StarDate

The largest moon of Neptune was an agent of chaos. It wasn’t born with Neptune itself. Instead, it was captured by the giant planet. As it spiraled in, it might have hit one of Neptune’s existing moons. And it scattered the others – knocking some of them out of orbit. Only one of them might have survived.

There’s a lot of evidence of that idea. Triton is about the same size and composition as Pluto. It contains more than 99 percent of the total mass of Neptune’s moons and rings. And it orbits in the opposite direction from Neptune’s rotation.

There’s no way for a moon to form in that kind of orbit, so Triton must have formed elsewhere in the solar system. It was captured when Neptune was young. It might have caromed off an existing moon. Or it might have had a companion. When the duo passed close to Neptune, it was ripped apart – Triton entered orbit, while its sibling was kicked off on its own. Triton then bludgeoned its way through the system.

A recent study says the only surviving moon is Nereid, Neptune’s third-largest moon. Researchers looked at it with Webb Space Telescope. They found that it’s made of the same materials as the moons of Uranus – not the other moons of Neptune. That suggests it was born with the planet, and survived Triton’s chaotic arrival.

Neptune is at its brightest this week. It’s in the east at nightfall, to the upper right of the bright planet Saturn. But you need a telescope to see it.

More tomorrow.

Script by Damond Benningfield

It’s not often that someone hands an astronomer a major discovery. But that’s what happened 180 years ago today. German astronomer Johann Galle, the director of Berlin Observatory, received a letter from Urbain Le Verrier. The French astronomer had calculated the likely position of a planet beyond Uranus, which at the time was the Sun’s most-distant known planet. Galle looked for the new planet that night – and found it: the planet Neptune.

Le Verrier had calculated that Uranus wasn’t orbiting the Sun as expected. He decided that it was being nudged out of position by the gravity of another planet, farther from the Sun. He calculated the planet’s location, then sent his results to Berlin.

British astronomer John Couch Adams had made similar calculations. He passed his results to his own colleagues. One of them searched for the planet, and saw it, in 1845 – but he didn’t realize it. So Adams usually is credited as a co-discoverer. But some research in recent years has suggested that his calculations were off, so there’s a debate about who discovered the Sun’s eighth planet.

Appropriately enough, Neptune is at its best this week. It lines up opposite the Sun, so it’s closest to Earth, and shining at its brightest. But it’s so far away that you need a telescope to pick it out. It’s not far to the upper right of the bright planet Saturn, which is low in the east at nightfall.

More tomorrow.

Script by Damond Benningfield

The Sun speaks to us. It tells us about conditions deep inside it, far below its surface. That helps scientists understand how the Sun is put together, how it works, and how it changes.

Listening to the Sun is called helioseismology. It works in the same way that seismology works on Earth.

Motions inside the Sun generate sound waves. Those waves can travel all the way around the Sun. And they can travel deep inside it. They cause the surface to vibrate. And scientists can measure the vibrations – the Sun’s voice.

The voice is complicated. It’s producing many frequencies of sound – like a diva singing many octaves of notes all at the same time. Some of the waves penetrate all the way to the core, where the Sun generates energy. Others stay close to the surface. Scientists have to unscramble this cacophony to understand what the waves are telling us.

One thing that scientists have learned recently is that an important region in its magnetic field has changed over the past four decades. Today, that region is much closer to the surface than it was 40 years ago, suggesting that our star is undergoing some changes.

The magnetic field is especially important to Earth. Magnetic storms can knock out satellites and power grids, disrupt radio waves, and cause other problems. So we may be able to better protect our technology by listening to the voice of the Sun.

Script by Damond Benningfield

Day and night will be just about equal the next few days for the entire world. That’s because fall arrives in the northern hemisphere tomorrow. It’s the September equinox – the moment the Sun crosses the equator from north to south.

At that moment, the Sun stands directly above the equator. So no matter where you are, the Sun rises due east and sets due west.

Equinox means “equal nights.” That tells us that day and night should be the same length. But that’s not quite the case. In the northern hemisphere, daytime – the interval from sunrise to sunset – lasts a few minutes longer than nighttime. They won’t balance out until a few days after the equinox.

One reason for the difference is the way we figure the moments of sunrise and sunset. For day and night to be equal, we’d have to think of them as the time the Sun is bisected by the horizon – when half is in view, and half is hidden.

Instead, of course, we consider sunrise as the moment the top of the Sun first peeks into view. And sunset is the moment when the Sun fully disappears. That adds a minute or so to the “daytime” side of the equation.

And we don’t actually see the Sun rise and set – at least not live. Earth’s atmosphere bends the Sun’s rays around the planet. So by the time you see the setting Sun touch the horizon, it’s actually already set. You’re seeing an extended version of things – stretching the daylight for a few extra minutes.

Script by Damond Benningfield

In Shakespeare’s play “Julius Caesar,” Caesar makes a bold proclamation: But I am constant as the northern star, / Of whose true-fix’d and resting quality / There is no fellow in the firmament.

Caesar turned out to be not so constant, of course. And neither is the northern star. In fact, Earth sees a cycle of North Stars – a cycle that lasts for 26,000 years.

The current North Star is Polaris. It stands almost due north. So from the northern hemisphere, all the other stars appear to wheel around it as Earth turns on its axis. But Polaris will slide away from that honored spot over the coming centuries. And about 1200 years from now, it’ll be replaced by Errai, in the constellation Cepheus the king.

We go through a sequence of north stars because of a slow wobble in Earth’s axis – the result of the gravitational pull of the Sun and Moon. When you combine a star’s brightness and its proximity to true north, Polaris may be the best of them all. Errai appears only about a third as bright as Polaris, and it won’t get quite as close to due north. Even so, it will reign as the North Star for about 2,000 years, before passing the crown to another star in Cepheus.

The king is high in the north at nightfall, to the upper right of Polaris. It looks like a child’s drawing of a house, although it’s upside down during the evening hours. Errai is the peak of the house’s roof – a future inconstant North Star.

Script by Damond Benningfield