of depth, and the line appears instead to soar upward as a seeming pillar of light whose color depends on the apparent color of the Sun and can range from deep red to white. Pillars, halos, and the like can also occur in blowing light snow.

The air itself is a refracting medium. When light from a star enters the atmosphere, it bends slightly downward. The night's stars thus always appear just a bit higher in the sky than they would without the air. The shallower the angle of entry (that is, the closer a star is to the horizon), the greater the effect. It is small at high elevations, toward the point overhead (the zenith), but not at all subtle near the horizon where the star is raised upward by half a degree, the angular diameter of the Sun and Moon. The effect is very noticeable in sunrises and sunsets. When you see the lower edge of the Sun sitting on the horizon, it is actually fully below it.

The rotation of the Earth causes the Sun to appear to move across the sky by its own angular diameter in two minutes. At the equator, where the Sun rises vertically, it comes up two minutes early and is delayed at sunset by the same interval, thereby extending the daylight hours. Advance and delay are longer at higher latitudes, where the Sun rises and sets at a sharp angle relative to the vertical. We tell our students that days and nights are of equal lengths at equinox passages, the first day of spring and fall when the Sun crosses the equator. But in reality, refraction and the fact that sunrise and sunset are counted at first and last glimpse of the solar disk cause the equinoctial day to be several minutes longer than the canonical twelve hours.

Because the degree of refraction increases toward the horizon, the lower edge of the Sun is refracted upward more than the upper edge. The rising or setting Sun is therefore squashed into an oval shape that is very evident to the naked eye (as long as the Sun is sufficiently dimmed to be comfortable to view). Add atmospheric layering -- sudden variations in temperature and density with altitude above the ground -- and you get stripes across the Sun and even pieces of the Sun that seem to hover above the main disk.

Because refraction means dispersion, the air also acts like a natural prism. Think of the Sun as consisting of completely overlapping colored disks, red through violet. Near sunset, the blue and violet disks must be refracted upward more than the red disk. However, violet and blue are fiercely absorbed by the air. The shortest wavelengths of light that come through are green. The narrow top edge of the setting Sun thus turns the color of new grass, which is not directly visible because of the Sun's brightness. Don't try to see it. But then the Sun actually sets, and the last thing to go down is this green edge. Its light, picked up by the air, produces the "green flash" which is particularly evident in Pacific Ocean sunsets. For the same reason, stars near the horizon observed telescopically will all be little vertical spectra, which can drive astronomers to distraction because they need to work with integrated starlight.

Now let the Sun again set, the Earth's shadow drawing above us to create the night, revealing the stars, the planets, and perhaps the Moon. The Moon circles the Earth about once a month, so it could be anywhere. Like Earth, the side of the Moon facing the Sun is in daylight, the side facing away in night. The Moon's phase depends on the angle it happens to make with the Sun, which controls how much of the daytime lunar hemisphere we see. If between us and the Sun, the Moon is new and we cannot see it. A week later, at first quarter after a 90-degree turn, we see half the daylight side and half the nighttime. Another week passes and the Moon is opposite the Sun (when it rises at sunset), allowing us to see the full sunlit face. Then after another week and a 270-degree circuit, it passes third quarter where again we see just half of lunar daytime. Between the new and the quarters the Moon takes on a crescent shape, showing us more night than day, while between the quarters and full it is gibbous, showing us more day than night. Round it goes, in nearly all cultures defining the month.

Under full moonlight you can see well enough outdoors to walk, almost to read. Transport yourself to the Moon. From there, the Earth goes through phases just like the Moon does from here, only in reverse. The Earth is four times the size of the Moon, and with its water and clouds highly reflective, a "full Earth" seen from the Moon eighty times brighter than the full Moon seen from Earth. Near the lunar crescent phase, where a "lunarian" would see a gibbous Earth, the nighttime side of the Moon becomes so brightly flooded with Earthlight that we can see it from here, the crescent outlining a ghostly lunar night that fills in the whole of the lunar disk: the "old Moon in the new Moon's arms."

Advance now to full Moon, seen rising huge and sometimes red, looming above the horizon. Among the most common questions any astronomer gets (other than "Aren't you that astrologer?") is "Why is the Moon so big when it rises?" It isn't. It's an illusion. Really. The Moon is half a degree across, whether on the horizon or high in the sky. Though the origin of the "Moon illusion" is subject to fierce argument, the most common explanation is that when rising, the eye-mind combination compares it to objects on the horizon, which makes it look bigger than it really is. The effect is seen in rising constellations as well. Orion the Hunter seems to become smaller as he treads his upward path. The Moon's redness has the same explanation as the reddened Sun: absorption of short-wave light by the atmosphere.

Because moonlight is just reflected sunlight, only fainter by about a million times for the full Moon, all the phenomena caused by the Sun have lunar analogues. Under full moonlight in a dark location the sky is not black but a subtle and easily photographed dark blue. Rare moonbows fall gloriously against a darkened sky, as do 22-degree halos and moondogs. Indeed, the latter are reported more frequently than their solar counterparts because people are much more inclined to look at the Moon than at the dangerous Sun. Even Moon pillars make their ghostly appearance.

You wear glasses and come into a warm room from the cold outdoors. Instantly the lenses fog over, and all the lights in the room have colored rings around them. For the glasses, substitute high clouds, and for the room lights, moonlight. And there again is the colored ring whose bluish inner edge wraps tightly around the Moon. Much smaller than the 22-degree halo, the ring's colors are also reversed. This ring is caused not by rerefraction but by diffraction. Put your thumb and index finger as close together as you can without touching to make a little slit. Put your eye to the slit and look at a light and you will see a pattern of lines -- fringes -- caused by light waves bending (diffracting) through the slit and falling on top of each other in one direction, canceling each other in another. A more careful experiment shows the fringes to be colored. Multiple slits create spectacular spectra, which in common experience are best seen reflecting from the grooves on the surface of a compact disk. Cloud particles create a natural diffracting medium -- and a spectral ring -- around the Moon, sometimes multiple rings. They are more common around the Sun, but the Sun's brilliance renders them nearly unobservable.

Many of these sights are dramatically enhanced by altitude. Fly and note the other passengers staring blankly at the seat backs, their magazines, or computers. Instead, you take a window seat on the sunny side of the aircraft and look down. From 30,000 feet, sunlight passing through higher icy cirrus clouds brings about spectacularly bright halos and sundogs. As your MD 80 passes over a layer of ice-cirrus below, you may see the Sun reflected from it, an oval "subsun" that is sometimes so bright you cannot bear to look. It's another celestial analogue to the romantic "setting Sun over the ocean" motif and is really an upside-down Sun pillar. It may even have an associated "sub-sundog." If the clouds are thin enough, they become invisible, and the subsun seems to float above the land.

The real winner, though, is the "glory." If you can sit now on the side of the aircraft facing away from the Sun, as you pass upward through a flat layer of clouds, you may see the airplane's shadow surrounded by a full circle of colored light, even several concentric rings. The phenomenon is best known in stories originating from the high peak in the Bavarian Alps called the Brocken. There, and from other high peaks, hikers can stand above a cloud deck and see their shadows thrown upon it, their saintly heads surrounded by colored rings, the eerie effect called the Specter of the Brocken. The complex origin of the glory, which has been related to reflection, refraction, diffraction, and interference of light waves by small water droplets in the clouds, is still not well understood. If you are far enough above the clouds, the shadow of the airplane is too small and diffuse to see, and only the rings remain to follow along with you on your journey. Sunset colors become brilliantly enhanced as well. When you fly into the night, watch for the Earth's shadow, which will appear as a dramatically dark wedge opposite the Sun. If the Moon is out, reflection of moonlight from clouds stretching out in a long line toward the Moon is enthralling.

Return to nighttime Earth. The Moon is now gone and the stars shine unhampered by any brightened sky. Look carefully at the brighter ones. Though many are white, others are subtly colored across the spectrum from orange-red to yellow-orange, even to sparkling bluish-white. The colors reflect their temperatures. Blue ones are hot, over 20,000 degrees Celsius, red ones cool, down to 3000 degrees; the yellow-white Sun is in the middle at just under 6000 degrees. Blue light carries almost twice as much energy as red, and it takes a hotter star to produce it.

Though seeming countless, over the whole sky only about eight thousand stars are visible to the naked eye. On average, just a third of them can be seen on any given night, their number cut below half the total because of absorption by the Earth's atmosphere close to the horizon, which is so strong that you cannot see stars actually rise and set. Their count, however, is highly variable and depends on the time of day and year. All the stars you see at night are part of our Galaxy, which is a disk-shaped assembly some 100,000 light years across, a light year being the distance light travels in a year, about 6 trillion miles. It contains over 200 billion stars and also holds the Sun. The combined light of the billions of stars of the disk make the famed Milky Way, an awesomely beautiful wide band of light that encircles the sky. When the Milky Way is nicely up in northern summer and fall evenings, we see huge numbers of associated stars, whereas in spring evenings, when we are looking perpendicular to the Galactic disk, the sky takes on a much quieter aspect, except for the ever-present twinkling.

One of the first nursery rhymes we learn is about twinkling stars. And we are back to ever-present refraction. Air is a turbulent medium. Within it float cells of higher or lower than average density and temperature that are blown about by winds. The cells act like non-focussing lenses that can temporarily brighten the light of a star, or defocussing ones that slightly dim it, all done quickly and at random. As a result, a bright star will seem to flicker at you. Related dispersion makes the brightest stars seem to flash in different colors. Through the telescope, the star also slightly but quickly shifts position, its image madly boiling like water on a hot stove. The effect, which severely blurs telescopic views, depends on the state of the local atmosphere and the elevation above the horizon, with low-lying stars twinkling much more than those near the zenith. Distant street lights twinkle for the same reason. Above the atmosphere, from space, twinkling entirely disappears, one reason for the existence of the Hubble Space Telescope, which can obtain exquisite images of stars and other celestial bodies that simply stare down at us.

The naked eye stars -- which mostly lie at distances in the tens to hundreds of light years and are local so far as the Galaxy is concerned -- are distributed more or less at random. Random, however, does not mean uniform. Like sugar tossed on a table or tea leaves at the bottom of a cup, they make patterns, some recognizable, some not. Nearly all cultures have looked to the heavens and have named the patterns, the constellations, often in very different ways. The so-called Western constellations go back thousands of years to the ancient Middle East, were adopted by the classical Greeks, and then were passed down to us, forty-eight or fifty of them depending on how you count. Another thirty-eight "modern" ones, constellations made of dim stars and of those in the deep southern hemisphere that could not be seen from classical lands, were added in our own times. Among the ancients is a set of twelve against which the Sun and planets seem to move, and since these moving bodies represented the gods, these constellations must be their homes. Perhaps the planetary configurations and positions tell us of the gods' dispositions, hence the rise of astrology. Of the twelve, all but one are animalistic, and the entire set is known as the Zodiac, the Greek "circle of animals."

The majority of constellations, though, simply tell tales. After a hard day herding the goats, the tribe settles down to drink and to be entertained by the storyteller, who weaves myths among the stars about why the gods placed Orion the Hunter in the sky, or who says, "Look, there is the crown of the princess, let me tell you why the gods took pity on her..."

While we no longer believe the myths, the constellations still retain their magic. Stand outside under a clear cool sky and find them, reflect back on their meanings to the ancient ones, the past allowed again to become alive. Learn the star names, say them aloud, hear from them a story of cultures moving from Greece to Rome to Arabia and back again as the tales are told and retold and cultural influences added to them. Admire if nothing else the sheer beauty of the magnificent Universe whose meaning is still unclear and may always be. Use them to count the days and months of the year: there are Leo and Virgo, it must be spring; look, Lyra the Harp, singing a song of summer; there is Fomalhaut in the Southern Fish, the trees will be turning soon; gaze at Orion the Hunter and his entourage of brilliant constellations, their light reflecting from winter snows. Enjoy, live among them, they are yours. Bring the children out to look, tell them the stories, introduce them to the natural world, and perhaps one will fall in love with it all and know that she will someday become an astronomer and learn what the stars really are and will solve some of the mysteries that they yet present.

Now add the planets. Five are bright and readily visible to the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn. Each moves around the Sun with a very different period, from eighty-eight days for Mercury to nearly thirty years for Saturn. As a result they continuously change positions, both among the stars and relative to each other. Each is also unique and immediately recognizable. But you do not have to know them to tell that this bright "star" is a planet. Unlike stars, they don't twinkle, or at least not as much. Stars are so far away that for all practical purposes they appear as points. Planets, however, though pointlike to the eye, are really small disks that you can easily see through the telescope. Twinkling takes place over a very small angle. While one point of a planet's surface will twinkle, changing apparent brightness and position in one direction, an adjoining point will twinkle oppositely. As a result the twinkling tends to average out, resulting in a quiet, steady light.

Best though that you learn the planets as individuals. So bright that it can be seen in a clear blue daylight sky, so bright it can cast shadows at night, Venus tops the list. Between us and the Sun, this planet is normally visible either in the evening after sundown or before morning dawn, depending on which side of the Sun it is on, turning it into the classic evening or morning "star." Cloud-covered and highly reflective, a shining creamy white, that it was named for the goddess of love and beauty is no surprise.

Closer to the Sun, Mercury, the smallest of the bright planets, is swift and elusive, seen only in twilight and hard to catch -- the fast messenger of the gods. Mars, reddish from its iron-oxide-loaded dusty surface, was readily related to the warrior god. Jupiter, the King, Zeus himself, second in brightness only to Venus, treads in stately fashion against the twelve zodiacal figures in twelve years, visiting one per year. Fainter (but still brighter than all but the brightest of stars), farther out and slower, is ringed Saturn, naturally named after Jupiter's father -- in Greek, the Titan Kronos. One can spend a lifetime watching them move, visiting one another and the stars, pairing their contrasting colors and brightnesses, then moving apart in an aesthetic dance. One can count one's life in Jupiter years, your author a mere child of age five and a half. Little is more charming than to see the crescent Moon move past Venus, the configuration appearing on several flags (although Venus can NOT be within the horns of the crescent).

Then the night is over, a western sunset replaced by a peaceful sunrise, the stars glimmering out as twilight grows brighter, then disappearing altogether as the Sun swells over the horizon, nested in a blue sky, to start another day for us to contemplate and admire. And we start the cycle all over again, day to night to day, a cycle that has been repeated for a million millennia and will continue to be repeated into the unimaginably distant future.