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Wake up to spring tomorrow and see the space station

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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

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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

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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

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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

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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”.

Winter solstice arrives today – don’t forget your party hat

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The winter solstice occurs today Dec. 21 at 11:30 p.m. CST when the sun reaches its lowest point in the sky for the year. Photo: Bob King

If you yearn for more sun and less darkness, you’ll get an early Christmas gift today. At 11:30 p.m. Central time, the sun hits bottom in the sky, hesitates a moment and then resumes its slow journey northward to summer’s endless days. Happy winter solstice!

The solstice, literally “sun stands still”, marks the beginning of winter and day of the year with the least amount of daylight for those living in the northern hemisphere. The standing still part stems from the fact that around the solstice the sun moves very little north or south compared to the spring equinox, when it’s climbing steadily north with every passing day. This causes both the sun’s height above the horizon and length of day to change very slowly over the next couple weeks.

The path the sun takes in the sky is caused by the tilt of Earth's axis (see diagrams below). In summer, the sun rides high and days are long; in winter its path is much closer to the horizon and days are short.

Here in Duluth, Minn. the day is a lean 8 hours and 32 minutes, barely long enough for tanning. I shouldn’t complain. At the Arctic Circle, that invisible circle of latitude at 66.5 degree north, the sun won’t even bother to rise on the solstice. North of there, the sun remains below the horizon longer and longer until we reach the North Pole, where it hasn’t shown its face since the fall equinox and won’t return until the first day of spring.

As you’re probably well aware, the South Pole experiences the exact opposite season. While we relish our precious allotment of daylight here in the North, Antarctica researchers are enjoying the midnight sun on what for them and the rest of the southern hemisphere is the summer solstice.

You’ll notice that the solstice occurs late on the 21st at 11:30 p.m. for the Midwestern states. For the Eastern U.S., Europe and much of the rest of the world, that switches over to the morning of the 22nd. The date of solstice varies not only because of what time zone you’re in, but also because a full revolution of the Earth around the sun takes 365 1/4 days rather than a nice neat 365. That quarter day is the reason we have to add a leap day every four years to our calendar – as we will in 2012 – otherwise the dates of the seasons would drift. Without leap days, after only 100 years, the calendar year would be 25 days ahead of the seasonal year, making the winter solstice begin in mid-January. We don’t want that to happen.

Still that extra 1/4 day and leap year days cause the seasonal start dates to vary by several days over a cycle of 400 years. The combined gravitational effects of the planets on Earth also cause a slight shifting of the season starts over centuries of time. Precession , which is the small cyclical wobble of the Earth’s axis caused by the gravitational attraction of the sun and moon on our planet’s equatorial bulge, also plays a part. Put it all together and the date of winter’s start can vary from December 20 to December 23. The outer dates – 20th and 23rd – are rare compared to the 21st and 22nd. The last Dec. 23 solstice occurred in 1903; the next happens in 2303. The next December 20 solstice won’t be until 2080. You can read more about the seasons and Earth’s orbit HERE.

The tip of Earth's axis causes the northern hemisphere in winter to face away from the sun and toward it in summer. Credit: NASA

Of course the changes of season are caused not by Earth’s varying distance from the sun but rather by the 23.5 degree tip of its axis. During winter, the northern hemisphere is tipped away from the sun, making it appear much lower in the sky with shorter days the consequence. Less sunlight means colder temperatures and snow instead of rain. In summer the situation is reversed and days are long and hot.

The winter solstice, a time of darkness but simultaneously holding the hope of returning light, never fails to bring out mankind’s party spirit. We string lights, put green trees in our homes, sing around the blazing bonfire and beat drums. Many towns across the world hold winter solstice celebrations. Tonight Dec. 21 starting at 5:30 p.m. at the lighthouse along the waterfront in Two Harbors, which is located a half-hour north of Duluth, Minn., the good folks there will hold their annual winter solstice celebration. It begins with a candle lantern launch and a torchlight procession to the bonfire in honor the event’s founder Ellen Anderson, who passed away recently. If the sky is clear, several amateur astronomers from the Arrowhead Astronomical Society will be on hand with telescopes to look at the night sky. All ages are welcome to join in the fun.

Saturn rises to meet dawn’s early light

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The ringed planet returns to view at dawn in early November. Created with Stellarium

Hey, we’ve got a new morning sky planet – Saturn! It rises around 6 a.m. daylight saving time and pops up in the southeastern sky near the bright star Spica in Virgo about 50 minutes before sunrise. The two are only 5 degrees apart tomorrow but will close to 4.3 degrees by mid-month. Look for them low in the southeastern sky about three outstretched “fists” to the lower right of the bright pinkish-orange star Arcturus.

As Saturn pulls away from the sun it will rise earlier and ascend a little bit higher in the east with each passing day. Many sky watchers are eager to get their scopes out for a look at the rings, which are tipped open 12.5 degrees. The tip increases through the month to 14 degrees or more than halfway to their maximum opening of 27 degrees in the year 2017.

Because Saturn's axis is tipped similar to Earth's, we see the ring tilt change during the planet's 29-year orbit. Credit: Tom Ruen

The rings are composed of nearly pure chunks of water ice and like ice anywhere, they’re good reflectors of light. That’s why as the rings open up, the planet’s brightness increases. The ring tilt changes over time because Saturn’s axis is tipped 27 degrees. As it revolves around the sun in its 29-year-long orbit, we see the rings from a slightly different angle year to year.

The thick atmosphere and shaking seeing conditions at the planet’s current low altitude will make the rings look pretty wobbly, but I imagine a few of us will be out just the same to welcome the ringkeeper back to the morning sky.

A little tilt is all it takes

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I hope the first couple days of spring have lived up to seasonal expectations where you live. They have here, but now a howling snowstorm is on its way, threatening to repave all the lovely, exposed ground with a foot of wet, white concrete.

The sun is shown at noon on the first day of spring (left) and 10 days later. The arrow shows the direction it's moving. Notice that the sun's path, called the ecliptic, intersects the imaginary arc of the celestial equator on the first day of spring. By the 30th, the sun has moved noticeably north of the equator and is higher in the sky. Created with Stellarium

In Sunday’s blog we discussed that the reason for the seasons has to do with the tip or inclination of Earth’s axis, rather than our planet’s distance from the sun. The tip causes first one hemisphere and then the other to alternatively face toward and away from the sun. This makes the sun’s apparent path across the sun swing from low to high to low in the year-long cycle of four seasons. Spring began when the sun crossed the imaginary projection of Earth’s equator – called the celestial equator – on its march northward.

This diagram shows the various tilts of the planets and dwarf planet Pluto. Credit: Image courtesy of Courtney Seligman / cseligman.com

What about the other planets in the solar system? Do they have seasons too? Well, to have seasons, you need to be tipped on your side. You can see that Mercury, Venus and Jupiter have little to no axial tip, so it follows they don’t experience the seasons. The sun follows the same path across the sky, neither dipping low nor ascending to the heights, the entire year round on these planets.

On Earth, and sometimes in Duluth, liquid water returns as winter transitions to spring. Photo: Bob King

Venus is a bit of an oddball. It’s tipped so far over – 177 degrees – that its north pole is where the south pole should be. This extreme tip means Venus is actually rotating backwards or clockwise as seen from above the plane of the solar system. From the same perspective, most of the other planets rotate counterclockwise.

Because Earth, Mars, Saturn and Neptune have similar inclinations, they all experience similar seasons. For instance, right now it’s late spring in Mars’ northern hemisphere and early spring in Saturn’s northern hemisphere.

Keep in mind that seasons on these outer planets last longer than those on Earth, because they take longer to orbit the sun. A typical season on Mars last twice as long as the equivalent on Earth. Neptune takes takes 165 years to journey once around the sun, making spring a tedious 41 years in length. Well, maybe you like spring more than I do.

Pluto and Uranus also have seasons, but surprising things happen when you’re rolling around the sun on your side.

The Hubble Space Telescope photographed Uranus and its rings between 2003 and 2007. The rings, which lie above the planet's equator, went from open presentation in 2003 to edge-on in 2007. Credit: NASA/ESA/M. Showalter

Uranus orbits the sun every 84 years. As the north polar regions face the sun for 21 years, the sun hugs the horizon at the equator and the south pole is in complete darkness. During the next 21 years, the equator faces the sun and all parts of the planet experience sunlight. The poles see sunlight too, but just barely, as the sun circles the horizon for years on end. This is followed by 21 years of sunlight at the south pole and an equally long night at the north pole. The Uranian year wraps up with the equator once again facing the sun with sunlight across the whole planet. Just weird.

Seasons on Earth keep life interesting, but when you add in the rest of the planets, you really get a feel for how much variation is possible from a simple little thing like tilt.

10 ways to enjoy tonight’s full moon

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The moon’s done it again. Ambled its way along the ecliptic to another full moon phase. Appropriately enough, this month’s full moon is called the Hunter’s Moon. It rises in the northeast in Aries the Ram and stands high in the south around midnight. While you’re out this evening, I’ve prepared a list of 10 things to see or reflect upon while you and the moon are beaming at one another.

Earth's atmosphere takes away the moon's blues, leaving it orange. Photo: Bob King

1. Dramatic colors – Moonrise for Duluth is 5:35 p.m. or about 35 minutes before sunset. If you can get to location with an open view to the east, try to catch the moon just as it’s clearing the horizon. Depending on the state of the atmosphere, the moon will rise in technicolor red or orange. Longer wavelength red light penetrates air and dust, while blues and greens are scattered or absorbed. Once the moon is a fist or so above the densest layer of air, it assumes its usual white color. You’ll recall that white light is a mix of all the colors of the rainbow spectrum. In the thinner air high above the horizon, all the colors – except a bit of the blue – make it through to give us back a pale moon.

The moon is very out-of-round (right) when rising compared to when it's higher up. Atmospheric refraction is the cause. Credit: Jim Schaff

2. Distorted shape – The atmosphere also acts like a lens and bends or refracts the moon’s light. There’s no time better to see this than during a full moon rise, when the moon’s lower half is closer to the horizon than the upper. Thicker air at the horizon “pushes” the lower half of the moon upward into the less-heavily refracted top half, causing the moon to appear flattened or squished. If you’re ever lucky enough to see the moon directly on the horizon, you’re only seeing an optical illusion. Refraction there is strong enough to lift the moon into view even before it’s truly risen!

The dusky band beneath the rising moon is the Earth's shadow. Photo: Bob King

3. Earth’s shadow – The moon rises along with Earth’s shadow tonight. You won’t see the shadow at first, because the sun won’t have set yet, but beginning at sunset and continuing for the next 20 minutes, you’ll notice a thick, blue-gray band in the east beneath the moon.  That’s our planet’s shadow. It extends all the way around the eastern horizon, and rises until it gradually merges with the darkening sky.

4. Eminently quotable – You’ll find many mentions of the moon, particularly the full moon, in literature, TV and other media. Here’s a sampling:

“Promises are like the full moon, if they are not kept at once, they diminish day by day.” – German proverb

“Moonlight is sculpture; sunlight is painting.” – Nathaniel Hawthorne

” Everyone is a moon, and has a dark side which he never shows to anybody.” – Mark Twain

Can you see the moon rabbit or the lady in the moon? Tonight's the night.

5. Make a face – You can see almost anything you want in the face of the moon. The dark spots, called lunar seas, contrast with the white regions known as the lunar highlands, creating patterns that give us everything from the familiar “man in the moon” to the lady of the moon, the alligator of the moon and anything else your imagination can conceive. As you making faces up there, consider that the lunar seas are impact holes that later filled with lava from below, while the white areas are ancient crust carpeted with craters from asteroid and meteorite bombardment in the early days of the moon’s formation.

Craters are labeled in white in this annotated photo of the full moon

6. Binocular exploration – Bring your binoculars tonight and see how many lunar seas and craters you can spot. The seas are obvious, while the four most prominent craters labeled at right require only a little extra effort.  Huge systems of bright rays surround each crater and make them easy to spot. The rays originated when boulders sent aloft from the crater impacts rained back down, striking the moon and creating thousands of mini-craters, each of which excavated fresh soil from the moon’s surface.

In this full moon photo, you can see the terminator scraping along the top or north edge of the moon. Nice shadow detail shows up in the craters on the moon's north edge, because the terminator there is the day-night border where shadows are long. Credit: NASA

7. Terminator madness – At full moon, the sun, Earth and moon are lined up in that order, and the sun shines squarely on the moon’s face, lighting it up completely. During other phases, we see only part of the moon; the terminator, or day-night line, separates the lit part of the moon from the unlit. At full moon, the terminator essentially disappears because the entire face of the moon is in sunlight. Truth be told, it doesn’t disappear completely. The lineup that I mentioned earlier is exact only in the case of a total lunar eclipse. The moon’s orbit is slightly inclined with respect to Earth’s orbit, so it’s usually a little north or south of the sun-Earth line. Tonight it’s far enough north of that line that telescopic observers will see the terminator running along the south edge of the disk below the prominent crater Tycho. To the naked eye, the moon will appear perfectly circular with no nibbling about the edges. By tomorrow, the terminator will have swung around to its usual post-full moon position on the west side of the disk. To see the “southern terminator” to best effect, look between 7-10 p.m. Central time. The exact moment of full moon is 8:36 p.m. CDT tonight.

The moon's black disk covers the sun during a total solar eclipse. In the distant future, this will be a thing of the past. Credit: Luc Viatour

8. Slip slidin’ away -The difference in the moon’s gravitational pull on the nearside versus the farside of Earth causes the two tidal bulges. Coastal residents are familiar with as the ocean tides. Some of the energy from this moon-Earth interaction causes our planet’s spin to gradually slow down. The spin energy is not wasted, but transferred out to the moon, causing it to move 1 1/2 inches farther from our planet each year. Over several billion years, the Earth’s spin will slow to eventually match the orbital period of the moon. On that far future day, we’ll be locked facing one another so only the inhabitants of the moon-facing side of the Earth will see the moon. Those living on the Earth’s “farside” will have to travel to the other hemisphere for a look-see at the moon. Weird to contemplate, but it’s inevitable. One other consequence of this tug of war is that a more distant moon will also appear smaller, and a smaller moon won’t be able to completely eclipse the sun. The days of the total solar eclipse will be over.

The Earth's tilted axis is stabilized by the moon's gravitational influence.

9. Thank you for being our friend – One of the wonderful things the moon does for our planet is keep its axial tilt stable. Without the moon, the Earth’s axis would wobble to more extreme tilts like Mars’ axis does, causing catastrophic climate changes. The moon stabilizes our axis and keep our planet’s alignment from going to extremes.

10. Moonlight on linen – After an evening with the full moon, leave the lights off for a few minutes when you return to your home or apartment. If you have an east or south-facing window, you’ll be treated to the sight of moonlight spilling on the floor, the bed and curtains. Though the moon only reflects 7% of the sunlight striking it, it seems strangely brilliant and alien. When I see that I’ve unwittingly allowed the moon into the house, it never fails to stoke my sense of wonder at how close the universe is.

Frosty and blazing, fall steps forward

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Frost lines a turning leaf. Photo: Bob King

Yikes, fall begins tomorrow night (Sept. 22). Is the autumnal equinox already upon us? Must be the lulling effect a long summer has on the mind, because I didn’t pay attention to the calendar until this morning, the last full day of summer. Here in Duluth, the waning of the season puts many of us in a wistful mood, knowing that we’ll soon be spending five months with our hands cupped around a single candle, snapping icicles off our beards and pulling frozen bodies off the sidewalk. Oh, it’s not like that, really. Well, maybe sometimes.

At 10:09 p.m. Central Daylight time Wednesday, the sun will cross the celestial equator moving south. The celestial equator is simply the earthly version expanded into the sky against the background of stars. If you happen to live on the equator, you’ll see the sun overhead tomorrow around noon, and if you look down at your feet, you’ll see they completely cover your shadow. For everyone on the planet, the sun will rise due east and set due west. Daylight and night will achieve a perfect balance at 12 hours apiece no matter where you live. That’s the meaning behind the word equinox, derived from the Latin words for “equal” and “night”.

Two views of the sun's path at the autumnal equinox which begins tomorrow. The left side shows the view from the equator where the sun passes overhead. The right shows the view from the northern U.S. Credit: Tau'olunga

Depending on your latitude – how far north or south of the equator you are – the sun’s altitude at your location will vary. It shines overhead at the equator, about halfway between the horizon and zenith for mid-northern and mid-southern latitudes and directly on the horizon at the poles. Since the poles are at +90 and -90 degrees latitude, the celestial equator hugs the horizon in all directions.

Because the sun continues moving south of the celestial equator in the days following the fall equinox, it soon disappears below the horizon at the north pole and won’t reappear for another six months. Deep cold will follow quickly enough for high northern latitudes. The situation is reversed for the south pole, where the sun slowly climbs higher and remains above the horizon 24 hours a day for six months.

During Earth's revolution around the sun, we alternately face toward and away from the sun. We're sidelong at the spring and fall equinoxes. Credit: Tau'olunga

This crazy Earth. These variations are all caused by the tip of our axis. In summer, the northern hemisphere is tipped toward the sun, vaulting it high in the sky and making for long hours of daylight and a bounty of heat. During the winter, we’re tipped away, and a low sun means less daylight and subsequent loss of heating. Fall and spring are in-between times when neither hemisphere is tipped toward the sun. We all face it from the side, hence the equal day-night lengths and sun’s east-west path.

Two views of the sun taken around 8 a.m. this morning by the Solar Dynamics Satellite, one in ordinary light that shows the big sunspot #1108, and the other in far ultraviolet light that exposes a large coronal hole. Credit: NASA

While we’re on the topic of the sun, the folks at the NOAA Space Weather Prediction Center, are forecasting minor solar storm levels for the next two nights which could mean a chance of seeing northern lights at high latitudes. The cause of this bit of unsettled weather is a hole in the sun’s atmosphere called a coronal hole. Solar plasma – electrons and protons – are free to stream into the solar system from such holes where they can interact with the magnetic fields surrounding many of the planets and stimulate auroras. There’s also a substantial sunspot group in the sun’s southern hemisphere, but thus far it’s produced no major flares.