StarDate

As seen from the eastern United States, there’s a “now-you-see-it, now-you-don’t” event in the early evening sky. The Moon will occult Regulus – passing in front of Leo’s brightest star and blocking it from view. The star will remain hidden for a few minutes. But its disappearance is almost instantaneous:

Regulus is there one second, then gone the next.

It does take a tiny fraction of a second for the Moon to cover the star. Astronomers make precise measurements of that timing. The length of time it takes a star to vanish reveals its apparent diameter – how big it looks in our sky. And that’s how the first good measurement of the size of Regulus came about.

In 1933, a French astronomer recorded an occultation of the star on a rapidly spinning photographic plate. That told him how long it took Regulus to disappear. From that, he calculated the star’s apparent diameter. And he was close to the modern value.

When astronomers combine that number with a star’s distance, they can calculate its true diameter. Regulus is 79 light-years away – and about four times the diameter of the Sun.

Tonight’s occultation is best seen from the eastern U.S. The Moon and Regulus will be in the sky as seen from the rest of the country as well. But at least part of the event will take place during daylight, when Regulus is too faint to see without help. The star will shine close to the Moon after the occultation ends.

Script by Damond Benningfield

It’s hard to ask for a better signpost for finding things in the night sky than the planet Venus. Right now it’s the brilliant “evening star,” low in the west as twilight fades. And it points the way to two other wonders: the planet Uranus and the Pleiades star cluster.

The Pleiades is fairly easy to find on its own. Its brightest stars form a tiny dipper shape. In fact, the Pleiades is often mistaken for the Little Dipper. But that dipper is in the north, anchored by the North Star.

Despite its prominence, the Pleiades is best appreciated with a technique known as averted vision – seeing it from the corner of your eye. And Venus offers a good chance to try it. Look at Venus, then see if you can see the sparkly cluster to its right. They’re separated by the width of a couple of fingers held at arm’s length.

Uranus is about one finger width below Venus. It’s the third-largest planet in the solar system. But it’s so far away that it looks tiny and faint. It’s an easy target for binoculars or a small telescope, though. It looks like a faint star.

A telescope reveals something interesting about Venus – it doesn’t look quite complete. That’s because it’s in a gibbous phase. If you watch the planet for months, you’ll see it get thinner and thinner. That’s because Venus will cross between Earth and the Sun in late October. Like the new Moon, it’ll be lost in the Sun’s glare no matter how you look at it.

Script by Damond Benningfield

Jupiter’s big moon Europa is one of the most likely bodies in the solar system to host life. The moon has a global ocean that holds more water than all of Earth’s oceans combined. The ocean might have sources of energy and chemical compounds that are needed to support microscopic life.

But getting to that ocean won’t be easy. It’s covered by a crust of ice. And a recent study says the ice is pretty thick.

The Juno spacecraft scanned part of Europa with an instrument that can probe conditions below the surface. It found that the average thickness of the ice is about 18 miles. That’s thicker than suggested by some earlier studies. Juno found many cracks in the ice. But they don’t penetrate anywhere close to the water. So there doesn’t appear to be a good way to get through the ice to study the ocean.

That also could be a problem for any organisms in the ocean. Jupiter’s radiation zaps material on the surface, transforming it into possible nutrients.

Without any holes or thin spots in the ice, there’s no direct way to flush the nutrients into the water. But another study found that large concentrations of nutrients could make blocks of ice denser than the surrounding ice. Over time, the heavier blocks could sink all the way through the ice – perhaps helping to sustain any life in Europa’s hidden ocean.

Jupiter is high in the west at nightfall, and looks like a brilliant star. The twins of Gemini stand above it.

Script by Damond Benningfield

In December of 2024, a region on Jupiter’s moon Io blew its top. Several huge volcanoes were erupting at the same time – the most powerful volcanic event ever seen anywhere in the solar system. The outburst covered an area the size of West Virginia. During the hour that a spacecraft was watching, it produced enough energy to power the entire United States for days.

Io is by far the most active body in the solar system. It has hundreds of cones, lava pools, and other volcanic features. They’re powered by a constant tug-of-war between Jupiter and some of its other big moons. They pull and stretch Io’s interior, heating it up.

The 2024 eruptions were observed by Juno, a spacecraft that’s orbiting through the Jovian system. The region on Io had been quiet when Juno last looked at it, about two months earlier. So the eruptions must all have started at about the same time.

That suggests they were powered by the same source of magma below the surface. The magma must have traveled through a network of underground plumbing, allowing it to power several eruptions at once. So Io’s interior might be like a sponge, with lots of open spaces – that are sometimes filled with molten rock.

Jupiter appears just above our moon tonight. It looks like a brilliant star. Through binoculars, Io and Jupiter’s other big moons look like tiny stars quite close to the planet.

We’ll talk about one of Jupiter’s icy moons tomorrow.

Script by Damond Benningfield

Two planets cross paths in the evening sky this week. One is brilliant, the other a little too faint to see without some help.

The brilliant one is Venus, the “evening star.” In all the night sky, only the Moon outshines it, so you can’t miss it. But you can miss Uranus. It’s a giant, but it’s so far away that it’s not easy to see.

Several factors control how bright a planet looks: the planet’s distance from both Earth and the Sun, its size, and how much sunlight is reflected from its surface.

The clouds that blanket Venus reflect much more sunlight than the clouds of Uranus do. And while Uranus is about four times the diameter of Venus, right now it’s almost 14 times farther. That makes it look smaller in our sky.

The distances are also important in another way. The farther an object is from the Sun, the feebler the Sun appears. At their average distances from the Sun, each square foot of Venus receives more than 700 times more sunlight than the same size patch of Uranus. At the same time, the farther an object is from Earth, the less of its light we receive.

When you put it all together, Venus looks more than seven thousand times brighter than its giant sibling.

Venus blazes into view as twilight fades. Uranus is a couple of degrees to its upper left, and it’s an easy target for binoculars. The two worlds will stand almost side by side on Thursday, just a fraction of a degree apart.

Script by Damond Benningfield