What would it look like if Mars, Saturn or Neptune replaced the moon?

The becomes part of the figure of the Winter Hexagon this evening. Most of the brightest stars of the winter sky form the hexagon with the exception of Orion’s Betelgeuse, which gets stuck in the middle of the figure.  The map shows the sky facing southeast around 8 o’clock local time. Created with Stellarium

Tonight the nearly full moon will become part of the Winter Hexagon, one of the biggest and easiest to see asterisms of the winter sky. You’ll find Luna twixt Procyon in Canis Minor and Pollux in Gemini along the eastern side of the figure which is comprised of six bright stars. When connected by imaginary lines, the stars form a hexagon 65° tall by 45° wide that reaches from the zenith to low in the southern sky.

Saturn at the moon’s distance from Earth rises majestically above the skyline. Appearing more than 33 times larger than the moon, its globe is 16.5° across while the rings extend nearly 40° from end to end or more than twice the size of the constellation Orion. Credit: Roscosmos

The moon’s 240,000 miles from Earth and 2,160 miles (3,475 km) in diameter. To get there by foot would take you 9 years. Seen from Earth, the lunar sphere spans just 1/2° of sky – you can more than cover it up with the tip of your pinkie finger.

At 7,500 miles across, Venus is only a few hundred miles smaller than Earth. It would appear very white and bright and just under 1.75° across or just shy of 4 times larger than the moon. Credit: Roscosmos

Yesterday we replaced the Sun with Polaris, Vega, Sirius, Alpha Centauri and Arcturus to get a feel for the real dimensions and appearance of what are otherwise tiny twinkling points of light.

Today we’ll replace the moon with several of the planets to better appreciate their true dimensions. Like the stars, all the planets look like dots of light to the unaided eye.

At 4,212 miles across, Mars is only about twice the size of the moon and would be 1° across. Your pinkie would just cover it. Credit: Roscosmos

Wouldn’t it be nice to just walk outside and see Jupiter’s Great Red Spot and all the wild weather happening within its many cloud belts? At the moon’s distance, all you’d have to do is look up on a clear day. Jupiter’s so big Earth would become another of its moons. We’d also be bathed in its dangerous radiation environment. Credit: Roscosmos

Uranus is four times the size of Earth and would appear a pale blue color in our sky. Credit: Roscosmos

Neptune’s deeper shade of blue is caused by methane gas in its atmosphere which absorbs light of longer wavelengths (yellow, orange and red). Neptune’s about 1,000 miles smaller than Uranus. Credit: Roscosmos

Covered in craters and just 1.5 times the size of the moon, Mercury would appear similar to our own satellite in the sky. Credit: Roscosmos

Now this looks weird. Earth in Earth’s sky? Of course this is how big the Earth would appear from the moon – 1.8° across or just shy of four times the size of the full moon. Credit: Roscosmos

And now for the video:

Looking for aurora tonight? Check out the Winter Hexagon + 2

This map shows the sky facing southeast around 8:30 p.m. local time tonight. The splendid gathering of stars – the Winter Hexagon – is just now coming into good view during early evening hours. The entire figure is quite large, starting with Sirius low in the south and topping out with Capella near the zenith. Stellarium

Update 9 a.m. Jan. 10: The blast of particles from the solar flare in big sunspot group 1944 was much weaker than expected. Some of it slid by Earth yesterday afternoon but only fired up auroras in Arctic latitudes that were in darkness at the time. There’s still one more chance for auroras tonight as the remainder of blast passes by.

– 12:05 a.m. Jan. 10: Still nothing visible from Duluth though there’s been a generally upward trend in activity over the past few hours. You can check the extent of the auroral oval HERE. The red line indicates the southern limit of aurora visibility. Though more technical, a good indicator of an impending aurora is the real-time Bz graph from the ACE spacecraft. If the red squiggly line dips sharply southward – toward the bottom of the chart (lower than -10) – be alert for potential northern lights.

While you’re out facing north hoping the aurora paints your sky tonight take a look around your backside to the south. Starting around 8-8:30 p.m. local time the complete Winter Hexagon – a beautiful hexagonal array of the brightest stars of the winter – tilts upward in the southeastern sky.

Each star or stars, as in the case of Castor and Pollux, which both belong to Gemini the Twins, heads up a particular constellation:

* Capella in Auriga the Charioteer
* Aldeabaran / Taurus the Bull
* Rigel / Orion
* Sirius / Canis Major the Great Dog
* Procyon / Canis Minor the Little Dog

What about the “+2″. These odd stars out – Betelgeuse and the planet Jupiter – aren’t part of the Hexagon but just happen to be fenced in by it. Count them all up and you’ve got nine shimmering sky objects, eight of which are first magnitude or brighter (Castor is magnitude 1.9) and located in the same tract of sky. What an attention grabber.

The Winter Hexagon with Jupiter in Taurus in 2012-13 (upper right) along with some insidious light pollution (lower right). Credit: Bob King

You might be surprised to know that winter skies are often more light polluted than those of other seasons. Streetlights and other forms of lighting reflect off the snow cover and bounce straight up into the sky. The difference is striking from where I live – the wash of light from the city reaches half again as high in the southern sky as during the fall.

The Hexagon’s concentrated radiance plus additional bright stars in the region leave the impression that winter is the clearest, darkest time of year when it may very well not be.

Parts of the world that don’t receive snow in the winter are better off, and if your haze and humidity levels are lower than as well, the winter stars may indeed sparkle that much brightly.

So enjoy the Hexagon tonight, and may a fine display of northern lights make you turn around the other way.

You absolutely must see the Winter Hexagon tonight

Check out the six bright stars that connected together form a large hexagon in the evening sky.  It’s up by 7 p.m. local time but best between 8 and midnight. Maps created with Stellarium

There’s a reason the winter sky is so full of sparkling stars. A great many are concentrated around the constellation Orion the form of a gigantic hexagon. How big is it? I made a fist and reached my gloved hand to the sky last night to measure – 6 fists high by 4 fists wide or 60 x 40 degrees. This six-sided figure of celestial real estate reaches from Sirius, low in the southern sky, all the way up to Capella, located nearly overhead from mid-northern latitudes.

What makes these stars special is how bright they are. They all shine at 1st magnitude (or brighter) and appear on the list of the Top 25 brightest stars. Joining the clan are Jupiter, more luminous than any of them, and Castor slightly fainter than the faintest.

Magnitude scale showing the limits of the eye, binoculars and telescopes. Credit: Dr. Michael Bolte, UCO/Lick Observatory

Astronomers use the magnitude scale to measure star and planet brightness. Each magnitude is 2.5 times brighter than the one below it. Aldebaran, which shines at 1st magnitude, is 2.5 times brighter than a 2nd magnitude star, which in turn is 2.5 times brighter than a 3rd magnitude star and so on.

A first magnitude star is 2.5 x 2.5 x 2.5 x 2.5 x 2.5 (about 100) times brighter than a 6th magnitude star.

The bigger the magnitude number, the fainter the star. On the other hand, if an object is really bright, it’s assigned a negative magnitude. Sirius, the brightest star sparkles at magnitude -1.4, Jupiter at -2-2 (currently) and Venus brighter yet at -4.4. The full moon reaches a magnificent -12.7, topped only by the sun at -26.7.

An object’s brightness has much to do with its distance from Earth. Small things like planets, the moon or even an asteroid can look bright if close, while a brilliant supergiant star can appear faint simply because it’s far away.

Photo taken last night Jan. 16, 2013 of the Winter Hexagon and Jupiter about 9 p.m. Photo: Bob King

To get a better appreciation of an object’s true or absolute brightness, astronomers assign it an absolute magnitude, based on how bright it would appear when moved to a distance of 10 parsecs (equal to 32.6 light years) from the sun. When stars are all placed at the same distance, absolute magnitudes show differences in true star brightness.

A parsec is the distance from the Sun to an astronomical object which has a parallax angle of one arc second – parallax second – against the background sky. Parallax, which is measured in arc seconds or tiny fractions of a degree, is the apparent shift of a nearby star against the distant background of stars as seen from either end of Earth’s orbit.

One parsec equals 3.26 light years. Click HERE for a blog I wrote explaining parallax. The main thing to remember is we’re comparing objects at the same distance of 10 parsecs from the sun.

Here are the apparent (what we see with the eye) and absolute magnitudes (in parentheses) of our featured stars::
* Sirius -1.5 (1.4)
* Procyon 0.4 (2.6)
* Pollux 1.1 (0.7)
* Capella 0.1 (0.4)
* Aldebaran 0.9 (-0.3)
* Rigel 0.1 (-8.1)
* Jupiter -2.2 (55)
* Betelgeuse 0.5 (-7.2)
* Castor 1.6 (0.5)
* Our sun -26.7 (4.8)

An illustration of how the Winter Hexagon and neighboring bright stars would appear if all moved to the same distance of 32.6 light years. We would see them at the absolute magnitudes. Notice anything missing?

Right away you’ll see some dramatic differences in intrinsic brightness. Rigel and Betelgeuse, both of which appear more than a magnitude fainter than Sirius to the eye, far outshine all the others. Seen from 10 parsecs, each puts out enough light to cast shadows at night. Why? They’re both extremely luminous supergiant stars. Jupiter, the big shot of the bunch, fades out of sight.

Sirius, only twice as big as the sun, dims to a rather meek mag. 1.4. It’s overtaken by otherwise mild-mannered Castor, a double star with suns 2.4 and 1.9 times larger than our own. How does our sun fare at 10 parsecs? Not so good. At magnitude 4.8, it would blend into the background of faint stars. Unless you looked carefully, you wouldn’t even notice it.

Knowing a star’s absolute magnitude gives us a true picture of a star’s brightness. What’s more, you can derive a star’s distance by comparing its apparent magnitude to the absolute magnitude. Want to have a little fun? Click on the Magnitude and Luminosity Calculator and play around with some of your favorite stars.

Moon and Winter Hexagon bright on bright tonight

The nearly full moon will be surrounded by a hexagon of bright stars tonight. The map shows the sky facing southeaster around 9 o'clock. Created with Stellarium

The moon’s been getting brighter and brighter with each passing night this week. Tonight we’re just a day away from the Full Wolf Moon, and if you look up around 9 o’clock or later, the moon will be surrounded by the Winter Hexagon, an enormous six-sided figure formed by the season’s brightest stars. The figure reaches from Sirius, low in the southeast, all the way up to Capella near the top of the sky.

People often remark that the stars of winter are so much brighter than those in other seasons and assume it’s because the sky is exceptionally clear. The real reason the winter sky seems sparklier is simple – there are more bright stars concentrated there than in any other region of the sky for observers at mid-northern latitudes. Nine of the 25 brightest stars are found in and around the constellation Orion. There are an additional nine 2nd magnitude stars if you add in Orion’s Belt, the two remaining stars in his outline and four in the nearby constellation of Canis Major the Greater Dog. That’s 18 sparklers in all. How can your attention not be drawn to the southern sky in winter? Toss in the moon and your retinas runneth over with radiance.

Sunday night’s full moon is named after the howling wolf packs heard on cold winter nights. Your calendars may show the full moon date as the 9th, but since full phase happens at 1:30 a.m. CST Monday, the moon will appear closer to perfectly round tomorrow night compared to Monday night.

During December's lunar eclipse NASA's LRO made temperature measurements of the moon's surface during its orbit. Seen from the moon, the Earth eclipsed the sun that morning. Credit: NASA with additions of my own

You’ll recall that last month’s full moon was eclipsed by Earth’s shadow for much of the central and western U.S., Canada, Alaska and points west. While some of us were waking up in the wee hours hoping to see the event, NASA’s Lunar Reconnaissance Orbiter (LRO) was busy measuring the temperature drop on the surface as the moon slid into shadow. We’ve all experienced an “eclipse temperature drop” when a cloud passes over the sun. Not only does the cloud eclipse the sun but it momentarily blocks some of the sun’s heat. If it’s early spring and there’s a breeze about, you can really feel the drop in temperature.

LRO's primary purpose it to map the moon incredible detail. In this photo we see the Apollo 17 lunar descent module, astronaut tracks and the parallel tracks made by the lunar rover driven by the astronauts. Credit: NASA

The same thing happens on the moon but with more dramatic effect, because there’s no atmosphere to hold onto the heat. LRO observations from the earlier June 15, 2011 total eclipse showed that temperatures dropped some 180 degrees F over parts of the moon’s surface in just 1 1/2 hours and then just as quickly shot back up when the sun returned at the end of the eclipse. Talk about extreme chill – nothing on Earth compares.

Since large boulders cool more slowly than a fine-grained or dusty surface, LRO’s Diviner temperature measuring instrument was able to see what areas are covered with boulders and what regions are blanketed by dust. The degree of cooling also depends on how rocky the surface is, how densely packed the soil is, and its mineral composition. Scientists hope the temperature data gathered from December’s eclipse will help us understand just a little bit more about that shiny thing in the sky tonight.

Winter Hexagon sparkles in the east

Have you noticed Vega in the evening like my mom? It's easily seen in the northwest in late twilight. Maps created with Stellarium

I called my mom the other night to tell her the International Space Station would be flying over her house in southwestern Wisconsin in an hour. She got out on time and enjoyed the sight, but had a question about that big bright star in the northwestern sky. That’s Vega, I told her, one of the stars in the Summer Triangle. You may have noticed Vega these late December evenings, especially if you’re on your way home from work around 5:30-6 p.m. The star’s low altitude makes it an easy catch through the car windshield if you happen to be driving north or northwest and encounter a swatch of unobstructed sky.

Vega is the brightest star in the Triangle, and the one most of us notice before the others two: Deneb of Northern Cross fame and fast-disappearing Altair in Aquila the Eagle. These summer stars will linger a few more weeks before they’re displaced by a wave of late fall and winter stars moving up from the eastern sky.

The Winter Hexagon reaches from Sirius, low in the southeast, to Capella, near the top of the sky. It's formed by the brightest stars of winter's most prominent constellations.

Vega sets around 10 p.m., not long after the entire Winter Hexagon clears the horizon on the opposite side of the sky. Seven of the nine brightest stars of winter compose this large and delightful asterism. Go out around 9 o’clock and start with Orion’s Belt. Below and right of the belt is Rigel. From there, drop down to Sirius, the brightest star in the sky, and then work your way back up through Procyon (PRO-see-on), the Gemini Twins and to Capella. Then come back down through Aldebaran in Taurus and return to Rigel. Or go another way altogether.

With all side of the Hexagon complete, let’s not forget Betelgeuse, the lone bull in a six-sided corral.