New comet C/2012 S1 (ISON) could be spectacular sight in fall 2013

Comet C/2012 S1 ISON was discovered on Sept. 21 in pictures taken with 15.7-inch reflecting telescope of the International Scientific Optical Network (ISON) in Russia. This photo was taken on Sept. 22. Credit: Ernesto Guido, Giovanni Sostero and Nick Howes

A new comet was discovered inching across Cancer the Crab in the morning sky may knock your socks off next fall. Based on a preliminary orbit, it could become a very bright object beginning in November 2013 for both northern and southern hemisphere sky watchers. C/2012 S1 (ISON), its formal name, was found by Russian amateurs Vitali Nevski and Artyom Novichonok of the International Scientific Optical Network (ISON), a network of observers who track man-made space debris.

When first photographed on Sept. 21 the comet was an incredibly faint 18.8 magnitude and appeared much like a star with only a “whiff” of a coma around its icy nucleus. It will slowly cruise through the constellation Gemini for many months while growing gradually brighter. And I do mean gradually. Even as late as next June, ISON will only shine at 14th magnitude; that’s scraping the bottom of the bucket for most amateur telescopes.

The situation improves next September when 8-10 inch scopes should pick it up as a small blob in Cancer around 11.5 magnitude. From late October through late-November, things get much cheerier with C/2012 S1 brightening sharply from 7th to 1st magnitude in the eastern sky at dawn. I’m sure I’ll be setting my alarm for a look Thanksgiving morning.

The sharply curving path of Comet C/2012 S1 (ISON) shown from mid-November through mid-December 2013. The comet will be only 1.1 million miles from the sun on Nov. 29. That’s only a little more one sun-diameter. Maps created with Chris Marriott’s SkyMap software

When closest to the sun at perihelion on Nov. 29 it’s predicted to shine a spectacular -7 magnitude or almost 10 times brighter than Venus. Before you say WOW, you need to know the comet will lie only 4.4 degrees north of the sun (very close!) on that date and probably be hidden in the solar glare. Then again, we might spy it in daylight through binoculars by taking proper precautions to keep the sun out of the field of view. Some of us saw the last daylight comet C/2006 P1 McNaught in January 2007 this way.

The comet will be visible low in the southwest just after sunset in bright twilight with an upward pointing tail. The numbers are its brightness or magnitude. For reference, Venus is -4.4, Vega is 0 and the Big Dipper stars are +2 or second magnitude.

A second round of excellent visibility commences immediately after perihelion as S1/ISON performs a  hairpin turn around the sun and banks north into Ophiuchus and Hercules in early December.

While the comet fades during this time, it’s likely to have a spectacular tail and be as bright as magnitude -4. Both hemispheres will get great views, with the northern favored as Christmas approaches. Indeed, northerners will see it at both dusk and dawn. The comet will pass nearest Earth at a distance of about 37 million miles in January 2014. On the 8th it will appear only 2 degrees from the North Star.

That makes two potentially bright comets in 2013 – the other is C/2011 L4 PANSTARRS. It’s forecast to shine as brightly as Vega next March. While I like what I see, it’s important to remember that comets don’t always perform as expected. Any prediction of a comet’s brightness is subject to change, sometimes radically. I’ve seen a few wax much brighter than expected, while others have gone nowhere but downhill. More needs to be known about C/2012 S1’s orbit before an accurate forecast can be made. Let’s just say things look very promising for now.

The Great Comet of 1680 over Rotterdam painted by Lieve Verschuier. Notice the lack of city lights. Some of the people are using cross-staffs to measure the comet’s altitude and tail length.

One other interesting tidbit about C/2012 S1 (ISON) is that its orbit appears very similar to the Great Comet of 1680 also known as Kirch’s or Newton’s Comet. The two may even be related. Kirch’s comet was discovered on November 14, 1680 by German astronomer Gottfried Kirch.  After passing extremely close to the sun, it brightened so much it was plainly visible to the naked eye in mid-afternoon in early December.

One eyewitness report described it as having a “very fiery tail” that stretched 70 degrees long or more than 2/3 the way from horizon to zenith. Newton was working on his great treatise “Principia” at the time and used the comet’s motions to test the predictions of his theory of gravity.

Will C/2012 S1 (ISON) become a Great Comet, too? I’ll look into my crystal ball when more data becomes available.

Morning mystery glow announces arrival of zodiacal light

The zodiacal light (left) points upward from the eastern horizon and “touches” the Milky Way (right) Sunday morning. The bright object is the planet Venus. Details: 16mm lens at f/2.8, ISO 800 and 2-minute exposure on a tracking mount. Photo: Bob King

When fall comes around I know it’s time to get up at dawn to see the glory of the zodiacal light. It may not be easy to set the alarm for 4:30 a.m. and wander out into 32-degree cold, but you take it a step at a time. Pull on your pants, zip up jacket, pack camera in car and drive to a dark location with an open view to the east.

I’ve never been able to shake the feeling of sneaking around or breaking the rules when I get up in the early morning hours. As a teenager I remember tiptoeing from the bedroom to the front door trying hard to avoid the “creaky spots” in the floor. The last thing you wanted to do was wake up dad.

Illustration showing the tapering wedge of zodiacal light extending upward from the eastern horizon to the Milky Way around 5 a.m. in late September. The Belt stars of Orion are at right. The light is centered on the ecliptic, the path the sun follows through the zodiac during the year. Created with Stellarium

Seen from a dark sky the zodiacal light is large wedge of ghostly light tilting up from the eastern horizon, reaching past the brilliant planet Venus all the way to the band of the winter Milky Way where it touches Orion’s shoulder. The bottom end is brighter and wider than the tapering tip. Many people are surprised when they see the light cone for the first time – it’s much bigger than they think. Brightness-wise the wedge looks like dawn itself at its base, but the top glows only as brightly as the fainter parts of the Milky Way.

The zodiacal light is so called because it’s centered on the zodiac, that ring of 12 constellations defined by the sun’s apparent path through the sky during the year. Right now the sun shines in Virgo; in December it will have moved to Sagittarius and next summer will ascend into Taurus.

Twice a year the zodiacal light makes a good show from mid-northern latitudes when it’s tilted up high at dusk in spring and at dawn in fall. This week it angles up through Leo, Cancer and Gemini.  Just like the sun, the angle of the zodiacal wedge to the horizon varies during the year. When tipped at a low angle, it’s obscured by thick air and haze. When tipped up high, you can’t miss it from a dark sky.

Comets like 168P/Hergenrother, currently visible in the fall evening sky, are the source of most of the material that creates the zodiacal light dust cloud. Credit: Michael Jaeger

The wonder of the zodiacal light is that it’s made of billions of dust particles shed by countless comets orbiting approximately in the plane of the solar system between Jupiter and the sun.

Heat from the sun vaporizes comet ices which are gummed up with dust and small rocks. Some of that liberated dust strikes Earth’s atmosphere, burning up as occasional random meteors we see on any night of the year. Much of it settles into a vast, rarefied cloud in the plane of the solar system where it’s illuminated by the sun like pollen shaken from a pine tree. Dust closer to the sun reflects its light more brightly; dust farther away less so. That’s why the zodiacal light is brighter at its base – which is closer to the rising sun – than at its tip.

Much comet dust slowly spirals into the sun over time. To keep a steady supply available, comets ancient and new have contributed the dribs and drabs that make the zodiacal light an arresting sight. To see it best, find a dark sky location with a great view to the east and start looking about 2 hours before sunrise. For my town, that’s between 5 and 5:30 a.m. 90 minutes before sunrise, you’ll still make out the glowing light but also notice dawn gaining ground.

Sunday morning was special. The cone of comet debris stretched all the way to the blizzard of stars comprising the band of the Milky Way. Comet dust, like all dust, is a gift of the stars. Seeing the eerie light literally reach for the stars hit me like a cosmic version of a baby touching her mother’s face.

Approximate view (not to scale) showing the dust cloud from comets that extends across the inner solar system. Lit by the sun, we see it as the zodiacal light. Illustration: Bob King

Tomorrow through Thursday morning are ideal times to see it for yourself. By Friday the moon will be up at dawn and spoil the view. The next moonless period begins on Oct. 13 and continues through the end of the month. Mark your calendar for an early morning adventure.

Lifestyles of the hydrogen-rich and erratically famous

A huge glacial erratic versus a small human on the Superior Hiking Trail this week. Photo: Bob King

Wait a minute. What is this thing? Asteroid, alien spacecraft, rock of doom? No, it’s only a 20-foot-high glacial erratic astride the Superior Hiking Trail 5 miles east of Finland, Minn. The guidebook indicated its location but I wasn’t prepared for the sight of this behemoth. Erratics are boulders plucked by glaciers and deposited miles from their source. This one , composed of the mineral anorthosite, appeared to have been dropped in the middle of nowhere, hence it’s otherworldly appearance.

A very tight closeup of lichens (center) and crystal structure (right) on a small patch of the erratic. Photo: Bob King

When seen up close up, the bland, gray exterior of the rock proved to be composed of coarse crystals. Tiny patches of tough lichens hid parts of the weathered surface.

The boulder probably parted company with the glacier 10,000 years ago. Anything around that long in one place becomes a time machine into the past. Through its dark crystal panes, we glimpse a long-gone world of mile-thick ice and numbing cold. Given enough time, life’s tiny fungal tentacles, working in tandem with nature’s freeze-thaw cycle, will reduce this titan to shards and finally soil.

The Dawn spacecraft used its gamma ray and neutron detectors to discover hydrogen hot spots on the asteroid Vesta this past year. Red indicates the greatest amount of hydrogen; gray the lowest. Credit: NASA/JPL-Caltech

Another big rock – this one in the asteroid belt – shares an even more ancient past than my erratic. Rather than rocks dropped by ice, Dawn was hit with hunks of water-rich asteroids from the asteroid belt called carbonaceous chondrites. The probe found Vesta’s equatorial zone laced with hydrogen from water chemically bound to the rocks as -OH, also called hydroxyl. Free water’s formula is OHH, described more simply as H2O.

“The source of the hydrogen within Vesta’s surface appears to be hydrated minerals delivered by carbon-rich space rocks that collided with Vesta at speeds slow enough to preserve their volatile content,” said Thomas Prettyman, the lead scientist for Dawn’s gamma ray and neutron detector (GRaND) at the Planetary Science Institute in Tucson, Ariz.

Hundreds of small pits inside Vesta’s crater Marcia may have formed when late bombardment heated earlier materials deposited by water-rich asteroids. Heated by impact, water bound in rocks escaped to create the pits. Credit: NASA/JPL-Caltech/University of Arizona/MPS/DLR/IDA/JHUAPL

If any ice itself were to survive, you’d think Vesta’s polar regions would be the best places for preservation just as on the moon. The moon’s rugged terrain and an axis tipped just 1.5 degrees to the plane of Earth’s orbit create permanently shadowed havens for ice in craters at its north and south poles. Unlike the moon, Vesta’s axis has a considerable 29-degree tilt. As it rotates and orbits the sun, both north and south polar regions are repeatedly exposed to sunlight just as they are on Earth. If ice once languished there, it’s long gone.

The Sutter’s Mill carbonaceous chondrite, which fell on April 22, 2012 in California, is dark colored like most of its class. Photo: Bob King

In fact, most of the hydrogen was found in darker-colored rocks encircling the equator. Since carbonaceous chondrites are themselves dark and water-rich compared to other meteorites, they’re a good match for what Dawn found on Vesta.

More evidence for ancient water comes from strange clusters of pits measuring about 100-800 feet across discovered in the 40-mile diameter crater Marcia. They resemble similar features on Mars that likely formed when water within the rocks vaporized explosively during an impact leaving behind pothole-shaped depressions.

Similar pits on Mars from water boiling from the surface by the heat of impact. Credit: NASA/JPL-Caltech

It’s thought that a second round of high-speed impacts accomplished the same on Vesta. Marcia’s center is pocked with pits and has very low levels of hydrogen, consistent with water boiling off into space when the crater was formed.

Water. It’s always at the center of the story when it comes to space exploration. Earth’s water is believed to have arrived the same way as Vesta’s through comet and “wet” asteroid bombardment. Much later, water would build the glacier that plucked the boulder that now reposes alongside a woodland trail. You never know what adventures may lie ahead when you go with the flow.

Uranus makes a rare pass by a star in Pisces

To get oriented, face east around 10 o’clock and find the Great Square of Pegasus. Uranus is located one “square width” below and in line with the Square’s left side and forms an isosceles triangle with Delta and Omega Piscium. Charts created with Chris Marriott’s SkyMap

Last week at a star party I aimed my telescope at Uranus for the first time this season and was pleasantly surprised to find it next door to 44 Piscium (PYE-see-um), a star of identical brightness. One by one people lined up for a look. We had fun comparing the two colors – Uranus an obvious blue against the yellow-orange of 44 – and trying to figure which was the planet and which the star. That was apparently too easy, since everyone had no trouble telling them apart.

As we zoom in a little, Uranus and 44 Piscium stand out better. Delta and Omega Piscium will guide binocular users to the pair. Uranus is magnitude 6 and just visible with the naked eye from dark rural skies. Stars are shown to magnitude 7 with the view facing around 10 this evening (22nd).

Since then, Uranus has been edging closer to the star night after night. Tonight (Sept. 22) and tomorrow they’ll be at their closest and form a striking “double star” through binoculars and telescopes. How close? Only 1.4 arc minutes  or about 1/20 the diameter of the full moon. Those with excellent skies will see the pair as a single, faint, unresolved star, while binoculars will show them as a pair of close-set “eyes” staring straight back at you. In the coming nights, the planet will slowly pull away to the west but remain near 44 through the end of the month. Finding the 7th planet and seeing it groove through the sky is a very worthwhile observing project that requires only the simplest of equipment.

Our final chart is close in, so you can use it to track Uranus’ movement to the right (west) over the coming nights. Positions are shown for 10 p.m. CDT. The blue dots show the planet on the nights of closest approach.

Uranus always appears identical to a star with the naked eye and binoculars, but a small telescope magnifying 60x or higher will not only show the color difference between star and planet but also reveal Uranus as a tiny disk. 44 Piscium will remain a flickering point of light even at high power – a fine side-by-side example of the difference in appearance between a star-like planet and a star. I wonder if the colors will be visible in binoculars? Only one way to find out. Good luck!

Happy equinox! Half-moon dabbles in the occult tonight

Yellow sugar maple leaves contrast with the bark of a paper birch yesterday. Photo: Bob King

Happy first day of fall! The sun crossed the celestial equator on its way south at 9:49 a.m. (CDT), sending a chill up the spines of those who love summer. Many of us look forward to cooler days and nights and that first frost. Here in Duluth, Minn. leaves began changing as early as the end of August. We’re now well into the color season with every hue from lemon to scarlet to brown falling to the ground.

Tonight all those colorful trees will cast shadows under a first quarter moon. First quarter refers to the moon having completed a quarter of its orbit around the Earth. This takes about 7 days. In another 7 days the half moon will fill out into a full moon. We get a special edition of the full moon this month – the Harvest Moon. I’ll have more on exactly what makes it special early next week.

Don’t pass up the chance to look at the moon through a small telescope over the next few nights.  The number of craters visible is incredible.

Illustration of the moon tonight (September 22) showing the star Mu Sagittarii hovering above the earthlit half of the moon minutes before occultation as viewed from Duluth, Minn. North is up. Created with Christ Marriott’s SkyMap software

Let’s return to tonight. The moon will occult the star Mu Sagittarii (Sa-ji-TARE-ee-eye) this evening around 8:20 p.m. (CDT). An occultation is the astronomical term to describe one celestial object covering up another.

Mu is a moderately bright star – magnitude 3.8 – and should be visible next to the moon this evening in binoculars. A telescope will show it plainly. If your sky’s free of haze you may even be able to make out the dim, earthlit half of the moon. Try putting the bright half out of the field of view to see it best.

The fun begins when the moon draws very close to the star. Minutes before the cover-up, you can see the moon move in real time as it moves in for the kill. With just seconds remaining,  Mu may seem to hover forever at the precipice, and then – PFFFT! – it’s gone. Whether you’re looking through telescope or binoculars, the star will blink out with surprising suddenness because the moon lacks an atmosphere.  If there was air up there, Mu would gradually dim and disappear. Even without special instruments, early astronomers could be certain there was little air on the moon by observing occultations.

An 8th magnitude star appears to sit on the moon’s edge moments before it’s occulted on April 21, 2007. Credit: Herbert Raab

Times for the occultation vary depending on where you live. In Duluth, Minn. it happens very close to 8:20 p.m., in Chicago 8:21 p.m. Central time, Atlanta at 9:21 p.m. Eastern time, Denver at 6:53 p.m. Mountain time and LA at 5:15 p.m. Pacific time. Depending on your latitude, Mu will disappear at a slightly different spot along the moon’s left or eastern edge.

In the northwestern U.S., the star will just miss the moon, appearing to graze its northern limb. If you’re keen to observe the event, write to me in the Comments section with the name of the nearest medium-sized or large city to your home, and I’ll send you back a time.

Is NASA hiding something? No, but the Earth is

Pictures of the sun snapped every 15 minutes by the orbiting Solar Dynamics Observatory. Data appears to be missing from the middle five frames. Credit: NASA

So what’s up with those blank squares? You’re looking at a screen capture of a page of pictures of the sun in photographed in ultraviolet light by NASA’s Solar Dynamics Observatory (SDO). The photos were shot 15 minutes apart starting Wednesday evening into Thursday morning this week.

Since SDO circles Earth in a geosynchronous orbit about 22,000 miles high, it “sees” the sun continuously both day and night from a vantage point high above Mexico and the Pacific Ocean. About 1.5 terabytes of solar data or the equivalent of half a million songs from iTunes are downloaded to antennas in White Sands, New Mexico every day. The space station, which orbits much closer to Earth, would make a poor solar observatory since Earth blocks the sun for half of every 90 minute orbit.

SDO’s eclipse season started around 1 a.m. September 6 when the observatory shot a photo of the Earth (top middle) cutting across the sun. Credit: NASA

Did I say SDO watches the sun continuously? Well, not quite. Twice a year for a period of about three weeks around the equinoxes, the Earth gets in the way of the sun from the space craft’s point of view, causing a total solar eclipse. The latest round of eclipses began on September 6 and will conclude on the 26th.

Now you know the reason for the blank frames – it’s a conspiracy by the Earth to block out the sun. The blackness is none other than the planet itself.

Normally the Earth is out of the way of the sun from SDO’s perspective but twice a year its orbit and Earth’s orientation to the sun cause Earth eclipses. Credit: NASA

Total eclipse happens every day between 1 and 2 a.m. local time (Mountain Daylight Time) when the Earth blocks the sun from SDO’s view. In similar fashion, we experience a solar eclipse on the ground when the moon covers up the sun. You can watch for pictures of the partial eclipse as Earth gets out of the way sometime next Tuesday the 25th by going to the SDO website. Follow these simple steps to find and view the images:

* Click on the Data tab and select AIA/HMI Browse Data
* Click on the Enter Start Date window, select a start date and click Done
* Click on Enter End Date and click Done
* Under Telescopes, pick the color (wavelength) sun you want
* Select Images in the display box
* Click Submit at the bottom and then browse the pictures

Not only does the Earth cross the sun from the observatory’s perspective, so does the moon (left) on occasion. The moon’s”bite” smaller and sharper. Earth’s atmosphere gives our planet a soft, diffuse edge compared to the airless moon’s. Photo at right was taken on September 6, 2012 at eclipse season start. Credit: NASA

While watching an eclipse of the sun by the Earth is one of the joys of living in the space age, there are other cool things to see from SDO’s perspective. Look at the drastic difference between the moon’s sharp outline and Earth’s fuzzy edge. Our planet “bites softly” into the sun because its substantial atmosphere grades from thick to thin, filtering the sunlight that passes through it. The moon’s a big baldy. With no air to grade and soften the light, the sun shines crisply right up to its edge.

Video of a partial eclipse of the sun by Earth. Refraction of light by Earth’s atmosphere causes the sun to bend at its edges. Credit: NASA SDO / Stanford University for HMI

We’ve seen how air can also bend or refract sunlight in strange ways, going so far as to “lift” the sun  into view when it’s still below the horizon.  You can see the same effect in a brand new way in this short video of an SDO partial eclipse. Watch the sun’s edge bend as the Earth rolls by. Compare it to a similar eclipse by the moon below.

Moon eclipsing sun via SDO 

SDO orbits about 22,000 miles above Earth, tracing out a figure-8 (called an analemma) above the Pacific and Mexico every 24 hours. Credit: NASA

SDO amazes with its spectacular pictures of the sun taken in 10 different wavelengths of light every 10 seconds; additional instruments study vibrations on the sun’s surface, magnetic fields and how much UV radiation the sun pours into space.

It’s the latest, greatest “Swiss Army knife” used by scientists to pry open the inner workings of the sun. The eclipses, while a gap in the data stream,  are a sweet bonus all their own.

Let the moon double your enjoyment tonight

The fat crescent in Scorpius tonight points the way to the pretty telescopic double star Graffias about 2 degrees away. Created with Stellarium

Tonight the thick crescent moon invites your eye to the sky. It’s in the constellation Scorpius the Scorpion not far from the beast’s red heart Antares (An-TARE-eez). Steadily held 7-10x binoculars will show crinkly textures of craters along the left side of the moon, where shadows created by low-angled sunlight snap them into view.

A small telescope will deliver views of hundreds of craters with much great clarity and definition. After Saturn, the moon remains the single most rewarding telescopic sight in the heavens.

Just to the moon’s upper right this evening you might spot the star Graffias, the constellation’s second brightest star shining at magnitude 2 1/2. Moon glare may make it a little tough to see with the naked eye, but binoculars will show it perfectly. Graffias, which means “claws” as in scorpion’s claws, is a beautiful double star for small telescopes. A magnification of 30x will easily cleave it. The companion star is magnitude 4.5 and just 14 arc seconds (close but easy to split) to the northeast. Every summer I make sure to stop by and admire this gemmy pair. With the moon nearby, it’ll be especially easy to find tonight.

How Graffias looks in a small telescope.  South is up. Credit: J. S. Schlimmer /

The stars of Graffias, also called Beta Scorpii, are hot, white suns nearly 10 times as massive as our own and some 400 light years from Earth. The pair also has a few secrets it keeps well hidden from amateur telescopes. The brighter of the two (the one visible with the naked eye) has two very close companion stars, while the fainter companion is also a tight triple star, making a total of six stars in all.

I hope you have clear skies tonight so you can have a look for yourself. Start close with the moon and then zoom to distant Graffias.

Curiosity to reach out and touch a rock named Jake

The drive by NASA’s Mars rover Curiosity during the mission’s 43rd Martian day, or sol, (Sept. 19, 2012) ended with this rock about 8 feet in front of the rover. The rover team consider it a suitable target for the first use of rover’s contact instruments on a rock. Credit: NASA/JPL-Caltech

Meet Jake. He’s about 10 inches tall, 16 inches wide and calls Mars home. Jake’s a rock named for the late Jacob Matijevic (mah-TEE-uh-vik), who was the surface operations systems chief engineer for the Mars Science Laboratory Project and the Curiosity rover.  Matijevic passed away on August 20 at age 64 just two weeks after Curiosity landed on the Red Planet. He was also a leading engineer the previous NASA Mars rovers Sojourner (1997), Spirit and Opportunity (2004).

the Alpha Particle X-Ray Spectrometer (APXS) on NASA’s Curiosity rover, with the Martian landscape in the background. Credit: NASA/JPL-Caltech/MSSS

The rover spotted the unusual rock during the drive to the Glenelg site. Curiosity’s parked about 8 feet from Jake and ready to “touch” it with the Alpha Particle X-Ray Spectrometer (APXS) mounted on its robotic arm.

APXS will be placed in contact with the rock and bombard it X-rays and alpha particles. Alpha particles are the nuclei of a helium atoms made of two protons and two neutrons.

When they interact with atoms in the rock, the atoms give off X-rays of specific energies depending on their type. APXS’s spectrometer analyzes the emissions to identify the composition of the rock’s minerals. The rover’s microscopic imager will also shoot a series of closeup portraits.

Schist boulder pitted by sand blast near Palm Springs Station. Credit: Walter Curran Mendenhall

Take a look at that gorgeous rock. Besides it’s interesting pyramidal shape, the erosion on the near side reminds me of similar rocks on Earth (also identified previously on Mars) that have been pitted, grooved and polished by blowing sand or ice. They’re called ventifacts and found in deserts where a lack of vegetation allows winds to drill away at the rock. Mars has sand and wind enough, so I wouldn’t be surprised if they’re responsible for Jake’s fluted front side.

Remember the picture of Phobos transiting the sun Curiosity snapped on September 13? Here’s a short video at a higher resolution.

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.

Mambo borealis – a little bit of green in my life

The Big Dipper reposes above a low arc of northern lights at 1:30 this morning. While prospects for the aurora are small tonight and tomorrow, activity is expected to pick up again Thursday the 20th. Click photo for a tuneful treat. Photo: Bob King

The north smoldered with a familiar glow last night. A restrained aurora lit up the bottom five degrees of sky a pale lime. Hidden to the eye but revealed in a time exposure, a band of pink haze arched above the visible display.

Space weather forecasters call for a pickup in auroral activity later this week on or about Thursday when gusty solar winds from coronal holes arrive. Let’s hope so. It’s been too quiet around here.

Material from a coronal hole (outlined) streams outward across the solar system. When aimed toward Earth, it can buffet our magnetic field and cause auroras. At right is a graph showing the number of geomagnetic storms (auroras) from 1875 to 1927 compiled by NASA solar physicist David Hathaway. There are clear peaks in March and September-October. Credit: NASA

Fall and spring are usually the best times for northern lights. The orientation of Earth’s magnetic field to the blobs of magnetized material streaming from the sun make it more likely the two will connect. When they do,  solar electrons and protons stream straight into our planet’s magnetic bubble and spark auroras in the upper atmosphere. I’m keeping my fingers crossed. Read more about seasonal auroras HERE.

This map shows the sky 20 minutes after sunset for the northern U.S. Binoculars will show Mars and possibly Saturn. Created with Stellarium

Tonight the 3-day-old crescent moon should be easily visible in the southwestern sky shortly after sunset. If you’d like a fun challenge, see if you can find Mars and Saturn nearby. I doubt most of us will spy them with the naked eye, but binoculars should do the job. Mars will be easier because it’s higher up in a darker sky. Saturn? Could be tricky.

If you’re an early morning person, you can catch plenty of passes of the International Space Station (ISS) this week. The times below are for the Duluth, Minn. region. For times for your town, type in your zip code on Spaceweather’s Satellite Flybys page or log in to Heavens Above. The ISS travels from west to east and looks like a brilliant yellow star.

* Wednesday Sept. 19 starting at 5:13 a.m. Brilliant high pass across the top of Orion
* Thursday Sept. 20 at 6 a.m. Another bright one but this time across the northern sky
* Friday Sept. 21 at 5:13 a.m. Appears out of Earth’s shadow high in the west and then crosses the top of the sky
* Saturday Sept. 22 (First day of fall) at 6 a.m. in the northern sky
* Sunday Sept. 23 at 5:13 a.m. in the northern sky