StarDate

The Moon has regular dates with the stars. It returns to the same position relative to the stars every 27 days and eight hours. As an example, the Moon cozied up to Regulus, the bright heart of the lion, on May 5th, and it does so again this evening – 27 days, eight hours later. This encounter is especially close as seen from the United States – the Moon and Regulus will appear to almost touch each other.

That time span is known as the lunar sidereal period – “sidereal” meaning “related to the stars.”

The planets have their own sidereal periods.

Mars, for example, returns to the same point relative to the stars every 22 and a half months. Tonight, Mars is well to the lower right of Regulus, and looks like an orange star. It’ll return to almost the same position in April of 2027. The match won’t be exact because our viewing angle to the Red Planet changes a bit from year to year.

The sidereal period is different from the period relative to the Sun – a difference caused by Earth’s own orbital motion. For the Moon, that period lasts 29 and a half days – the length of a cycle of phases. And for Mars, the Sun-related period is almost 26 months. That’s how long it takes Mars to return to the same angle from the Sun – part of the precise but sometimes confusing motions in the night sky.

More about the motions of the planets tomorrow.

Script by Damond Benningfield

To the more poetic among us, summer is a time of soft breezes, warm nights, and fireflies: The livin’ is easy, the breeze makes us feel fine, the warm Sun shines kindly upon us.

But there’s less poetry in the summers on Mars – especially in the northern hemisphere, where summer began on Thursday. It stays cold, and the only fireflies are occasional meteors blazing through the night.

Like the seasons on Earth, those on Mars are caused by the planet’s tilt on its axis. Northern summer begins when the north pole dips most directly toward the Sun. But Mars’s orbit is much more lopsided than Earth’s, so there’s a much greater change in the planet’s distance from the Sun. Mars is farthest from the Sun during northern summer. So the summer stays fairly cool.

Summers and winters tend to be quiet times in the planet’s thin atmosphere. Big dust storms fire up in spring and fall, sometimes covering the whole planet. But they settle down by the start of summer. Mars does see more “dust devils” during summer – whirlwinds that can tower miles high.

Northern summer will last for 178 Mars days – not giving way until the start of autumn exactly six months from now.

Mars is close to the upper left of the Moon at nightfall, and looks like a fairly bright orange star. The true star Regulus is farther along that line. More about this lineup tomorrow.

Script by Damond Benningfield

The United States plans to send astronauts to the Moon later in this decade, aiming toward a permanent lunar base. But experience shows that plans come and go. In fact, if all the plans for lunar exploration had actually come about, we’d be skittering all across the Moon today.

In 1958, for example, the Air Force developed Project LUMAN, a comprehensive plan for human spaceflight. It would culminate with a single astronaut landing on the Moon. Later, the service developed another plan – LUMEX. It called for three astronauts to travel to the Moon using a giant new booster and a streamlined spaceship. The Army developed its own plan, involving a space station and other steps.

All of those plans died – in part because human spaceflight was turned over to a new civilian agency: NASA.

And NASA had its own false steps. It studied using its two-man Gemini spacecraft for lunar missions before settling on Apollo. And even then, some of its plans were scuttled; the final three Apollo missions were scrapped, in 1970.

President George W. Bush proposed lunar missions as part of the Constellation program. It was nixed by President Obama. But some of its hardware has been kept for Artemis – which plans to send astronauts to the Moon in the next few years.

Look for the Moon in the west at nightfall. The twin stars of Gemini stand to its lower right, with Mars to its upper left – another planned destination for human explorers.

Script by Damond Benningfield

Globular clusters are the oldest members of the galaxy. They’re tight balls of hundreds of thousands of stars, most of which were born when the universe was no more than a couple of billion years old. Their most-massive stars have long since died. And most of the stars that remain are cool and faint. So a globular tends to be fairly quiet and calm. But that doesn’t mean that things don’t change.

Consider Messier 13. It’s in Hercules, which is high in the east at nightfall. Under dark skies, the cluster is just visible to the unaided eye, looking like a faint, fuzzy star.

The cluster is about 25,000 light-years away, and it contains up to half a million stars. But the stars at the edge of the cluster aren’t held as tightly as those in the middle. So the gravity of the rest of the galaxy can pull some of them away. In fact, astronomers have identified a few dozen stars that appear to be escapees from M13.

But the cluster also can grab stars from the space around it. One especially young star probably became a member of the cluster that way.

And stars inside the cluster can change. Some of them merge, forming bright, blue stars that look much younger. And stars die. M13’s brightest member is dying right now. It’s about as massive as the Sun, but it’s puffed up to dozens of times the Sun’s diameter. Soon, it’ll blow away its outer layers, leaving only its tiny, dead core – one more change in an ancient family of stars.

Script by Damond Benningfield

Three of the four big moons of Jupiter appear to have something in common: oceans of liquid water below their crusts. For Europa, the ocean is considered a slam dunk. The case is also strong for Ganymede. But the case for the third moon, Callisto, is the weakest.

Callisto is about 3,000 miles in diameter – bigger than our own moon. And it’s more heavily cratered than any other large body in the solar system. That indicates that the surface of Callisto is pretty much dead. But things might be different far below the surface.

In the 1990s, the Galileo spacecraft flew near Callisto eight times. Its measurements of the magnetic field around the moon hinted that a salty ocean was sloshing around inside. But those observations also could be produced by an electrically charged layer of Callisto’s thin atmosphere.

In a recent study, though, scientists looked at all of Galileo’s observations, and used computer models to understand them. The work suggested that there is an ocean. It could be dozens of miles deep. But it’s buried beneath an icy crust that could be hundreds of miles thick.

Two spacecraft that are en route to Jupiter will fly close to Callisto many times. Their observations should tell us for sure whether an ocean is sloshing below Callisto’s battered surface.

Jupiter stands below our moon in the early evening twilight. It looks like a bright star.

Script by Damond Benningfield

If stars had trophy cases, Vega’s would be packed. The leading light of Lyra was the first star other than the Sun to have its picture taken, the first to have its spectrum taken, and the first with a published measurement of its distance.

Vega is impressive in many ways. It’s more than twice the size and mass of the Sun, and about 50 times the Sun’s brightness. It’s encircled by a wide belt of dust, produced by collisions between big chunks of ice and rock. In about 12,000 years, it’ll serve as the Pole Star. And in about 200,000 years it’ll become the brightest star in the night sky.

Vega first had its picture taken in 1850. In 1872, an astronomer took a picture of its spectrum, spreading its light into its individual wavelengths. A spectrum reveals a star’s composition, motion, and much more.

In 1838, Russian astronomer Friedrich von Struve published Vega’s parallax – the first publication for any star. He plotted its position when Earth was on opposite sides of the Sun. That revealed a tiny shift against the background of more-distant objects. That shift revealed a distance of about 26 light-years – just one light-year off the modern measurement.

Vega is low in the east-northeast at nightfall and climbs high overhead during the night. It’s the fifth-brightest star in the night sky, so you can’t miss it – a beautiful star with a case full of trophies.

Script by Damond Benningfield

One of the major beauties of the summer sky dangles in the northeast this evening like a piece of cosmic jewelry – the constellation Lyra. Its brightest star is Vega – the fifth-brightest star in the night sky. It sparkles like the diamond stud in an earring. The rest of Lyra hangs to its lower right like the rest of the earring. It forms a parallelogram – a slanted rectangle. Under fairly dark skies, it’s easy to see.

Lyra represents a lyre – a small harp. In skylore, it was sometimes shown being held by a large bird – an eagle or vulture. In fact, the name “Vega” comes from an Arabic phrase that means “the falling eagle.” But mainly the lyre was associated with the story of Orpheus.

His music was legendary. When he accompanied Jason and the Argonauts, his playing silenced the Sirens – evil creatures who lured sailors to their doom.

Orpheus married Eurydice. But she was bitten by a snake and died. Orpheus begged Hades, the god of the underworld, to let Eurydice return to him. His music was so beautiful that Hades agreed. But there was one condition: Orpheus couldn’t look back until they were outside. But he couldn’t resist – he looked too soon, and Eurydice vanished into the underworld forever.

Orpheus was heart-broken. He roamed aimlessly across the countryside, playing sad but beautiful music on his lyre – an instrument commemorated in the stars.

We’ll have more about Vega tomorrow.

Script by Damond Benningfield

The names of the stars are a cultural mash-up. The names come from Greek, Latin, Arabic, and other cultures. And some names combine words from different languages. Two examples are the stars Tania Borealis and Tania Australis. “Tania” comes from an Arabic phrase that means “the second.” Borealis and Australis come from Latin, and mean northern and southern. Combined, the stars represent the second leap of the gazelle – a bit of skylore from Arabia.

Skywatchers there saw three close pairs of stars as the leaps of a gazelle. All three pairs are at the edge of the modern constellation Ursa Major, the great bear. The “Tanias” are above the stars that form the outer edge of the dipper’s bowl.

Tania Borealis is a single star that’s a good bit bigger, brighter, and heavier than the Sun. It’s at the end of the prime phase of life, so it’s undergoing big changes in its core. That’s causing its outer layers to begin to puff up to giant proportions.

Tania Australis is a binary – two stars bound together by their mutual gravitational pull. One of the stars is similar to the Sun. The other is more than six times the Sun’s mass, and it’s already reached the “giant” phase of life. It’s puffed up to about 75 times the Sun’s diameter, and it shines about a thousand times brighter. So Tania Australis looks a bit more impressive than its northern cousin – the brighter half of the second leap of the gazelle.

Script by Damond Benningfield

A gazelle leaps past the feet of the great bear. In ancient skylore, in fact, it made three leaps – each marked by a pair of stars.

The stars that mark the first jump are known as Alula Borealis and Alula Australis – the northern and southern first leaps. As night falls this evening, they’re high in the northwest. They’re far to the left of the Big Dipper, which has the most prominent stars of the great bear. The “alulas” are close together, so they resemble a pair of eyes.

Alula Australis holds an important distinction in the history of astronomy.

A telescope reveals it’s a “double” star – two stars that are close together. By measuring the motions of the stars, in the late 18th century, astronomer William Herschel showed that they’re bound to each other. That made the system the first confirmed binary – two stars that move through space together, tied by their mutual gravitational pull.

A few decades later, it became the first binary to have its orbit accurately measured. The stars orbit each other once every 60 years, at an average distance of more than 20 times the distance from Earth to the Sun.

In more modern times, astronomers found that both stars are binaries on their own – each has a small, faint companion in a tight orbit. So the first leap of the gazelle consists of at least four stars, leaping through the galaxy as a family.

We’ll talk about the second leap tomorrow.

Script by Damond Benningfield

About 44 lightning bolts flash through the skies of Earth every second. And for a while, it seemed there might be more flashes than that on Venus.

Telescopes on Earth and spacecraft in orbit saw flashes of light in the planet’s clouds, or heard the sounds of lightning in the planet’s radio waves. One estimate said Venus could see several times more lightning bolts than Earth.

But studies in recent years have suggested that lightning on Venus might be quite rare.

Venus has a much thicker atmosphere than Earth does, topped by clouds of sulfuric acid. But there’s very little water vapor in the clouds. And water is a key ingredient for lightning on Earth and other worlds where it’s been confirmed. So that led to some skepticism about claims of lightning on Venus from early on.

A Sun-watching spacecraft has swung close to Venus several times. It’s listened for radio waves like those produced by lightning on Earth. It found them. But they were headed in the wrong direction – toward the ground, not into space, as they are on Earth.

And a Venus orbiter has spent hundreds of hours looking for lightning flashes, but hasn’t seen a thing. In fact, one recent study said some of the flashes seen from Earth might really be meteors burning up in Venus’s atmosphere. So lightning might be rare on our neighboring planet.

Venus is the bright “morning star.” Tomorrow, it’s close to the crescent Moon.

Script by Damond Benningfield