StarDate

Many stars have great names. But one of the best has to be Ra’s al-Ghul — better known as Algol, “the head of the ghoul” or the demon star. It represents Medusa, a monster from Greek mythology. Its head was covered in snakes, and one glance at her instantly turned the viewer to stone.

The name probably was bestowed because Algol does something that skywatchers in ancient times found a bit scary: It periodically gets a lot fainter. The heavens were thought to be eternal and unchanging, so such a star was considered a big problem.

The cause of its behavior isn’t scary at all, though.

Algol includes two stars that are locked in a tight orbit around each other — they’re just a few million miles apart. One of the stars is big and bright — almost 200 times brighter than the Sun. The other star is much smaller and fainter. Every two days, 20 hours, and 49 minutes, the fainter star passes in front of the brighter one. That causes the system to drop to just a third of its normal brightness. An eclipse lasts about 10 hours, with about four hours at minimum brightness. After that, the fainter star moves out of the way, and Algol roars back to full brilliance.

Algol’s next eclipse will take place tomorrow night. The star is high in the east-northeast as night falls, in Perseus the hero — the character who killed Medusa. If you can find Algol, check it out tonight, then again late tomorrow night — to see the action of a “ghoulish” star.

Script by Damond Benningfield

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In 1969, the Pentagon found itself with a stack of large telescope mirrors that it couldn’t use. It had planned to launch them on Manned Orbiting Laboratory, a small space station. Astronauts would use the six-foot mirrors to spy on the Soviet Union and its allies. But improvements in robotic spy technology made the lab obsolete.

So six of the mirrors eventually were given to the University of Arizona. It combined them to make a single telescope — the third-largest in the world at the time. The Multi-Mirror Telescope operated from 1979 to 1998, when the mirrors were replaced by a single piece of glass.

Defense agencies have given several other big gifts to astronomy as well.

In 1976, for example, the Air Force gave a whole observatory. It was built on a mountain peak in New Mexico after World War II to study the Sun. The Pentagon wanted to know how the Sun might affect radio communication, radar, and other technologies, so it built telescopes to find out. The observatory is still in operation.

And NASA is preparing the Nancy Grace Roman Space Telescope for launch later this decade. It uses one of two spy satellites the National Reconnaissance Office gave to NASA a decade ago. Its primary mirror is the same size as the one in Hubble Space Telescope. But Roman’s field of view is a hundred times wider. The telescope will study dark energy, exoplanets, and other leading topics — thanks to an astronomical gift.

Script by Damond Benningfield

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The night sky offers a few gifts to everyone this week, including a string of planets, a beautiful Moon, and the pre-eminent constellation of winter.

Shortly after sunset tonight, as the colors of twilight get bolder, look toward the southwest for three planets. They’re strung out like bulbs on a strand of Christmas lights.

The brightest of the three is Venus, the brilliant “evening star.” Fainter Saturn stands to the upper left of Venus, by about one-and-a-half times the width of your fist held at arm’s length. And Jupiter — second only to Venus in brightness — is about the same distance to the upper left of Saturn.

Venus drops from view as the sky gets fully dark. By then, Orion is in view in the east. It’s perhaps the most beautiful of all the constellations, and the easiest to find. Look for its three-star “belt” extending up from the horizon. It’s flanked by bright orange Betelgeuse to the left, and blue-white Rigel to the right.

A couple of hours later, the brightest true star in the night sky climbs into view below the belt: Sirius, the Dog Star.

Finally, the Moon heaves into view by 11 or 11:30. Regulus, the bright heart of the lion, is far to the upper right of the Moon.

Almost everything will look the same tomorrow night, with the exception of the Moon. It’ll rise later and stand farther below Regulus, and it won’t be quite as full or bright.

So enjoy the view of the night sky — an extra present for this holiday week.

Script by Damond Benningfield

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It’s easy to track the phases of the Moon. They’re on just about every printed calendar, plus countless apps and web sites — including ours. That allows you to find the phase on any date far into the future.

People have been devising ways to track the Moon for thousands of years. One of the earliest could be a bone found in Africa. Columns of notches could represent the phases of the Moon.

The Ishango bone was found in present-day Congo, in the remains of a village that was buried by a volcanic eruption. The village was inhabited about 20,000 years ago.

The bone is about four inches long. It’s the leg bone of a baboon, topped by a crystal of quartz. The bone contains three columns of notches. The columns are divided into groups, with each group containing a different number of notches.

The notches could be mathematical. They could represent a counting system, or even serve as a calculator, helping the owner do simple addition or multiplication.

On the other hand, the notches could represent the phases of the Moon. The notches in each group are a different length, so they could represent the waxing and waning of the Moon, covering about six months of phases — an early “calendar” for tracking the Moon.

Tonight, the Moon is about three-quarters full, so it’s nice and bright. And it has a bright companion: Regulus, the heart of the lion. They climb into view in late evening, and soar high overhead later on.

Script by Damond Benningfield

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Many ancient cultures thought of the stars as fixed and unchanging. Any change they did see was a bad sign. A star in Perseus that gradually fades and brightens, for example, was called the demon star.

Goodness only knows what they would have called Betelgeuse, the orange shoulder of Orion. It’s normally the 10th-brightest star in the night sky. In late 2019 and early 2020, though, it dropped out of the top 20. It faded to only about a third of its normal brightness, although it eventually recovered.

Betelgeuse is a supergiant. It’s much bigger, heavier, and brighter than the Sun. The exact numbers are uncertain, mainly because its distance is uncertain. A recent study put it at about 550 light-years — closer than earlier studies.

Betelgeuse is only about nine million years old, yet it’s near the end of its life. It “burns” through its nuclear fuel at a furious rate. When it’s through, it’ll explode as a supernova — a blast that’ll be visible in the daytime for weeks.

Astronomers expect that to happen in the next hundred thousand years or so. So when Betelgeuse faded, there was some speculation that it was a prelude to the final blast. Most studies, though, say the star faded because it blew a giant blob of gas into space. The blob cooled and darkened, making Betelgeuse fade away.

Still, we know it’s only a matter of time until the star’s brilliant end — an event that would have been really scary for long-ago skywatchers.

Script by Damond Benningfield

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Mars is beginning to creep into the dawn sky. It’s quite low in the southeast during twilight, so it’s tough to spot. The planet is rising a little earlier each day, though, so it’ll be a much easier target by the end of the month.

The Perseverance rover has been trundling across the Red Planet since February. It landed in Jezero Crater — a big hole in the ground that once was filled with water. Perseverance’s early work has shown that the water probably stuck around for a while — long enough to make Jezero an inviting home for life.

All that water disappeared billions of years ago. So did most of the rest of the water on Mars, which was warm and wet in the distant past. Scientists have been trying to figure out where it all went.

A lot of it escaped into space. Mars’s gravity is weak, so water molecules are easily carried into the upper atmosphere. There, they can be split apart by sunlight, and the individual atoms blown away by the solar wind. And a recent study found that Mars loses a lot more water when it’s closest to the Sun, and when it has giant dust storms, which heat the air and carry water high into the sky.

But the rate at which Mars loses water to space isn’t nearly enough to account for everything it’s lost. Other studies have concluded that a lot of it was chemically bound with minerals on the surface. And still more probably trickled deep below the surface — where it’s awaiting discovery by future explorers.

Script by Damond Benningfield

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If you stand outside the next couple of days at local noon — the time the Sun appears highest in the sky — and face away from the Sun, you might notice that your shadow is especially long. In fact, it’s the longest noontime shadow of the entire year.

That’s because tomorrow marks the winter solstice — at 9:59 a.m. Central Standard Time. It’s the shortest day of the year in the northern hemisphere, and it marks the beginning of winter. The Sun scoots lowest across the sky on the solstice, which is why you cast an especially long shadow.

The solstice is set up by Earth’s tilt on its axis. At this time of year, the north pole tilts away from the Sun. That brings those of us north of the equator especially short days.

And the farther north you go, the shorter the days. Miami, for example, will see about 10 and a half hours of sunshine. For Dallas and Los Angeles, which are a little farther north, it’s about a half hour less. For Seattle, it’s just seven and a half hours. And for Anchorage, it’s a meager five and a half hours of sunshine.

The numbers stay about the same for several days. That’s because the Sun appears to “stand still” now — it rises and sets at the same point along the horizon. In fact, “solstice” means “the Sun stands still.”

Before long, though, it’ll begin to move more quickly. Its rising and setting points will shift, and it’ll pass a little higher across the sky — bringing longer days, but shorter shadows.

Script by Damond Benningfield

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Many people celebrate their birthdays under the stars. But when Ambrose Swasey turned 89, he celebrated under a machine his company was building to study the stars — the first telescope for McDonald Observatory.

Swasey was born 175 years ago today, in a small town in New Hampshire. He quickly showed a knack for designing and building things. As a teenager, he went to work in a machine shop, where he met Worcester Warner. They formed their own company, Warner & Swasey, which they soon moved to Cleveland.

The company mainly built machine tools. But both founders were interested in astronomy, so they started building telescopes and their protective domes as well — work that made the company well known.

Warner and Swasey built a big new telescope for Lick Observatory in California, and a bigger one for the University of Chicago’s Yerkes Observatory — still the biggest telescope of its type in the world.

In 1923, Chicago astronomer Otto Struve named an asteroid discovered with the Yerkes telescope in Swasey’s honor. Later, Struve became the Yerkes director. And when Chicago signed a deal with the University of Texas to operate its new McDonald Observatory, Struve became its director as well. He hired Warner & Swasey to build it, including its first big telescope — the second-largest in the world.

The telescope has moved down the rankings since then. But it’s still in service — one of the legacies of Ambrose Swasey.

Script by Damond Benningfield

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Romantics and werewolves take note: There’s a lot of moonlight the next few nights — more than at any other time of the year. The middle of the country, for example, will see about 14 and a half hours of moonlight the next couple of nights, after the day brings just nine and a half hours of sunlight.

That extra lunar glow is the result of the Moon’s phase and the time of year. The Moon will be full tonight. It’s known as the Moon Before Yule. It lines up opposite the Sun, so it rises around sunset and sets around sunrise.

And we’re just a few days away from the winter solstice — the shortest day of the year in the northern hemisphere. The shortest days are accompanied by the longest nights. So December’s full Moon is known as the Long-Night Moon.

Not only does the Moon remain in view for a long time, it also climbs highest across the sky for the year. That’s because the Moon lies near the ecliptic, which is the Sun’s path across the sky. At this time of year, the ecliptic arcs low across the sky during the day because Earth’s north pole is tipped away from the Sun. But the full Moon is on the opposite side of Earth, so the north pole tips toward it. As a result, the ecliptic climbs high across the northern sky — and so does the full Moon.

The exact moment of the full Moon, by the way, is 10:35 p.m. Central Standard Time — the time of the Long-Night Moon.

Tomorrow: a special line of business.

Script by Damond Benningfield

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The Sun will change addresses tonight. It’ll move from Ophiuchus to the next constellation over, Sagittarius. That’s where it’ll reside at the winter solstice, next week.

Sagittarius hasn’t always been the Sun’s home for the solstice, though. A couple of thousand years ago, the Sun was in Capricornus. And a century and a half from now, it’ll move into Ophiuchus.

The change is caused by precession — a slow “wobble” in Earth’s axis. It causes the Sun to slide through the background of stars over the millennia. It also causes the north pole to trace a circle on the sky. It takes almost 26,000 years to make one circle. And during that time, Earth has different “north stars.

An artist used precession to honor Hoover Dam. Oskar Hansen designed Monument Plaza. It includes a 14-story flagpole, two 30-foot statues, and a terrazzo floor that indicates the date of the dedication — September 30th, 1935 — with the help of the stars.

Today, the North Star is Polaris. A line from the flagpole through a wide ring around it points to the position of Polaris at a precise time on the night of the dedication. Another line points to Thuban, the pole star when Egypt built the Great Pyramid. In fact, that was one of Hansen’s inspirations: Architects used Thuban to align the pyramid — another great work of human history.

And yet another line points to the bright star Vega. It will serve as the pole star in 12,000 years — thanks to precession.

Script by Damond Benningfield

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