Gemini has been around for thousands of years. Its roots trace back to ancient Babylon, as do those of many other constellations. All of them were recorded in one of the most important works of astronomy in the ancient world. Known as the Almagest, it was written by Claudius Ptolemy around the year 150.

Ptolemy studied many fields, including astronomy, astrology, geology, and music theory. The Almagest is perhaps his most famous work. In it, he recorded the positions of a thousand stars, and included details on the motions of the Sun, Moon, and planets. He also discussed everything from eclipses to the length of the year.

The book listed 48 constellations that were visible from northern skies – Gemini among them. The constellations weren’t given any borders – just the regions of the sky in which they appeared. And some barren regions weren’t part of any constellation.

Over the centuries, astronomers shifted things around some. And they created constellations for southern skies as well. Finally, in 1930, they created an “official” list of 88 constellations. Each one was given precise borders. That gave every star a home – its own “address” in the universe.

Script by Damond Benningfield

That spin is a result of the Coriolis effect. It’s caused by a combination of Earth’s rotation and its shape.

Because Earth is a sphere, locations on the equator move more than 24,000 miles in 24 hours. Locations off the equator move a much smaller distance in the same time. So, as a storm moves across the northern hemisphere, its southern edge moves faster than its northern edge. This causes the storm to rotate counter-clockwise.

The Coriolis effect is much more pronounced on the giant worlds of the outer solar system – especially Jupiter. It’s the biggest planet, and it has the fastest rotation – one turn in less than 10 hours. That combination deflects what normally would be north-south winds into east-west winds. They can blow at hundreds of miles per hour. They separate Jupiter’s atmosphere into wide bands. Individual storms – some the size of continents or bigger – spin through the bands, or along their boundaries – monster storms spinning through alien skies.

Jupiter stands to the upper left of the Moon this evening. It looks like a brilliant star. The twins of Gemini are above the Moon, and we’ll have more about them tomorrow.

Script by Damond Benningfield

We can’t see it, but the Moon is moving farther from us – by about an inch and a half per year. It’s been moving away since it was born, when Earth was young. In fact, that shift was one of the clues that led to the leading theory of how the Moon was born.

In the chaotic conditions of the early solar system, Earth was walloped by a planet about the size of Mars. That blasted debris into orbit around Earth. Much of that material quickly coalesced to form one or more moons. Today’s Moon is the only survivor.

The collision caused Earth to spin much faster, so a day was much shorter than it is now. Gravitational interactions between Earth and Moon have slowed us down. But they’ve also caused the Moon to slide farther away. The process isn’t smooth – the Moon speeds up and slows down. And it won’t stay smooth in the future.

Given enough time, the Earth-Moon system would reach a point when the same hemisphere of Earth would always face the Moon, and the Moon would stop moving away. But that time may never come. It could be so far in the future that the Sun will have expired – perhaps destroying Earth and its slip-sliding Moon.

Script by Damond Benningfield

A good example is Messier 13, the Great Hercules Cluster. Under especially dark skies, it’s just visible to the unaided eye, so people have known about it forever. It looks like a faint, hazy star. But during the 1700s, the cluster was “discovered” several times.

The first discovery was made by Edmond Halley. Using a small telescope, he came across it in 1714. He described it as “a little patch.” Charles Messier saw it a half-century later. He described it as “round, beautiful, and brilliant.” But, he wrote, “I am sure it doesn’t contain any star.” He made it the 13th object in his catalog.

In 1779, though, William Herschel contradicted Messier. M13 “is a most beautiful cluster of stars,” he wrote.

Many other discoveries have followed. They’ve told us that M13 contains hundreds of thousands of stars packed into a tight ball. And the cluster is ancient – 12 billion years old or older.

Messier 13 is 25,000 light-years away. In early evening, look in the east-northeast for the Keystone of Hercules – a lopsided “square” of stars. M13 is between the two stars at the top of that pattern, a bit closer to the one on the left – a giant cluster that’s still producing amazing discoveries.

Script by Damond Benningfield

But a few follow their own paths. An example is a star at the tip of the Guitar Nebula. The nebula is a bubble of gas with an outline that resembles a guitar.

It’s in Cepheus, which is low in the north at nightfall. The king’s brightest stars form an outline that resembles a child’s drawing of a house. Don’t look for the nebula, though – it’s so faint that it wasn’t discovered until 1992.

The guitar was sculpted by a pulsar – the crushed corpse of a mighty star. It spins once every two-thirds of a second, emitting a beam of energy that sweeps past Earth on each turn.

The pulsar was born when the star exploded as a supernova. The explosion must have been off-center, so it gave the dead core a powerful kick. The pulsar is plowing through clouds of gas and dust at almost two million miles per hour. It leaves an expanding wake behind it, like a ship traveling across the ocean.

That wake is what we see as the Guitar.

But there’s more to the nebula than meets the eye. X-ray telescopes in space reveal a long, high-speed “jet.” It’s firing away from the tip of the nebula at a right angle to the nebula itself. The jet most likely is powered by the pulsar’s magnetic field, which funnels charged particles away from the pulsar – an interesting note from a celestial guitar.

Script by Damond Benningfield

Like the Milky Way, Messier 82 is a thin disk, with spiral arms wrapping around a dense core. It’s less than half the size of the Milky Way.

M82 is a starburst galaxy. It had a close encounter with another galaxy within the past hundred million years or so. That caused huge clouds of gas and dust to collapse, triggering the starbirth.

The new stars are concentrated in the center of the galaxy, where astronomers have cataloged more than a hundred super star clusters. Each one contains hundreds of thousands of stars. Many of the stars are especially hot and massive, which makes the clusters especially bright.

A strong “wind” of hot gas races away from that region. It squeezes the surrounding clouds, giving birth to more stars. But within another hundred million years, all the gas and dust will have been used up. Then, M82 will settle down to the same quiet life as the Milky Way.

M82 is in Ursa Major. As night falls, it dangles below the upside-down bowl of the Big Dipper. It’s an easy target for small telescopes. We see it edge-on, so it looks like a small, bright slash.

Script by Damond Benningfield

Alpha Crucis is 320 light-years away. To the eye alone, it looks like a single point of light – the 13th-brightest star in the night sky. But binoculars or a telescope show two stars. Both of them are at least a dozen times as massive as the Sun, and thousands of times brighter. They’re so far apart that it takes about 1300 years for them to complete a single orbit around each other – a slow turn across the dance floor.

But one of those stars is actually two stars on its own. They’re so close together that not even the biggest telescopes can see them individually – the second star reveals its presence only to special instruments. But it’s also bigger, heavier, and brighter than the Sun. The two stars are dancing to a faster tempo – one turn around each other every 76 days.

Those three stars might have three more companions. They’re a long way from the first trio, and they’re not as impressive. But they appear to share a common motion through the galaxy with the brighter trio. That means the two groups could be gravitationally bound to one another – dancing a waltz that would require a hundred thousand years to complete one turn across the floor.

Script by Damond Benningfield

One of the treasures we miss is Crux, the southern cross. It’s the smallest of the 88 constellations. But it’s also one of the prettiest and most prominent.

Four of its stars are fairly bright, and they do form a shape that looks like a cross. If you include one more star in the pattern – the faintest of the five – the pattern looks more like a kite. It points the way to the south celestial pole.

Not surprisingly, that pattern has played a big role in the skylore of many southern-hemisphere cultures. Several saw the cross as the footprint of a big bird. Others saw it as a stingray, the anchor of a giant canoe, or some other prominent object or animal.

Today, Crux is featured on the flags of Australia, New Zealand, and Brazil. It’s also on the flag of the European Southern Observatory – which has a great view of the southern cross.

From the United States, Crux is barely visible from the Florida Keys, far-southern Texas, and Hawaii. At this time of year, it’s quite low above the southern horizon in early evening – pointing the way to the celestial pole.

More about Crux tomorrow.

Script by Damond Benningfield

Researchers have been trying to explain some oddities in the Saturn system. The planet itself is tilted far more than it should be, for example. The biggest moon, Titan, follows a more lopsided orbit than expected. And the moon is moving away from Saturn by about four inches per year.

A few years ago, a team proposed that Saturn once had another big moon, which the team called Chrysalis. Interactions between the moons might have kicked Chrysalis so close to Saturn that it was ripped apart, forming the rings.

But this year, another team came up with a slightly different scenario. It, too, involves a second moon. It collided with Titan a few hundred million years ago, changing Titan’s orbit. Debris from the impact formed the present-day moon Hyperion.

The activity caused two other moons to ram together as well. Both moons quickly re-formed, with the leftovers spreading out to form the rings as recently as 50 million years ago. This model explains many of the system’s oddities – bringing order to a chaotic arrangement.

Look for Saturn near our moon at dawn tomorrow. The planet looks like a bright star, low above the horizon.

Script by Damond Benningfield

But Venus exploration could tick up over the next few years. Several missions are being developed. Most of them are big and complicated, so they won’t be ready until the next decade. But a craft the size of a beachball could head for Venus as early as this summer. It’ll probe the planet’s clouds for signs of organic compounds – the chemistry of life.

Venus Life Finder is a project of Rocket Lab and MIT – the first commercially developed mission to the planet. It’s a small, blunt cone. When it arrives at Venus, it will plunge through the planet’s clouds, shining a laser on the way down. The reflected light will reveal details about the cloud particles. Bits of organic matter might be set aglow.

Some recent observations have hinted that the clouds could contain microscopic life. Life Finder won’t actually search for life, but it could tell us if the building blocks of life lurk inside the planet’s clouds.

Venus is the “evening star.” It’s sneaking up on the star Elnath, at the tip of one of the horns of the bull. Tonight, the star is a little to the upper right of Venus. The planet will slip past it during the week.

Script by Damond Benningfield