Monthly Archives: September 2010

Space travel on the cheap

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Just for fun, I zoomed in on Jupiter during this 30-second time exposure taken last night. Nothing like traveling at warp speed while standing perfectly still. Details: ISO 3200, f/2.8. Photo: Bob King

With moonrise after 10 o’clock, I drove out to the country last night to take in some dark sky and silence as well as to check out how Comet Hartley 2 was doing. In a previous blog, I wrote that the comet would become a binocular object by this time. Although fuzzy and still without a tail, I can report that the comet appears as a softly-glowing puff through 10×50 binoculars among the stars of Cassiopeia. Tomorrow I’ll provide a chart you can use to find it. It’s hard to say if the comet would be visible from bright suburban skies, but from rural areas I was surprised at how easy it was to see.

Venus will be low, really low, in the southwest in the coming weeks. This is exactly how it looked yesterday at 7:15. I almost had to stand on tiptoes. Photo: Bob King

10 minutes after viewing Hartley in the scope, clouds came rushing in, swallowing the stars in great gulps. I stayed a while longer looking through the available holes and taking pictures. Earlier in the evening, I stopped along a road with a view to the west and hoped to find Venus. The time was 7:15 or 25 minutes after sunset. I thought it would be a snap to see, but the planet was so low in the southwest I couldn’t believe it. If you should try the same, go out 15 minutes after sunset and look far to the left or south of the sun to spot it.

Venus is quickly becoming a thin crescent as seen in a telescope or pair of 10x binoculars. It’s also moving back toward the sun’s direction and setting earlier. By the end of October the planet will disappear from evening twilight and move into the morning sky. We’ll have to wait until mid-November before Venus once again becomes easy to view, this time at dawn.

The last quarter moon, framed by maple leaves, was plain to see against the deep blue sky this morning in Duluth. Photo: Bob King

Goldilocks might find new planet ‘just right’

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This artist's conception shows the inner four planets of the Gliese 581 system and their host star. The large planet in the foreground is the newly discovered Gliese 581 g, which has a 37-day orbit right in the middle of the star's habitable zone and is only three to four times the mass of Earth, with a diameter 1.2 to 1.4 times that of Earth. Credit: Lynette Cook

Better watch out for the three bears, Goldilocks. A team of exoplanet hunters from University of California-Santa Cruz, and the Carnegie Institution of Washington announced their exciting discovery today of a potentially habitable planet around the star Gliese (GLEE-zeh) 581 in the constellation Libra the Scales. They used a high resolution spectrometer on the Keck I telescope in Hawaii over a period of 11 years to dig out the subtle gravitational tugs the new planet, named Gliese 581 g, exerted on its host star. After detailed analysis, the scientists determined the planet has a mass three to four times that of Earth and orbits within the “goldilocks” or habitable zone over a period of 37 days. Habitable in this case means it’s at the right distance from its sun for liquid water to exist on its surface. It’s also massive enough to hold onto an atmosphere. Both are key ingredients for life.

“Our findings offer a very compelling case for a potentially habitable planet,” said Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz and co-leader of the Lick-Carnegie Exoplanet Survey. “The fact that we were able to detect this planet so quickly and so nearby tells us that planets like this must be really common.”

The planetary orbits of the Gliese 581 system compared to those of our own solar system. The star is orbited by six planets labeled from b through g. G orbits fourth from the star. Image Credit: National Science Foundation.

The sun (left) and star Gliese 581. Credit: R.J. Hall/Kevin Heider

Gliese 581 is a red dwarf star with only 1/3 the mass of the sun and a much cooler surface temperature — 5300 degrees F vs. 11,000. Red dwarfs are ideal candidates for searching for extrasolar planets because with cooler temperatures, their habitable zones lie closer to the star. A planet orbiting close to a small sun exerts a stronger, more easily detectable tug than one orbiting within the more distant habitable zone of a massive, hotter star.

The planet’s gravitational pull causes the star to wobble a bit back and forth, and that’s what astronomers detect with sophisticated equipment. Gliese 581 g orbits just 14 million miles from Gliese 581, more than six times closer than Earth to the sun. Its close proximity has slowed the planet’s rotation so only one side faces the sun, while the other is in shadow. The team estimates that temperatures range from 160 above on the sunny side to 25 below on the back. In the twilight zone, along day-night border, it would be comfortable enough to stroll about without a coat.

Gliese 581 is well known among the exoplanet crowd – four previous planets have been discovered around it. The Lick-Carnegie team added two more, making it the most exoplanet-rich star of any thus far discovered. The total of extrasolar planets currently stands at 490 planets. Amazing!

Read more about the new discovery HERE.

Space out with the ISS

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Expedition 25 Commander Doug Wheelock (top) and Flight Engineer Shannon Walker perform maintenance on the International Space Station's treadmill this week. Credit: NASA TV

The International Space Station (ISS) is again buzzing the sky at dawn, and sunrise is late enough for many of us to see it. Some passes happen around 6:30 a.m., when commuters stuck in traffic on their way to work might gaze longingly at the craft from their windows, wishing they had their own express lane like the astronauts above.

There are only three crew members aboard the station at the moment, but three more will board the Russian new Soyuz TMA-01M transport craft next Thursday, October 7 and rocket up to join them. Right now the crew is performing maintenance on the treadmill and water system. It’s one of several exercise devices the astronauts use as part of a daily exercise regimen to reduce the loss of bone density and muscle mass that typically occurs during long-duration spaceflight. Three of their crew mates departed the station in the older Soyuz TMA-18 transport this past Saturday and landed safely in the steppes of Kazahkstan. Take a look at the video. I found it interesting that the tracking crew had to roll the capsule like a big snowball before pulling the astronauts through the hatch.

The Soyuz TMA-18 spacecraft is seen as it lands near the town of Arkalyk, Kazakhstan on Saturday, Sept. 25, 2010. Russian Cosmonauts Skvortsov and Kornienko and NASA Astronaut Caldwell Dyson, are returning from six months onboard the International Space Station. Credit: NASA/Bill Ingalls

The dawn space station passes will all be in the southern and southeastern sky below the constellation of Orion and not far from the brilliant, twinkling star Sirius. The Central Daylight times shown are for Duluth and region. For times for your town, please click HERE and put in your zip code or head over to Heavens Above.

If you’ve never seen the ISS before, it’s looks like a brilliant star on the move. At best, it nearly equals Venus in brightness. A typical pass takes about four to five minutes as the craft travels from southwest to southeast.

* Tomorrow morning Sept. 30 starting at 6:39 a.m.
* Saturday Oct. 2 at 5:58 a.m.
* Sunday Oct. 3 at 6:24 a.m. Brilliant pass across the top of the sky. The best!
* Monday Oct. 4 at 5:19 a.m.
* Tuesday Oct. 5 at 5:45 a.m. Another spectacular high pass


The undocking from the space station and landing of the Soyuz transport vehicle in 2008

Date with a dolphin

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A bottlenose dolphin crests with a wave. Credit: NASA

Dolphins are beloved by humans for their intelligence and sense of play. Thanks to the ancient Greek seafarers, who were also well acquainted with dolphins, one splashes across the night sky throughout the summer and fall months. Delphinus (del-PHY-nis) was placed in the sky by the sea god Poseidon out of gratitude for helping him woo his future wife Amphitrite. The dolphin’s friendly ways coaxed her back to the god after his rough advances had sent her running.

Find Delphinus the next clear night. This map shows it, the Summer Triangle and the hazy band of the Milky Way as you face south around 8:30 in the evening. Altair is about halfway between the overhead point and the southern horizon. Charts created with Stellarium

Delphinus is ideally placed due south at nightfall in late September and early October, and you can use the Summer Triangle to point you to it. While small, with not a single bright star to its name, its form is so distinctive, the constellation stands out better than many brighter ones.

Go out around 8:30 and look high in the south to find the three bright stars that form the Summer Triangle. Deneb and Vega will both be nearly overhead, while Altair is further south and easier on the neck. Now reach a clenched, horizontally-held fist out to the sky and put Altair on the right side. On the left side of your fist and a little above, you should be able to spot the five equally bright stars of Delphinus. It’s such a cool little constellation. Not only does its shape resemble a dolphin, it also appears to be leaping out of the frothy Milky Way nearby.

The diminuitive stellar dolphin Delphinus and its two (in)famous stars.

Take a look at the names of Delphinus’ Alpha and Beta stars, which mark the top and west side of the constellation. Since many star names are Arabic in origin, they might look as ancient and convincing to western eyes as Deneb or Betelgeuse.

These two are different though. In what must be one of the few cases of stars being named after an individual and recognized by the professional astronomy community, Sualocin and Rotanev are Nicolaus and Venator spelled backwards. They were named after Nicolaus Venator, the Latinized name of the assistant director of the Palermo Observatory back in the early 1800s. Venator worked with Giuseppe Piazzi, who discovered the very first asteroid Ceres in 1801. No one’s sure whether it was Piazzi or Venator that inserted the name in a star catalog at the time, but somehow they survived to the present day. You’ll see those names on star charts and sky-charting software used round the world. Pretty sneaky, I say.

In our time, various star-naming services have popped up that for a fee will name a star in the sky after whomever you like. You pay your money, give them a name like Yram Senoj – or Mary Jones if you prefer – and you’re assigned a dim star that’s only visible in a telescope. The names are published in a book and locked in a vault for safekeeping. There’s one downside: no one outside of you and the person you named the star after will recognize your claim. It would be like me going to the Rocky Mountains and assigning names (for a fee) to all the minor, unnamed foothills. I don’t think I’d have much luck getting the locals, not to mention mapmakers, to abide by my naming system.

When people ask me about getting a star named after a loved one, I tell them to pick one that’s nice and bright. Before you pick, you may even want to look up some facts about the star to see if something about its character recalls a personality trait.  Additional layers of meaning and purpose can be added to any star any time we choose. Cost to you: nothing.

Moon and Pleiades a sweet sight in binoculars tonight

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The waning gibbous moon lines up closely with the Seven Sisters star cluster tonight. The bright star Capella in the constellation of Auriga the Charioteer is about four outstretched fists to the left of the moon. Using Capella and the moon, try to find the stars Mirfak and Algol in Perseus. Later this week we'll revisit them. Created with Stellarium

Tonight’s moon comes up in the northeastern sky in Taurus the Bull. Since it’s five days past full phase, its light has diminished enough so that sky watchers in rural areas can again see the Milky Way slicing across the southern sky. We’re in for a treat because the moon will be very close to the Pleiades or Seven Sisters star cluster. It rises relatively early around 8:45 p.m., but the best time to see this fine pairing is at 10 p.m. or later, when they’ve cleared trees and buildings. While not a great naked eye event, due to the lunar glare, the view in binoculars will be outstanding. Both will fit easily in the same field of view and stand in striking contrast to one another.

As the moon revolves around the Earth, the angle it makes between us and the sun widens. This exposes more of the moon to sunlight and its phase fills out. After full, the angle narrows and the phases play out in reverse. Credit: NASA

This past Saturday night my older daughter had to determine the moon’s phase for a homework assignment. She hesitated but then got the correct answer: waning gibbous.

The moon fills out from the time we first see it as a thin crescent in evening twilight up until full moon, when the side facing us is 100% illuminated by the sun. The line separating the daylight from the nighttime portion of the moon is called the terminator. Night after night, the terminator moves to the left or east exposing more of the moon. Picture the terminator as the line of advancing sunrise. Features that were previously in darkness rotate into the sunshine as the angle between the moon, Earth and sun widens with each passing night. Unlike Earth, where the nights last a matter of hours, a typical lunar night lasts almost two weeks. That’s because it takes the moon almost a month (27.3 days) to rotate once on its axis. A night on the moon is a very chilly experience. With no atmosphere to speak of and plenty of darkness to go around, temperatures plummet to an average of 250 below Fahrenheit.

When waxing toward full moon, the terminator or sunrise line advances to the left and exposes fresh moonscapes. After full moon, the terminator becomes the advancing line of sunset. It once again moves to the left (east) night after night until nothing's left but a thin crescent at dawn. Photos: Bob King

When the moon is full, it’s directly opposite the sun and all lit up the same way someone might shine a big, bright light directly into your face. The terminator disappears briefly at full but soon reappears along the moon’s western edge the day after.

Have you noticed the moon looks a little out of round the past couple nights? The western edge is missing because the sun is now setting there after the long lunar day. Slice by slice, the terminator, which is now the advancing line of lunar sunset, gobbles up the moon, converting it from full to waning gibbous and then to a half. The half or 3rd quarter moon refers to the moon having completed 3/4 of its orbit around the Earth since new moon.

The moon's phase depends on its position in relation to the sun and Earth. This diagram looks down on Earth from north. Sunlight is coming in from the right, as indicated by the yellow arrows. Over a month, we see the moon cycle from new moon to full and back to new again. Times shown are when the moon is due south and highest in the sky. Credit: Minesweeper

Several nights later, the moon’s phase will have narrowed to a thin crescent, and by October 7 – new moon – we’ll see no moon at all. It will lie almost directly between us and the sun in the daylight sky. The side facing the sun will be fully illuminated, but the side facing Earth will be in complete shadow. A day later the cycle begins anew with a fresh and thin crescent low in the west at dusk.

In darkness you may find light

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This false-color composite image, constructed from data obtained by NASA's Cassini spacecraft in the near-infrared light, shows the glow of auroras streaking out about 600 miles from the cloud tops of Saturn's south polar region. More information below. Click the photo for a wallpaper version. Credit: NASA/JPL/U. of Arizona/U. of Leicester

I had a craving for General Tso’s chicken last night and stopped at the Beijing Restaurant to pick up an order. As I was writing the check, the fellow behind the counter remarked about the early darkness. “It’s only 8, and, look, it’s already dark outside,” he said. And he was right. With the sun setting now around 7 and twilight over by 8:30, access to the night sky has expanded into the prime TV viewing hours. Looking for an excuse to step away from the TV? Hint, hint.

Earlier nights mean young children can go out for a look at the stars and planets before bedtime. If you haven’t already, consider taking your child for a walk at night. If the moon is out, it provides a friendly glow to light the way. Even if you’re only familiar with the Big Dipper or Orion, there’s much joy in sharing thoughts and speculations about the stars with children. They’ll have dozens of questions of course, each one an opportunity to share knowledge or honestly admit ignorance. I used to love it when my daughters saw or heard something that scared them. They’d give a little shout and then cling to my side for a minute or two. If that doesn’t make you feel like a dad, I don’t know what does.

When children grow older and become teens, walks under the stars are a chance for both you and them to share what you did that day or talk over school, boy problems and all the rest. Darkness provides a cloak of anonymity that encourages kids who are reticent to feel more at ease. And there’s nothing like the whole universe over your head to give you the space you need to wrangle through an issue. Since they’re nearly all grown up and have plans of their own, night walks with my girls are becoming fewer and fewer, but I remain hopeful. I always ask if they’d like to join me, and still occasionally get a yes.


This movie, made from data obtained by NASA’s Cassini spacecraft, shows Saturn’s southern aurora shimmering over approximately 20 hours as the planet rotates.

NASA released a new set of photos and a video this week of auroras in the south polar atmosphere of Saturn. Like Earth, Saturn’s magnetic field channels particles from the solar wind toward the planet’s north and south poles where they collide with atoms in the tenuous upper atmosphere. When the atoms settle back down, each emits a brief flash of light. Multiply those flashes by billions of atoms and you get great glowing curtains and rays of dancing light in the Saturnian sky. Unlike Earth, Saturn’s auroras can also get started when one of its moons moves through the dilute electron and proton soup in Saturn’s magnetic field, setting off rippling waves of energy that stimulate an auroral storm in the atmosphere below.

The green color in the video and photo is light from hydrogen atoms photographed in near-infrared light or light that’s just beyond the reddest thing we can see with our eyes. Even though invisible, we sense it as heat. The heat from your oven burner is a powerful infrared emitter. The rings are blue because they reflect sunlight at a particular infrared wavelength, while the globe is red because of heat rising from deeper within the atmosphere. Those dark features striping the globe are clouds and storms seen in silhouette against the heat energy coming from below. “Fascinating,” as Star Trek’s Spock might say.

Planets line up, world doesn’t end

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The sun and the three largest planets line up with Earth late this month. Orbits are simplified to circles and sizes are not to scale. All the planets revolve counterclockwise around the sun as viewed from above the north pole of the Earth. Illustration: Bob King

We’ve talked over the past week about Jupiter and Uranus being near one another in the sky, and how that makes this a good time to find the more remote and fainter planet with binoculars. As it turns out, those aren’t the only two planets lining up this month. Naturally, you have to add the Earth and sun, but there’s also Saturn. The ringed planet will be in conjunction on September 30 when it will be in the same line of sight as the sun just the way Uranus is in the same line of sight as Jupiter – with one difference. Saturn is on the other side of the sun from the Earth.

If you could zoom high above the plane of the solar system right now and look down over the planetary scene, you’d see that Saturn, the sun, Earth, Jupiter and Uranus are in an approximately straight line. Mars, Mercury, Venus and Neptune are either not quite in alignment or well off to one side, so I’ve excluded them from the illustration.

You might wonder if having the four largest bodies in the solar system lining up on either side of Earth would give our modest planet a hard time. All that pushing and shoving, you know. While the gravitational field of a planet, star or what have you, extends across light years, its strength is inversely proportional to the square of the distance from the body’s center. This bit of mathematical talk is called the inverse square law, and what it means is that when you double your distance from the Earth’s surface for example,  gravity drops to a quarter of its strength. By the time you’re 100 times farther away from planet center, the Earth’s gravitational pull is 1/10,000 of what you felt on the surface. In other words, gravity, while felt strongly nearby, rapidly becomes weaker with distance.

All the planets “feel” each other’s gravitational pull, but the distances between them are so vast, they produce barely measurable effects here on Earth. The exception is the sun which contains 99.85% of all the matter in the solar system. That’s one of the main reasons we and the rest of the planets circle it rather than say, Mercury or Neptune. Not to suggest the planets are gravitational wimps. 63 moons obey Jupiter’s gravitational domain and if a comet should pass too near the planet, its powerful gravity will change the comet’s orbit or even knock it completely out of the solar system. Closer to home, the smaller moon circles the larger, more gravity-rich Earth every month.

Large planet lineups occur now and again over our lifetimes, and while some might have you believe they can wreak havoc here on Earth, their pull is too puny to make a real difference. They do however make for pretty alignments in the sky and wonderful “wow!” moments when a guy or a girl looks up.

Curious dark rock on Mars may be visitor from the asteroid belt

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NASA's Mars Exploration Rover Opportunity used its panoramic camera to capture this view of a dark rock that may be an iron meteorite. Credit: NASA/JPL-Caltech/Cornell University

Last Thursday, the Opportunity Rover on Mars took a photo of the landscape on its way to Endeavor Crater and turned up something interesting. Mission controllers spotted a curious rock about 18 inches across some 102 feet in the distance which they suspected to be a meteorite. “”The dark color, rounded texture and the way it is perched on the surface all make it look like an iron meteorite,” said science-team member Matt Golombek of NASA’s Jet Propulsion Laboratory, Pasadena, Calif.

A color closeup of the Red Island rock, a likely iron meteorite. Credit: NASA/JPL-Caltech/Cornell University

The rock has been given the informal name of “Oileán Ruaidh” (ay-lan ruah) or Red Island, the Gaelic name of an island off the northwestern coast of Ireland. It’s about the size of a crockpot. Since its discovery, scientists have directed the rover into position for a closer look.

Opportunity has found four iron meteorites during its 14.5 miles of travel since arriving on Mars in early 2004. Judging by appearance alone, this one is likely the 5th. Look at the dimpled texture and the torn edges along the deep fissure on the left side – all classic textures found in many meteorites but irons in particular. Yes, I think I would like to add this one to my collection. Which would be rarer I wonder – a meteorite found on Mars or a piece of Mars itself? Because the planet is closer to the asteroid belt than Earth, and the belt is where many of the meteorites on Earth come from, Mars gets pelted even more often by space rocks than we do. Combine that with an atmosphere nearly devoid of oxygen and water vapor and lots of open, desert-like landscape, and you’ve got the perfect place in the solar system to hunt for meteorites.

Hunks of harder petrified wood lie perched at the top of softer clays and mudstone in the South Unit of the North Dakota Badlands. The wood is eroding too, but not as fast as the clay. Photo: Bob King

If scientists find iron and nickel in the rock with the rover’s x-ray spectrometer – telltale elements in many meteorites – they’ll clinch it.  There’s something else curious about this rock. Red Island is sitting on the surface just as if someone picked it up and set it there. No impact mark, no hole. My hunch is that it fell so long ago that the erosive forces on the planet have completely stripped away the underlying sand and rock, leaving it all to itself. A hunk of iron would hold up much better to erosion by wind than softer rock. It’s also possible it was moved from its original location by a flash flood a couple billion years ago when Mars is postulated to have been a much wetter planet. All speculation I know, but check out what similar erosion processes have done in the North Dakota Badlands. I was there this summer and hiked into a small canyon along the Paddington Creek trail. There, eerie pedestals of clay held chunks of petrified trees 10 feet above the ground. The softer clay eroded much faster than the trees and rocks, leaving them perched in the air. The tougher wood and rocks also help shield the clay immediately beneath. The two processes worked in tandem to create the strange pillars. Eventually the clay will melt away entirely, and petrified wood and harder rocks will lie scattered about the ground. An analog to our possible meteorite?

A 3-D picture of the new potential Red Island meteorite on Mars. Credit: NASA/JPL-Caltech/Cornell University. 3-D by Stuart Atkinson

To learn more about the new potential meteorite discovery, I encourage you to visit the Mars Rovers website and follow the excellent Road to Endeavour blog by Stu Atkinson. Stu has lots more photos, including several 3D versions, one of which I’ve posted. All you need is a pair of those red-blue glasses, and you’ll feel like your standing right next to Opportunity.

Don’t miss Jupiter and the Harvest Moon tonight

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Jupiter appears briefly in a crack in the clouds last night to the left of the overexposed moon. The bright colored disk around the moon is a corona. Coronas form when light from the sun or moon is diffracted by minute water droplets inside clouds. Photo: Bob King

Have you been watching the moon’s nightly progress toward Jupiter this week? Well, tonight’s the big night. We’ll have a full Harvest Moon and it will sit right atop Jupiter. The night’s two brightest characters join together for a wonderful naked eye sight all night long.

Although the actual moment of full moon occurs Thursday morning, it will be fullest during evening hours tonight. The full moon nearest the autumnal equinox is called the Harvest Moon. Because the angle of the full moon’s path to the horizon is very shallow in September and October, the time difference between successive moonrises is only about 20 minutes instead of the usual hour. That means the moon comes up not long after sunset several nights in a row. No doubt this helped farmers harvest their crops into the night back in the days before electricity. Early moonrises at full moon guaranteed nearly continuous light from sunset to sunrise.

Jupiter and the moon will be neighbors in the sky tonight, shining even brighter than Hollywood celebrities. It's also the first night of fall and the full Harvest Moon - an astronomical triple whammy! Maps created with Stellarium

The moon, planets and sun all travel along the ecliptic, an invisible circle in the sky that defines the plane of the solar system. During early fall, the ecliptic is tipped at a narrow angle to the eastern horizon. As the moon scoots along the ecliptic at the rate of about one outstretched fist to the east each day, it rises later each night … but not by much. If you’re a lover of moonlight like the old-time farmers, or just enjoy more light for an evening walk, you’ll appreciate the difference an angle can make.

The angle the full moon makes to the eastern horizon around the time of the fall equinox is shallow, making the difference in time between successive moonrises only 20 minutes or so.

The easiest way to understand the rising time difference is to compare the full moon’s path in fall versus that of spring. The moon is only a little lower below the horizon each night around the fall equinox, so only a short time has to pass between successive moonrises. In spring, the ecliptic cuts the horizon at such a sharp angle, even though the full moon still moves just a fist eastward night to night, it has to “climb” a long way up to the horizon between successive moonrises.

Here's the moon's path along the ecliptic in spring, when the angle it makes to the horizon is very steep. The times between successive moonrises are at their maximum.

The difference in rise times is amazing. For Duluth, Minn., it’s 19 minutes on the first day of fall and one hour and 20 minutes around the first day of spring. You’ll see for yourself what the farmer looked forward to in times past if good weather prevails in the coming nights. Happy moonbeam harvesting!

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.