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.

Happy New Baktun and a joyous solstice to all

Circumscribed halo around last night’s half moon. Photo: Bob King

Today the Mayan calendar rolls over to a new Baktun or 144,000 day cycle as it has for centuries. Coincidentally, it’s also the first day of winter in the northern hemisphere and summer for folks down under. Winter tiptoed in at 5:12 a.m. (CST) this morning while many of us were snug in our beds. Looking out my window, the world looks much the same as it did yesterday – with a difference. It’s sunny!

Come join Duluth’s celebration of the solstice at the University of Minnesota-Duluth’s planetarium.

That means a clear sky tonight and a chance to celebrate the new season. If you live in the Duluth, Minn. region, the Marshall Alworth Planetarium will feature a special “End of the World – Winter Solstice” party with shows on the half-hour in the dome, telescope viewing, pizza, cider, a raffle and a free 2013 calendar. Cost is $8 per person or $15 per family. The event starts at 6 p.m. and runs until 9. More information HERE.

Only 8 hours and 32 minutes separate sunrise and sunset in Duluth, Minn. today. The rest belongs to the night. Solstice is combination of two Latin words – sol for sun and sistere to stand still. That’s what it feels like for a week or two at the time of the summer and winter solstices, when the sun reaches its highest and lowest points in the sky.

The seasons are caused by the 23.5 degree tilt of our axis. In summer, Earth’s north polar axis is tipped toward the sun, causing it to appear higher in the sky and making for longer days. Half an orbit later in winter, the north polar axis is tilted away from the sun, making for a low sun and short days. In spring and fall, the axis is tilted neither toward nor away and day and night are equal. Credit: Tau’olunga with additions by Bob King

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 yellow orb of day climbs to just under 6 degrees before slinking back toward the west. My dear brother Mike who lives there must wait until 10:14 a.m. for the sun to rise today. With sunset at 3:42 p.m., he’ll need to be vigilant to catch sight of it. Buildings and trees could easily block the sun from view. .


Excellent, short video on how the seasons happen

These extremes of daylight and night are brought on by Earth’s tipped axis. If it ran straight up and down, much as Jupiter’s axis does, sunrise and sunset times would barely vary for your location. The sun would rise in the east and set in the west 12 hours later every day of the year. No variation and no seasons. Who wants that?

Thanks to the Earth’s tipped axis we experience the joys winter and the ice it brings. These are air bubbles trapped in pond ice near my home yesterday. Photo: Bob King

The tip ensures that the northern hemisphere of the planet tilts toward the sun in the summer and away in the winter. As a consequence, the sun appears very high in the sky in summer. Its longer, steeper path naturally means longer days and more intense heat. In the winter, we’re tipped away from the sun. Slanted, less intense solar rays and short days follow.

Vesta shown at 9:30 p.m. (CST) every 5 days now through Jan. 10, 2013 as it glides near the Hyades cluster. 97 Tauri is mag. 5.  Stars shown to 7.5 magnitude. Created with Chris Marriott’s SkyMap software

If you’re looking for an interesting astronomical treat in the night sky this solstice, face east anytime during the evening hours and find the brightest “star” you can see. That’s the planet Jupiter. Just below Jupiter is the bright star Aldebaran and a V-shaped pattern of stars called the Hyades star cluster. Not far from the cluster is the famous asteroid Vesta. You’ll recall it was was orbited and studied by NASA’s Dawn spacecraft this past year.

Vesta shines at magnitude 6.5 (just under the naked eye limit), as bright as it gets this year. The star-like asteroid is super easy to see right now in binoculars, especially with Jupiter to help point you there. Take a look the next clear night.

Thank God it’s finally autumn this weekend

As the days grow shorter, green chlorophyll breaks down in these sugar maple leaves unmasking yellow and orange pigments that have been there all along. Reds are produced in late summer and early fall from excess sugar in the leaves. Photo: Bob King

Summer’s slipping away. Back in July, when every day was sunny and hot, many of us couldn’t wait for fall to get here. Our fondest hopes will materialize this Saturday September 22 at 9:49 a.m. (CDT) when autumn finally comes a-knockin’.

Astronomers call the first moment of the new season the autumnal equinox. It’s one of two times a year when the sun’s path intersects with the celestial equator, a projection of Earth’s equator onto the sky. The spring or vernal equinox is the other.

As seen from the equator, where the celestial equator is directly overhead, the sun will be overhead at local noon. People there who look down at their feet will discover they’re standing directly on the shadow of their head! From mid-northern latitudes, the celestial equator arcs approximately midway between the overhead point and the horizon at noon. Up at the north pole the celestial equator it’s a hula-hoop encircling the entire horizon. If you were standing there this Saturday, you’d see the sun circle the horizon for 24 hours straight, never rising higher.

The orientation of Earth’s axis to the sun changes during our yearly orbit – the reason for the changing seasons. Notice that the tilt of Earth’s axis remains fixed in space and does not flip-flop back and forth. Credit: National Weather Service

Seasons are caused by the 23.5 degree tilt of the Earth’s axis. As we orbit the sun during the year, the north-south position of the sun changes because of the changing orientation of our axis. When the north polar axis is pointed toward the sun, our star reaches its most northerly point in the sky and we experience long days and summer heat.

During northern hemisphere winter, our axis points away from the sun and our star is southernmost and lowest in the sky. Shorter days and a low sun make for cold weather.

The sun’s been sliding south in the sky each day since the beginning of summer. This Saturday it’s exactly halfway between its highest point (June 20) and lowest (December 21). Photo: Bob King

The first day of fall is special because Earth’s axis points neither toward nor away from the sun. Instead, we’re broadside to the sun, and day length is approximately equal to night nearly everywhere across the planet. If you’re into equality of light for all, the equinoxes are your symbols of emancipation.

The word equinox comes comes from the Latin words for equal and night because both day and night are approximately 12 hours long. Prior to September 22, days are longer; after the 22nd they get shorter. Shorter days are caused by the sun dropping farther south in the sky (lower altitude). The lower the sun, the less time it spends crossing the sky and the shorter the hours of daylight.

Interestingly, day and night are not exactly equal at the equinoxes. Yes, it’s true that the center of the sun sets exactly 12 hours after it rises on the first day of fall. Problem is, we determine sunrise at the first sighting of the sun, when its upper edge (not center) breaches the horizon. Similarly, sunset occurs when the last bit of sun disappears below the horizon. That adds about two minutes to daylight’s tally.

The sun in this beautiful sunrise photo is an illusion caused by the thick atmosphere bending the real sun (below the horizon) into view. Credit: Lyle Anderson; illustration: NOAA

We get another few minutes thanks to atmospheric refraction. That’s our atmosphere’s freaky ability to act like a prism and bend the sun’s rays upward into view when it’s still below the horizon. If you’ve ever seen the sun directly on the horizon at sunset or sunrise, you’ve witnessed one of nature’s grandest illusions. The sun’s not really there. The air is thick enough across your sightline to “lift” the sun into view about two minutes before it rises for real.

As astronomer George Greenstein, who worked for years at the Old Farmer’s Almanac, once said: “If the Sun were to shrink to a starlike point and we lived in a world without air, the spring and fall equinoxes would truly have ‘equal nights.’” To whittle away those excess minutes of daylight gained by these parlor tricks, we have to wait until September 25 for day and night to momentarily be equals.

Any planet with a decent amount of axial tilt will experience seasons. How many do? All but Venus, Mercury and Jupiter. Venus’ axis is tipped nearly 180 degrees and rotates backwards compared to the other planets, Mercury’s is 0 degees and Jupiter just 3. Mars’ axis is tilted closest to Earth’s at 25.2 degrees, but since that planet is about 2/3 farther from the sun than ours, its seasons are that much longer.

One order of spring please, sunny side up

The sun rises over Lake Superior last week. Today marks the first day of spring when the sun rises due east and sets due west. Photo: Bob King

I celebrated the transition from winter to spring by taking a walk with my dog in the rain. We stepped out shortly after midnight this morning, me with umbrella in hand and Sammy nosing the ground. At 12:14 a.m. CDT at the stroke of spring I strained to listen for the first frogs. None were heard. They don’t usually begin calling until mid-April, but with temperatures in the 70s the past few days and snow vaporizing faster than a comet’s nucleus, my expectations were high.

Spring is when day and night are nearly equal across the entire planet. That’s because Earth’s axis is oriented neither toward nor away from the sun. If the southern hemisphere is the planet’s feet and northern hemisphere its head, today we’re 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 sun to pay for it. In summer, we’re tipped toward the sun with long days, a high sun and heat to spare.

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

Spring and fall are the ‘tween times when temperatures moderate and the sun rests for a brief moment between extremes. And don’t forget the bonus alignment: at the equinoxes the sun rises due east and sets due west. If you’ve never been sure of your directions, this week is the time to get reacquainted. Face the sunset and stick out your arms. Your right arm points due north, your left south and your back faces east. Couldn’t be easier.

Where the sun is lower in the sky in the polar regions, its energy is more spread out and heats the ground and water much less than when it's high in the sky (b) and its rays are more concentrated. That's one of the reasons winter is so much colder than summer. Credit: Peter Halasz

Spring is that astronomical moment when the sun’s path crosses an imaginary circle in the sky called the celestial equator. The celestial version is an extension of the Earth’s equator into the sky. That’s why the sun is exactly overhead at the  real equator. An observant equatorian might notice that flagpoles or power poles cast no shadows at noon, because the poles literally stand right on top of them.

From here on out, the sun continues moving northward in the sky, which for those living in the northern hemisphere, means the sun gets higher and higher and daylight hours longer and longer until maxing out on June 20, the solstice. Not so for those living south of the equator, where the seasons run in exactly the opposite direction. It’s the autumnal equinox down under. The sun’s headed lower in the sky, bringing with it shorter days and longer nights.

A low sun at the North Pole photographed on April 8, 2008. The temperature at the time was 14 below F. Click image to webcam page. Credit: NOAA/North Pole webcam

The sun’s apparent movement north or south in the sky is a result of the Earth’s axial tilt of 23.5 degrees, which in turn is amount the Sun moves north or south of the equator during a year.

If you’re trekking to the North Pole today, situated at latitude 90 degrees north, the sun will make its first appearance of the year on your horizon at local noon … and it won’t set for the next six months! It doesn’t matter what direction you look either since it’s up all night and day.

At 90 degrees north, the celestial equator rings the horizon. Not until the sun reaches this point – which happens on the first day of spring – does it finally return for observers at the pole. Conversely, today is the last day the sun is up for the next six months for an observer at the South Pole. I hope your spirits rise today like the sun in the new season. Happy equinox!

Before and after pictures of supernova 2012 aw in M95. The image at left was taken in April 2008 before the explosion; the other just a few days ago. Credit: William Wiethoff

The new supernova in M95 in Leo, shining at magnitude 13.1, now has a name – 2012 aw. It doesn’t sound like much but if you add an “e” you’ve got an eyeful of “awe”.