StarDate

The full Moon won’t look quite itself for part of tomorrow night. That’s because it’ll pass through Earth’s long shadow, creating a partial eclipse. At its peak, the shadow will cover all but a sliver of the lunar disk. That will darken the surface, with sunlight that filters through Earth’s atmosphere adding an orange or red tint.

Lunar eclipses occur only at full Moon, when the Moon lines up directly opposite the Sun. The Moon’s orbit around Earth is tilted a bit, so most months the Moon passes above or below the shadow. But at least twice a year, the geometry is just right, and the Moon plunges into the shadow.

This eclipse is only partial, not total. That means the angle isn’t quite perfect, so the Moon won’t be fully immersed in the shadow. But it’ll be hard to tell much of a difference – the shadow will cover 96 percent of the lunar disk.

At least part of the eclipse will be visible from almost the entire United States. It gets started at 8:24 p.m. Central Daylight Time, when Earth’s outer shadow first touches the Moon. It’s so faint that you might not even notice it. But you will notice the partial eclipse, when the Moon enters the dark inner shadow. That starts at 9:34 p.m., peaks at 11:13, and ends at 12:52 a.m. on Friday.

More about the eclipse tomorrow.

Script by Damond Benningfield

As the roster of known planets in other star systems grows, astronomers learn a lot more about planets and stars in general. One key finding is that almost all stars are still infants when their planets form.

A planet is born from a disk of gas and dust around the newborn star. Solid particles stick together to form larger and larger bodies. They eventually form a chunk of rock and metal that can be much more massive than Earth. If these embryos are far enough from the star, where conditions are cold, they may sweep up vast amounts of leftover gas to form giant worlds.

Observations of infant stars and planets suggest that most planets take shape within 10 million years or so. By then, the star’s radiation and winds have cleared away most of the raw ingredients for making planets. That leaves only the planets and some leftover “building blocks.” The gravity of the giant planets kicks many of the building blocks out of the system. But others remain, forming asteroids and comets.

The planetary system isn’t in its final form by then. Gravitational interactions can cause a planet to move toward or away from the star, change the angle of its orbit, or even get kicked out of the system. And the asteroids and comets can pound the young planets – perhaps giving birth to moons.

So while planets form quickly, they continue to evolve throughout the lifetime of their star system.

Script by Damond Benningfield

Of the thousands of planets that have been discovered in other star systems, only a few orbit stars that are easy to see with the eye alone. Most of the stars are so faint or so far away that they just fade into the firmament.

One exception is Hamal, the brightest star of Aries, the ram. It climbs into view, in the east-northeast, by mid-evening. It’s about 66 light-years away, but it’s bright enough that it’s easy to see from all but the most light-polluted cities.

Hamal is so bright because it’s completed the “prime” phase of life – a phase known as the “main sequence.” Now it’s in the next phase, as a giant. It’s puffed up to about 15 times the diameter of the Sun, making it much brighter.

Hamal has one confirmed planet – Hamal b. It, too, is a big ‘un – it’s almost twice as massive as Jupiter, the giant of our own solar system. But the planet is only a little farther from Hamal than Earth is from the Sun. So every square inch of the surface receives about 10 times more energy than the same-sized patch of Earth.

As a result of that bombardment, Hamal b is extremely hot. That makes it highly unlikely to host any form of life – at least not like any life on Earth. And if it ever did have life, it probably perished as Hamal expanded and brightened. So Hamal b isn’t a good place to go hunting for neighbors.

We’ll have more about planets in other star systems tomorrow.

Script by Damond Benningfield

Jupiter is climbing away from the Sun, moving a little higher into the dawn sky day by day. Right now, it’s quite low in the east as twilight paints the sky. But it’s also quite bright, so if you have a clear horizon, you’ll be able to pick it out.

Despite appearances, Jupiter isn’t really close to the Sun at all. It looks close only because of the relative positions of Jupiter and Earth.

As seen from Earth, Jupiter passed behind the Sun in late July, so it was hidden in the Sun’s glare for several weeks. But Earth follows a smaller, faster orbit around the Sun. Now, we’re looping around toward Jupiter. We’ll catch up to it and pass it next February. So Jupiter will rise earlier and remain in view longer every day until then.

Jupiter repeats this cycle every 13 months. In other words, wherever it appears in the sky now, it’ll be in a similar position 13 months later.

But it won’t appear against the same background of stars. It takes Jupiter almost 12 years to orbit the Sun, so it takes that long for it to complete one full circle through the constellations. On average, it shifts eastward by about one constellation per year. So while it’s currently in Cancer, by the next time it graces the dawn sky, next September, it’ll be one constellation over, in Leo.

Again, look for Jupiter low in the east during the dawn twilight, and climbing higher into the sky morning by morning.

Script by Damond Benningfield

The Sun narrowly skirts by the heart of the lion today and tomorrow – the star Regulus. At their closest, they’ll be separated by just a fraction of a degree. After that, the Sun will slide across Leo for almost four weeks before moving into Virgo.

That may surprise those whose astrological sign is Virgo. The Sun is supposed to cross into Virgo tomorrow. That highlights two points: the difference between the “signs” and the constellations, and the Sun’s changing position relative to both.

The constellations of the zodiac were drawn thousands of years ago. They were based on connect-the-dots patterns of stars, not formal boundaries.

The constellations are different sizes, so the Sun spends different amounts of time crossing each one. On average, though, with 12 months and 12 constellations, the Sun spent a month in each. So astrology divided the zodiac into 12 equal slices. That meant the Sun spent an equal amount of time in each sign, regardless of the size of the constellation itself.

But the Sun shifts position relative to the background of the stars. So today, the signs and constellations are out of sync by about a month.

And in the early 20th century, astronomers assigned formal borders to the constellations. So the Sun spends from about a week to more than five weeks crossing each constellation. And the way the borders are set up, it actually crosses 13 of them – including Leo.

Script by Damond Benningfield