Winter solstice brings longest night, warm comforts

A wintry scene along the Superior Hiking Trail in northern Minnesota photographed earlier this week. Credit: Bob King

You can kiss the fall goodbye starting at 5:03 p.m. (CST) tonight. That’s when the Sun arrives at its southernmost point in the sky in the constellation Sagittarius. For those of us in the northern hemisphere it rises late, never climbs very high in the south and sets early, making today the shortest day and tonight the longest night of the year.

Earth’s tipped axis is responsible for the seasons. On one side of Earth’s orbit, the northern hemisphere is tilted away from the Sun and we experience winter; on the other side it’s tilted toward the Sun and we experience summer. Fall and spring are in-between times when the entire planet is broadside to the Sun and all places on the globe receive equal amounts of sunshine. From places like Australia, which is bisected by the Tropic of Capricorn, the Sun is high in the sky. For them and all other southern hemisphere locations, today marks the first day of summer. Credit: Wikipedia with additions by the author

Seen from the Tropic of Capricorn, that imaginary circle touching every location with a latitude of 23.5 degrees south, the Sun will be directly overhead at noon today. On the summer solstice, anyone living on the Tropic of Cancer at 23.5 degrees north latitude sees the Sun overhead at noon. The number 23.5 is special because it’s how many degrees the Earth’s axis is tilted from the vertical.

Because of the 23.5 tilt of Earth’s axis, the altitude of the sun varies with the seasons. In winter it’s 23.5 degrees below the celestial equator and shines over the Tropic of Capricorn, while in summer it’s 23.5 degrees above and shines over the Tropic of Cancer.  Source: Stellarium

At the winter solstice, the northern hemisphere is tilted away from the Sun, which makes it appear low in the sky. Not only are the Sun’s precious rays more spread out (less direct), but the days are short. Cold soon follows. In summer we experience exactly the opposite – the top of the globe is canted in the Sun’s direction. With our star high overhead, days are long and temperatures steamy.

Map of the continental U.S. showing the time of the average coldest day of the year. Click to enlarge. Credit: NOAA

While the start of winter can be cold, it’s rarely the coldest time of the season. Most places see their coldest days in mid to late January even as the days slowly grow longer and the Sun climbs higher. This seasonal delay occurs because the land is still losing more heat than what the feeble Sun can resupply, while the oceans, which effect climate worldwide, take more time than the land to cool down and warm up.

Likewise we don’t feel the hottest day of summer until the land and oceans heat up from day after day of a high-rolling Sun. That happens in July.

Because the Sun’s at its lowest point in the sky, your noontime shadow is longest this time of year. And if you’re paying close attention, you’ll notice that the earliest sunset occurred two weeks ago – not on the shortest day. However, the Sun will continue to rise later up to about January 4th.

The earliest sunsets happened two weeks ago. The sun sets about 3 minutes later today than it did in early December. Credit: Bob King

The reasons for the discrepancy have to do with both the tilt of the Earth’s axis and the planet’s varying speed due to its oblong, non-circular orbit. For a nice explanation of the phenomenon, head over to Prof. Kirk Korista’s Sunrise, Sunset and the Solstice page or HERE.

Christmas and other important holidays and celebrations happen at this darkest time of year to keep our spirits up and fan our hopes for the return of the light. They’re a time to revisit our deepest beliefs, spend time with family or just fall asleep in a soft chair next to a roaring fire.

A fire in the woodstove on the winter solstice. What could be better? Credit: Bob King


Quasars Mysteriously Align Across Billions of Light Years

Artist’s rendering of ULAS J1120+0641, a very distant quasar powered by a black hole with a mass two billion times that of the Sun. Some of the matter falling into the hole gets beamed back into space along the axis of the spinning disk’s rotation. Credit: ESO/M. Kornmesser

Quasar. One of the coolest words ever. It’s really shorthand for “quasi stellar radio source”. When radio telescopes were developed in the latter half of the 20th century, astronomers eagerly used them to find new objects in the sky, many invisible in visible light. One of the things they found were these star-like objects glowing brightly in radio waves but appearing only as undistinguished stars in optical telescopes.

Further study revealed they were extremely distant – billions of light years – as far as the most remote galaxies. For such a tiny object to shine across such vast distances, it must be powered by something extraordinary.

We now know that quasars are galaxies with very active supermassive black holes at their centers. Matter falling a black hole gets spun into a whilring disk, releasing tremendous amounts of energy before it finally goes “down the drain” for good. Some the incandescent matter beams back into space in long jets perpendicular to the disk.

This artist’s impression shows the mysterious alignments between the spin axes of quasars and the large-scale structures that they inhabit that observations with ESO’s Very Large Telescope have revealed. These alignments are over billions of light-years and are the largest known in the universe. The large-scale structure is shown in blue and quasars are marked in white with the rotation axes of their black holes indicated with a line. This picture is for illustration only and does not depict the real distribution of galaxies and quasars. Credit: ESO/M. Kornmesser

Today, according to a news announcement by the European Southern Observatory (ESO), a European research team has found that the rotation axes of the central supermassive black holes in a sample of 93 quasars are parallel to each other over distances of billions of light-years. The team has also found that the rotation axes of these quasars tend to be aligned with the vast structures in the cosmic web in which they reside. We see the 93 quasars at a time when the universe was just a third of its current age.

Admittedly, these sounds like pretty obscure stuff, but why should these objects that are not only far from us but billions of light years from each other, be connected? It’s such an intriguing mystery, but before we look at why, let’s stop to examine the large scale structure of the universe.

This simulation, created by the Millennium Simulation Project, represents a 2 billion- light-year-wide chunk of the universe and more than 20 million galaxies. The purple strands represent dark matter around which normal matter (the bright yellow clumps of galaxies) has clustered into filaments of billions of galaxies surrounded by empty voids of space. Credit: Millenium Simulation Project

When we sit back and take in the really, really big picture, the billions of galaxies out there are arranged in dense filaments and strands resembling a pile of spaghetti or neurons in the human brain. The strands in turn are clustered about the still-mysterious dark matter, of which there’s far more of than the bright stuff like stars and galaxies. To refresh your memory, the universe is composed of 73% dark energy, 23% dark matter and only 4% bright matter.

Large pockets of relatively galaxy-free space called cosmic voids lie betwixt and between the strands. The entire texture strikingly shown in the simulation (and visible on smaller scales in maps and photos) is linked to dark matter, which though invisible, is both plentiful and makes its presence known through gravity. Dark matter forms the backbone for all the beautiful galaxies we see in our telescopes and in photos taken by the Hubble. It’s the coral reef and the galaxies are the fish, crabs and all the rest.

Artist concept of a supermassive black hole powering a quasar. Black holes that form from the collapse of a star during a supernova explosion are only a few miles across. Supermassive ones, built up over billions of years from matter straying too close the hole’s event horizon, are the size of the solar system. Our Milky Way harbors such a supermassive black hole, but it’s currently not active like those seen in quasars. Credit: Wiki

The new VLT results indicate that the rotation axes of the quasars tend to be parallel to the large-scale structures in which they find themselves. So, if the quasars are located in one of the spaghetti noodle, then the spins of the central black holes will point along the axis of that noodle. The researchers estimate that the probability that these alignments as simply the result of chance is less than 1%.

Take a journey through the large-scale structure of the universe in this video version of the photo above.

By the way, the research team, led by Damien Hutsemékers from the University of Liège in Belgium, could not see the rotation axes directly but inferred them by measuring the polarization (light waves vibrating in a preferred direction) of the quasars’ light.

So the “why” of these spooky alignments is this: we don’t know … yet:

The alignments in the new data, on scales even bigger than current predictions from simulations, may be a hint that there is a missing ingredient in our current models of the cosmos,” concludes Dominique Sluse.

Are they following the dictates of the unseen dark matter? Hmmm … questions as always. This is the beauty of going where no one has ventured before.

Summer solstice 2014 – welcome to iced tea, mosquitos and BBQ

The first day of summer begins tomorrow June 21 at 5:51 a.m. CDT. It’s also the longest day of the year. Credit: Lyle Anderson

It’s all about the sun. Always has been, always will be. Our lives depend upon the unstoppable nuclear fire that burns in its heart. At no time of year do we feel closer to that fire than at the summer solstice, when the sun reaches its highest point in the sky in the northern hemisphere.

The difference in altitude between the sun at the summer solstice vs. winter is dramatic. Extra height means both longer days and more intense sunlight – the key reasons summer’s so much hotter than winter. Stellarium

At 5:51 a.m. tomorrow morning June 21 – just after sunrise for my little town – summer begins. If you’ve been paying attention, you’ve noticed the sun creeping higher and higher since the time you last shoveled snow. Well, the buck stops at summer. That’s when the sun stands 23.5 degrees above the imaginary circle in the sky called the celestial equator, an extension of Earth’s equator onto the sky.

If we could remove the atmosphere tomorrow, we’d see the solstice sun high in the constellation of Taurus. The sun’s path across the sky and the celestial equator are shown. Stellarium

For anyone living along the equator, its celestial counterpart starts at the eastern horizon, passes directly overhead and then arcs down to the western horizon. In mid-northern latitudes, the celestial equator crosses the southern sky about halfway between the horizon and zenith. Add in 23.5 degrees or about two fists held at arm’s length against the sky, and that’s where the solstice sun stands around 1 p.m. daylight saving time.

23.5 is a familiar number. You’ll recall that’s the tilt of Earth’s axis. No coincidence there. The sun’s yearly swings from its summer peak at 23.5 degrees above the equator to 23.5 degrees below the equator at the winter solstice is merely a reflection of that tilt. In reality, the sun’s not moving at all – it’s the Earth’s doing.

In northern hemisphere summer (left), the north polar axis tilts in the sun’s direction, causing the sun to appear high in the sky and the days to be long. When it points away, it’s winter and the sun rides low in the sky. At the fall and spring equinoxes, the planet is tilted neither toward nor away and day and night are equal. Credit: Tau Olunga

On the summer solstice, Earth’s north polar axis tilts toward the sun, ‘lifting’ it 23.5 degrees above and beyond the equator. Not only is the sun high in the sky, it’s up for many more hours than during the winter. Days reach their maximum length and the sun’s high angle means the energy per unit area it pours over Earth’s surface is more than twice as intense as during the winter. Add it all up and you’ll start feeling … sweaty.

See the seasons unfold before your eyes. This is an animation using monthly global images from the NASA Earth Observatory taken from January to December 2004.

Enjoy the best the sun can bring to the game these next three months. Happy solstice!

Happy equinox! Time to tip your hat to Earth’s tipped axis

One of the earliest of spring flowers, a crocus blooms on a sunny spring afternoon. Credit: Bob King

Such a beautiful flower. Look as hard as you like and you won’t find a single one in my town where more than two feet of snow still blankets the good brown earth. I’m not worried. Two weeks from now, the spring sun will reduce it all to puddles.

Today’s the vernal equinox, the start of spring in the northern hemisphere. It began at 11:57 a.m. CDT, the instant the center of the sun’s blazing disk crossed the imaginary circle in the sky called the celestial equator. If you live on the real equator, the celestial version passes directly overhead. That means no shadows at noon today for residents of places like Quito, Ecuador and Kampala, Uganda.

North Pole webcam 2013

Travel north of the equator and the celestial equator drops lower and lower in the southern sky. At the north pole, it sits exactly on the horizon 360 degrees all around. If you could stand there today, you’d be seeing your first sunrise since the autumnal equinox last September. It would also be the start of six months of uninterrupted daylight. By the way, the weather’s fantastic there today – sunny with a high of 24 degrees!

Because of the 23.5 tilt of Earth’s axis, the altitude of the sun varies cyclically across a year. In winter it’s 23.5 degrees below the celestial equator, while in summer it’s 23.5 degrees above. At the equinoxes, it straddles the equator. Created with Stellarium

Most of live between the pole and equator, where the sun stands roughly halfway up in the southern sky at local noon. That’s a far cry from winter, when the sun stood 23.5 degrees (a little more than two fists held at arm’s length) below the equator. Its rays were less direct and intense, and the time it spent above the horizon relatively brief, the two key factors that make a winter.

In summer, we experience just the opposite. The sun stands 23.5 degrees above the celestial equator; its rays are more direct and it spends many more hours above the horizon. Long days and short nights are a delight for many  … including the bugs.

The sun’s cyclic journey above and below the celestial equator all goes back to Earth’s tipped axis. As Earth travels around the sun in a year, the north polar axis tilts toward the sun in summer, taking it 23.5 degrees above the equator, and away from the sun in winter for a ride 23.5 degrees below the equator.

The tip of Earth on its axis causes the seasons. On the first day of spring or vernal equinox, the axis is perpendicular to the sun and days and nights are equally long in both northern and southern hemispheres. Notice the axis doesn’t “flip-flop” but remains pointed in the same direction. It’s the Earth’s orbital travel that causes it to point toward and away from the sun. Credit: Tao-olunga with my own additions

On the first days of spring and fall, the axis is oriented neither toward nor away from the sun. Day and night across the planet are paired up at 12 hours apiece. After today, daylight slowly gains the upper hand by 2-4 minutes a day. Doesn’t sound like much, but like snow, it quickly adds up. By June the mid-latitudes will have gained some four additional hours of solar photons.

What spring looks like where in Duluth this season – a high sun but plenty of snow to go around. Photo taken March 16, 2014. Credit: Bob King

You’ve probably heard that you can balance an egg on its end on the first days of spring and fall. Like water going down the bathtub drain in different directions depending on your hemispher this is an urban myth. It’s hard to balance an egg ANY time of year. Just try it.

I think we all relate to the new season for the same reasons generations of humans before us have. Rebirth, renewal and the return of warmth and light capture the essence of spring. We tip our hats to the random impact at the dawn of the solar system that set Earth’s axis askew.

Daylight forces the hand of night as we surge toward spring

Animation showing the Earth – with tipped axis – revolving around the sun. Seasons are shown for the northern hemisphere.

It happens every mid-winter. I wake up earlier and earlier, unconsciously responding to the daylight that spills beneath the window shade as the pace of the season quickens.

We’ve been putting seconds and minutes in our sunny-day piggy bank every since the winter solstice last Dec. 21. Those deposits are now accumulating rapidly as February gives way to March. Where I live, days were as short as 8 hours 32 minutes in late December. Today that time has swelled to 10 hours 24 minutes.

As Earth revolves around the sun, its 23.5 degree-angled axis points toward, perpendicular to and away from the sun over the year to make the seasons. Credit: Tao’olunga with additions by B. King

While a half hour of extra light may not be enough to notice, 1 hour and 52 minutes is a revelation. Many of us now drive home in bright twilight at the end of a work day. This has beneficial effects like seeing more sunsets and full moon rises. We also feel more connected with the world because we can see it. Humans weren’t born to live as troglobites in dark caves. We crave sunlight as much as clear,dark nights.

I like the extra daylight for hiking and skiing. Shorter nights also mean less time for the Earth to loose heat and the temperature to dip below zero. If you’d like to see how your day/night account is coming along, check out the UNSO’s Duration of Daylight/Darkness Table.

All things warm and fuzzy (and cold and spiky) come our way because of Earth’s axial tilt. The axis remains fixed at an inclination of 23.5 degrees, but as the planet revolves about the sun during the year, the northern hemisphere tilts toward the sun in summer and away in winter. These are the extremes. In between, we have the spring and fall equinoxes, when both hemispheres are “face on” to the sun and receive equal amounts of daylight and night.

There are mini-seasons too. Mid-February is as good a time as any to call by that name. We’re moving away from winter toward spring with night on the run and daylight gaining the upper hand. A month from today, on the verge of the spring equinox, daylight will have increased an additional 1 1/2 hours to 12 hours. For a moment day and night will balance. The next moment day surpasses night and won’t relinquish its lead until after the fall equinox.

The sun’s always high in the sky at low tropical latitudes, so the seasons don’t vary much. This diagram shows the sun’s position around noon on the winter and summer solstices and equinoxes. Stellarium

Daylight length depends upon your latitude. If you took a tropical vacation this winter, you probably noticed that the sun rose around 6 a.m. and set around 6 p.m. Closer to the equator, the sun’s path is steeply inclined to the horizon every day of the year with little change in sunrise and sunset times. The sun’s always high in the sky there at the noon hour, bringing with it those consistently warmer temperatures we’re willing to pay big bucks for.

A mid-winter sun shines through an icicle formation on Lake Superior. Credit: Bob King

At mid and high latitudes, the yearly variation in sun’s position in the sky puts it high in the sky during summer and low in the sky during winter. Low means less time above the horizon, shorter daylight hours and cold temperatures.

To better understand this, consider that on the first day of spring and fall on the equator, the sun rises due east, passes directly overhead and sets due west. On the first day of summer, the sun at noon passes 23.5 degrees ( a little more than two fists held at arm’s length) north of the overhead point, while on the winter solstice it’s 23.5 degrees south of overhead. No matter the season, the sun will always shine down from a high altitude at noon.

This view shows the sun from a mid-northern latitude city like Minnepolis, Minn. Notice how the sun’s yearly elevation spread take it much closer to the horizon (wintertime) and also quite high (summertime). The scale of this map is different from the one above because it doesn’t need to include as much sky near the overhead point. Stellarium

In Minneapolis, halfway between the equator and north pole at latitude 45 degrees north, the sun is 45 degrees high at noon on the first day of spring and fall or halfway between the overhead point and southern horizon. Come the first day of summer, it’s way up there at 68.5 degrees and roasts the back of your neck, but on the winter solstice it peaks out at just 21.5 degrees high. Better protect that neck with a scarf.

The full range of the sun’s yearly motion – 23.5 degrees north to 23.5 degrees south of the celestial equator – is the same no matter where you are on Earth, but if you live far from the equator, the sun’s altitude reaches greater extremes, making the seasons more pronounced.

Lunar secrets? How to see the moon’s hidden seas tonight

The combination of the slow rocking back and forth of the moon called libration brings into view three lunar maria or “seas” that are normally hidden around the backside – Mare Humboldtianum, M. Marginis and M. Smythii. To find them, you can use the easy-to-spot Mare Crisium. Credit: Virtual Lunar Altas

If everything revolved in perfect circles and all planet and moon orbits were concentric, the solar system wouldn’t be nearly as much fun. Consider the moon. Orbiting in a circle rather than ellipse,its distance from Earth would never vary. There’d be no “super moons” or full moons at the time the moon is closest to the Earth.

Simulated views of the Moon over one month, demonstrating librations in latitude and longitude. Credit: Tom Ruen

The moon’s orbital speed would also be constant and never get out of sync with its rotation rate. Because the moon moves slower when farthest from the Earth (and faster than average when closest), we can peer around the east and west limbs of the moon for a few days each month to see craters and lunar seas that are otherwise hidden. This apparent rocking back-and-forth of the moon, called libration, exposes an extra 7.9 degrees of lunar longitude for our viewing pleasure.

Similarly, if the moon’s orbit were exactly concentric with Earth’s and the moon’s axis straight up and down, we’d never be able to peek over and under its north and south polar regions. We’re grateful that the combination of the 5.1 degree tilt of the moon’s orbit and the 1.5 degree inclination of its axis exposes an extra 6.8 degrees of latitude. As you might guess, this tippy business is called libration of latitude.

Add in 1 degree of diurnal libration caused by our changing perspective at moonrise vs. moonset, and altogether we’re able to see 59% of the moon. Pretty cool, eh?

You can see the effects of libration tonight through next week if you have a pair of 10x binoculars or small telescope.

Here’s what the moon will look like on Feb. 14 when it will be full. Because of libration, two of the three featured lunar seas have now disappeared behind the moon’s eastern edge. Credit: NASA

Three lunar seas that normally are absent or appear as little more than skinny stripes along the extreme eastern edge of the moon are in good view this evening – Mare Humboldtianum (Sea of Humboldt), Mare Marginis (the Border Sea) and Mare Smythii, a sea named in honor of 19th century British astronomer Admiral Smyth.

Watch in the coming nights as the rock n’ rollin’ moon whisks them away.

Winter solstice offers hope in our darkest hour

Ah, winter. Water dripping from a building rooftop in downtown Duluth grew into a shape resembling a perched bird this week. Credit: Bob King

The nights are long. You never seem to warm up. It must be winter. Or it will be anyway at 11:11 a.m. (CST) tomorrow Dec. 21 when the sun bottoms out in its yearly circuit of the sky like a cigarette crushed in an ashtray.

But every winter solstice has a silver lining; after tomorrow the sun begins moving northward again, chipping away at the darkness as it rises higher with each passing day.

Winter takes getting used to which is why we still call it fall in November and much of December. By the time the solstice rolls around on the 21st, we’ve long accepted the cold, snow and driving home at 5 with the headlights on.

The fundamental facts of life all revolve about the tilt of Earth’s axis. If our planet rotated straight up and down like Mercury, we’d have no seasons. Mid-latitudes would experience eternal spring with the sun forever stuck halfway between its summer high point and winter low. Some of you might like this … for a while.

The Earth’s tilt combined with its yearly revolution of the sun tip the northern hemisphere toward the sun in summer and away in winter. Credit: Tao’lunga / Wikipedia with my annotations

Meanwhile, those living at the equator would see the sun directly overhead at noon every day of the year, while  polar explorers and researchers would watch it skirt the horizon and never rise higher. For everyone the sun would rise and set at nearly the same time every day.

The sun’s noontime elevation changes from season to season thanks to the 23.5 tilt of the Earth’s axis. Stellarium

But no. The 23.5 degree tip of the Earth’s axis combined with our planet’s revolution around the sun break the monotony and create the seasons. The tilt ensures that the northern hemisphere of the planet nods toward the sun in summer and away in winter when we’re on the other end of our orbit.

Ethel O’Leary of Duluth deals with the consequences of Earth’s tilted axis as she clears the sidewalk in front of her house Dec. 5, 2013. Credit: Bob King

As a result of that nod, the sun appears high in the sky in summer. Its longer, steeper path across the sky means longer days and more intense heat. In the winter, the northern hemisphere “leans back” from the sun. Slanted, less intense solar rays and short days follow.

On Dec. 21 the sun reaches its lowest altitude above the southern horizon at noon for the year. Here in Duluth, that’s about 20 degrees or two fists held at arm’s length. For Chicagoans, it’s 25 degrees, a bit higher. But if you live in Anchorage, the solar disk climbs to just under 6 degrees before slinking back toward the southwestern horizon.

The sun’s path during the year hits a low in winter and a high in summer. Around the winter solstice, the sun travels little in the northward direction and appears to “stand still” in the sky. The same happens at the summer solstice. Come late January,the sun’s path is more steeply inclined to the horizon and it moves northward and higher in the sky. Longer days result. Credit: Dr. John Lucey, Durham University

Solstice literally means “sun stands still” and refers to the fact that around the solstice sunrise and sunset times change very little and the sun seems stuck in the same low spot in the sky. In December the sun sits at the “bottom” of its yearly path around the sky. Most of its daily motion is to the east and very little to the north. For the sun to get higher in the sky (and days to grow longer), it needs to spend more time moving “upward” or to the north. That starts happening in late January and accelerates during the spring when the sun’s path is more steeply angled to the horizon.

We’ve spent the last three months watching the sun glide to the cold bottom of the celestial sphere. Beginning tomorrow there’s nowhere to go but up. The next time you grab that snow shovel and heave a chunk of winter over the bank, know that the sun – starting tomorrow afternoon – will be on your side.

Celebrate summer’s start ’round about midnight Friday

A radiant sun shines through a cluster of Norway pine needles. This Thursday-Friday marks the summer solstice or first day of summer.  The season begins at 1:04 Eastern time June 21 and 10:04 p.m. Pacific  June 20. Credit: Bob King

“Summer afternoon—summer afternoon; to me those have always been the two most beautiful words in the English language.”  - Henry James

So it’s always seemed to me at the start of a summer vacation. Endless time laps ahead like a wave that never breaks. Those splendid hot stillnesses are returning. Come Friday June 21 at 12:04 a.m. CDT summer tiptoes through the dark to quietly unseat spring; the next three months belong to the high sun, iced drinks and late evening light.

Whichever end of Earth’s axis points toward the sun, it’s summer in that hemisphere. In June, the north polar axis tilts that direction and we experience summer (left). When it points away, it’s winter. At the fall and spring equinoxes, the planet is tilted neither toward nor away and day and night are equal. Credit: Tau Olunga

That’s in the northern hemisphere of course. Way down south it’s the first day of winter and the sun is never lower in the sky than on June 21. Here in the north, the sun beams from its highest point in the sky. Since it spends a great deal of time climbing to this lofty perch and an equally long time descending, summer days are exceptionally long. Daylight squeezes night into a narrow slot fewer than 9 hours long.  With darkness beginning after 10 o’clock, skywatchers are forced to choose between sleep and stars.

A mosquito from early Miocene times (~ 20 million years ago) frozen in time in Dominican amber. Credit: Didier Desouens

If the choice is stars, you’ll be sharing it with tiny, whining friends of the night. Mosquitos have been around for millions of years; our most distant human ancestors slapped and batted them away just like you and I do every time we look up in wonderment without protection on a pleasant June evening. But there are fireflies too and owls and frogs about, making a clear summer night as much a sonic experience as a visual feast.

All this summer stuff happens for one reason – the tilt of Earth’s axis. Simple as that. No need to bring in the experts, no special app required. Earth circles the sun tilted 23.5 degrees from vertical. Every June 20 or 21 the northern hemisphere points toward the sun, causing it to appear high in the sky. Not only do the days reach their maximum length, the sun’s high angle means the energy per unit area it pours over Earth’s surface is more than twice as intense as during the winter.

Six months later the north tilts away from the sun. A low sun and less intense surface heating means wintry consequences.

Male fireflies flash as they fly over the ground looking for a mate on a June night. Credit: Bob King

Spring and autumn fall between winter and summer extremes with Earth broadside to the sun and neither axis tilted toward or away. Day and night briefly agree to share the clock equally before charging off to the next season.

So yes, I’m ready for summer. Bring on the sweet smells of morning air, those endless afternoons and nights of fireflies tearing across the sky like biological meteors.

Wake up to spring tomorrow and see the space station

Harry Nynas of Duluth heaved shovels fresh snow on top of the high banks that have accumulated over the season along his sidewalk yesterday. Photo: Bob King

After shoveling another 8 inches of snow after a winter of white, the banks along my walkway are now nearly at eye level. If there’s a lawn under there, I’m gonna need a team of archaeologists to find it. No matter, that won’t stop spring.

Tomorrow morning at 6:02 a.m. (Central time) the sun quietly slips over the line into the northern half of the sky. We call this the vernal equinox or start of spring. For me it will be a matter of faith in the cyclical movement of the sun. For you, the zephyrs of the new season may already be blowing through your hair.

The tip of Earth on its axis causes the seasons. On the first day of spring or vernal equinox, we face the sun from the side and days and nights are approximately of equal length in both northern and southern hemispheres. Credit: Tao-olunga

On the first day of spring, Earth’s axis is oriented neither toward nor away from the sun. If the southern hemisphere represents the planet’s feet and northern hemisphere its head, tomorrow we’ll be showing the sun our belly or profile if you like. In winter, the northern hemisphere is tipped away from the sun with short days and a low, chilly sun. In summer, we’re tipped toward the sun with long days, a high sun and more heat than most of us need. But during the vernal and autumnal equinoxes, neither hemisphere has the solar advantage (or disadvantage) and equality rules. Days are 12 hours long, nights are 12 hours long.

The rising sun tomorrow will bring with it the start of the spring season in the northern hemisphere. Credit: Rick Klawitter

The sun also also rises due east and sets due west. If you’ve ever been puzzled by which direction is which in your neighborhood, face the sunset sun around the time of the equinoxes and stick out both your arms at your sides. Your right arm points due north, the left due south. Pretty handy, eh?

On the first day of spring the sun crosses the celestial equator, an imaginary extension of Earth’s equator onto the sky, moving north. As the sun moves north, it climbs higher and higher in the sky – with increasing daylight hours – until it’s highest on the first day of summer. Illustration: Bob King

Spring and fall are the in-between times when temperatures moderate and the sun rests for a brief moment between extremes. For folks living on the equator, tomorrow the sun will rise in the east and pass directly overhead at noon before declining in the west. Equatorial skywatchers will stand in their own shadows at local noon.

Take an imaginary flight to Earth’s south pole and tomorrow means something quite different. There the sun will hover along the horizon 24 hours straight, neither rising nor setting. Starting March 21, it won’t breach the horizon for another 6 months. What marks the start of spring for northerners means the beginning of fall for Australians and a temporary end of sunshine for itinerant Antarcticans.

As you’d expect, the situation is just the opposite at the north pole, where 6 months of daylight begins with tomorrow’s sunrise.

The sun sets due west tomorrow on the first day of spring in the northern hemisphere. Photo: Bob King

Our planet’s tilted axis combined with its yearly orbit makes such strange things happen here on the ground. Just think how monotonous the weather and daylight-length would be if our axis were straight up and down with no tilt. Our skewed planet is like an artist looking at the world from varied and surprising perspectives.

Spring also coincides with a series of fine morning passes of the International Space Station (ISS) for at least the U.S. and Canada. Less than an hour before spring’s start, the station will pass over northern Minnesota tomorrow morning. To find times when it’s visible from your location, log on to Heavens Above (which also provides excellent maps of its path in the sky) or key in your zip code at Spaceweather Satellite Flybys page. The ISS first appears in the western sky and moves eastward, appearing like a very bright, moving star.

Space Station times for Duluth, Minn. region:

* Tues. March 20 starting at 5:14 a.m. “Magically” appears out of Earth’s shadow high in the southern sky and moves east. Brilliant pass!
* Weds. March 21 at 5:58 p.m. across the northern sky
* Thurs. March 22 at 5:09 a.m. Exits Earth’s shadow at 5:09 a.m. above the North Star and moves eastward
* Fri. March 23 at 5:52 a.m. across the northern sky
* Sat. March 24 at 5:03 a.m. Exits Earth’s shadow just below the North Star and moves east
* Sun. March 25 at 5:46 a.m. across the northern sky

23.5 cheers for daylight

The sun returns! We know it’s always been there, but soon we’ll really start to feel its presence. Photo: Bob King

I know it tends to cut into nighttime sky watching, but humans like daylight. Heck, even I do. Every January there comes a time when we start to notice the days getting longer. This happened to me two evenings ago when I got off work around 6 p.m. under a twilit sky. A month ago that same sky would have been twinkling with stars.

A quick check on sunset and sunrise times reveals that we’ve gained a full 40 minutes of daylight since December 21, the shortest day. The earliest sunset occurred around Dec. 8 (4:20 p.m. here in  Duluth, Minn.) and the latest sunrise (7:53 a.m.) around Jan. 3.  Those times are now 5 p.m. and 7:42 a.m. The sunrise lags behind sunset due to a combination of factors involving the angle of the sun’s path in winter and Earth’s orbital speed.

40 minutes is nothing to sniff at, but we’re just getting going. Each day, we add an additional 1 to 2 minutes of evening sunshine and 1 minute of morning light. What began as a trickle is rapidly becoming a cascade as we set our sights on March 20, the first day of spring.

The 23.5 degree tip of our planet, probably imparted by some long-ago impact in its formative years, is responsible for the ever-changing length of daylight during the year. Credit: Tau’olunga with my annotations

Those extra minutes are doled out day by day as the sun climbs ever northward on its yearly path around the sky. Its low position (along with short days) happens in winter, because that’s when the Earth’s northern hemisphere is tipped away from the sun. Summer happens when we’re tipped toward the sun.

The full range of the sun’s north to south movement in the sky is 47 degrees or nearly five fists held vertically against the sky. Why 47? If you divide it in half you get 23.5 degrees, and that’s the angle at which our planet’s axis is tipped. You can see that the difference between the high summer sun and low winter sun is simply a reflection of our planet’s tilt.

The sun’s position is shown at noon on the first days of winter, spring and summer 2012-2013. The sun’s path – known as the ecliptic – climbs up or northward starting on the first day of winter. The separation between each position is 23.5 degrees, equal to the tilt of Earth’s axis. Created with Stellarium

Astronomers divide the sky in half with an imaginary circle called the celestial equator. This is really nothing more than a projection of Earth’s actual equator into the sky above. For an observer on the equator, the celestial equator begins at the due-east horizon point, passes directly overhead and ends at the due-west point. For mid-northern latitudes, the celestial equator starts and ends at the same points but passes midway between the southern horizon and the zenith (overhead point). At the north pole the celestial equator runs right along the horizon from due east to due west.

The sun spends 23.5 degrees below the celestial equator from the first day of fall until the first day of spring and 23.5 degrees above the equator from the first day of spring until the first day of fall. At the equinoxes, the sun’s path momentarily crosses the celestial equator as it moves north or south.

If you’re looking for something to thank for providing the precious daylight you’ve been looking forward to for months, give a nod to Earth’s tilt.