StarDate

Flat-Earthers occasionally show up in the headlines. They raise a bit of a ruckus, demand proof that Earth isn’t flat, then vanish again. But humanity has known that Earth is round for about 2500 years. And some of the first proof was provided by Aristotle, a Greek philosopher and scientist.

Other Greeks had proposed a round Earth before Aristotle’s time. But they didn’t provide any evidence to back it up. In a book published around 350 BC, Aristotle did.

One of his ideas was an early concept of gravity. Based on his observations of how things work, he reasoned that every portion of Earth is pulled toward a common center. That would naturally result in the creation of a sphere, not a flat plane.

Aristotle also knew that if you watch the night sky from different locations, you see different stars. Observers in Egypt and Syria could see stars peeking above the southern horizon that were never visible to viewers who were farther north. That could happen only on a curved surface.

Finally, Aristotle talked about lunar eclipses. As Earth’s shadow passes across the face of the Moon, the edge of the shadow is curved — an indication that our world is round, and not flat at all.

You can see that bit of evidence for yourself late tomorrow night because there’s an almost-total lunar eclipse — Earth’s shadow will cover almost the entire Moon. It’ll be visible across all of the United States. More about that tomorrow.
 

Script by Damond Benningfield

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Sagittarius is wrapping up its annual evening appearance. It’s low in the southwest as night falls, and will drop from view over the coming weeks. Some of its stars form the outline of a teapot. As it drops toward the horizon, the spout is tilted down, as if it’s ready to pour its brew onto the ground below.

The constellation can be a little tough to find when it’s this low in the sky, especially through the fading twilight. But it’s pretty easy to find right now because a brilliant visitor is moving across it: Venus, the “evening star.” The planet far outshines every other point of light in the night sky, so you can’t miss it.

Tonight, Venus passes within a whisker of the star Phi Sagittarii, which connects the teapot and the top of the handle. Although the star is billions of years younger than the Sun, it’s already at the end of its “normal” lifetime. It’s moving from the prime of life into its “giant” phase. Changes in its core are puffing up its outer layers. Over the next few million years, that will make the star much bigger and brighter.

On Friday evening, Venus will pass just as close to the star at the corner of the handle. Known as Nunki, it’s the second-brightest member of the teapot. It’s still in the prime of life, but it won’t be for long. It’s so massive that it, too, will live only a short while. Then, like its neighbor, it’ll expand and shine brilliantly — making the teapot much more prominent.
 

Script by Damond Benningfield

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 A quiet but steady meteor shower is building toward its peak tomorrow night. The gibbous Moon will steal most of its luster, though. Only a few “shooting stars” will shine through the moonlight.

The shower is called the Leonids because its meteors appear to “rain” into the sky from Leo. The lion climbs into view in the wee hours of the morning.

But there’s no relation between the meteors and the constellation. Instead, the meteors are the offspring of a comet.

The comet was first seen by Ernst Tempel, in December 1865, and Horace Tuttle a few weeks later, so it carries both of their names — Tempel-Tuttle.

Not long after the discovery, several astronomers noted a similarity between the orbit of Tempel-Tuttle and the Leonid meteor shower. They realized that the comet must somehow spawn the shower.

And they were right. A comet is a ball of frozen water and gases mixed with bits of solid rock. When it gets close to the Sun, some of the ice vaporizes, releasing solid particles. When Earth crosses the comet’s path, these particles slam into the atmosphere, forming the glowing streaks known as meteors.

The Leonids are best when the comet is closest to the Sun, and when Earth passes through a dense part of the stream of comet dust. Neither of those will happen this year, so the shower is pretty weak. Even so, if you have a dark, safe viewing spot, it’s worth looking for those few meteors — the offspring of a comet.
 

Script by Damond Benningfield

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About eight million years ago, a big space rock slammed into the Atacama region of Chile, near the Pacific coast. The impact melted the sand and dirt and blasted it high into the sky. As the debris fell back to Earth, it formed dark balls of glass about the size of your little fingernail. Today, thousands of them are scattered across hundreds of square miles of desert.

Geologists discovered the glass balls a few years back, and confirmed their origin this year. The beads contain a lot of iron and other elements that likely came from the space rock.

Researchers haven’t found the impact crater that was gouged by the impact, even though it should be at least a mile across. It might have been filled in by blowing sand.

Wind, rain, and the motions of Earth’s crust probably have erased many other impact craters as well. Yet scientists have logged about 200 confirmed craters, and they’re still finding new ones. The craters are tough to see. But drilling into them reveals that rocks below the surface have been crushed and churned by a powerful blow.

One possible recent discovery is the Brushy Creek structure in Louisiana, which is more than a mile wide. Another is Ora Banda in Western Australia, which is three miles wide.

Neither has been confirmed as an impact crater. But scientists are doing more research to prove that both were created by collisions with space rocks — like the one that sprinkled round beads across Chile eight million years ago.
 

Script by Damond Benningfield

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When Mariner 9 entered orbit around Mars 50 years ago today, the view was awful. A dust storm blanketed the entire planet. About all that Mariner’s cameras could see were yellow clouds of dust and a few tall mountains rising above them.

By a couple of months later, though, the dust had settled, and the first spacecraft to orbit any planet other than Earth got busy. By the end of 1972, it had snapped more than 7,000 pictures of the Red Planet. They showed giant volcanoes, polar ice caps, and other wonders. They made Mariner one of the most successful space missions in history.

Earlier missions had simply flown by Mars. They’d seen only a fraction of the planet — relatively boring regions that revealed little about Mars’s true character.

Mariner 9 went far beyond that, however. It photographed almost the entire planet. It found that Mars is a dynamic world with lots to study. It showed us channels that once carried mighty rivers, suggesting that Mars was much more comfortable in the distant past. It saw volcanoes larger than anything on Earth. And it discovered a canyon system that would stretch across the entire United States. Scientists named it Valles Marineris — Mariner’s Valley.

Although it’s been silent for almost half a century, Mariner 9 continues to orbit Mars — but probably not for much longer. Its orbit could decay as early as next year — sending it plunging to the surface of the planet it taught us so much about.
 

Script by Damond Benningfield

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Cosmic rays can be dangerous. They can damage electronics on the ground and in space, and harm travelers in space and in aircraft.

But they also can be helpful. Scientists use them to monitor the amount of water in the soil — an important measurement for agriculture and other applications. And it may be possible to use those readings to look for harmful outbursts.

Cosmic rays are particles from beyond the solar system. They’re created by exploding stars and other powerful events. The number that hit Earth is controlled by the Sun. When the Sun is active, it blocks most of them. When it’s quiet, it lets them through.

When an outburst of cosmic rays reaches Earth, it can zap high-altitude aircraft. There’s a global warning network, but it has only a few dozen stations, so much of the world isn’t covered.

Cosmic rays seldom make it all the way to the surface. They ram into atoms and molecules in the atmosphere, creating showers of other particles that then reach the surface. Some of the particles hit water molecules in the soil, creating their own reactions. Networks of sensors use those reactions to measure the soil’s water content.

British scientists studied those networks. They found that the sensors recorded a couple of big cosmic-ray outbursts over the last decade. And with a little work, they could be adapted to create a real-time alert system — providing an extra layer of protection against a cosmic hazard.
 

Script by Damond Benningfield

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The giant of the solar system tags along with the Moon tonight. Jupiter, the largest of all the planets, stands close to the upper right of the Moon at nightfall, and to the lower right of the Moon as they set, around midnight. The planet looks like a brilliant star.

A dozen people have walked on the Moon, and others may lope across its surface later in this decade. But it’ll be a long time before we can send anyone to Jupiter.

In part, that’s because it’s so far away — it takes years to get there. We’ll probably need new types of rockets — perhaps with nuclear power — to cut down the travel time.

Also, there’s no place on Jupiter to land. The planet’s outer layers are made of gas, so there’s no surface. And as you descended through them, temperatures and pressures would quickly destroy any craft. So visitors to Jupiter itself probably will float through the clouds in its atmosphere.

And just getting close to Jupiter is also a problem. The planet’s powerful magnetic field generates radiation belts that are thousands of times stronger than Earth’s. Robotic spacecraft are protected by heavy shielding. And without similar shielding, it wouldn’t take long for an astronaut to absorb a dose that could be harmful — or deadly. And the radiation is a problem even if you stay away from Jupiter itself and land on one of the planet’s big, icy moons.

So the Sun’s biggest planet isn’t likely to receive human visitors for many decades.
 

Script by Damond Benningfield

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If you wanted to set up a base camp for exploring Earth, the south pole is about the last place you’d consider — it’s one of the toughest environments on the planet.

On the Moon, though, the south pole is one of the best locations. It can provide abundant sunshine, a narrower swing in temperatures, and even water.

That’s why NASA is considering the pole as the landing site for its first astronauts to visit the Moon in half a century. In particular, it’s looking at the rims of some deep craters.

One of these is Shackleton Crater, which nestles right next to the pole. The crater is about a dozen miles in diameter and two and a half miles deep. Some points on its rim see the Sun about 90 percent of the year, which makes them good spots for solar panels. And the temperature range isn’t as extreme as at most locations on the Moon.

Perhaps most important, most of the crater floor is in perpetual darkness — it never sees the Sun. That keeps the temperature at 300 degrees below zero or colder. As a result, frozen water may be mixed with the dirt there. The ice could provide drinking water, oxygen, and rocket fuel — everything you need to live on the surface of the Moon.

And the Moon has two bright companions tonight. The brilliant planet Jupiter stands to the upper left of the Moon at nightfall. The fainter planet Saturn is closer to the upper right of the Moon. More about the Moon and Jupiter tomorrow.
 

Script by Damond Benningfield

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Black Gap Wildlife Management Area in West Texas recently won a special honor. It was designated a dark sky sanctuary by the International Dark-Sky Association. That indicates it has especially pristine night skies, and it makes an effort to keep them that way. It joins two other dark-sky sites in that region, which still has some of the darkest nights in the country.

Dark skies are getting harder and harder to find across most of the United States, though. As the population grows and cities and towns expand, the night sky is filled with more and more light. That drowns out the view of faint stars, the Milky Way, meteor showers, and more. It can interfere with bird and turtle migrations and other wildlife movements, and cause health problems for people.

Local regulations sometimes help. They limit the types and placement of outdoor lighting. Some cities have adopted streetlights that cast less light into the sky while also using less electricity.

And you can still find some fairly dark skies in many regions — even not too far from cities. The Dark-Sky Association has certified scores of dark-sky sites across the country. And many state and national parks are far enough from city lights that they offer good views of the night as well. Many of them offer skywatching programs to help you navigate those dark skies — an increasingly rare treat for much of the American public.

Tomorrow: looking for dark areas on the Moon.
 

Script by Damond Benningfield

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A space rock as long as a football field or bigger will glide past Earth tonight. 2019 XS will pass about 350,000 miles away — half-again the distance between Earth and the Moon.

Such encounters happen all the time. This one was forecast well in advance because the asteroid was discovered several years ago. Astronomers have observed it many times since then, allowing them to plot its orbit well into the future. But many visiting asteroids aren’t discovered until they’re right on top of us — usually because they’re quite small.

Astronomers have been searching for close-approaching asteroids for decades. If an asteroid as big as 2019 XS hit Earth, it could cause extensive damage. And if a bigger one hit us, it could threaten life across the whole planet. Finding and tracking such planet-killers far enough in advance might give us time to mount a defense.

We’d need to know as much as possible about the specific asteroid — not just its size and speed, but what it’s made of, how it’s put together, how it spins, and other details. So when asteroids pass close by, scientists try to observe them.

In the case of 2019 XS, a NASA station that tracks missions to the planets will bounce radio signals off the asteroid from Thursday through Sunday. It’ll “catch” the reflections from the asteroid to make images. Scientists will study the pictures to learn more about this close-flying neighbor.

Tomorrow: looking for darkness.
 

Script by Damond Benningfield

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