Powerful cold fronts move across North America at this time of year. These blankets of dry, cold air push away the clouds and haze, providing some amazingly beautiful blue skies. That color is produced by the interaction of sunlight with Earth’s atmosphere. The Sun is classified as a yellow star because its energy output peaks at yellow wavelengths. And if we could see the Sun from a distance of a few light-years, where it would appear as only a pinpoint of light, it would have a yellow hue. But from close range, the Sun is so intensely bright that we see its light as a mixture of all the colors of the rainbow. As a result, it looks white. As the Sun’s light enters Earth’s atmosphere, it’s subjected to a number of effects. Most of the time, the most important effect is Rayleigh scattering. It’s named for a British scientist who studied the effect in the late 19th century. Blue light waves are shorter than waves of red light. That makes them the right size to bounce off molecules of nitrogen and oxygen in the atmosphere. That scatters them in random directions. Since the blue wavelengths are scattered across the entire sky, the sky looks blue. Molecules in the air actually scatter a lot of violet light as well. But our eyes are more sensitive to blue wavelengths, so we see the sky as distinctly blue – the frosty color of clear winter days. Tomorrow: the oddball month of February. Script by Damond Benningfield

The constellation Gemini consists of two long lines of stars capped by two of the brighter stars in the night sky. Many cultures have seen these stars as two men. But the legend that endures is the Greek story of Castor and Pollux. The two bright stars bear their names. In the story, the twins had the same mother – Leda, the queen of Sparta. But they had different fathers. Castor was the son of the king – a mortal – while Pollux was the son of Zeus, the king of the gods. The boys were inseparable. They had many adventures together. They joined Jason and the other Argonauts in the search for the golden fleece, and saved their legendary boat during a nasty storm. But during a later battle, Castor was killed. Pollux was inconsolable. He begged Zeus to let him die so he could join Castor in the underworld. Moved by Pollux’s love for his brother, Zeus agreed to keep them together for all time. They would spend half of their time in the heavens, and the other half in the underworld – just like the stars of Gemini. The twins appear near the Moon the next couple of nights. The Moon aligns along the body of the twins tonight. Castor, the fainter twin, is to the left of the Moon at nightfall, with Pollux to the lower left. The giant planet Jupiter is passing through the constellation as well. It looks like an especially brilliant star. Tonight, it’s a little closer to the Moon than the twins are. Script by Damond Benningfield

Gene Cernan was the last American to walk on the Moon. As he prepared to leave it, he expressed optimism that his colleagues would return soon. CERNAN: As I take man’s last step from the surface for some time to come, but we believe not too long into the future… Well, it’s probably been a little longer into the future than Cernan expected, but NASA is preparing to send astronauts back to the Moon. The Artemis II mission is scheduled to launch in the coming weeks. It will carry four astronauts to the Moon. They won’t land, or even go into orbit. But it will be the first time anyone has come close to the Moon in more than half a century. The astronauts will follow a looping path to the Moon. They’ll fly behind it, coming within about 6500 miles of the surface. The Moon’s gravity will sling them back toward Earth. They’ll splash down in the Pacific Ocean. During the 10-day mission, the astronauts will check out all of the systems on the Orion spacecraft. They’ll also conduct a few experiments, and make some observations of the Moon. Artemis II has been delayed by several years. Among other problems, during the unmanned Artemis I mission, in 2022, the life support system and heat shield didn’t work as planned. Astronauts are supposed to land on the Moon during the next mission. Issues with the lander and other problems may delay that until 2028 or beyond – adding to the gap between moonwalks. Script by Damond Benningfield

The Moon is a tale of two faces. The side we see – the nearside – features giant volcanic plains and a fairly thin crust. The far side features more mountains and craters and much thicker crust. And the differences might go even deeper. The layer below the crust – the mantle – might be cooler on the farside – or was cooler billions of years ago. That difference is suggested by samples returned to Earth by a Chinese lander – the first samples from the farside. Some of the samples formed from molten rock. It cooled and solidified 2.8 billion years ago, deep inside the Moon. Details about the samples suggest the molten rock was much cooler than the same layer on the nearside – by about 200 degrees Fahrenheit. That’s probably because the far side has fewer radioactive elements, which heat the interior as they decay. Just why that’s the case isn’t clear. A smaller moon might have splatted into the lunar farside when the Moon was young. Or a giant asteroid impact might have moved things around. The pull of Earth’s gravity might have played a role as well. Whatever the cause, there’s a big difference in the lunar hemispheres – which may be more than skin deep. The gibbous Moon is passing through the constellation Taurus tonight. Aldebaran, the bull’s eye, is to the right of the Moon at nightfall. And Elnath, at the tip of one of the bull’s horns, is closer to the lower left of the Moon. Script by Damond Benningfield

In Greek mythology, Chiron was the wisest of the centaurs – creatures who were half human and half horse. He taught other centaurs about medicine, botany, and other sciences. Today, the astronomical Chiron is teaching scientists about the formation and evolution of ring systems. Chiron is one of about a thousand known centaurs – chunks of ice and rock between the orbits of Jupiter and Neptune. It’s one of the larger ones, at an average diameter of about 125 miles. Even so, it’s so far away that it’s tough to study. But it sometimes passes in front of a distant star. Such a passage allows scientists to measure its size. It also allows them to study the space around Chiron. Rings cause the light of the background star to flicker. Observing that effect from different locations, and at different times, provides a profile of the rings. A study last year reported some changes. Scientists already knew of three rings. The new study reported evidence of a fourth ring. It’s so far out that Chiron’s weak gravity might not be able to hold it. The scientists also found a wide disk of dust. The rings and disk might be debris from a small moon, or the result of an outburst from Chiron itself. Chiron is moving closer to the Sun. As it warms up, it could produce more outbursts. So the system could undergo more big changes in the years ahead – teaching us much more about the evolution of rings around the small bodies of the solar system. Script by Damond Benningfield

The realm of the giant outer planets is like a transit station for some smaller bodies. They come from beyond the orbit of Neptune, the solar system’s most remote major planet. And like passengers at a hub airport, their destinations are all over the map. These objects are called centaurs. Like the half-human, half-horses of myth, they’re hybrids – they look like both asteroids and comets. Most of them are quiet chunks of rock and ice, like asteroids. But some have haloes or tails of gas, like comets. Centaurs orbit the Sun between Jupiter and Neptune. And their orbits cross those of at least one of the giant planets. They’re small and far away, so they’re hard to find. Even so, astronomers have discovered about a thousand of them. And there could be as many as a hundred thousand that are at least a kilometer across. Centaurs come from a belt of debris beyond Neptune. They’re nudged inward by Neptune’s gravity. None of them will spend more than a few million years in the realm of the giants, though. Instead, the gravity of the planets will give them a kick. Some will be booted out of the solar system. Others will be pushed into the inner solar system. And others will slam into a planet. The biggest centaur is Chariklo. It’s about 160 miles in diameter, and it has a couple of rings. The first centaur ever seen, Chiron, also has rings. And it’s growing new rings even now. More about that tomorrow. Script by Damond Benningfield

Many of the features on the Moon are named for astronomers. So are features on Mars and other planets and moons. And hundreds of asteroids are named for astronomers as well. But you won’t find many features named for astronomers here on Earth. Quite a few streets and schools are named after them. But when it comes to major features, the list is pretty thin – especially in the United States. One of the few is Mount Langley, a 14,000-foot summit in California. It’s named for Samuel Pierpont Langley, who was a long-time director of the Allegheny Observatory. To see more features named for astronomers, though, you need to head south – to Australia, New Zealand, and even Antarctica. In Australia, for example, a river and an estuary are named for Thomas Brisbane, an early governor of the state of New South Wales. And so is the city of Brisbane, the capital of Queensland. In addition to his government duties, Brisbane was an astronomer. He set up Australia’s first major observatory. In New Zealand, several peaks in a large mountain range are named for astronomers, including Galileo and Copernicus. And an entire range is named for Johannes Kepler. In Antarctica, many features are named for James Ross, an early explorer. But Ross himself named several features for astronomers, including Cape Smyth and Mount Lubbock – down-to-earth features named for men who studied the stars. Script by Damond Benningfield

When a dying Sun-like star exhales its final breath, it’s a doozy. The star blows its outer layers of gas into space. That surrounds the star’s dying core with a colorful bubble. The bubble can last for tens of thousands of years before it fades away. One of those bubbles is on the edge of Gemini, which is well up in the east at nightfall. Known as the Medusa Nebula, the bubble is about 1500 light-years away, and it spans more than four light-years. It’s named for one of the Gorgons of Greek mythology. That’s because some of its tendrils of gas have reminded skywatchers of the snakes on Medusa’s head. Those tendrils have been expanding into space for thousands of years. They began their journey when their star could no longer produce nuclear reactions in its core. Gravity squeezed the dying core tighter, making it smaller and hotter. The radiation of the hotter core pushed away the layers of gas around the core. Today, they’re moving outward at more than 30 miles per second. Ultraviolet light from the core “energizes” the gas in the nebula, making it glow like a fluorescent bulb. Different elements glow in different colors. That tells astronomers about the original star, and about the process of its demise. The fate of the Medusa Nebula is shared by all Sun-like stars. So billions of years from now, the Sun will create its own nebula – a colorful bubble blown with its dying breath. Script by Damond Benningfield

There just aren’t enough superlatives to describe the galaxy OJ 287. It’s a quasar – an especially bright object powered by two supermassive black holes. One of them is about 150 million times as massive as the Sun. The other is 18 billion times the Sun’s mass – one of the heaviest black holes yet seen. They team up to produce outbursts that are a trillion times brighter than the Sun – brighter than all the stars in the Milky Way Galaxy combined. OJ 287 is always bright. But every few years, it flares up – the result of interactions between the black holes. Each of them is encircled by a giant disk of gas. As the gas spirals in, it gets extremely hot. That makes the disks extremely bright. The smaller black hole orbits the larger one every 12 years. The orbit is tilted. So every six years, the black hole plunges through the disk around the larger black hole. That can heat some regions to trillions of degrees, producing the flare-ups. Astronomers recently used radio telescopes to take a picture of the system. They saw a long “jet” of particles from the smaller black hole. The jet is twisted by the interactions between the black holes – confirming the profile of this amazing system. OJ 287 is in Cancer, which is low in the east at nightfall. Even though it’s billions of light-years away, OJ 287 is bright enough to see through most amateur telescopes. Script by Damond Benningfield

Saturn’s rings are among the most beautiful features in the solar system – and the most mysterious. Scientists continue to debate how and when the rings formed, and how much longer they might hang around. But the rings aren’t Saturn’s only beautiful and mysterious feature. An almost perfect hexagon of clouds wraps around the planet’s north pole. And scientists continue to debate how it formed and what keeps it going. Saturn is the second-largest planet in the solar system – nine-and-a-half times the diameter of Earth. So the hexagon is giant as well – more than twice as wide as Earth. And it’s long-lasting – it was first observed in 1981. There are several ideas about what shapes the hexagon. Perhaps the leading idea says that winds deep in the atmosphere blow at different speeds at different latitudes. As these jet streams rub against other, they create waves that ripple to the top of the clouds. Those clouds form regular patterns – the sides of the hexagon. There’s one other mystery about the hexagon: It appears only at the north pole, not the south. So scientists are still working to explain this beautiful feature at the top of a beautiful planet. Saturn poses near the Moon the next couple of evenings. It looks like a bright golden star. It’s to the upper left of the Moon tonight, and a bit farther below the Moon tomorrow night. Tomorrow: a system that defies description. Script by Damond Benningfield