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

With a tip of its rings, Saturn greets Earth on opposition day

The north face of the rings are tipped nearly wide open toward Earth this year, making for wonderful views of the planet through a small telescope. Notice that Saturn’s south polar region barely pokes out below the ring plane. This is a fun detail to try and see in a telescope. Credit: Anthony Wesley

Saturday is truly Saturn’s day this year. We mark the ringed planet’s opposition today, the time when it’s closest to Earth and brightest for 2014.

Opposition occurs when Earth passes between Saturn and the sun. When both planets lie on the same side of the sun, they’re almost 175 million miles closer than when they’re on opposite sides. That translates to a bigger, brighter Saturn. Because Saturn travels a little ways around its 29.5 year orbit every year, Earth requires about 13 days to catch up to it at each succeeding opposition. We’ll line up again next year on May 23.

Earth and Saturn are lined up with the sun today and 173 million miles closer than they’ll be in about six months when Saturn is in conjunction with the sun. Illustration: Bob King

The word opposition refers to Saturn being opposite the sun in the sky, rising when the sun sets and setting at sunrise. In a word, it’s visible all night long. Just about anytime you feel like pointing your telescope Saturn’s way, it awaits your gaze.

But before we talk telescope views, let’s take a minute to pinpoint the planet’s location in the evening sky. While it rises at sunset, it doesn’t clear the low trees until about an hour and a half later. Skywatchers at mid-northern latitudes will find it low in the southeastern sky around 10 o’clock well to the lower left of Mars, due south at that hour.

This map shows the sky around 10 p.m. local time tonight May 10 facing south-southeast. Saturn is smack in the middle of the dim constellation Libra below and to the left of Mars and Spica. Stellarium

Tonight the gibbous moon won’t be far from Mars, making it exceptionally easy to find the Red Planet. Swing down to the lower lower left of Mars to spot Saturn. You shouldn’t have trouble spotting it – at magnitude +0.1 it’s nearly as bright as Vega and a pale yellow-white. The ring bearer’s out all spring and summer, so there will be many opportunities to see it.

The ring bearer calls the dim zodiac constellation Libra the Scales home in 2014. Because the moon’s waxing toward full, it’s tricky at the moment to see Libra’s dim stars. Wait till after May 16 for a better view. Maybe then you’ll notice the whimsical “Saturn Cross” like I’ve been seeing the past couple weeks.

A whimsical “Saturn Cross” formed from Libra’s brightest stars with Saturn at its center. Libra precedes the gangly Scorpius the Scorpion with its bright star Antares. Viewing time shown is 1 a.m. in mid-May. Stellarium

The “Cross”, which just happens to be oriented north-south like the constellation Crux a.k.a. ‘Southern Cross’, is simply a different way to see Libra’s four most prominent stars. Saturn marks the center of the crossbeam.

If you’ve never seen the real Southern Cross, this might serve as a cheap, no-airplane-travel-required substitute. Mostly I bring it up as an easy way for you to add a new and rather faint constellation to your life list.

As Saturn travels around the sun in its 29.5 year orbit, we see one side of the rings for about 15 years, followed by an edgewise presentation. The rings – made of dirty water ice – are huge at some 155,000 miles wide (250,000 km), but they’re only about 30 feet thick and virtually disappear when seen edge-on.

That last happened in 2009. Since then they’re re-opened with the north face visible for some 15 years.

Saturn on April 6, 2014. Its clouds belt are less contrasty than Jupiter’s and except for the prominent north equatorial belt not easy to see. A small telescope and magnification as low as 30x will show the rings. Higher power will show the wide B-ring and thinner, outer A-ring. Also visible is the dim C-ring, the dusky band in the foreground crossing in front of the planet. Credit: Efrain Morales Rivera

This year the ring plane’s tipped open 21-22 degrees, nearly the maximum of 27 degrees which occurs in 2017. A large tip exposes lots of ring ice particles to sunlight, boosting the planet’s brightness.

Views of Saturn at different ring plane inclinations taken by the Hubble Space telescope. Rings are labeled in the top image. Credit: NASA/ESA

That’s all good news for both visual and telescopic observation. Even a 2.4-inch telescope will show the rings, with a larger instrument providing a brighter, larger picture and sharper resolution of the three brightest rings. I love the planet’s subtle colors through the eyepiece – the globe looks pale brown or butterscotch to my eye and the rings distinctly brighter and whiter.

Saturn and its brightest moons around 10 p.m. CDT tonight. Titan is brightest at magnitude 9 and very easy to see. North is up. Credit: Meridian software

Small scopes will also show the brightest moons including Titan, Rhea, Dione, Tethys and Iapetus. To find where the moons are on a given night and time, check out Sky and Telescope’s handy Saturn javascript utility, free Meridian software (used to make the diagram above) or download the $2.99 app SaturnMoons.

Saturn’s the best. No other sight in the sky elicits the wonder and amazement of guests at the telescope. Look at the ringed planet every night you can, and the more friends and family your share it with, the better.

Earth rises from the moon … again!

This image, captured Feb. 1, 2014, shows the Earth from the moon-based perspective of NASA’s Lunar Reconnaissance Orbiter. Credit: NASA/Goddard/Arizona State University

Who can get enough earthrises? I mean, look at how beautiful we are. NASA just released a photo and short video of our blue planet lifting off the limb of the moon on Feb. 1 this year. It was taken by the Lunar Reconnaissance Orbiter (LRO) which experiences 12 earthrises every day.

Only thing is, LRO is normally busy imaging the lunar surface; only rarely does the opportunity arise for it to focus its gaze up and beyond the lunar limb. On Feb. 1, the probe pitched forward while approaching the moon’s north pole allowing the wide angle camera to capture Earth rising above the 112-mile-wide (180 km) crater Rozhdestvenskiy crater.

This animation of LROC WAC observations shows the rising Earth from the moon’s limb. Multiple filters were used to make the photo including one for blue light. You can see how much brighter Earth looks because of its blue oceans in the final seconds before it leaves the frame. Credit: NASA/Goddard/Arizona State University

Occasionally LRO points off into space to acquire observations of the lunar exosphere – a atmosphere so rarefied it contains mostly atoms from the solar wind – and perform instrument calibration measurements. During these slews sometimes the Earth (and other planets) pass through the camera’s field of view and dramatic images like this one can be taken. Otherwise it’s ‘nose to the grindstone’ with its eye focused on what’s below.

Unlike an ordinary camera, LRO’s wide angle camera builds up an image frame-by-frame using multiple filters. In the animation the gaps between each frame that create a ‘venetian blind’ effect are from actual gaps between the filters on the camera’s CCD chip. Combined, they form one continuous image.

Another earthrise taken on April 6, 2008 by the video camera aboard the Japanese Kaguya orbiter. Click to enlarge. Credit: JAXA/NHK

The color photo is a composite of six black and white (grayscale) frames of the moon, while the color view of the Earth required three exposures through filters of three different wavelengths that were combined to create a natural color view. This is just how we’d see if we could hitch a ride there.

Notice how much brighter the Earth is compared to the moon. The moon only appears brilliant at night because we see it against a dark sky. Perspectives like these always amaze. They show how fertile and purposeful Earth is in a cosmos that hardly notices.

Forbidding Planet: Scientists find remains of monster asteroid impacts on early Earth

Artist’s view of Earth several billion years ago during the Late Heavy Bombardment, when the planet is thought to have been battered by impacts of comets and asteroids. Credit: Chris Butler/SPL

Earth 3.5 billion years ago was a terrifying place. Picture a rocky landscape pounded by meteorites and asteroids with a surface resembling that of the moon. Volcanoes spewed water vapor but also a toxic mix of carbon dioxide, sulfur dioxide and methane. If you could whisk yourself back to this world by time machine, you’d need to be fully protected by a spacesuit and lucky enough to not get picked off by a falling space rock. Oh, and bring a boat too. Hot-water oceans likely covered a fair portion of the planet back then.


This time-lapse illustration of the Nice (pronounced ‘neece’) model of solar system evolution shows how outer planet migrations kick asteroids into the inner solar system

Scientists call the period from about 3.8 billion to 1.8 billion years ago the Late Heavy Bombardment (LHB), a time when the number of asteroids and their fragments pelting the inner planets and their moons spiked. Why then? No one’s absolutely certain, but the leading theory posits that the migration of the giant outer planets to their present positions “stirred the gravitational pot”, slinging boatloads of asteroids into the inner solar system, where they rained down on Earth and its neighbors in hellish monotony for millions of years.

Anyone with a small telescope can see resulting devastation to this day. Just take a long look at the moon’s battered and cratered surface and thank your lucky stars you’re around during a more peaceful time. Finding Earth’s craters is trickier because water and wind erosion, along with the continual recycling of much of our planet’s crust through plate tectonics, has erased much of our violent past.

The Vredefort Dome – these concentric hills, which rebounded after the impact that created Vredefort Crater – are what remains after an asteroid about 3-6 miles wide struck Earth 2 billion years ago. Credit: NASA

About 180 craters are known on Earth today, but we’re aware of only three resulting from the Late Heavy Bombardment. The oldest, estimated at 3 billion years old and 62 miles (100 km) wide, is also the most recently discovered. Found in western Greenland in 2012, all that remains of the impact are rocks rattled by the massive shock wave that penetrated 15 miles (25 km) deep within Earth’s crust.

You can still see the remains of the impacts that formed the 112-mile-wide (180 km) Vredefort Crater in South Africa, which is 2 billion years old, and the youngest LHB member, the 155-mile (250-km) Sudbury crater in Canada dated at 1.85 billion years.

Map of South Africa with the Barberton greenstone belt shown in red. Shock waves from the impact of an asteroid 3.26 billion years ago created telltale formations within the belt. No one knows yet where the impact happened.

Now, a group of scientists have announced they’ve found evidence for an even older impact, one that occurred 3.26 billion years ago and left its signature in a South African region known as the Barberton greenstone belt.

A recent press release describes the huge impactor as between 23 and 36 miles wide (37- 58 km). Colliding with the planet at 12 miles per second, the jolt delivered was bigger than a 10.8 magnitude earthquake and propelled seismic waves hundreds of miles through the Earth, breaking rocks and setting off other large earthquakes. Tsunamis thousands of feet deep swept across the oceans that covered most of the planet at that time.

A graphical representation of the size of the asteroid thought to have killed the dinosaurs (left), and the crater it created, compared to an asteroid thought to have hit the Earth 3.26 billion years ago and the size of the crater it may have generated. A new study reveals the power and scale of the event some 3.26 billion years ago which scientists think created geological features found in a South African region known as the Barberton greenstone belt. Credit: American Geophysical Union

“We knew it was big, but we didn’t know how big,” Donald Lowe, a geologist at Stanford University and a co-author of the study, said of the asteroid.

The collision would have blasted out a crater some 300 miles (500 km) wide, filled the atmosphere with fiery rock vapor and set the surface of the ocean a-boil. We’re talking serious cataclysm. Somehow life found a way through the heat and crater-punching to gift us with the rolling green hills, coral reefs and forests that characterize Earth today.

Table from the book “Near Earth Objects – Finding Them Before They Find Us” by Donald Yeomans showing average asteroid impact results and probabilities by size. Credit: Donald Yeomans

I try to imagine the dark days of the LHB to help me appreciate these calmer times. Yet we know in our gut – and in fact, thanks to probability – that we’ll never truly be out of the woods. Asteroids lurk in the deep that could one day cause a similar scenario. Don’t let it worry you too much – the chance of a 10-mile-wide space rock striking Earth is once every 89 million years. You’ve still got time to take a nap, catch a show and enjoy a few nights out on the town. Probably.

Happy equinox! Time to tip your hat to Earth’s tipped axis

One of the earliest of spring flowers, a crocus blooms on a sunny spring afternoon. Credit: Bob King

Such a beautiful flower. Look as hard as you like and you won’t find a single one in my town where more than two feet of snow still blankets the good brown earth. I’m not worried. Two weeks from now, the spring sun will reduce it all to puddles.

Today’s the vernal equinox, the start of spring in the northern hemisphere. It began at 11:57 a.m. CDT, the instant the center of the sun’s blazing disk crossed the imaginary circle in the sky called the celestial equator. If you live on the real equator, the celestial version passes directly overhead. That means no shadows at noon today for residents of places like Quito, Ecuador and Kampala, Uganda.


North Pole webcam 2013

Travel north of the equator and the celestial equator drops lower and lower in the southern sky. At the north pole, it sits exactly on the horizon 360 degrees all around. If you could stand there today, you’d be seeing your first sunrise since the autumnal equinox last September. It would also be the start of six months of uninterrupted daylight. By the way, the weather’s fantastic there today – sunny with a high of 24 degrees!

Because of the 23.5 tilt of Earth’s axis, the altitude of the sun varies cyclically across a year. In winter it’s 23.5 degrees below the celestial equator, while in summer it’s 23.5 degrees above. At the equinoxes, it straddles the equator. Created with Stellarium

Most of live between the pole and equator, where the sun stands roughly halfway up in the southern sky at local noon. That’s a far cry from winter, when the sun stood 23.5 degrees (a little more than two fists held at arm’s length) below the equator. Its rays were less direct and intense, and the time it spent above the horizon relatively brief, the two key factors that make a winter.

In summer, we experience just the opposite. The sun stands 23.5 degrees above the celestial equator; its rays are more direct and it spends many more hours above the horizon. Long days and short nights are a delight for many  … including the bugs.

The sun’s cyclic journey above and below the celestial equator all goes back to Earth’s tipped axis. As Earth travels around the sun in a year, the north polar axis tilts toward the sun in summer, taking it 23.5 degrees above the equator, and away from the sun in winter for a ride 23.5 degrees below the equator.

The tip of Earth on its axis causes the seasons. On the first day of spring or vernal equinox, the axis is perpendicular to the sun and days and nights are equally long in both northern and southern hemispheres. Notice the axis doesn’t “flip-flop” but remains pointed in the same direction. It’s the Earth’s orbital travel that causes it to point toward and away from the sun. Credit: Tao-olunga with my own additions

On the first days of spring and fall, the axis is oriented neither toward nor away from the sun. Day and night across the planet are paired up at 12 hours apiece. After today, daylight slowly gains the upper hand by 2-4 minutes a day. Doesn’t sound like much, but like snow, it quickly adds up. By June the mid-latitudes will have gained some four additional hours of solar photons.

What spring looks like where in Duluth this season – a high sun but plenty of snow to go around. Photo taken March 16, 2014. Credit: Bob King

You’ve probably heard that you can balance an egg on its end on the first days of spring and fall. Like water going down the bathtub drain in different directions depending on your hemispher this is an urban myth. It’s hard to balance an egg ANY time of year. Just try it.

I think we all relate to the new season for the same reasons generations of humans before us have. Rebirth, renewal and the return of warmth and light capture the essence of spring. We tip our hats to the random impact at the dawn of the solar system that set Earth’s axis askew.

Daylight forces the hand of night as we surge toward spring

Animation showing the Earth – with tipped axis – revolving around the sun. Seasons are shown for the northern hemisphere.

It happens every mid-winter. I wake up earlier and earlier, unconsciously responding to the daylight that spills beneath the window shade as the pace of the season quickens.

We’ve been putting seconds and minutes in our sunny-day piggy bank every since the winter solstice last Dec. 21. Those deposits are now accumulating rapidly as February gives way to March. Where I live, days were as short as 8 hours 32 minutes in late December. Today that time has swelled to 10 hours 24 minutes.

As Earth revolves around the sun, its 23.5 degree-angled axis points toward, perpendicular to and away from the sun over the year to make the seasons. Credit: Tao’olunga with additions by B. King

While a half hour of extra light may not be enough to notice, 1 hour and 52 minutes is a revelation. Many of us now drive home in bright twilight at the end of a work day. This has beneficial effects like seeing more sunsets and full moon rises. We also feel more connected with the world because we can see it. Humans weren’t born to live as troglobites in dark caves. We crave sunlight as much as clear,dark nights.

I like the extra daylight for hiking and skiing. Shorter nights also mean less time for the Earth to loose heat and the temperature to dip below zero. If you’d like to see how your day/night account is coming along, check out the UNSO’s Duration of Daylight/Darkness Table.

All things warm and fuzzy (and cold and spiky) come our way because of Earth’s axial tilt. The axis remains fixed at an inclination of 23.5 degrees, but as the planet revolves about the sun during the year, the northern hemisphere tilts toward the sun in summer and away in winter. These are the extremes. In between, we have the spring and fall equinoxes, when both hemispheres are “face on” to the sun and receive equal amounts of daylight and night.

There are mini-seasons too. Mid-February is as good a time as any to call by that name. We’re moving away from winter toward spring with night on the run and daylight gaining the upper hand. A month from today, on the verge of the spring equinox, daylight will have increased an additional 1 1/2 hours to 12 hours. For a moment day and night will balance. The next moment day surpasses night and won’t relinquish its lead until after the fall equinox.

The sun’s always high in the sky at low tropical latitudes, so the seasons don’t vary much. This diagram shows the sun’s position around noon on the winter and summer solstices and equinoxes. Stellarium

Daylight length depends upon your latitude. If you took a tropical vacation this winter, you probably noticed that the sun rose around 6 a.m. and set around 6 p.m. Closer to the equator, the sun’s path is steeply inclined to the horizon every day of the year with little change in sunrise and sunset times. The sun’s always high in the sky there at the noon hour, bringing with it those consistently warmer temperatures we’re willing to pay big bucks for.

A mid-winter sun shines through an icicle formation on Lake Superior. Credit: Bob King

At mid and high latitudes, the yearly variation in sun’s position in the sky puts it high in the sky during summer and low in the sky during winter. Low means less time above the horizon, shorter daylight hours and cold temperatures.

To better understand this, consider that on the first day of spring and fall on the equator, the sun rises due east, passes directly overhead and sets due west. On the first day of summer, the sun at noon passes 23.5 degrees ( a little more than two fists held at arm’s length) north of the overhead point, while on the winter solstice it’s 23.5 degrees south of overhead. No matter the season, the sun will always shine down from a high altitude at noon.

This view shows the sun from a mid-northern latitude city like Minnepolis, Minn. Notice how the sun’s yearly elevation spread take it much closer to the horizon (wintertime) and also quite high (summertime). The scale of this map is different from the one above because it doesn’t need to include as much sky near the overhead point. Stellarium

In Minneapolis, halfway between the equator and north pole at latitude 45 degrees north, the sun is 45 degrees high at noon on the first day of spring and fall or halfway between the overhead point and southern horizon. Come the first day of summer, it’s way up there at 68.5 degrees and roasts the back of your neck, but on the winter solstice it peaks out at just 21.5 degrees high. Better protect that neck with a scarf.

The full range of the sun’s yearly motion – 23.5 degrees north to 23.5 degrees south of the celestial equator – is the same no matter where you are on Earth, but if you live far from the equator, the sun’s altitude reaches greater extremes, making the seasons more pronounced.

Curiosity rover snaps 1st photos of Earth and moon from Mars

Earth is the brightest “star” in Mars’ western evening sky as seen and photographed by the Curiosity Rover on Jan. 31, 2014. As seen through Martian eyes, Earth is in the constellation Pisces near its brightest star Al Rischa. Click to enlarge. Credit: NASA/JPL-Caltech/MSSS/TAMU

How many people can you fit inside a pale blue dot? Try 7 billion. We’re all there along with nearly 9 billion other species in these first photos ever taken of planet Earth by the Curiosity rover.

In this scene, the inset photo shows enlarged view revealing the fainter moon close by. Click to enlarge. Credit: NASA/JPL-Caltech/MSSS/TAMU

The pictures were made on Jan. 31, 2014 from the sandy dunes of Dingo Gap inside Gale Crater and show the Earth setting in the evening sky over the crater’s rim. If you could be there in person, the home planet would appear as a pale blue “star” shining at magnitude -1, a little fainter than Sirius, the brightest star in the skies of both planets.

Standing on the ground next to Curiosity’s location at 4.5 South latitude an observer would face west during twilight this evening to see a brilliant blue Earth in the constellation Pisces. Click to learn more about Earth in the Martian sky. Stellarium

The moon would also be visible very close to the planet and much fainter at around magnitude 2.7. Observers with keen vision might see the two tightly-spaced worlds with the naked eye, but a pair of binoculars would come in handy for most of us.

Earth in a telescope in early February as seen from Mars. Stellarium

If you happened to pack your telescope along and pointed it at Earth, you’d be delighted to see our planet as a thick crescent and near its greatest brilliancy. Because Earth orbits the sun inside Mars’ orbit, it passes through phases exactly like Venus and Mercury do as seen from Earth.

These aren’t the first photos of Earth from Mars. The Spirit Rover took a portrait of the home base in 2004 and NASA’s Mars Global Surveyor did the same in 2003 and Mars Reconnaissance Orbiter in 2007.

Every one of these images is a great treasure. They remind us that Earth swims in a cosmos vast beyond imagination.

Time exposure photo of the starry sky taken by Curiosity on Jan. 31, 2014. Do you recognize any? Click to enlarge. Credit: NASA/JPL-Caltech/MSSS/TAMU

The view inside Gale Crater from sandy Dingo Gap yesterday Feb. 6, 2014. Credit: NASA/JPL-Caltech/MSSS/TAMU

Rise and shine! Yutu wakes up / New photos of Earth from the moon

Planet Earth seen from the moon photographed by the Chang’e 3 lander on December 24, 2013 at 12:15 p.m. CST. No stars are visible because the picture was exposed to capture the bright Earth. To record stars, the Earth would have been greatly overexposed. Credit: Chinese Academy of Sciences

The Chang’e 3 moon lander and Yutu rover got their way up call this weekend. Roused from hibernation after more than two weeks chilling in the long lunar night, they’re back in business taking pictures and examining the moon’s crust and minerals.

With temperatures dipping to -292 F (-180 C) and no sunlight to generate electricity with their solar panels, mission control powered down the pair on December 26. The sun will shine over rover and lander for another two weeks until setting again on Jan. 25.

Earth’s plasmasphere photographed in far ultraviolet light by the lander camera on December 16, 2013. The plasmasphere is the inner part of Earth’s vast magnetic bubble called the magnetosphere and consists of dense, cold plasma or ionized atoms and molecules. Click to learn more. Credit: Chinese Academy of Sciences

We’ve seen precious few quality, high resolution images yet from the mission – most have been screen-grabbed off TV and video. Here are few new ones, including a couple featuring a favorite planet.

360-degree panorama of the landing site around the landing site taken by the Chang’e 3 lander on Dec. 17 and 18, 2013. Multiple images have been combined to create the view. Click to enlarge. Credit: Chinese Academy of Sciences

360-degree panorama around the Yutu rover. Click to enlarge. Credit: Chinese Academy of Sciences

 

Chang’e 3 lander photographed by the Jade Rabbit “Yutu” rover on Dec. 16, 2013. Click to enlarge. Credit: Chinese Academy of Sciences