StarDate

The gibbous Moon takes aim at the planet Uranus tonight. And for skywatchers in the western part of the United States, it’ll catch it — the Moon will pass in front of the giant planet, briefly hiding it from view.

Uranus is the Sun’s third-largest planet — four times the diameter of Earth. Most of its bulk consists of a slushy mix of water, methane, and other compounds, with a solid core of rock and metal.

Scientists know about the interior in several ways. They can measure the planet’s gravitation pull on other planets and on its own moons. That reveals the planet’s mass. They also know its distance, which reveals its size. From that, they calculate its density — less than any other planet except Saturn.

Next, they use computer models to simulate what materials came together to make the planet, and how the planet might have evolved over the eons. And that gives them a rough idea of its composition and structure.

To get more detail about the planet’s structure, we’ll need to send more spacecraft. As they pass close to the planet, they can produce much better maps of its gravitational field — providing a good look at what’s going on inside this giant planet.

Tonight, the Moon will pass in front of Uranus around 1 a.m. Central Time, and will hide it for a few minutes. For most of the country, the two will just miss each other. Through a small telescope, Uranus will look like a tiny star sliding just below the Moon.
 

Script by Damond Benningfield

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The light display from a rocket launch may hang around long after the rocket itself is gone. But the display is visible only from narrow strips of Earth’s surface — mainly from Canada, Scandinavia, and Russia.

The rockets may create bands of noctilucent clouds — Latin for “night shining.” The clouds form at altitudes of about 50 miles — the highest clouds of all. Water vapor freezes around grains of dust from meteors, forming ice crystals. The clouds are visible only about an hour before sunrise or after sunset, when the Sun is at the right angle to light up the clouds but not the ground.

Noctilucent clouds were first reported in the late 1800s. At first, they were seen only fairly close to the poles. In recent decades, though, they’ve crept toward lower latitudes. And they’ve become a lot more common.

Much of the increase may be the result of climate change, which pumps more water vapor into the upper atmosphere. But a recent study says that rocket launches are boosting the clouds as well — especially launches that take place in the morning. That’s when high-altitude winds are most efficient at pushing rocket exhaust toward the poles. And the launch-influenced clouds are most common in July, when the combination of high-level water vapor and temperature is just right.

With the number of launches reaching record levels, so may the reports of noctilucent clouds — beautiful displays in the twilight.
 

Script by Damond Benningfield

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The roster of constellations includes some weird and wonderful creatures. There’s a dragon, a unicorn, and two centaurs. But perhaps the weirdest of all is Capricornus, the sea-goat. It’s half goat and half fish.

In Greek mythology, it’s associated with the god Pan, who was half goat and half man. The story says that he was about to be attacked by the monster Typhon, so he jumped into the water to escape. At the same time, he tried to transform himself into a fish to speed his getaway. But he botched the spell, and turned his human half into a fish, but kept the half that was a goat.

The constellation is low in the southern sky as darkness falls at this time of year. Its brightest stars form a wide triangle. None of the sea-goat’s stars is especially bright, though, so you need a fairly dark sky to make them out.

The brightest forms the left point of the triangle. It’s known as Deneb Algedi — “the tail of the goat.” It’s actually a system of at least two stars. The leader is about twice as big and heavy as the Sun, and shines several times brighter. The other is quite similar to the Sun.

The two stars orbit each other about once a day. As they do so, each star passes in front of the other for a bit. When the fainter star crosses the brighter one, the system’s overall brightness drops by about a quarter. That’s just enough for a skilled observer to notice with the eye alone — a slight flicker for one of the night sky’s oddest creatures.
 

Script by Damond Benningfield

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There’s a full Moon tonight, with the brilliant planet Jupiter hanging close by.

As the first full Moon after the Harvest Moon, this one’s known as the Hunter’s Moon. It might not hold the same place in the American heart as the Harvest Moon, but in centuries past, it was just as important. Its light made it easier for hunters to find game in the bare fields, allowing them to stock up for the coming winter.

It was important for cultures all across the northern hemisphere — especially at higher latitudes, where the winters were darker and colder. But the name of the Hunter’s Moon came from the native cultures of North America. In fact, that’s where the names of almost all the full Moons came from.

All the names reflect the season in which the Moon appears. So in winter we have the Frost Moon and Wolf Moon — times of cold and want. And in summer — a time of warmth and abundance — we have the Strawberry Moon, Grass Moon, and Honey Moon.

European settlers adopted the names and brought them back to their homelands. So the name “Hunter’s Moon” first appears in Europe in the early 1700s.

Here in the U.S., the name has become more popular in recent decades. It’s been the title of several movies and TV episodes, novels, songs and albums, and video games. There’s even a Hunter’s Moon festival in Indiana — a throwback to centuries past, when grateful people celebrated the bounty made possible by the Hunter’s Moon.
 

Script by Damond Benningfield

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The night sky is filled with beautiful objects with beautiful names — names from ancient Arabic, Greek, and Latin. But the most prominent object in the night sky doesn’t have one of those names. It’s the Moon — a name that means — well, the Moon. The name is a little more lyrical in other languages — Luna in Spanish, Lune in French — but they all mean the same thing. There’s no real story behind any of them.

That’s not the case for the moons that orbit the brilliant planet Jupiter, which is to the left of the Moon early this evening.

Jupiter’s first four moons were discovered in 1610. Since they attended Jupiter — the king of the Roman gods — eventually they were given names associated with Jupiter. Callisto, Europa, and Ganymede were all seduced by Jupiter, while Io was driven mad by his jealous wife.

The next Jovian moon wasn’t discovered until almost three centuries later, but astronomers stuck with the naming theme. They chose Amalthea, after a nymph who had nursed the young Jupiter.

Later discoveries also kept the theme, but widened it a bit — they included characters related to Zeus, the Greek god whom the Romans adopted as Jupiter. So that gave us names like Sinope and Lysithea.

That tradition continues today, although it takes a little more work. Even though Jupiter and Zeus were grand characters with rich histories, even they eventually ran out of nursemaids and partners.
 

Script by Damond Benningfield

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Like a young child peeking over the edge of the kitchen table, the planet Mercury just peeks over the eastern horizon the next few mornings. It’s quite low in sky 30 or 40 minutes before sunrise, so you need a clear horizon to find it. But it looks like a bright star, so it should hold in view for a few minutes as the dawn twilight grows bolder.

Mercury is tougher to spot than the other four naked-eye planets because it’s closer to the Sun. In fact, it’s the closest of all the Sun’s planets. So from Earth, it’s never visible for more than a short time — either after sunset or before sunrise.

Mercury will stand farthest from the Sun for its current morning appearance on Saturday — about twice the width of your fist held at arm’s length. After that, it’ll start to drop back toward the Sun. It’ll pass behind the Sun — and into the evening sky — in about a month.

But as Mercury moves closer to the Sun, our viewing angle changes. As a result, Mercury will get a little brighter day by day for the next few weeks. On the 13th, for example, it’ll be twice as bright as it is tomorrow. That will make it a little easier to spot for a few days. But as it slides back toward the Sun, it’ll eventually drop so low that you won’t be able to find it at all.

So for now, enjoy elusive little Mercury as it peeks over the horizon and into view shortly before sunrise.

Tomorrow: naming some “jovial” moons.
 

Script by Damond Benningfield

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Saturn is made almost entirely of hydrogen and helium — the same ingredients that make up the Sun. But like an apple, it’s topped by a thin “skin” — in this case, layers of clouds that contain water, ammonia, and other ingredients. They’re a few hundred miles deep. But compared to Saturn’s diameter of 72,000 miles, that’s not much.

As you plunge into those layers, the temperature changes dramatically. At the top of the clouds, it averages about minus-275 degrees Fahrenheit. The temperature actually drops a bit below that altitude because the top of the atmosphere absorbs a little of the Sun’s energy.

At a pressure of one bar — the equivalent of the surface pressure on Earth — the temperature jumps to about minus-220. By comparison, on Earth the surface temperature is about 60 degrees.

Clouds made of water ice start below that level and extend for scores of miles. At the bottom of that layer, the atmosphere has warmed up to almost freezing.

Finally, the lower layer, made of water and ammonia, goes much deeper. The temperature at the bottom of that layer may reach 135 degrees.

And thanks to the pressure inside Saturn, plus a possible “rain” of helium drops, the temperature keeps going up until you reach the center — a toasty 21 thousand degrees.

Saturn stands close to the upper right of the Moon as night falls. It looks like a bright star — thanks in part to its cold, cold clouds.
 

Script by Damond Benningfield

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Vega, one of the brighter stars in the night sky, stands high overhead as twilight fades away this evening. It’s the leading light of Lyra, the harp. A possible message to alien life may target that same constellation. And if it does, October 4th is one of the two dates that are the best times to send it.

A team of scientists has crafted a new message based on one that was transmitted in 1974. Known as “Beacon in the Galaxy,” the binary message consists of basic text and illustrations.

It’s made of 13 parts. The first would use math and physics to establish a common language. Another would “timestamp” Earth’s location in the galaxy using star clusters as reference points. And others would provide information about Earth, the solar system, and humans. All of it would be squeezed into just 26 kilobytes — the size of a small text file.

The message would be transmitted from two radio telescopes. One is a giant dish in China. The other is an array of smaller dishes in California. When the team combined the locations of the telescopes, the location of the target, and other factors, they concluded that October 4th and March 30th are the best times to send the message.

There’s no guarantee that the message will ever be dispatched. Many scientists say it’s too risky — it could bring hostile aliens to attack us. In any event, there would be a long wait for a reply: the target region is thousands of light-years away.
 

Script by Damond Benningfield

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From 1876 to 2001, the Homestake mine in South Dakota yielded almost 1400 tons of gold. It shut down when costs went up and gold prices went down.

Today, though, the mine is just about as busy as ever. Hundreds of scientists use its tunnels as laboratories. Shielded from radiation from space, it’s a good site for trying to catch exotic particles — especially dark matter.

In fact, the world’s most sensitive dark-matter detector finished its first test run this year. It didn’t find any dark matter, but it did show that it’s ready to look for it.

Dark matter exerts a gravitational pull on the normal matter around it. And it appears to account for about 85 percent of all the matter in the universe. But it produces no energy, so we don’t know what it is. The leading idea has been that it’s a type of heavy particle. But no experiment has seen any such particles.

The new experiment is LUX-ZEPLIN — L-Z for short. It’s almost a mile below the surface. It consists of a tank filled with seven tons of ultra-pure liquid xenon, surrounded by a much larger tank of water. If a particle of dark matter hits an atom of xenon, the impact should produce a flash of light. A second flash would come from an electron released in the collision. Studying the flashes in detail should reveal what caused them — perhaps a particle of dark matter.

L-Z will run for several years — looking for scientific “gold” far below South Dakota.
 

Script by Damond Benningfield

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One of the most colorful star systems around climbs high across the sky on autumn nights. A telescope reveals two stars — one yellow orange, the other blue.Almach is one of the brightest stars of the constellation Andromeda the princess. It’s in the northeast as night falls, and passes high overhead in the wee hours of the morning.The system is about 350 light-years away. At that great distance, the eye alone sees only a single point of light. It takes a telescope to reveal Almach’s double identity.The orange star is nearing the end of its life, so it’s going through some big changes. Its core has gotten smaller and hotter, so its radiation pushes harder at the surrounding layers of gas. That’s caused the star to puff up to giant proportions — it’s several dozen times wider than the Sun. And that expansion has caused the star’s surface to get cooler, which is why it looks yellow orange.The blue star actually consists of three stars. One is visible through a small telescope, a second comes into focus with a bigger telescope, and the third reveals its presence only to special instruments.All three stars are bigger and more massive than the Sun. They’re also hotter, so their surfaces are white with a hint of blue. Seen next to their orangey companion, their combined light looks even bluer than it really is — making Almach one of the most beautiful double stars in the galaxy.Tomorrow: mining astronomical gold. Script by Damond Benningfield

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