Floating through the clouds at Jupiter’s equator sounds like a celestial carnival ride. The equator spins at about 28,000 miles per hour – 28 times faster than Earth’s equator. So the Sun, moons, and stars would zip across the sky in a hurry.

Jupiter is the largest planet in the solar system – 11 times Earth’s diameter. It also spins faster than any other planet – so fast that it bulges outward at the equator. At that speed, a day on Jupiter is less than 10 hours long. So the equator always sees about five hours of daylight followed by five hours of darkness.

It might not sound right, but Jupiter spins so fast because it’s so big. As it swept up more material while it was taking shape, gravity compressed it, making it smaller. The planet had to spin faster to balance the books – like a skater spinning faster as it pulls in its arms.

Some studies have suggested that Jupiter might actually have been slowed down early on by its magnetic field. The young planet was encircled by a disk of gas and dust that gave birth to its moons. As the gas swirled through the magnetic field, some of it developed an electric charge. The charged-up gas grabbed on to the field, acting like a brake – slowing down the solar system’s biggest and still fastest planet.

Jupiter looks like a brilliant star below the Moon at dawn tomorrow. The twin stars of Gemini are closer to the left and lower left of the Moon.

Script by Damond Benningfield

If you’d like to thank your lucky stars for a bit of good fortune, we have two stars for you to look at. They’re the brightest stars of Aquarius. Both of them have names that mean “lucky.”

The brighter of the two is Sadalsuud. The name comes from an Arabic phrase that means something along the lines of “luckiest of the lucky.” When the name was bestowed, the star first appeared in the dawn sky around the spring equinox. The days were getting longer and warmer, and spring rains were settling in – bringing life-giving water to the fields. So the star was considered a sign of good fortune.

The other lucky star is Sadalmelik – “luck of the king.” The exact reason for its name is unclear, although it, too, may relate to the seasons.

Both stars are class-G supergiants. They’re about the same temperature and color as the Sun, but much bigger, heavier, and brighter. Both stars have passed through the prime phase of life, so their luck is running out – they’re nearing the end. Each will shed its outer layers and leave behind a massive white dwarf – a corpse about as heavy as the Sun, but only as big as Earth.

Aquarius is in the southeast at nightfall. The “lucky” stars line up parallel to the horizon, with Sadalmelik on the left. The stars are separated by about the width of your fist held at arm’s length. But they’re so far from us that they don’t look all that bright – a bit of bad luck for skywatchers.

Script by Damond Benningfield

Stars like the Sun go through several distinct phases of life, from embryo to corpse.

Consider Aldebaran, the bright eye of Taurus, which accompanies the Moon tonight. It’s more than six billion years old – older than the Sun. And it’s well into “old age.”

Aldebaran was born when a cloud of gas and dust collapsed. For millions of years, it shined as a result of the heat generated by that collapse – its “embryonic” phase. Eventually, its core got hot enough to ignite the fires of nuclear fusion, and Aldebaran entered the prime phase of life – fusing hydrogen to make helium.

A few hundred million years ago, it used up the hydrogen in the core. The core got smaller and hotter, and Aldebaran began fusing the hydrogen in a shell around the core. At the same time, its outer layers puffed up, so Aldebaran is more than 40 times wider than the Sun. This is the giant phase of life.

Eventually, the core will get hot enough to fuse the helium to make carbon and oxygen. But when the helium is gone, fusion will stop. The core will get smaller and hotter, and its radiation will push the star’s outer layers into space. Only the hot, dead core will remain – a white dwarf.

Even that isn’t the end, though. The white dwarf will cool and fade. Hundreds of billions of years from now – and perhaps much longer – it’ll stop producing any visible light at all. That will make it a black dwarf – the final stage for the eye of the bull.

Script by Damond Benningfield

The Moon barrels through the Pleiades star cluster this evening. It’ll pass directly in front of the cluster, briefly blocking most of its stars from view.

The Pleiades is the most famous of all star clusters. It’s also known as the Seven Sisters, but under dark skies – with no Moon in the way – you might actually see nine stars or more.

But that’s only the beginning. The cluster contains more than a thousand stars. In fact, it was the first cluster to be recognized as a cluster – a group that’s moving through the galaxy together.

That recognition came in 1767. John Mitchell, a clergyman and scientist, was looking at several tightly packed groups of stars. He studied the Pleiades in the greatest detail. And he calculated that there was only a one-in-500,000 chance that the grouping could be random. Instead, something had to be holding the stars together.

His idea was confirmed when astronomers measured the motions of the cluster’s stars. They’re all moving in the same direction, and at the same speed.

Today, we know that’s because they were born together, from a single giant complex of dust and gas. They’re bound to each other by their mutual gravitational pull. But they won’t stay together. The cluster is being pulled apart by the gravity of the rest of the galaxy. So the Pleiades probably will dissipate in about 250 million years – with its member stars going their own way.

Script by Damond Benningfield

An embryonic star may be about to vanish – perhaps for a century. It’s not going anywhere. Instead, it’ll be cloaked by a dense cloud that encircles two companions.

T Tauri is the prototype for a class of proto-stars. The gravity of such a star is causing it to collapse, making it hot and bright. But its core isn’t hot enough to ignite the fires of nuclear fusion, so it’s not yet a true star.

The star we see as T Tauri is about twice as massive as the Sun. It’s encircled by a disk of gas and dust – the raw materials for making planets. And it might already have given birth to at least one planet.

T Tauri is a member of a triple-star system. Its companion stars are close together, encircled by their own disk. It’s so thick that it hides the stars at visible wavelengths – we see them only in the infrared.

Now, the companions and their disk are starting to slide between us and the brighter star. The star has faded a good bit in recent years. Eventually, it may be hidden behind the disk as well. And it could take a century for the disk to move out of the way – allowing the brightest star of the T Tauri system to shine through once more.

Taurus is low in the east and southeast by late evening. T Tauri is just above Aldebaran, the bull’s brightest star, far to the lower left of the bright Moon. The young star is visible through a telescope – for now.

More about the Moon and the bull tomorrow.

Script by Damond Benningfield

The bull is charging into the evening sky. Taurus is in full view by about 11 o’clock, low in the east. He stands high in the south before dawn. He’s rising earlier each night, and will be in view all night long by about Thanksgiving.

All the stars rise four minutes earlier each night – a result of Earth’s motion around the Sun. Earth makes one full turn on its axis against the background of distant stars every 23 hours and 56 minutes. So, if you looked at the sky every 23 hours and 56 minutes, and you could see through the daytime glare, you’d always see the same stars in the same position.

But during that period, Earth moves along its orbit around the Sun. The distance it covers means the planet has to turn four extra minutes for the Sun to reach the same position in the sky. That makes a day 24 hours long. And it also means that the background stars rise and set four minutes earlier on our 24-hour clock.

As a result, every star and constellation is in prime evening view at different times of the year. For Taurus, it’s fall and early winter – the time the bull charges across the evening sky.

For now, look for Taurus beginning in late evening. Its brightest star is Aldebaran, the bull’s eye. His face is outlined by a V-shaped pattern of stars to the upper right of Aldebaran. And his shoulder is the sparkly little Pleiades star cluster, well above Aldebaran.

More about Taurus tomorrow.

Script by Damond Benningfield

The Moon is full tonight, and it’s especially bright as well. And to top things off, it’s the most famous full Moon of them all – the Harvest Moon.

Harvest Moon is the full Moon closest to the fall equinox, so most years it falls in September. But once every five years or so it skips into October. This year, September’s full Moon came 15 days and 10 minutes before the equinox, which took place on the 22nd. This month’s full Moon comes 14 days, 9 hours, 29 minutes after the equinox, so it barely takes Harvest Moon honors.

The Harvest Moon was important in earlier times because it shined over the fields when crops were ready to be brought in. Its light allowed farmers to work into the night. And because of the angle of the Moon’s path at this time of year, the full Moon rises only a few minutes later each night as seen from more northerly latitudes. So it’s almost like having a full Moon for several nights in a row.

People often think that the Harvest Moon must be especially bright, but that isn’t usually the case. This year, however, it is. That’s because it comes less than a day and a half before the Moon is closest to Earth for its current orbit – roughly 15,000 miles closer than average. That provides some especially bright nights for farmers – and the rest of us, too.

Tomorrow: the bull charges into the evening sky.

Script by Damond Benningfield

It’s pretty easy to measure the length of a day on Mars or most other solid bodies. Just pick a feature on the surface and see how long it takes to spin back into view.

It’s not so easy for planets that don’t have a solid surface. We can track bands of clouds, but different bands can move at different speeds.

That’s been an especially tough problem for Saturn, the second-largest planet in the solar system. Scientists have been trying to pin down its rotation rate – the length of its day – for centuries.

When the twin Voyager spacecraft flew past Saturn in the 1980s, they measured the planet’s magnetic field to reveal the rotation rate of its interior. But when the Cassini spacecraft orbited Saturn decades later, its observations showed the day was about six minutes longer.

At the end of its mission, Cassini flew between Saturn and the inner edge of its rings. Measuring waves in the rings and tiny changes in the planet’s gravitation field produced yet another length: 10 hours, 33 minutes, and 38 seconds.

That’s not necessarily the final answer. Scientists continue to study the giant planet to know how to set their Saturn clocks.

And Saturn is in great view tonight. It looks like a bright star quite close to the lower right of the Moon at nightfall, and below the Moon as they set, before dawn.

Script by Damond Benningfield

Saturn and Venus bracket the pre-dawn sky now. As Saturn drops from view in the west, Venus nudges into view in the east. Saturn looks like a bright star, while Venus is the brilliant morning star.

The planets are both sliding eastward against the background of distant stars.

Saturn lined up opposite the Sun a couple of weeks ago. For a few months around that point, the planet looks like it’s “backing up” against the background of stars – a result of the relative motions of Saturn and Earth.

Earth is closer to the Sun than Saturn is, so our planet moves faster. It overtakes Saturn every 13 months, making Saturn appear to shift into reverse. It’s actually still moving in its usual direction – only our viewing angle is changing. It’s like passing another car on the highway. For a while, the other vehicle looks like it’s moving backward against the background of buildings and trees. When you move far enough past it, though, it appears to resume its normal forward motion.

Saturn will end its backward motion and shift back into forward at the end of November.

Venus, on the other hand, is about to pass behind the Sun as seen from Earth, so it’s dropping closer to the Sun every day. That’s also a result of the orbital motions of the two planets. Venus will disappear in the twilight in December, and cross behind the Sun in January – depriving us of the “morning star.”

More about Saturn tomorrow.

Script by Damond Benningfield

Scientists don’t know what dark matter is. But they have some ideas of what it isn’t. And they took a big step in ruling out some possibilities with the release of a study last year.

Dark matter produces no energy – the reason it’s described as “dark.” But we know it’s there because its gravity pulls on the visible matter around it. In fact, it appears to make up about 85 percent of all the matter in the universe.

The leading idea says dark matter consists of some kind of subatomic particle. A top candidate is called a WIMP – a weakly interacting massive particle.

One experiment is LUX-ZEPLIN. It’s in a former gold mine, almost a mile below the town of Lead, South Dakota. The rock above it blocks other types of particles from reaching the experiment.

Its detectors are inside a vat filled with about 8,000 tons of liquid xenon. The hope is that a WIMP will hit a xenon molecule and trigger that spark of light.

Project scientists conducted 280 days of observations. And they didn’t find any indication of WIMPs. But their test was the most sensitive yet for certain types of WIMPs. So the experiment rules out some candidate particles – narrowing the possibilities for dark matter.

Script by Damond Benningfield