StarDate

The Boyne River, in County Meath, Ireland, may be the site of an ancient astronomical computer. Giant mounds at three prehistoric complexes appear to track the motions of the Sun and Moon. And one of those mounds could be the home to the oldest known map of the Moon.

The mound at Knowth is more than 200 feet in diameter and three stories tall. Two long passages — one aimed east, the other west — lead to a central chamber painted with elaborate geometric shapes.

At one of the other complexes, passages align with the sunrise and sunset on the equinoxes. At Knowth, though, the Sun shines down these passages a few days or weeks before or after the equinoxes. Some researchers argue that the passages were designed to align with the Moon instead.

Many of the pictures inside the mound appear to have a lunar connection. The calendar stone is decorated with circles and crescents that may represent the cycle of lunar phases. The layout may be an effort to depict a 19-year cycle in which the motions of the Sun and Moon synchronize.

Another stone contains three groups of curved lines and dots. These may be depictions of the lunar disk, with the lines and dots representing the Moon’s dark markings. Since the site is about 5,000 years old, that would make the painting the oldest known lunar calendar.

Look for the Moon early tomorrow, with a bright companion close by: the planet Jupiter. They’re side by side at first light.

Script by Damond Benningfield

The night sky is a time machine. All of the stars are light-years away, so we see them as they looked years ago. But one star also shows us the future. It gives us a glimpse of what the Sun will look like in perhaps a billion years or more.

Chara is the second-brightest member of Canes Venatici, the hunting dogs. The constellation is in the east-northeast at nightfall. Chara is above its brightest star, Cor Caroli.

Chara is a solar analog — a star that’s almost identical to the Sun. It’s a couple of percent more massive than the Sun, and a little bit bigger and brighter. But it’s almost the same color and temperature as the Sun. So it looks a lot like the Sun would for anyone at Chara looking our way.

The small differences in size and brightness are mainly due to the star’s extra weight. Heavier stars burn their nuclear fuel more quickly than lighter stars, so they’re bigger and brighter.

But some of the difference is a result of Chara’s age. Most estimates say it’s at least several hundred million years older than the Sun, and perhaps as much as a billion and a half. As a star ages, the nuclear reactor in its core must work harder to produce energy. That makes the core hotter, which causes the star’s outer layers to puff up.

And that’s what’s ahead for the Sun. In a billion years, in fact, it may be so big and bright that it will have boiled away Earth’s oceans and atmosphere — leaving nothing but a cosmic cinder.

Script by Damond Benningfield

Although it looks like a single pinpoint of light, the orange heart of the scorpion actually consists of two stars. Both of them are far bigger and heavier than the Sun, making both of them among the galaxy’s most impressive stars. Yet the stars face different fates. One will explode, while the other will fade more gently into the night.

The stars form the system known as Antares. It will stand to the lower left of the Moon at first light tomorrow.

Only one of its stars is bright enough to see with the unaided eye — a red supergiant that’s about 15 times as massive as the Sun.

This star is already nearing the end of its life. Sometime in the next million years or so, it’ll explode as a supernova, briefly outshining billions of normal stars.

When the fireworks end, only the star’s dead core will remain — a neutron star. Although it’ll be two or three times as massive as the Sun, gravity will crush it so thoroughly that it’ll be only a few miles across.

Antares’s other star, which is only about half as heavy, should survive the supernova blast just fine, then live millions of years longer. At the end of its life, its outer layers will blow off into space as well, but in a much gentler process. Gravity will crush its now-dead core, forming not a neutron star, but a white dwarf — a ball as massive as the Sun, but about the size of Earth. It’s the same fate that awaits the Sun — in the far-distant future.

Script by Damond Benningfield

If you like spring, then you might want to head for Mars about now. That’s because today marks the beginning of spring in the planet’s northern hemisphere. And it’s the longest season of the year, lasting 194 Mars days.

Mars has seasons for the same reason Earth does — the planet is tilted on its axis. So as Mars orbits the Sun, the northern and southern hemispheres take turns receiving more and less sunlight. That creates the cycle of seasons.

The Martian year is almost twice as long as an Earth year. As a result, the seasons are about twice as long as well — on average. Mars’s orbit around the Sun is much more lopsided than Earth’s is, so the planet’s orbital speed varies by quite a bit. That makes the seasons uneven.

Northern spring, for example, occurs when Mars is near its farthest point from the Sun, so Mars is moving slowly. That stretches the season out. Northern autumn, on the other hand, takes place when Mars is close to the Sun, so the season is shorter — just 142 Mars days.

A Mars day, by the way, is about 40 minutes longer than an Earth day. Because of the different length, scientists usually call a Mars day by a different name — a “sol,” for solar — the time it takes the Sun to make one loop across the Martian sky.

And Mars continues to highlight our sky during our own spring season. It’s about a third of the way up the western sky as night falls, and looks like a bright orange star.

Script by Damond Benningfield

One of the most accomplished astronomers of the Middle Ages was born on today’s date in 1394 — 625 years ago. He built a university and an observatory and compiled a major star catalog. But he was also a political leader, and that brought about an early and unfortunate end to his career.

Ulugh Beg was the grandson of Tamerlane, a famous ruler who built an empire in the Middle East. When Tamerlane died, Ulugh Beg’s father took over the empire. And he installed Ulugh Beg, who was just 16 years old, as the governor of Samarkand, a major city in present-day Iran.

Over the decades, Ulugh Beg built a major university there, which he staffed with mathematicians and astronomers. And he did his own astronomical research.

To further that work, in 1428 he built an observatory at Samarkand. It included a device for accurately measuring the positions of stars. He used those positions to compile a catalog of more than a thousand stars. It corrected some errors made in the most-used catalog of the time, which had been compiled more than a thousand years earlier. He also calculated the length of the year, and the angle of Earth’s tilt on its axis.

After his father died, in 1447, Ulugh Beg faced challenges from many quarters, including his own son. He eventually surrendered to his son, which was a bad idea. The son had Ulugh Beg killed — ending the life and work of one of the ancient world’s great astronomers.

Script by Damond Benningfield

By Earthly standards, a hard vacuum surrounds the surface of the Moon. Yet it’s not a complete vacuum — the Moon has a thin atmosphere.

Much of the material that makes up the atmosphere comes from the surface. It’s kicked off of rocks and dirt by the solar wind or by impacts by tiny space rocks.

And some of the material may come from inside the Moon — mainly from dormant volcanoes.

Billions of years ago, volcanic activity might have given the Moon a much thicker atmosphere. Bombardment by giant space rocks punched holes in the crust. That allowed molten rock to spill onto the surface. The magma contained hydrogen, oxygen, and other gases. Researchers say that some of the gases — including water vapor — could have formed an atmosphere that was thicker than that of present-day Mars. And it could have lasted for up to 70 million years.

The Moon’s gravity just isn’t strong enough to hold on to that much “air,” though. The solar wind stripped most of it away, blowing it off into space.

But some of the water may have fallen back to the surface. And some of it may still be on the Moon today. It’s hidden in the bottoms of craters at the lunar poles, which are always in shadow — icy traces of a vanished atmosphere.

Look for the Moon climbing into good view by about 10 or 10:30 the next couple of nights. The bright star Spica will stand to the lower right of the Moon tonight, and to the upper right tomorrow night.

Script by Damond Benningfield

Spring begins in the northern hemisphere today, as the Sun crosses the equator heading north. For many of us in modern times, that moment — the spring equinox — is little more than a notation on the calendar. But to many cultures in bygone centuries, it was a time for celebration, as the Sun warmed the earth and banished the long, cold nights of winter.

For the Lakota of the Great Plains, the equinox marked the beginning of a new year. They commemorated the event with the start of a journey across the Black Hills. Sometimes, only medicine men or tribal elders made the trek; at other times, entire tribes were involved.

The spring journey celebrated a connection between the stars and the land. Several Lakota constellations represented prominent features in the Black Hills. In fact, one large constellation represented the entire mountain range.

The spring journey began when the Sun entered Cansasa Ipusye — the Dried Willow. The constellation includes stars of present-day Aries, which is high in the southeast as night falls. It culminated at the Hill of the Bears Lodge, a tower of volcanic rock in Wyoming that’s also known as Devils Tower. It’s represented in the sky by the stars of Gemini. Sun Dance ceremonies there affirmed the connection between the people and the sky and celebrated the completion of one full circle of life — and the beginning of another.

Tomorrow: giving the Moon some air.

Script by Damond Benningfield

Sometimes, you just can’t trust your lying eyes. If you observe the sky, for example, you might see the Sun rising and setting every day and conclude that it circles around Earth. And that’s what most people thought until about five centuries ago. They were wrong, of course — their eyes deceived them.

Not everyone agreed with that idea, though. More than 2400 years ago, a Greek philosopher proposed that Earth, Sun, Moon, and planets all orbit the “central fire” of the universe. And a century later, Aristarchus of Samos concluded that the Sun was bigger than Earth. From that, he suggested that Earth orbited the Sun, not the other way around.

But neither man could produce evidence to back his idea. Since the eye suggested that the Sun was moving around Earth, that model held on for a long time.

It began to give way in the 16th century. Nicolaus Copernicus more fully developed the idea of a solar system with the Sun in the middle. And he described it with geometric equations.

Many educated people accepted the idea. And later, Johannes Kepler added better mathematical descriptions of how it works. So the idea of an Earth-centered universe gave way to one in which Earth orbits the Sun — regardless of what our eyes may tell us.

We’ll reach a key point in our annual orbit tomorrow — the vernal equinox. The Sun will cross the equator from south to north, marking the start of spring in the northern hemisphere. More tomorrow.

Script by Damond Benningfield

The evolution of a star is dominated by one thing: the star’s mass. Heavy stars live short but brilliant lives. Lightweight stars are less showy, but they live a long time. And when there are two stars in a single system, a difference in mass can change the evolution of both.

A prime example is Regulus, the brightest star of Leo, the lion, which is just a whisker away from the Moon tonight.

The bright star we see as Regulus has a tiny companion — a white dwarf. It’s the dead core of a once-normal star. But when the stars were born, the companion was the more impressive one.

Recent studies, in fact, suggest that the companion originally was about 2.3 times as massive as the Sun. Today’s bright star was only 1.7 times the Sun’s mass. The extra weight squeezed the companion’s core tightly, speeding up its nuclear reactions. So the star evolved more quickly.

As it used up the nuclear fuel in the core, it puffed up to many times its original size. The two stars were so close together that the expanding star began dumping gas on the other. When that process was done, the heavier star had transferred most of its mass to the present-day bright star. So while the star we see is now almost four times the Sun’s mass, its flashier companion is a faint ember — just one-third the mass of the Sun.

The transfer had another important effect. It made bright Regulus spin much faster — almost fast enough to rip itself apart.

Script by Damond Benningfield

The Sun exerts a mighty influence on the space around it. Our star holds on to eight major planets, perhaps hundreds of dwarf planets, and a trillion or more smaller chunks of rock and ice.

The key to this domination is the Sun’s mass. The Sun contains 99.9 percent of all the material in the solar system — more than 300,000 times the mass of Earth.

That great heft generates a powerful gravitational pull. In fact, the Sun’s gravity dominates a “bubble” of space that spans more than two light-years.

The most obvious members of the Sun’s family are Earth and the other major planets. The most distant of those is Neptune, 30 times farther from the Sun than Earth is. At that range, the Sun’s pull is only about one-tenth of one percent as strong as at Earth. Yet that’s plenty strong to hold Neptune in place.

Beyond Neptune is the Kuiper Belt — a doughnut-shaped region that contains millions of objects, including Pluto and many other dwarf planets.

The Sun’s influence doesn’t end there, though. It reaches all the way to the Oort Cloud — a shell of mountain-sized iceballs. The cloud may extend more than one light-year in every direction.

At that distance, an object barely “feels” the Sun’s pull — it’s only about one four-billionths as strong as at Earth. Unless another star wanders close by, though, that tiny pull holds objects in the Oort Cloud in orbit — extending the Sun’s dominance deep into space.

Script by Damond Benningfield