StarDate

You can tell whether a train or a police car is moving toward or away from you just by listening. Its horn or siren changes pitch – higher if it’s moving toward you, lower if it’s moving away.

The same principle applies to the stars. Their light is shifted to longer or shorter wavelengths. Measuring that shift reveals the star’s motion – its “radial velocity.”

An example is Pollux, the brightest star of Gemini. It’s close to the Moon at first light tomorrow. Its radial velocity tells us that the star is moving toward us at more than 7,000 miles per hour. That sounds fast. But at the star’s distance of almost 34 light-years, it would take millions of years to reach us. Pollux’s orbit is also carrying it sideways, so the star will never even get close.

By measuring its radial velocity, astronomers at McDonald Observatory discovered a planet orbiting Pollux. As it orbits, the planet tugs at the star, changing its motion toward us by a few miles per hour.

Precise measurements of that change revealed some details. The planet is a bit more than twice as massive as Jupiter, the giant of our own solar system. It’s a little farther from Pollux than Mars is from the Sun. And it orbits the star once every 19 months.

Astronomers named the planet Thestias – a version of Leda, the mother of Pollux – a planet discovered by measuring a tiny shift in the star’s light.

Script by Damond Benningfield

When Mars was young, the planet was much warmer and wetter than it is today, with a fairly thick atmosphere. Most of that atmosphere leaked away into space. And scientists are still trying to understand how that happened.

They should learn more from twin spacecraft that are scheduled to launch soon. The mission is called Escapade, while the individual craft are designated Blue and Gold. If they launch on time, they’ll enter orbit around Mars a year from now.

The probes will study the weak Martian magnetic field and how it interacts with the solar wind – a “breeze” of charged particles from the Sun.

Because Martian gravity is much weaker than Earth’s gravity, molecules in its early atmosphere drifted to the top of the atmosphere. There, radiation from the Sun split the molecules apart. The solar wind then carried off some of the residue. Over the eons, that depleted the atmosphere. Today, it’s less than one percent as thick as Earth’s.

Many of the details of that process are unclear. The Escapade probes will monitor that process from different perspectives, offering a 3-D view of what’s going on. That should help scientists fill in the blanks – providing a much better understanding of how Mars lost its air.

Mars perches close to the Moon early tomorrow. They climb into view by 1 or 1:30 a.m., and stand high in the sky at first light. Mars looks like a bright orange star to the lower right of the Moon.

Script by Damond Benningfield

NOTE: Since this program was recorded NASA has delayed the launch of Escapade until at least 2025.

Jupiter might have “blinked” for more than a century. A storm in its southern hemisphere might have winked out, with another eventually taking its place – today’s Great Red Spot.

Scientists first reported a spot in Jupiter’s southern hemisphere as early as 1632, and definitely by 1665. But there were no records of it from the early 1700s to the early 1800s. So there’s been a debate about whether the spot disappeared, or astronomers simply missed it.

A recent study argues that the original spot vanished. Researchers combed through all the records of Jupiter they could find – including those by some of the most accomplished observers of the time. And there were no reports of a big, dark spot. That suggests the original storm fizzled out, and a new one fired up in the same location.

The current version was first reported in 1831. Since then, it’s changed dramatically. At first, it was an oval about three times as wide as Earth. Today, it’s rounder, and about the same size as Earth.

The new study says the Great Red Spot probably sprang from the interplay of two powerful jet streams. Today, winds at its perimeter reach almost 300 miles per hour. And they’re getting faster as the spot shrinks – perhaps to once again blink out.

Jupiter stands to the right of the Moon as they climb into good view, around midnight. It looks like a brilliant star. A small telescope will reveal the Great Red Spot.

Script by Damond Benningfield

Three bright companions cradle the Moon early tomorrow: the planet Jupiter and the brightest stars of Taurus. The whole group climbs into good view by midnight, and stands high in the sky at dawn. The star Elnath is to the left of the Moon at that hour, with brilliant Jupiter to the lower left of the Moon, and Aldebaran a little farther below the Moon.

There’s a big gap in the distances to the four bodies. Yet they’re all among our closest neighbors.

On average, the Moon is the closest of all. Tonight, it’s just 230,000 miles away. Small asteroids occasionally pass closer than that. But most of the time, those bodies are much farther.

Jupiter is the next-closest member of the group. Right now, the Sun’s largest planet is 445 million miles away. At that distance, it takes years to get there. Even so, nine spacecraft have traveled to Jupiter. Another mission is en route, with one more scheduled for launch next month.

Aldebaran is about 65 light-years away – more than 850,000 times farther than Jupiter. And Elnath is double that distance. That puts them far beyond our ability to reach them. Even so, they’re quite close as stars go. Our home galaxy, the Milky Way, spans a hundred thousand light-years. And all the other major galaxies are millions or billions of light-years away. So the Moon and tomorrow’s companions are all close neighbors – just down the cosmic block.

Script by Damond Benningfield

In many ways, fall has already arrived. The fall term has started for schools. Football season is underway. And September 1st marks the beginning of the fall weather season. Astronomical fall catches up to them tomorrow. Autumn officially arrives at 7:44 a.m. Central Daylight Time – the moment of the fall equinox.

The change in seasons occurs as the Sun crosses the equator from north to south. The Sun rises due east and sets due west as seen from the entire planet. And the length of day and night are about equal.

That balance won’t last long. In the northern hemisphere, the days will grow shorter and the nights longer until we reach the winter solstice, in December. As that happens, the sunrise and sunset points will slide southward.

As the season changes, so do the stars. At nightfall, Arcturus, the brightest star of summer nights, is dropping lower in the west. The signature star pattern of the season, the Summer Triangle, is moving to the western side of the sky. And the scorpion is getting ready to disappear in the southwest.

In the meantime, the Great Square of Pegasus is in the east at nightfall, ready to climb high across the sky during the night. Constellations associated with the flying horse are moving into view as well. And so are the constellations of the Celestial Sea – water-related constellations like the fish, the sea goat, and the water boy – stellar sights for the longer nights of autumn.

Script by Damond Benningfield

When Voyager 2 flew past Neptune, 35 years ago, its list of discoveries included six moons, a set of rings, a giant storm, and an odd magnetic field. Mission scientists converted some of its observations of the field to sound. And scientists are still trying to understand the magnetic field today.

Neptune is the Sun’s most remote major planet. It’s about four times Earth’s diameter. Methane in its upper atmosphere absorbs red light, giving the planet a blue-green color.

Neptune is tilted on its axis at about the same angle that Earth is. Also like Earth, its magnetic poles don’t line up with its geographic ones. But on Neptune, there’s a much bigger angle between the two – about 47 degrees, versus about 11 degrees for Earth.

And while Earth’s magnetic field is generated near the center of the planet, Neptune’s is generated closer to its cloud tops than its core.

Scientists haven’t settled on an explanation for how Neptune’s field is generated. It may be powered by “bubbling” motions inside a hot, slushy layer of ammonia, methane, and water. However it’s generated, Neptune’s magnetic field is powerful and complicated – befitting one of the giants of the solar system.

Neptune is at its best this week. It rises at sunset and remains in view all night, in Pisces. It’s brightest for the year as well. But it’s so far away that you need strong binoculars or a telescope to see it.

Script by Damond Benningfield

The giant planets of the outer solar system are like mini-solar systems on their own, because each has a large entourage of moons. Consider Neptune, the solar system’s fourth-largest major planet, which is putting in its best showing of the year. It rises at sunset and remains in the sky all night. It’s also at its brightest for the year. But it’s still so faint that you need a telescope to see it, in the constellation Pisces.

Until 1989, astronomers knew of only two moons orbiting Neptune. But 35 years ago, the Voyager 2 spacecraft discovered a half-dozen more moons as it flew past the planet. And since then, astronomers have used telescopes on the ground to find even more, bringing the total to 16.

Some of the moons probably formed with Neptune, four and a half billion years ago. But some of the more recently discovered moons may have been asteroids that were captured by Neptune’s gravity. And others may be fractured remains of a larger moon that split apart when it was hit by an asteroid or comet.

Neptune’s biggest moon may be a newcomer as well. Triton orbits in the opposite direction from most of Neptune’s moons, suggesting that the planet captured it in the distant past. And billions of years in the future, Triton will move so close to Neptune that the planet’s gravity will rip it apart. That will create a giant but short-lived ring around this giant planet.

More about Neptune tomorrow.

Script by Damond Benningfield

A hidden giant lurks between the Moon and the bright planet Saturn tonight. Neptune is the fourth-largest planet in the solar system, and it’s putting in its best showing of the year this week. But it’s so far away that you need help to see it.

Neptune is about four times the diameter of Earth. But it’s the Sun’s most-remote major planet – 30 times farther from the Sun than Earth is. As a result, it looks faint. It’s so faint, in fact, that it wasn’t discovered until September of 1846. And it was found because astronomers calculated that another planet was pulling on Uranus, the most-distant planet known at the time.

This week, Neptune is at opposition – it lines up opposite the Sun in our sky. That means it rises around sunset and is in view all night. The planet’s also closest to us for the year, so it shines at its brightest.

It peaks at a magnitude of about 7.7. That sounds bright. But in the magnitude scale, brighter objects have lower numbers. So Neptune would have to be several times brighter to be visible to the unaided eye even under the absolute best viewing conditions. Under moonlight or city lights, it would have to be even brighter.

Tonight, Neptune is to the upper right of the Moon at nightfall. It’s about half way along toward Saturn, which looks like a bright star. Neptune is a big but faint target for good binoculars or a small telescope.

Script by Damond Benningfield

The Moon has a lot going on tonight. For one thing, it’s full. And it’s the best-known full Moon of the year – the Harvest Moon. It’ll stage a minor eclipse that’ll be visible across almost all of the United States. And a couple of giant planets lurk near by – one of them especially close.

The Harvest Moon honor goes to the full Moon that’s closest to the fall equinox. The equinox is coming up on Sunday – just five days after the full Moon.

The lunar eclipse begins at 7:41 p.m. Central Daylight Time. That’s when the Moon makes contact with Earth’s outer shadow. But that part of the shadow is so faint that it’s hard to tell much difference.

A couple of hours later, the Moon will barely dip into the darker inner shadow. It’ll cover only a tiny fraction of the Moon, so it’ll look like something took a nibble from the disk. At least part of the eclipse will be visible from the entire United States except western Alaska.

Not long after the eclipse ends, the Moon will stage its own eclipse: It’ll pass directly in front of Neptune, briefly blocking the planet from view for most American skywatchers. Neptune’s too faint to see without binoculars or a telescope, so most of us won’t even notice it. We’ll have more about Neptune tomorrow.

And if all that lunar activity isn’t enough, the planet Saturn stands close to the Moon all night. It looks like a bright star – leading the Harvest Moon across the night sky.

Script by Damond Benningfield

On March 2nd of the year 1105, the Moon barely dipped its toe into Earth’s outer shadow, the penumbra. The eclipse was so faint and short that it probably went unnoticed. But it marked the start of a cycle that will last 1300 years and feature 73 eclipses. And the next one takes place tomorrow night. The full Moon – the Harvest Moon – will pass through the penumbra, and barely touch the darker inner shadow, the umbra.

Eclipses occur in cycles known as Saros. This eclipse is part of Saros 118.

The Moon starts a cycle at the far north or south of our planet’s shadow. Over the centuries, it moves through the shadow, until it exits at the opposite pole. So a Saros begins with penumbral eclipses, then moves through partial eclipses, total eclipses, then back again. The next eclipse in this Saros will take place in September of 2042.

This eclipse is number 52 in the series. It’ll begin at 7:41 p.m. Central Time, when the Moon first touches the penumbra. Later, the Moon will barely dip into the umbra, so it’ll look like a celestial dragon took a tiny nibble from the Moon. At least part of the eclipse will be visible from the entire United States except western Alaska.

So look for the almost-full Moon tonight, with the bright planet Saturn close by. Then tomorrow night, see if you can follow the subtle shading of a faint lunar eclipse.

Script by Damond Benningfield