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.

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

 

Earth and Moon captured together in amazing new photo

Chang’e 5 took this splendid photo of Earth and Moon together while it passed over the lunar far side on October 28, 2014. The Moon reflects far less light than Earth and appears darker.  Click to grab a large version. Credit: CNSA / Xinhua News Agency

A friend alerted me to this wonderful photo of Earth and Moon in the same single image taken by China’s Chang’e 5 lunar test vehicle. The spacecraft is conducting an 8-day mission to the Moon and back to refine the technology needed for a planned sample return mission in 2017. Launched on October 23, this is China’s fourth volley to the Moon; the spacecraft will return to Earth on November 1 according to Xinhua News.

View of Earth taken by the Chang’e 5 test vehicle on October 28 after rounding the far side of the Moon. Australia is easy to see in the clearing. Credit: CNSA / Xinhua News Agency

As it swung high above the far side of the Moon – the hidden half of the lunar globe out of sight from Earth – the solar array monitoring camera on the craft snapped this incredible image. While not the first ever taken of the pair, it’s one of the best composed images and possibly the first to clearly feature the lunar far side along with Earth. You can easily see how much more cratered the Moon’s hidden hemisphere is. And that dark splotch? That’s Mare Moscoviense (Sea of Moscow), one of the very few dark maria or seas on the far side.

View of the Moon by Chang’e 5 on October 28 shows the dark lunar “sea” called Mare Marginis. This patch is visible along the western edge of the moon from Earth. Credit: CNSA / Xinhua News Agency

Chang’e 5 did not enter lunar orbit but kept its camera humming to shoot separate close-ups of Earth and Moon. Like seeing Earth and Moon from afar? Check these out:


Earth and Moon dance a pirouette in these images taken by the Jupiter-bound spacecraft Juno on Oct. 9, 2013

The European Space Agency’s Mars Express captured this image of Earth and the Moon on July 3, 2005 when it was 5 million miles ( 8 million km) away. Credit: ESA

Earth and Moon in 1992 as Galileo photographed the duo on its way to Jupiter. Credit: NASA

Earth is the brightest “star” in Mars’ western evening sky as seen and photographed by the Curiosity Rover on Jan. 31, 2014. Credit: NASA

A single frame from high-definition video of the full Earth over the lunar limb taken by Japan’s Kaguya spacecraft on April 6, 2008. Credit: JAXA/NHK

Earth and Moon from Mars, imaged by Mars Global Surveyor on May 8, 2003. Credit: NASA

Earth rises over the barren lunar landscape photographed by the Apollo 8 crew on December 24, 1968. Credit: NASA

Earth and Moon become a single dot in this photo taken by the Voyager 1 spacecraft from a distance of 4 billion miles (6.4 billion km) on February 14, 1990. Credit: NASA/JPL

Fleet of foot Mercury appears at dawn

Mercury comes into good view the remainder of October and the first week of November low in the eastern sky during morning twilight. This map shows the sky from the central U.S. (Champaign, Ill. in particular) tomorrow morning October 28 about 40 minutes before sunrise. Also shown is the planet’s orbital path in the sky and the bright star Arcturus, which you can use to help you find the planet. Source: Stellarium

Mercury is the solar system’s hot sports car. Not only is it the smallest planet, but it rips around the Sun once every 88 days, faster than any of the others. That’s 4 revolutions for every one the Earth makes. As you read this, Earth’s toting you around the Sun at 66,600 mph. Mercury’s got the pedal to the metal at nearly106,000 mph.

Now through the early November you have a chance to watch this speed demon in morning twilight. Six times a year the fleet planet reaches greatest elongation from the Sun, when it’s highest above the horizon during twilight and easiest to spot. This season that date is November 1st, but you can look for Mercury anytime now through about Nov. 10th.

Mercury has phases like the Moon because of the changing angle it makes to the Sun as viewed from Earth during its 88-day orbit. The dates show inferior conjunction between Earth and Sun (Oct. 16), greatest western elongation (Nov. 1), superior conjunction (Dec. 8) and greatest eastern elongation (Jan 14) when the planet returns to good evening sky viewing. Credit: Bob King

Unlike the outer planets, which orbit beyond the Earth, Mercury orbits between our planet and the Sun. That’s why it never strays far from the Sun in the sky and only puts in an appearance after sunset at dusk or before sunrise at dawn. Because it’s in such an orbital hurry, we usually only get to see the planet for a couple weeks during each favorable elongation.

Mercury shows phases like the Moon. This is approximately how the planet will appear in the next few mornings. Source: Stellarium

To the eye, Mercury looks like a fairly bright star (magnitude 0 and brightening to -0.7 in the next two weeks), but through a small telescope it shows phases just like the Moon and Venus.

Right now it’s a fat croissant but it will fill out and brighten in the days ahead.

Take advantage of the late morning sunrises in the days before we lose Daylight Saving Time to find Mercury at a reasonable hour (around 6:40-7:15 a.m. from many locations).  Look “one fist” above the eastern horizon about 45 minutes before sunrise.

 

Feel the bliss, don’t miss Thursday’s partial solar eclipse

The solar crescents show how much Sun will be covered at maximum for various locations across the U.S. and Canada during the October 23rd (Thursday) partial solar eclipse. Credit: Jay Anderson

Doing anything Thursday afternoon? Have a few minutes to spareThere’s a partial eclipse of the Sun visible across much of North America and of Mexico you might like to catch. For observers in the U.S. and Canadian West the whole event begins and ends in the afternoon before sunset. Those living east of the Great Plains will see the Sun set while still in eclipse.

During a solar eclipse, the orbiting Moon passes between the Sun and Earth, completely blocking the Sun from view as shown here. In Thursday’s eclipse, the moon will pass a little north of a line connecting the three orbs, leaving a portion of the Sun uncovered. To view a partial solar eclipse, a safe solar filter is necessary. Credit: Wikipedia

Solar eclipses occur when the Moon glides between the Earth and the Sun, temporarily blocking it from view. Total solar eclipses get most of the attention because the Earth- Moon-Sun alignment is perfect. Like a snug lid on a pot, the Moon blanks out the Sun completely to create a dramatic spectacle of a black, fire-rimmed disk set in a plush solar corona.

Partial eclipses happen because the Moon’s orbit is tipped a few degrees to the Sun-Earth line. Most months, it passes north or south of the Sun and misses it completely. But during a partial eclipse, the Moon’s close enough to that line to partially block the Sun from view. Unlike a total eclipse, all phases of a partial eclipse are unsafe to view unless you use a safe solar filter or view it indirectly via projection.

Map showing times and percentage of the Sun covered during Thursday’s partial solar eclipse. Times are Pacific Daylight – add 1 hour for MDT, 2 hours for CDT and 3 hours for EDT. Interpolate between the lines to find your approximate viewing time. The arc marked A shows where the eclipse begins at sunset; B = Maximum eclipse at sunset and C = Eclipse ends at sunset. Credit: NASA, F. Espenak,with additions by Bob King

As you can see from the map, nowhere will this eclipse be total. Maximum coverage will happen in Nunavut Territory in northern Canada where the musk oxen might catch sight of a fat solar crescent 81% covered by the moon at sunset. The farther north you live in the U.S. or Canada, the deeper the eclipse. Northern U.S. states will see around 60% covered compared to 40% in the deep south.

In Duluth, Minn. for example, the eclipse begins at 4:21 p.m., reaches a maximum of about 65% at 5:33 p.m. and continues into sunset at 6:06 p.m. Since the sun will be low in the western sky from many locations, be sure to get a spot with a wide open view in that direction.To find out times and coverage for your city, use these links:

* U.S. Cities
* Cities in Canada and Mexico 

Some of the different kinds of safe solar filters available. They work by reflecting or absorbing most of the light from the Sun, allowing only a fraction through to the eyes. NEVER LOOK DIRECTLY AT THE SUN without one. Click photos for a supplier of eclipse glasses. Credit: Bob King

Solar filters come in a variety of styles from inexpensive eclipse glasses that use an optical polymer to glass welder’s filters to caps you place over the front end of a telescope. It’s important to use the correct kind – don’t stack a bunch of sunglasses and figure “it’ll do” or look through smoked glass. They still allow dangerous UV and infrared light to pass through and will mess up your retinas for life.

Because we’re on the heels of the eclipse, if you don’t already have a pair of eclipse glasses I recommend a #14 welder’s glass. It’s my favorite actually because it’s easy to stuff in a pocket and heavy-duty enough to take a few dings. You can pick one up for a few dollars at a welding supply shop. Only buy a #14 – lower numbers won’t cut it.

A piece of aluminum foil, a pin and a cardboard box are all you need to build a pinhole projector. The tiny hole creates a small image of the eclipsed Sun inside the darkened box which you place over your head. Remember to look at the projection of the sun on the inner wall of the box – not through the pinhole itself.

Projection provides a fine alternative to using a filter. You can mount a pair of binoculars (or small telescope) on a tripod and project the Sun’s image on a sheet of white paper or build your own pinhole projector using the instructions above.

You can mount binoculars on a tripod, cover one lens with a lenscap and project the sun’s image safely onto a sheet of white cardboard. Credit: Bob King

If leaves still cling to your trees this season, the narrow spaces between the leaves act like natural pinholes and will cast multiple images of the eclipsed Sun on the ground below.

You can even place one hand atop the other and let the sun shine through the gaps between your fingers to see the eclipse. Low tech as it gets, but works in a pinch.

Here are some other things to watch for during the eclipse:

* If you live where half or more of the sun will be covered, you may notice a change in the quality of daylight. To my eye, the light becomes “grayer”. What do you see?

* Telescope users will see the mountains and crater rims along the moon’s edge as tiny bumps and projections against the brilliant solar photosphere. You’ll also notice how much blacker moon is compared to sunspots. Guess what? We’ve got a huge sunspot out there right now – Region 2192. Perfect for comparison!

Partially eclipsed sun just before sunset seen from Island Lake north of Duluth in May 2012. Credit: Bob King

*  Those living where parts of the eclipse happen at sunset will get an extra special view of the sun with a big bite out of it right sitting atop the southwestern horizon.

I wish you excellent weather – good luck!

 

Earth and Mars, space pals forever

This single shot of Earth and Mars together was taken on May 24, 2014 with NASA’s Lunar Reconnaissance Orbiter spacecraft as it orbited the moon. Click to see full, hi-res photo. Credit: NASA/GSFC/Arizona State University

Yesterday we watched the total lunar eclipse from Mercury. Today, NASA’s Lunar Reconnaissance Orbiter (LRO) expands our gaze to encompass both Earth and Mars together in space.

LRO’s viewing post was none other than the moon located 240,000 miles from Earth. On May 24th, instead of staring down at the lunar surface, NASA engineers sent commands to the spacecraft to point its Narrow Angle Camera toward Earth. On that date the two worlds were in conjunction from LRO’s perspective.


Mars and Earth from lunar orbit

Mars was about 70 million miles away (112.5 million km) away at the time or 300 times farther away from the Moon than the Earth. That’s why it’s only a tiny dot in the sky.

Moon-facing hemisphere of Mars on May 8, 2014 seen from lunar orbit. Instruments on LRO sometimes use stars and planets for calibration or other special observations. During one of these off-Moon observations, LROC imaged Mars. The planet is so small in LRO’s camera it could only make out the two larger features shown above. Credit: NASA/GSFC/Arizona State University

I know a commercial photographer who takes pictures of babies when they’re asleep. She has to invest a lot time into each of her photos, much of it spent waiting for the children to fall asleep! Likewise the LRO team. To make sure they got the timing and exposure right, the team practiced on Mars weeks in advance.

Seeing the two planets in the same frame seems to shrink the distance between them and tempt us to shove off from home on an exploratory visit.

The LRO folks put it this way:

“The juxtaposition of Earth and Mars seen from the Moon is a poignant reminder that the Moon would make a convenient waypoint for explorers bound for the fourth planet and beyond! In the near-future, the Moon could serve as a test-bed for construction and resource utilization technologies. Longer-range plans may include the Moon as a resource depot or base of operations for interplanetary activities.”

Ever seen a lunar eclipse from Mercury? Me neither … till now


Wednesday’s lunar eclipse photographed by NASA’s MESSENGER spacecraft at Mercury

As millions of us awoke at dawn and trundled outside to watch the total lunar eclipse this week another set of eyes was keeping tabs from afar. 66 million miles away, NASA’s MESSENGER spacecraft turned its camera toward Earth to capture several images of the moon disappearing into our planet’s shadow. Laced together, they make for a brief but fascinating glimpse of planetary bodies in motion.

Two of the still images showing Earth and moon before and during Wednesday morning’s total eclipse. Credit: NASA

The animation was constructed from 31 images taken two minutes apart from 5:18 to 6:18 a.m. EDT. The images start just before the Moon entered the umbra, the darkest part of the Earth’s shadow.

“From Mercury, the Earth and Moon normally appear as if they were two very bright stars,” noted Hari Nair, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory, in Laurel, Md. “During a lunar eclipse, the Moon seems to disappear during its passage through the Earth’s shadow, as shown in the movie.”

MESSENGER photographed Earth and moon on May 6, 2010 from 114 million miles (183 million km) away. Credit: NASA

Because the moon is so much darker than Earth its brightness has been increased 25 times to show its disappearance more clearly. I’ve included another picture of the Earth and moon against the starry backdrop of deep space also photographed by MESSENGER. Sure puts things in perspective. While not as breathtaking as photos of Earth taken by the Apollo astronauts, seeing our tiny home floating in the void effectively communicates how improbable our existence is. Thank goodness life got a grip and kept it. After 3.5 billion years of evolution the double helix has proven itself a force with which to be reckoned.

The 133-mile-wide double ringed crater Vivaldi captured at sunrise. The low sun highlights valleys and chains of secondary impact craters radiating away from it. Credit: NASA

MESSENGER has been in orbit around Mercury since March 2011 studying the chemical composition of the surface, measuring planet’s magnetic field, mapping polar ices and of course taking pictures. Enjoy a few recent ones.

Hollows on the floor of an unnamed crater on Mercury. Hollows may be areas “eaten away” by the ceaseless bombardment of particles in the solar wind. Or they may form when heat from volcanic activity melts away softer rocks. No one knows for sure. Credit: NASA

How long would it take to drive to the sun?

My old Subaru achieved lunar orbit when the odometer hit 238,000 miles several years back. Credit: Bob King

I spend way too much time in the car, mostly on the job as a photojournalist. Every day, there are places to be at this time and that. Like many who drive around for a living,  I’ve accumulated a few miles on my vehicles.

Once, in an older Subaru, I achieved a one-time dream of reaching the moon. The odometer rolled past the 238,000 mile mark – just under the average lunar distance but easily within perigee range. I would have pushed the vehicle further, but the brakes seized up and soon after I sold the car. I recall it leaving the driveway on a flatbed like a patient being wheeled away to the emergency room.

The sun is some 387 times farther from Earth than the moon. Credit: Bob King

The years of driving it took to “get to the moon” got me wondering how long it would take to drive to the sun, which lies some 93 million miles (150 million km) from Earth or 387 times farther away than the moon.  According to the Guinness Book of World Records, the record vehicle mileage goes to a 1966 Volvo P-1800S with more than  2,850,000 miles (4,586,630 km). Owned by Irvin Gordon of East Patchogue, New York, the car is still driven daily.

A commercial jet flying at 550 mph would need 19 years to reach the sun. Credit: Bob King

While that trashes my record, it’s still only 3% of the way to the sun, a nice start but barely there. Instead, let’s drive non-stop at 60 mph (97 kph). How long would it take before we would complete our journey? An amazingly long time – 177 years. Strange, isn’t it? The sun seems so close because we can feel its warmth and watch it ripen our tomatoes. But it’s out there, w-a-y out there.

Even in a commercial jet flying at 550 mph (885 kph) it would still take 19 years. I’m afraid I just don’t have that kind of time or patience. Even the 5-hour trip to Hawaii from Los Angeles made me twitchy. The Helios probes, the fastest moving space vehicles ever, reached speeds of 157,000 mph as they orbited around the sun sensing the solar wind. At that rate, the sun could be reached in just 24.7 days.


Bill Nye demonstrates the distances between the planets.

How about a planet? Let’s choose picturesque Saturn, now low in the southwestern sky at dusk. Its average distance from the sun is 891 million miles (1.4 billion km) or 1,695 years in a car. That means if we started driving in 320 A.D. when ancient Rome still dominated the western world, we’d finally arrive today. Aw heck, I’d rather take a plane and get there in just 185 years.

Maps showing the planets and layout of the solar system give a false impression of sizes and distances. But you can hardly blame the creators. There’s just too much empty space between the planets compared to their tiny sizes to squeeze it all a useful diagram. Credit: NASA

Even in the solar system, never mind the stars, distances are so immense we can hardly comprehend them. If we reduced the sun to the size of a grapefruit, Earth would be a poppy seed 35 feet (10.7 m) away, Saturn a pea at 335 feet (102 m) and the nearest star system, Alpha Centauri, a pair of grapefruits 1,800 miles (2,900 km) away. There’s so much emptiness and so little stuff, it’s mind-boggling.

Sunrise and sunset – nature’s most beautiful illusions

Earth turns on its axis to greet the sun at sunrise each morning of the year. Credit: Bob King

Every day the sun rises, crosses the sky and sets. And it does it again and again and again like the perpetually repeating cycle of events in the movie Groundhog Day.

Except perhaps for a few remaining Flat-Earthers, we know what’s going on here. The sun’s not doing the moving. Instead, the Earth’s rotation causes the apparent motion of the sun across the sky. Yet the sense of the sun’s movement is so powerfully ingrained in our experience you might balk if I told you it’s essentially sitting still in the sky.

Every day the turning Earth causes the nearly static sun to rise in the east at sunrise and set in the west at sunset. Credit: Canadian Space Agency

For you to see a sunrise, Earth must rotate on its axis until your location faces the sun as it crests above the planet’s curvature. The following morning, when Earth rolls around after another 24 hours, the sun is very nearly in the same place in the celestial sphere as the previous morning. Once again, we see the sun ‘rise’. Ditto for the next morning and the next. It’s like turning over in your bed each and every morning and seeing your spouse in the same spot. Or very nearly.

If the Earth spun but stood in one spot never circling the sun, we would meet the rising sun at precisely the same time and place every day ad infinitum – a true Groundhog Day scenario. But the Earth orbits or revolves around the sun as surely as it rotates. Just like our daily spin, our planet’s revolution is reflected in the sun, which appears to slowly crawl across the sky, inching its way from one background zodiac constellation to the next, during the course of a year.

The orbiting and titled Earth cause slow but continuous changes in the times of sunrise and sunset during the course of a year. Credit: Thomas G. Andrews, NOAA Paleoclimatology

The ever-changing times of sunrise and sunset stem from the Earth’s orbital travels combined with the shifting seasonal tilt of the planet. From December 21 until June 21, as the amount of daylight increases in the northern hemisphere, the sun appears to travel slowly northward in the sky and we meet its welcome rays a couple minutes earlier each morning.

The sun’s yearly motion across the sky during the year traces out a path called the ecliptic. The top of the curve, at right, is the sun’s position during the summer. The low part of the curve is the sun’s location during winter. The up-and-down path is a reflection of the 23 1/2-degree tilt of the Earth’s axis. Illustration and animation by Dr. John Lucey, Durham University

Then from June 22 to December 20, Earth’s orbital motion causes the north polar axis to slowly point away from the sun. The sun appears to slide south as the hours of daylight wane, and we meet the sunrise a minute or two later each morning.

The sun, located some 26,000 light years from the center of the Milky Way galaxy, takes about 220 million years to make one revolution around its core moving at 483,000 mph. Credit: ESO

Earth moves along its orbit at an average speed of 67,000 mph (108,000 km/hr).

How about the sun? If I left the impression that it’s totally static I apologize. Yesiree, it’s moving too – at the astonishing speed of 483,000 miles per hour (792,000 km/hr) around the center of the galaxy.

Don’t look now, but you and I are going on the ride of our lives.The only reason stars remain static in the sky over the span of many generations despite the sun’s hurry is because nearly all of them are too far away to show a shift in position with the human eye. Telescopes, which magnify everything including motion, do show very subtle changes in the positions of nearby stars over much shorter time intervals.

Rising each morning to the same old sun, I try to remind myself that with every rotation comes a new opportunity to spin some joy into the day.

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!

Magnetic collapse makes Saturn’s auroras dance the cha-cha

Images of auroras over Saturn’s north pole in ultraviolet light with the Hubble Space Telescope capture moments when Saturn’s magnetic field is affected by bursts of particles streaming from the Sun. Click to enlarge. Credit: NASA, ESA, Jonathan Nichols (University of Leicester)

Saturn shines brightly in the late May evening sky. You can see it now at nightfall by following an arc starting with fiery Mars and passing through Spica. Continue and you’ll end up at Antares, the alpha luminary in Scorpius. Did you know that it also shimmers with auroras too just like the Earth?

Recent Hubble Space Telescope photos taken in ultraviolet light, where the aurora shines brightly, show bursts of light shooting around Saturn’s polar regions traveling at more than three times faster than the speed of the gas giant’s roughly 10-hour rotation period.

Saturn’s auroras shine brightly in UV but would appear deep red at the bottom and violet at top with the naked eye. That’s because hydrogen gas dominates the planet’s atmosphere and emits light in different colors when bombarded by the energetic electrons in the solar wind. On Earth, excited oxygen and nitrogen molecules produce the more familiar greens, reds and blues of northern lights.

A magnetosphere is that area of space around a planet that’s controlled by the planet’s magnetic field. The shape of the Earth’s magnetosphere is the direct result of being blasted by solar wind, compressed on its sunward side and elongated on the night side forming a magnetotail. Saturn’s is similar. Credit: NASA

University of Leicester researchers recently discovered an amazing connection between Saturn’s and Earth’s auroras. Both planets are surrounded by teardrop-shaped magnetic domains called magnetospheres generated by the churning of materials within their cores. In each case, the side facing the sun is compressed and flattened, while the other side is drawn out into a long tail called a magnetotail.

“Our observations show a burst of auroras that are moving very, very quickly across the polar region of the planet. We can see that the magnetotail is undergoing huge turmoil and reconfiguration, caused by buffering from solar wind,” said Jonathan Nichols, of the University of Leicester’s Department of Physics and Astronomy, who led the Hubble observations.


NASA’s THEMIS Spacecraft See magnetic reconnection / collapse in Earth’s magnetotail

What’s happening – and you can see it clearly in the video above – is that the incoming solar wind connects to and ‘peels back’ a portion of the magnetic field on the dayside of both Earth and Saturn. When the lines pinch together and reconnect on the back or magnetotail-side, a torrent of solar electrons is funneled into the upper atmospheres of both planets. Voila – aurora borealis! Here’s another video showing it from a slightly different perspective.


Dance of Saturn’s auroras

“The particular pattern of auroras that we saw relates to the collapsing of the magnetotail,” Nichols added. “We have always suspected this was what also happens on Saturn. This evidence really strengthens the argument.”

Cool beyond cool. Earth and Saturn are auroral buddies.

Follow the arc from fiery Mars in the south through Spica to find Saturn. Keep going all the way to Antares. All four are magnitude 1 or brighter. The map shows the sky around 10:30 p.m. local time facing south. Stellarium