StarDate

You don’t have to be an astronaut to see the far side of the Moon – at least some of it. A little bit of that hemisphere wobbles into view every month – an effect known as “libration.” So we can actually see 59 percent of the lunar surface.

The Moon is “locked” in such a way that the same hemisphere always faces Earth. That means we see the same features all the time – the same dark volcanic plains, and the same lighter-colored jumbles of mountains, valleys, and craters. Almost.

As the Moon orbits Earth, we get some peeks at the other hemisphere – the far side. The Moon is tilted a bit on its axis, for example. So as it orbits Earth, it appears to “nod” up and down a bit – just as Earth appears to nod as seen from the Sun. That allows us to see just beyond the north and south poles.

Also, the Moon’s distance from Earth isn’t constant – it varies by almost 30,000 miles. As a result, the Moon’s orbital speed goes up and down. But the rate at which the Moon spins on its axis remains constant. That allows us to see a little bit around the eastern and western edges – glimpses of a little bit of the lunar farside.

As darkness falls this evening, we’ll see a little bit beyond the north pole. We’ll also see a little bit beyond the western limb – or would if that part of the lunar disk weren’t in darkness. As a bonus, the Moon has a close companion: Antares, the bright heart of the scorpion.

Script by Damond Benningfield

The stars that represent the head of the scorpion are impressive families within an even more impressive clan. Each of them looks like a bright point of light. But each one consists of two or more stars, all of which are much bigger, heavier, and brighter than the Sun.

The stars align near the Moon this evening. From the top, the stars are known as Pi, Delta, and Beta Scorpii.

Pi Scorpii consists of three stars. The most massive of the trio is destined to blast itself to bits as a supernova sometime in the next few million years. And another member appears to be just massive enough to become a supernova as well.

Delta is a binary. One of its stars is also a future supernova, while the other is along the dividing line – it might be heavy enough to become a supernova, but it might not.

Beta is the brightest of the three stars. It’s also the most complicated. It consists of six stars that orbit each other in a complex gravitational dance. At least two of the stars will become supernovas, and perhaps as many as four.

All three systems were born from the same giant complex of gas and dust. And all of their stars are quite young – no more than about 15 million years old, compared to four and a half billion years for the Sun.

Another sibling is Antares, the bright orange star that marks the scorpion’s heart. The Moon will snuggle up to it tomorrow night, and we’ll have more about that tomorrow.

Script by Damond Benningfield

Scorpius is immersed in the Milky Way – the hazy band of light that outlines the disk of the Milky Way Galaxy. Because of that, the constellation is home to a dense variety of star clusters. Some of them are young, so they hold some especially bright stars.

Two examples are Messier 6 and 7.

M7 is the brighter of the two. Under dark skies, it’s fairly easy to see with the unaided eye. It’s about a thousand light-years away, and it contains hundreds of stars.

M7 appears to be about 200 million years old. At that age, all of its most-massive stars have long since blasted themselves to bits. That’s because heavy stars use up their nuclear fuel in a hurry. But the cluster still contains some stars that are a good bit bigger, brighter, and heavier than the Sun.

M6 may be just half as old as M7, so some of its stars are more impressive than any in M7. But astronomers have cataloged fewer stars there. And the cluster is hundreds of light-years farther than M7, so it’s harder to see – a faint family of stars in the Milky Way.

Look for the clusters quite low in the southern sky at nightfall. They’re to the upper left of the stars that form the “stinger” of the scorpion. M7 is about half way between the stinger and the “spout” of the teapot formed by the next-door constellation Sagittarius. Fainter M6 is a little higher in the sky. Both clusters are good targets for binoculars.

Script by Damond Benningfield

A few star patterns are easy to pick out. There’s Orion, with his prominent “belt.” There’s the Big Dipper. And on summer evenings, there’s Scorpius. It really does look like a scorpion skittering along the horizon.

Some of the stars that outline the scorpion are related – they were born from the same giant complex of gas and dust. That includes Antares, the scorpion’s bright orange heart.

Antares and the other family members we can see with our eyes alone are all supergiants – many times the size and mass of the Sun. And all of them are quite young – no more than 10 million to 15 million years old. None of these showoffs will stick around much longer, though. Such heavy stars “burn” through their nuclear fuel in a hurry. At the end of their short lives, they explode – briefly outshining the combined light of most of the galaxy’s other stars.

But the supergiants aren’t the only stars in this impressive family. Thousands of smaller and fainter stars have been born in the same complex. And thousands more are taking shape today – less-flashy stars that will outlive their brilliant siblings.

Scorpius is in the south at nightfall, quite low above the horizon. It has a curving body with a prominent “stinger” at the end, like a big letter “J.” Some cultures have seen it as a fishhook. Either way, it’s one of the easiest star patterns to pick out in all the night sky.

More about Scorpius tomorrow.

Script by Damond Benningfield

The Moon gets especially cozy with the star Spica this evening. From much of the United States, in fact, the Moon will pass in front of the bright star, blocking it from view for a while. Astronomers will keep an eye on the event – called an occultation – to learn more about Spica.

In earlier days, occultations also helped scientists map the Moon. The way a star disappeared and reappeared could reveal the contours of lunar mountains, valleys, and craters. We still see those features during occultations, but they’ve been well mapped.

Scientists have been making maps of the Moon for centuries. Perhaps the earliest known map – a truly accurate chart of lunar features – was created by William Gilbert, a British scientist and a physician to Queen Elizabeth I. He drew it no later than 1603, before the invention of the telescope, using his eyes alone. But it wasn’t published until 1651.

By then, Polish astronomer Johannes Hevelius had published a whole atlas of the Moon, made with a telescope. He named many of the lunar features. Eventually, though, the names were superseded by those created just a few years later by Giovanni Riccioli, a scientist and Jesuit priest. And most of his names are still in use today.

Many more names have been added in recent decades, thanks to the detailed pictures taken by orbiting spacecraft. All of those names are found on modern maps of the Moon.

Script by Damond Benningfield

It would take a couple of hours to fly from Chesapeake Bay, on the coast of Virginia, to the forests and swamps of eastern Texas. But about 35 million years ago, some brownish-green bits of glass made the trip in just minutes – the result of a massive collision between Earth and a space rock.

The rock was an asteroid or comet, and was perhaps two or three miles in diameter. It slammed into the coastline at tens of thousands of miles per hour. The impact gouged a crater about 50 miles across and a mile deep, which later became Chesapeake Bay.

The collision vaporized the space rock and some of the ground it hit. It also blasted millions of tons of water, mud, and granite high into the sky.

The heat of the impact melted sand and some of the other debris. Molten droplets blasted through the air at thousands of miles per hour. Many of them were sculpted into aerodynamic shapes, like teardrops or spheres. As they cooled, they fell to Earth as pieces of glass known as tektites.

A major field of them has been found in Georgia. Another is in Texas. Its tektites are known as Bediasites. They’re named for a small town, which took the name from a native tribe that inhabited the region long ago.

The first Bediasites were discovered in 1936. The largest is nicknamed the “Star of Sabine” for the location where it was found. It weighs about seven ounces – a large chunk of colored glass that crossed half a continent in a hurry.

Script by Damond Benningfield

A dwarf planet far from the Sun may spend most of its time outside the Sun’s influence. In fact, it might have come from interstellar space – from another star.

Sedna takes more than 11,000 years to orbit the Sun. It’s passing through the Kuiper Belt – a wide “doughnut” far beyond the orbit of Neptune, the Sun’s most distant major planet. Right now, Sedna’s almost eight billion miles from the Sun – about as close as it ever gets. At its most distant, it’s more than 10 times farther. That places it far beyond the Kuiper Belt – and well outside the “bubble” produced by the Sun’s magnetic field.

Scientists have speculated that Sedna started in the Oort Cloud – a big “shell” of rocky, icy bodies that surrounds the Sun. It could have been pushed into its elongated orbit by the gravity of a passing star. Or it could have been a member of another star system that was pulled away by the Sun.

There’s one other oddity about Sedna’s orbit: It seems to be influenced by the gravity of a much larger but unseen body far from the Sun – a possible “Planet Nine.”

Because Sedna’s so far away, we don’t know a lot about it. It’s probably about 600 miles in diameter. And it’s quite red – an indication that radiation has been zapping its surface for a long, long time. No one has found a moon yet, so it’s hard to measure Sedna’s mass. So we still have a lot to learn about this remote little world far, far from the Sun.

Script by Damond Benningfield

Pluto is the largest known member of the Kuiper Belt – a wide zone beyond the orbit of Neptune, the Sun’s most remote major planet. But Pluto isn’t the most massive member of the belt. That distinction goes to Eris – a fellow dwarf planet that may be a lot like Pluto.

Eris was discovered almost 20 years ago. It wasn’t noticed earlier because it’s a long way from the Sun – almost three times Pluto’s distance. At that range, it looks quite faint. And it creeps along against the background of stars, so it took a while to realize that it’s a member of the solar system.

Eris is a tiny bit smaller than Pluto. But it’s a good bit more massive. That means it’s denser than Pluto – it has a higher ratio of rock to ice. But like Pluto, it’s probably still geologically active.

One indication of that is that the surface of Eris is almost pure white – it reflects almost all of the sunlight that strikes it. That suggests that fresh ice is erupting from its interior, repaving the surface.

Another indication is the chemistry of methane ice at the surface. Observations by Webb Space Telescope showed that the methane probably formed from chemical reactions between water and carbon inside the little world. Such material must be constantly renewed – oozing to the surface through cracks or holes in the icy surface of Eris – the heaviest of the Sun’s dwarf planets.

We’ll talk about another dwarf planet tomorrow.

Script by Damond Benningfield

The planets of the solar system come in three varieties: the rocky planets, like Earth; the giant planets, like Jupiter; and dwarf planets, like Pluto. The first two groups are well known. Except for Pluto, though, the third group is pretty much anonymous to everyone except scientists who study them.

Astronomers created the dwarf-planet category in 2006, after they began finding bodies similar to Pluto in the outer system. A dwarf planet must orbit the Sun and not some other object, and it must be mostly round. Today, the count varies. But most lists include at least eight members. And some say that scores of known objects could fit the category, with hundreds more awaiting discovery.

One definite dwarf planet is Ceres, in the asteroid belt, between the orbits of Mars and Jupiter. All the others are beyond Neptune, the most remote major planet. Most of them are in the Kuiper Belt – a doughnut that extends billions of miles beyond Neptune.

Pluto is the largest dwarf planet, and the closest to the Sun. The second largest is Eris. It’s actually a little heavier than Pluto, which means it’s also a little denser. We’ll have more about Eris tomorrow.

Most of these distant little worlds have icy surfaces. But a few could have active interiors, with possible oceans of liquid water. That means at least some of them are possible habitats for life – in the deep freeze of the outer solar system.

Script by Damond Benningfield

If astronomers at the star Regulus could pick up radio broadcasts from Earth, right about now they’d be hearing something like this: [1945 audio clips]

They’d just now be hearing about the final days of World War II because Regulus is a bit more than 79 light-years away. That means it takes more than 79 years for light – including radio waves – to travel from Earth to Regulus, or from Regulus to Earth. So the light we see from Regulus tonight actually left the star in 1945. And if anything dramatic were to happen to the star tonight, we wouldn’t know about it until 2103.

79 light-years is a huge distance. Light travels at 670 million miles per hour. That means Regulus is almost 500 trillion miles away – a five followed by 14 zeroes.

Such vast distances make many people skeptical that we could ever have a chat with another civilization. It might take generations from the time one side said “hello” until the other could respond. So some day we might hear a broadcast from another civilization. But we’ll probably never become best buds.

Look for Regulus to the upper left of the Moon in the evening twilight – a close neighbor that’s not close enough for a neighborly chat.

Script by Damond Benningfield