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The surface of Mercury is crawling with snakes — more than two dozen of them. The largest is more than 165 miles long. You don’t need to worry about their bite, though — they’re remnants of ancient volcanoes.

The features are known as faculae — a Latin word that means “little torches.” They’re patches of bright material around dead volcanoes. The volcanoes erupted billions of years ago, when Mercury was young. They blasted out huge amounts of lava that contained a lot of water and similar compounds.

Planetary scientists have selected different themes for naming the surface features on Mercury and the other planets. Mercury’s faculae have been named for the words for “snake” in different languages. Some of the features look like snakes, and the god Mercury often was depicted with a snake on the end of his staff.

The largest snake is Nathair Facula, from the Irish word for snake. It’s about 165 miles in diameter. Other names include Inyoka, from the Zulu word for snake; Ular, from Malaysia; Abeeso, from Somalia; and Orm, from Sweden.

Not all faculae have been named yet. So we can look forward to more snakes wriggling across Mercury in the years ahead.

Mercury is in good view in the early morning now. It’s farthest from the Sun in our sky, so it’ll hang around for a few days. It’s in the southeast at first light, and looks like a bright star. But it’s so low that you need a clear horizon to spot it.

Script by Damond Benningfield

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Orion is filled with so many bright stars that some of them get ignored. An example is Tabit, the star at the middle of Orion’s shield.

The star is class F6V — a designation that tells us a lot.

Astronomers categorize the stars with the letters O, B, A, F, G, K, and M. O and B stars are the biggest, hottest, and brightest. They shine blue or white. But they’re also the least common stars — they account for only a tiny fraction of one percent of all the stars in the galaxy.

K and M stars are cool and orange. They include the most common stars in the galaxy, known as M dwarfs.

A, F, and G stars are in the middle. They shine white or yellow. And the list of G stars includes the Sun.

At F6, Tabit outranks the Sun, but not by much. It’s more than a thousand degrees hotter, so it looks whiter than the Sun. It’s a little bigger and more massive than the Sun, and about three times brighter. And the “five” at the end of its classification tells us that, like the Sun, it’s still in the prime of life.

More than 200,000 years ago, Tabit would have looked about twice as bright as it does now. It was only about 15 light-years away, versus its present distance of 26 light-years.

At nightfall, Orion’s three-star belt is high in the southeast, pointing almost straight up. If you follow that line upward, the first fairly bright star you come to is Tabit — the center of the shield of Orion.

Script by Damond Benningfield

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Middlesboro is a small town in the Cumberland Gap of southern Kentucky. About 300 million years ago, it was the bullseye for a cosmic missile: a small asteroid that gouged an impact crater about four miles wide. Evidence of that high-speed collision is preserved in the rocks below Middlesboro.

Middlesboro is one of about 30 confirmed impact sites in the United States. Most craters have been erased by wind, rain, and the motions of Earth’s crust. And many of the remaining ones aren’t visible — the only traces are found far below the surface.

The best preserved is Barringer Crater in Arizona. It formed about 50,000 years ago. It’s less than a mile wide, but it’s easy to identify as an impact crater. In fact, it’s the first confirmed impact structure on Earth. It helped geologists determine that most of the craters on the Moon were formed by collisions instead of volcanoes.

The largest American crater is Chesapeake Bay. It’s about 55 miles in diameter, and formed about 35 million years ago.

A few craters are easily accessible — you can drive through them. Examples include Middlesboro, as well as Crooked Creek and Decaturville in Missouri.

One of the easiest to see as a crater is Sierra Madera, in West Texas. It’s about eight miles across. A state highway crosses it, climbing over the northern rim, then passing near its central mountain peak — the scar of a collision almost a hundred million years ago.

Script by Damond Benningfield

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A planet that played a big role in measuring the speed of light stands just above the Moon this evening.

Brilliant Jupiter is the largest planet in the solar system. And it has a giant family of moons, including four that are about the size of our moon or bigger. The smallest and nearest is Io. It’s closer to Jupiter than our moon is to Earth. It orbits Jupiter once every 42 and a half hours. And that’s the key to the measurement of lightspeed.

In the 1670s, Danish astronomer Olé Roemer was trying to refine the measurements of Io’s orbit. Io passes “behind” Jupiter on each orbit, so it vanishes in Jupiter’s long shadow. The timing reveals the details of Io’s orbit.

After a few years of measurements, though, Roemer noticed something odd. When Earth was moving toward Jupiter, Io vanished a few minutes earlier than expected. And when Earth was moving away from Jupiter, it vanished a few minutes later.

Roemer decided the change was a result of the changing distance between Earth and Jupiter. When the distance was greater, it took longer for light to make the trip. And that meant the speed of light was limited — something that scientists weren’t sure about before.

Roemer calculated that speed at about 500 million miles per hour. That’s about 25 percent off modern calculations. Still, it was the first reasonable estimate of the speed of light — made with the help of Jupiter and one of its many moons.

Script by Damond Benningfield

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The population of our home galaxy, the Milky Way, includes about 150 globular clusters. And for a long time, astronomers thought all of these giant balls of stars were pretty much alike. But in the last couple of decades, they’ve discovered that each one is unique.

One example is NGC 2419. It’s about 300,000 light-years away, making it one of the galaxy’s most remote globulars. When its distance was first measured, in fact, astronomers thought the cluster was too far to be tied to the Milky Way. So they called it the Intergalactic Wanderer. But it really is part of the Milky Way — far beyond the galaxy’s disk, in a region known as the halo.

The stars in most globulars were all born at the same time. But NGC 2419 has two different populations of stars. The stars in one group have a lot more helium and a few other elements than those in the other. The helium-rich stars are clumped in the cluster’s core.

So far, astronomers aren’t sure what that means. There doesn’t seem to be a way to make two separate groups of stars in a globular. That could mean that NGC 2419 formed in a different way from other globulars — a unique formula for a big star cluster.

NGC 2419 is in the constellation Lynx, which is in the northeast as night falls. The cluster is close to the upper left of Castor and Pollux, the twin stars of Gemini. Despite its distance, it’s an easy target for small telescopes.

Script by Damond Benningfield

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We may be living inside a football. A recent study measured the Milky Way Galaxy’s halo — a shell of stars that surrounds the galaxy’s bright disk. And it found that the halo is shaped like a lopsided football or zeppelin. That could be the result of a collision billions of years ago.

The stellar halo extends hundreds of thousands of light-years from the center of the galaxy. It contains lots of stars. But they’re widely spread, so the halo is faint. That makes it hard to map.

The new study used telescopes in space and on the ground to measure the motions of thousands of halo stars. That was enough to reveal the shape. It also revealed that the halo is tilted compared to the Milky Way’s disk. That off-kilter alignment suggests the halo is locked in place by a halo of dark matter. We can’t see the dark matter, but its gravity reveals its presence.

The stellar halo may have formed billions of years ago, when another galaxy slammed into the Milky Way. That splattered many of its stars like droplets from a water balloon thrown against a brick wall. In this case, the stars entered wide orbits around the Milky Way — forming the lopsided halo.

The Milky Way’s faint disk arcs high across the sky this evening. It stretches from Sirius, the brightest star in the night sky, in the southeast, over to W-shaped Cassiopeia high in the northwest, and down to the Northern Cross just above the horizon. The halo is nowhere in sight.

Script by Damond Benningfield

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A thin crescent Moon slides past a tight pair of planets in the early evening sky the next couple of nights. The Moon will be hard to find tonight, but much easier tomorrow night.

Venus is the brighter planet — the spectacular “evening star.” Its companion is Saturn. Tonight, the two planets look like they’re about to touch each other. They’re low in the southwestern sky in early twilight.

The Moon is below them. It’s so low in the sky that anything along the horizon will block it from view. And even if your horizon is clear, the Moon might be tough to find. That’s because it’s just more than a day past “new,” when it crossed between Earth and the Sun. Only about two percent of the lunar hemisphere that faces our way is in sunlight, so it’s the barest of crescents.

The entire lunar disk should be visible, though, because it’s lit up by earthshine — sunlight reflected from our own planet. If you were standing on the Moon and looking back in our direction, you’d see an almost-full Earth. It would shine more than 40 times brighter than a full Moon. So the lunar night would be quite bright.

By tomorrow night, the Moon will have moved past Venus and Saturn, which will have opened up a little separation between themselves. The Moon will be a slightly fatter crescent, so it’ll be easier to find and follow. The three worlds will form a beautiful trio as the color of twilight fades away.

Tomorrow: a lopsided halo.

Script by Damond Benningfield

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The Big Dipper wheels around the North Star like the hour-hand of a giant clock, ticking off the hours of the night. And winter is a good time to watch it, because it’s in good view most of the night.

Tonight, for example, the dipper stands clear of the northeastern horizon by an hour or two after night falls. It’s perched on its handle, with the bowl highest in the sky.    

Over the following few hours, the dipper wheels counterclockwise around and over the North Star. It’s highest in the sky around 3 or 4 in the morning, stretching high across the north. The bowl is upside down, so it looks as though it’s spilling its contents toward the northern horizon.

After that, the dipper begins its descent across the northwest. It’s still quite high at first light, with the leading edge of the bowl higher than the North Star.

As the dipper turns across the sky, you can use the bowl to find the North Star. Line up the two stars that form the outer edge of the bowl, then extend the line away from the bowl. The first moderately bright star you come to is the North Star, Polaris.

Despite its important position in the sky, the North Star isn’t all that bright. So it’s good to have some well-known stars to point the way — like those of the Big Dipper: the great hand of a celestial clock that’s ticking off the hours of a long winter’s night.

Tomorrow: The crescent Moon and a tight pair of planets.

Script by Damond Benningfield

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The diversity of the planets in our own solar system is amazing. They range from bare rocks roasting in the sunlight to giant balls of gas with monster storms churning through their skies.

Consider Venus and Saturn, which are staging a close conjunction in the early evening sky. Venus is about the size of Earth, and its atmosphere is hot enough to melt lead. Saturn is almost 10 times wider than Earth, and its atmosphere is stretched into globe-circling bands by Saturn’s high-speed rotation.

Venus is the second planet out from the Sun, so it receives a lot of sunlight. Its atmosphere is made mainly of carbon dioxide, and it’s about 95 times thicker than Earth’s atmosphere. That traps a lot of solar energy, heating the surface to more than 850 degrees Fahrenheit.

Saturn is the sixth planet from the Sun. It receives much less sunlight, so its atmosphere is cold. But it generates a lot of internal heat — more than it gets from the Sun. That powers storms that roll through its bands of clouds. And every few decades, it produces a monster storm that’s as big as Venus, and can eventually wrap around the entire planet.

Look for Venus and Saturn quite low in the west-southwest in the early twilight. Venus is the “evening star.” Tonight, Saturn stands a little above it. They’ll be closer tomorrow evening, and appear to almost touch each other on Sunday — a close pairing of quite different siblings.

Script by Damond Benningfield

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Sirius, the brightest star in the night sky, arcs across the south tonight. And if you live in the far southern latitudes of the United States — south of about Phoenix or Atlanta — the second-brightest star sneaks into view as well. Canopus rises about 10 p.m., and stands due south an hour or two later. It’s just a few degrees above the horizon. It’s so bright, though, that you shouldn’t have much trouble spotting it.

Canopus is the brightest star of Carina, the keel. The constellation represents the bulk of the Argo, the ship that carried Jason and the Argonauts on their adventures.

Canopus may be named for a sailor from another vessel from mythology.

According to the story, Canopus was the helmsman for Menelaus, the king of Sparta. Canopus piloted the boat on an expedition to retrieve the king’s wife, Helen of Troy. On the way back, a storm blew the boat off course, and it landed in Egypt. When Canopus went ashore, he was bitten by a cobra and died. Helen and Menelaus buried him, and Menelaus established a town there. He named both the town and a bright star that was in the sky then in the helmsman’s honor.

That star looks only about half as bright as Sirius. But that’s only because of its greater distance — it’s more than 300 light-years away, compared to nine light-years for Sirius. If you lined them up at the same distance, Canopus would shine more than 500 times brighter.

Script by Damond Benningfield

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