StarDate

The winter solstice is the shortest day of the year – the shortest gap between sunrise and sunset. After the solstice, the days gradually get longer, all the way until summer solstice, in June. But the weather doesn’t begin to warm up right after the solstice. On average, in fact, the coldest days of the year in the United States come in February. The weather doesn’t immediately respond to the extra sunlight for several reasons. For one thing, there’s still more darkness than daylight. And for another, the Sun is still hovering at a low angle in the sky. That means the atmosphere blocks more of the Sun’s energy than at other times of the year. Perhaps the biggest factor, though, is the response of the land and oceans. They absorb the Sun’s energy during the longer, warmer times of the year, then release it back into space quite slowly. It’s like putting a roast in the oven. Even after you turn off the heat, the roast continues to get hotter for a while. And when you take it out of the oven, it doesn’t immediately cool off to room temperature – it slowly radiates its heat into the kitchen. The land cools off more quickly than the oceans. But both of them continue to “cook” the atmosphere for a while as they gradually radiate their heat – a process that continues for weeks. And just the opposite happens after the summer solstice – it takes extra weeks to crank up the heat, creating the oven-like conditions of summer. Script by Damond Benningfield

Winter will spring on the northern hemisphere in the wee hours of tomorrow morning – the winter solstice. That makes today and tomorrow the shortest days of the year – the shortest interval from sunrise to sunset. That makes the nights the longest of the year. Over the centuries, many cultures have held special events to commemorate this changing of the seasons. In China, for example, the event has been marked by a festival known as Dongzhi – a name that means “winter arrival.” It’s a celebration of the coming victory of light over darkness, as the Sun reverses course and brings longer days. In ancient Rome, the event coincided with Saturnalia – a festival dedicated to the god Saturn. It was one of the most important events of the year. It began on December 17th and continued for a week. Schools, businesses, and most government activities closed down. People held big banquets, exchanged gifts, and lit their homes with special candles. Scandinavians held the Feast of Juul. They made sacrifices to the hammer-wielding god Thor. They also lit bonfires to celebrate the returning sunlight, and placed a long-burning log in their hearths – the Yule log. Many of the features of Saturnalia and the Feast of Juul have been incorporated into the modern celebration of Christmas – another event that roughly coincides with the winter solstice – a dark time with the promise of sunnier days ahead. Script by Damond Benningfield

Space agencies have been sending missions to the Moon for more than six decades. More than a hundred of them have succeeded. And each success has told us a little bit more about our satellite world. One of the more recent lessons came from a Chinese mission. Chang’e 5 landed in the Ocean of Storms – a giant volcanic plain that forms one of the dark patches on the lunar surface. It scooped up about four pounds of rock and dirt, and brought them to Earth four years ago this week. Earlier this year, scientists reported a big discovery from those samples: Three tiny glass beads were produced by a volcanic eruption about 120 million years ago – far later than any previously known eruption. In fact, the finding suggests that the Moon could still pop off today. Until recently, the best evidence said the last major volcanic activity on the Moon came about two billion years ago. A lunar satellite has photographed some regions that look like they were formed by more recent activity. But that’s hard to confirm without sampling those regions. The Moon lost most of its interior heat long ago. But regions with large amounts of radioactive elements might generate their own heat. That could power volcanoes even now – a new insight from our continued exploration of the Moon. The Moon rises in late evening, with Regulus, the brightest star of Leo, close by. They stay close together as they climb high across the sky later on. Script by Damond Benningfield

If you were camping on Mars right now, you’d see a couple of bright lights low in the west at nightfall. The brighter and higher of the two is Venus. Just as it does for those of us on Earth, it shines as Mars’s brilliant Evening Star. It’s the brightest object in the planet’s night sky other than its two tiny moons. Just below Venus you’d see the other bright object: our own planet Earth, looking like a blue-white star. Earth is quickly dropping lower in the Martian sky, and will disappear in the faint twilight within a few days. That’s because Earth is about to pass between Mars and the Sun. Over the next few Martian evenings, a fainter light will briefly move into view quite close to Earth: the Moon. Tonight, Earth and the Moon will stand so close that you’d probably need binoculars to separate them. But the two bodies are moving apart a bit as viewed from Mars, allowing them to be seen as separate objects with the eye alone – a beautiful sight in the Martian sky. From here on Earth, Mars is putting in a great appearance almost all night. It looks like a bright orange star, and it climbs into good view in the east-southeast by about 8:30 or 9 o’clock. The Moon rises below it, about 45 minutes later. They climb high across the sky during the night, and are high in the west at dawn – a beautiful sight in Earth’s night sky. We’ll talk about the Moon and another bright light tomorrow. Script by Damond Benningfield

It’s generally a good idea to make sure your potatoes are dirt-free before you dig in. If you’re on Mars or the Moon, though, it might be good to mix them with dirt. The combination might make an especially strong form of concrete. Scientists and engineers have spent years thinking about ways to build habitats on other worlds. It’s too expensive to carry everything from Earth, so habitats will have to be built mostly from local materials. But some ingredients will have to come from home. So scientists have experimented with everything from fungus to urine to cow’s blood. One team even used a ground-up bit of a meteorite to make Lego blocks. A team in England mixed dehydrated starches with simulated Mars and Moon dirt. They tried several plants – rice, corn, tapioca, and potatoes. And the spuds made the best concrete – a mix they called StarCrete. The blend was about twice as strong as regular concrete. And it could be made from potatoes grown in the habitats. It’ll be quite a few years before anyone is actually living off-world. So there’s plenty of time to dream up new ways to build habitats on Mars and the Moon. The two worlds appear quite close together tonight. As they climb into view, in early evening, bright orange Mars is just below the Moon. The Moon will move closer to Mars during the night, and they’ll appear to almost touch in the wee hours of the morning. We’ll have more about Mars tomorrow. Script by Damond Benningfield

To the eye alone, the star Castor looks like a single pinpoint of light. It’s one of the “twins” of Gemini. But Castor takes the “twins” business to an extreme. Instead of a single star, it appears to be a system of six stars, consisting of three binaries – three sets of twins. The sets are known as Castor A, B, and C. The members of each pair are so close together that not even the biggest telescopes can see them as individual stars. Astronomers use special instruments to determine the details of each star. Castor A and Castor B each consist of a star that’s bigger, heavier, and brighter than the Sun, plus a small, faint companion. A and B orbit each other once every 450 years or so. Astronomers have been watching the two binaries for hundreds of years. That means they’ve seen enough of an orbit to know for sure that the two binaries are gravitationally bound to each other. Both of the stars of Castor C are small and faint. The system appears to orbit the other two binaries about every 14,000 years. But astronomers haven’t been watching Castor C as long as the others. So they still can’t be completely certain that Castor C is tied to the others – making one of the “twins” of Gemini a family of three sets of twins. Look for Gemini near the Moon at nightfall. Castor is to the left of the Moon, with brighter Pollux to the Moon’s lower left. Mars rises well below them, and we’ll talk about it tomorrow. Script by Damond Benningfield

Call it “The Revenge of the Scrawny.” The Procyon star system was born with two stars. One was bigger and heavier than the other, so it shined much more brightly. Because of that showiness, though, it burned out much more quickly. Today, all that’s left of the star is its tiny, dead core – while the scrawny companion shines on. Procyon climbs into good view, due east, by about 9 p.m. That’s just before Sirius, the Dog Star, which rises to the lower right of Procyon. In fact, the name “Procyon” means “before the dog” – an indication that it leads the Dog Star into the sky. What we see of Procyon is the scrawny survivor, Procyon A. But “scrawny” is relative – the star is bigger and heavier than the Sun. But when the system was born, the other star, Procyon B, was much more impressive. Because of its greater mass, it “burned” the nuclear fuel in its core much more quickly. That made it much brighter than Procyon A. It used up its fuel in a hurry, though. It cast its outer layers into space, leaving only its hot but dead core – a white dwarf. It’s more than half the mass of the Sun, but only about as big as Earth. It’ll shine faintly for billions of years. Procyon A is about to follow the same path. It’s nearing the end of its “prime” phase of life. Soon, it’ll puff up to giant proportions. Then it, too, will cast off its outer layers. That will leave Procyon with two “dead” stars – glowing embers in the cosmic night. Script by Damond Benningfield

Moon lovers, rejoice! Tonight’s full Moon will be in view longer than any other full Moon of the year – the Long-Night Moon. The full Moon lines up opposite the Sun, so it does just the opposite of what the Sun does in the daytime sky. The winter solstice – the shortest day of the year – is just a week away, so the Sun is putting in its most feeble appearance` of the year. It rises late, sets early, and scoots low across the south during the day. So the full Moon does just the opposite – it rises around sunset, climbs high across the sky during the night, and sets around sunrise. The difference is more dramatic as you go farther north. San Antonio, for example, will see about two-and-a-half hours more moonlight than sunlight. But from Seattle, the difference is about seven hours – ten-and-a-half hours of sunlight, followed by seventeen-and-a-half hours of moonlight – a long night to watch the silvery glow of the full Moon. And as you watch the Moon, look for some bright companions around it. The brilliant planet Jupiter stands to the right of the Moon at nightfall. The star Aldebaran, the bright “eye” of Taurus, the bull, is about the same distance to the right of Jupiter. And the star Elnath, the tip of one of the bull’s horns, is closer to the lower left of the Moon. The Moon will slide especially close to Elnath a few hours later – adding to the beauty of a long night of moonlight. Script by Damond Benningfield

Friday the 13th brings a bit of bad luck for skywatchers this year. The Geminid meteor shower is at its peak, and it’s generally one of the year’s best, with peak rates of more than a hundred meteors per hour. But the Moon is almost full, so it’ll overpower all but the brightest meteors. A meteor forms when a bit of debris from a comet or asteroid rams into the atmosphere at tens of thousands of miles per hour. That generates intense heat, so the particle burns up, leaving a streak of light. Most of the streaks vanish in a few seconds. But some of them leave “persistent meteor trains” – glowing trails that can last for a few minutes to more than an hour. A recent study says that most of these long-lived trains are formed by particles that pass deeper into the atmosphere. Scientists recorded thousands of meteors above New Mexico. They found that about five percent of meteors left a train that was visible for at least five minutes – a much higher fraction than in earlier studies. There wasn’t much correlation between the persistence of the train and either the speed or brightness of the meteor. But there was a correlation with the meteor’s altitude. Those that descended to about 55 miles were much more likely to create long-lasting trains. At that altitude, metals in the chunk of space debris may react with ozone in the atmosphere long after the meteor burns up – creating a glowing trail in the night sky. Script by Damond Benningfield

Life has a way of adapting when things change. An impact by a giant asteroid wiped out most of the life on Earth about 66 million years ago. Within years, though, life began to rebound – especially plant life. Dense forests developed. And a recent study says that, in the western hemisphere, the winners included grapes and other viny plants. The asteroid was about six miles in diameter. According to another recent study, it was born in the early solar system, beyond the orbit of Jupiter. An encounter with another body kicked it toward the Sun – and Earth. When the asteroid hit Earth it created a huge fireball and a shockwave that destroyed everything within hundreds of miles. It also gouged a giant crater, which blasted billions of tons of water and rock into the sky – enough to blot out the Sun for years. It also created acid rain across the globe and caused other major problems. So most life perished. But research in recent years has shown that new plant life quickly thrived. One example was found by researchers who were studying fossilized grapes in the Americas. They discovered the oldest grape seed ever seen in the western hemisphere – 60 million years old. The researchers suggested that, without giant animals around to knock down trees, forests could have grown taller and thicker. Viny plants that climbed the trees also developed – including grapes. We’ll have more about space rocks tomorrow. Script by Damond Benningfield

Antarctica has been a target in a cosmic shooting gallery over the past few million years. At least three big asteroids have exploded above the continent. And two of them were close enough to the ground to cause major damage. In fact, such an explosion over a populated region today could kill millions. Scientists have found evidence of the explosions in the Antarctic ice. Thirty years ago, for example, teams looking for meteorites atop the ice found a dark streak in the ice. The streak contained a lot of dust. The minerals in the dust grains were like those found in the most common type of meteorite. But they’d been changed by interactions with the ice. Scientists concluded the grains were embedded in the ice when an asteroid exploded above Antarctica about two and a half million years ago. A powerful shockwave slammed into the surface, showering it with dust grains. Similar groups of dust grains have revealed two other explosions, both within the past half-million years. One of the asteroids weighed about a hundred thousand tons when it plunged into the atmosphere. It exploded so high that the damage was minimal. The other weighed a million tons. It exploded deep in the atmosphere. Heat from the blast vaporized a lot of ice, while a shockwave would have gouged a crater in the ice. Bits of the asteroid slammed into the surface – of the Antarctic shooting gallery. More about asteroid impacts tomorrow. Script by Damond Benningfield

Several scientists plan to spend the next few weeks snowmobiling across the ice sheets of Antarctica. It’s not a vacation or an extreme-sports event. Instead, they’ll be hunting for meteorites – bits of space rock sitting atop the ice. About two-thirds of all the meteorites found on Earth have been recovered from Antarctica. They aren’t more common there, but they are easier to find. There aren’t many Earth rocks on the ice, and meteorites look different. Many of them have been frozen below the surface, then exposed as the ice vaporized. Others are exposed when ice piles up at the base of mountains. Scientists find about a thousand meteorites every southern summer, during expeditions that last a few weeks. But a recent study says that pace might not hold in the decades ahead – a result of our changing climate. Under warmer conditions, the dark meteorites could heat and melt the ice below them – then sink into the ice, where they’d be lost from view. Under the temperatures predicted for the rest of the 21st century, about a quarter of the Antarctic meteorites could drop from view by 2050. And three-quarters could be hidden by the end of the century. Scientists say there may be hundreds of thousands of meteorites sitting atop the ice today. But future expeditions might have to spend more time on their snowmobiles to find them. We’ll have more about Antarctic meteorites tomorrow. Script by Damond Benningfield

Making planets is messy. It involves giant clouds of gas and dust, and frequent collisions between planetary building blocks. And it takes tens of millions of years to sort out. A good example of how it works is seen in the star system Beta Pictoris. The star is only about 20 million years old. It has two known planets, both of which are much more massive than Jupiter, the giant of our own solar system. But the star also is encircled by huge disks of gas and dust – the raw materials for making planets. And the supply is constantly changing. In one example, Webb Space Telescope revealed a ribbon of dust that looks like a cat’s tail. The tail might be debris from a collision between two big space rocks a century ago as seen from Earth. And earlier this year, scientists reported evidence of a more recent collision. They compared observations made by Webb to those from another space telescope a couple of decades ago. They found that two clumps of dust had vanished. That suggests there was a collision between two giant space rocks as recently as 20 years ago. The impact blasted out huge clouds of dust, which were seen by the earlier telescope. But the star has blown away most of that dust – part of the messy process of making planets. From the far-southern U.S., Beta Pictoris just climbs into view around midnight, quite low in the south. It’s to the upper right of Canopus, the second-brightest star in the night sky. Script by Damond Benningfield

Astronomers were scattered across the globe 150 years ago today to watch a black dot move across the face of the Sun – a transit of the planet Venus. They’d use their observations to measure the distance to the Sun, revealing the scale of the entire solar system. Astronomers already knew that distance to within a few million miles. But they wanted a more accurate reading. They’d get that by watching the transit from many locations. Venus would cross the Sun at a slightly different angle from each location. Measuring the difference would reveal the distance to both Venus and the Sun. So the nations of the world mounted dozens of expeditions. They spread out from Cairo to Honolulu, from Cape Town to Tasmania. Each trip required years of planning, plus months of travel. The scientists carried tons of equipment, including prefab observatories and housing. They spent weeks setting up. And they had to calculate the precise longitude of each site, which was a key factor in measuring the angles to Venus and the Sun. Clouds blocked the view from many of the sites. Even so, the astronomers still managed to snap thousands of pictures of the event, and take many more measurements visually. Then came years of tedious calculations. And the results were disappointing. The transit didn’t improve the known distance to the Sun at all. So astronomers fanned out once again for the next transit, eight years later. Script by Damond Benningfield

The Moon barrels toward the planet Saturn tonight. Saturn looks like a prominent star to the upper left of the Moon at nightfall. As they drop lower in the sky, the gap between them will close. From most of the country, they’ll be a couple of degrees apart as they set – the width of a finger held at arm’s length. From Alaska and Hawaii they’ll be even closer – almost touching each other. Although Saturn is bright, it’s a little fainter than average now. That’s because of our viewing angle on the planet’s rings. Most of the time, the rings are tilted enough to add quite a bit to Saturn’s luster. But right now, we’re seeing them almost edge-on. At that angle, they just about disappear. The rings will be precisely edge-on early next year. But Saturn will appear so close to the Sun that it’ll be hidden in the Sun’s glare. Saturn’s rings turn edge-on to Earth roughly every 15 years. It takes almost 30 years for Saturn to orbit the Sun. Because of the planet’s tilt on its axis, its north and south poles take turns dipping toward the Sun – just like Earth’s poles. The changing angle brings a change in seasons – also like Earth. The rings turn edge-on to Earth and the Sun at the equinoxes – the start of spring and fall. Northern fall arrives next year. So the rings will look like a thin ribbon across the planet for a good bit longer. After that they’ll begin to open up – providing a better view of Saturn’s majestic rings. Script by Damond Benningfield

Jupiter is like a mini solar system. The Sun’s largest planet has a family of almost a hundred known moons. Most of them are little more than cosmic flotsam – chips of rock and ice no more than a couple of miles wide. But a few rank among the most interesting worlds in the solar system. The four largest moons are Io, Europa, Ganymede, and Callisto. They account for more than 99.9 percent of the combined mass of all of Jupiter’s moons. They probably formed along with Jupiter itself, from dust and gas that encircled the newborn planet. Io is the most volcanically active body in the solar system. It’s covered by hundreds of volcanoes, lakes of molten lava, and other features. The other three big moons appear to have oceans of liquid water buried below their icy crusts. And Europa’s ocean is considered one of the most likely homes for life in the solar system. Some of Jupiter’s little moons could have formed at the same time as the big guys, then perhaps moved into different orbits. But most of them appear to be the remains of asteroids that Jupiter captured when they wandered close. The asteroids were blasted apart in big collisions, leaving small chunks of ice and rock in orbit around the planet. Jupiter is low in the east-northeast at nightfall, and climbs high overhead during the night. It looks like a brilliant star. Through binoculars, the four big moons look like tiny stars lined up near the planet. Script by Damond Benningfield

Jupiter is backing up. The planet is in the “retrograde” portion of its trip across the sky. Instead of its usual eastward motion against the background of stars, it’s moving to the west. And right now, it’s about half way through that “reverse” period. That’s because Jupiter is at opposition – it lines up opposite the Sun in our sky. Some people see a planet’s retrograde period as an omen. What kind of omen depends on the planet and the person making the prognostication. In reality, retrograde is just an optical illusion. The planet doesn’t move backward at all. It only appears to do so because of the relative motions of Earth and the other planet. Now, for example, Earth is moving past Jupiter in our smaller, faster orbit around the Sun. As we do so, Jupiter appears to stop and back up for a while. The effect is like passing a slower car on the highway. For a while, the other car appears to move backward against the background of buildings and trees. But when you get far enough away it appears to resume its forward motion. The car itself hasn’t actually changed direction – it only looks that way. Jupiter entered retrograde in early October. It’ll resume its normal course in early February. Jupiter is low in the east-northeast as darkness falls, and climbs high across the sky during the night. It looks like a brilliant star, with Aldebaran, the brightest star of Taurus, close by. More about Jupiter tomorrow. Script by Damond Benningfield

The Moon and the planet Venus – the brilliant “evening star” – put on a spectacular display tonight. They’ll be separated by the width of a couple of fingers held at arm’s length. They’ll be easy to spot not long after sunset, and they’ll remain in good view for more than two hours. Venus and the Moon are in almost opposite “phases” tonight. The Moon is a thin crescent – the Sun lights up only about 14 percent of the hemisphere that faces our way. It’s nighttime across the rest of that hemisphere. That tells us that the Moon is lining up only a little way to the side of the line between Earth and the Sun. Over the next week and a half, the Moon will move farther from the Sun as it travels along its orbit. Eventually, it will line up opposite the Sun. The Sun will shine on the entire Earth-facing hemisphere, so the Moon will be full. Venus, on the other hand, is in a “gibbous” phase – the Sun lights up two-thirds of its Earth-facing hemisphere. That tells us that Venus is on the far side of the Sun. But it’s looping over to our side, and will pass between Earth and the Sun in March. As the planet slides our way, the Sun will light up less and less of the portion we can see. Before Venus disappears in the Sun’s glare, it’ll be a thin crescent. Yet for most of that time, Venus will grow brighter, because it’ll be getting closer – setting up more beautiful encounters with the crescent Moon. Script by Damond Benningfield

The days are continuing to get shorter here in the northern hemisphere. That provides a lot of time to get out and enjoy the night sky. But over the next few weeks, one part of that trend will head the other way: The Sun will begin to set a little later each night – even though the shortest day of the year is still weeks away. The shortest day is the winter solstice, December 21st. So between now and then, the total length of time between sunrise and sunset will shrink. Yet the date of earliest sunset comes before the solstice. There are a couple of reasons for that. One is Earth’s tilt on its axis. It causes the angle and location of sunrise along the horizon to change from day to day. The other reason is Earth’s lopsided orbit. It causes the Sun to return to the same point in the sky a little earlier or later in the day depending on whether we’re close to the Sun or farther away. When you add these factors together, you find that the date of earliest sunset works its way from south to north. For the latitudes of Honolulu and Miami, the earliest sunsets happen about now. For higher latitudes, such as Denver, they’ll take place in a few days. And for cities that are even farther north, such as Anchorage, earliest sunset happens just a few days before the solstice. To balance things out, the date of the latest sunrise comes after the solstice, working from north to south – a few more minutes of darkness before dawn. Script by Damond Benningfield

Earth is about to catch up to Jupiter, the solar system’s largest planet. We’ll pass between Jupiter and the Sun on Saturday. As a result, Jupiter is putting on its best performance of the year. It’s in view shortly after sunset and stays in view all night. Jupiter is brightest for the year as well. Jupiter is the fifth planet out from the Sun – about five times farther from the Sun than Earth is. At that distance, it takes almost 12 years for the planet to complete a single turn around the Sun. Since Earth is much closer to the Sun, it zips past Jupiter every 13 months. Jupiter lines up opposite the Sun during those passages – a point called “opposition.” That’s when the two planets are closest. This week, Jupiter will be just 380 million miles away. The combination of that proximity and the good viewing angle means that Jupiter is brightest at opposition. It’s almost always the third-brightest object in the night sky – only the Moon and the planet Venus regularly outshine it. But it’s especially bright right now – it looks like a brilliant star, shining all night long. Look for Jupiter low in the east-northeast at nightfall. It’s between the horns of Taurus. The bull’s bright orange eye, the star Aldebaran, glows to the upper right of Jupiter. The planet climbs high across the sky during the night, and is in the west at dawn, with Aldebaran below it. We’ll have more about Jupiter during the week. Script by Damond Benningfield