StarDate

The Moon will run a gauntlet of bright stars and planets the next few mornings, from the twins of Gemini to the heart of the lion. The twins are up first. Pollux and Castor will perch to the upper left of the Moon at dawn tomorrow. Pollux is the brighter of the two, and it’ll be closer to the Moon. On the following mornings, the Moon will slide past Mars, Jupiter, and Regulus.
These encounters are possible because all of these objects are near the ecliptic – the Sun’s path across the sky. The stars all maintain the same position relative to the ecliptic from month to month and year to year. Regulus, for example, is less than half a degree away from it – less than the width of a pencil held at arm’s length.
The planets, on the other hand, can shift from one side of the ecliptic to the other. That’s because their orbits around the Sun are tilted a bit.
The Moon’s orbit is tilted as well, by about five degrees. So the Moon can swing a little farther from the ecliptic. Tonight, it’s a few degrees north of it. By Wednesday, when it’s closest to Regulus, it’ll be just south of it.
Despite that wiggle room, the Moon passes close to each of these bright pinpoints during every month-long cycle of phases. And when the geometry is just right, it can even cover them up – which it will do to Jupiter on Tuesday. We’ll have more about that later in the week, and more about the Moon and Mars tomorrow.

Script by Damond Benningfield

Saturn is a “superior” planet. Among other things, it’s the second-largest and second-heaviest planet in the solar system. Technically, though, what makes it “superior” is its location. It’s the sixth planet from the Sun, so its orbit is outside Earth’s orbit.

Superior planets can line up opposite the Sun in our sky – a point known as opposition. And that’s what Saturn is doing now. It’ll reach that point early Sunday. So for a while, it’ll be in the sky all night, and shine brightest for the year. Only a few planets and stars will outshine it.

A planet looks especially bright at opposition for a couple of reasons. For one thing, it’s closest to Earth. Saturn, for example, will be about a hundred million miles closer than average.

Another reason is the viewing angle. At opposition, a planet is reflecting more sunlight directly toward Earth than at any other time. In effect, the planet is a more efficient “mirror.” We see the same effect with the Moon. A full Moon – which also is in a “superior” position – is about six times brighter than a half Moon. The change in the Moon’s brightness is complicated by other factors, but the principle is the same – a superior Moon puts in a superior show – just like the superior planet Saturn.

Saturn is low in the east at nightfall. It looks like a bright star. It climbs high across the south during the night, and is low in the west at dawn.

Script by Damond Benningfield

This isn’t a sound effect from a sci-fi movie. Instead, it’s the “voice” of the auroras on the planet Saturn – radio waves that have been shifted to wavelengths we can hear. They were recorded by the Cassini spacecraft as it closed in on Saturn two decades ago.

The radio waves are produced by the complex interplay between Saturn’s magnetic field and the solar wind – a steady flow of charged particles from the Sun.

Motions deep inside the planet generate the magnetic field, which is about as strong as Earth’s. But because Saturn is much bigger than Earth, its magnetic field is much bigger as well – it fills a huge volume of space.

The magnetic field forms a teardrop-shaped “bubble” around Saturn. That bubble deflects much of the solar wind. But some of the particles make it through. The lines of magnetic force guide some of them toward the magnetic poles. They spiral in, emitting radio waves as they do so. And when the particles hit the upper atmosphere, they create auroras – shimmering curtains powered by the Sun.

Saturn is putting in its best appearance of the year. It’s at opposition – it lines up opposite the Sun. It’s closest to Earth, so it shines brightest. And it’s in view all night. The giant planet looks like a bright golden star. It’s low in the east at nightfall, and climbs high across the south later on.

Script by Damond Benningfield

Saturn puts in its best appearance of the year over the next few nights. On Sunday, it’ll reach opposition – it will line up opposite the Sun. It’s closest to Earth at opposition, so it shines brightest. And it’s in view all night. It looks like a bright golden star. It’s low in the east at nightfall, and climbs high across the south later on.

Saturn reaches opposition every 12 and a half months – the result of the combined motions of Saturn and Earth. Earth orbits the Sun at an average speed of about 67,000 miles per hour. Saturn is more than nine times farther from the Sun. And thanks to the laws of orbital motion, it moves only a third as fast as Earth.

Johannes Kepler formulated those laws four centuries ago. He determined that the planets follow elliptical orbits; instead of perfect circles, the orbits are stretched out. There’s a relationship between the planet’s distance and its orbital period – the time it takes to make one full turn around the Sun. And a planet moves fastest when it’s closest to the Sun, and slowest when it’s farthest.

Earth’s distance varies by only about three percent, so there’s not much change in its orbital speed. But Saturn’s distance varies by more than 10 percent, so there’s a bigger change in its speed.

All of this works together to bring Saturn into alignment every 12 and a half months – shining at its best.
More about Saturn tomorrow.

Script by Damond Benningfield

To modern eyes, the stars of Sagittarius form the outline of a teapot. But in Greek mythology, the constellation was far more extensive. It represented a centaur – half-man, half-horse – holding a bow and arrow. And the brightest star in the constellation plays a role in both of those pictures.

Kaus Australis is at the lower right corner of the teapot. It also represents the southern end of the bow – in fact, that’s the meaning of its name.

It’s actually a binary – two stars in a wide orbit around one another. One of the stars is like the Sun. But from the system’s distance of about 145 light-years, it’s much too faint to see with the eye alone.

The star we can see is much bigger and heavier than the Sun, and about 500 times brighter. And it spins in a hurry – once every 1.6 days, compared to almost four weeks for the Sun. If it were spinning just a little faster, it would rip itself apart. In fact, it’s closer to that self-destruct point than any other star yet seen.

As a result of its rotation, the star is squashed – it’s about a third wider through the equator than the poles. And because they’re closer to the star’s core, the poles are thousands of degrees hotter than the equator.

Astronomers can’t explain the star’s high-speed rotation. The star could be siphoning gas from a much-closer companion that’s hidden from view – spinning up the tip of the archer’s bow.

Script by Damond Benningfield