StarDate

Saturn is almost 10 times farther from the Sun than Earth is. At that distance, the Sun looks only about one percent as bright as it does from Earth, so

Saturn is cold and dark. But it’s not quite as cold as you might expect. Saturn actually puts out more than twice as much energy as it receives – heat radiating from deep inside the planet.

Saturn is made mostly of hydrogen and helium – the lightest and simplest chemical elements. But its core contains a lot of rock, metal, and other heavy materials – more than 15 times the total mass of Earth. Gravity squeezes the core tightly, causing it to shrink. That produces heat, which rises to the surface and shines out into space.

But that doesn’t explain all of Saturn’s heat. The rest may come from an odd type of rainfall. Droplets of liquid helium may fall toward the core. As they fall, they produce friction, which produces heat.

Saturn’s internal heat drives much of the weather in its atmosphere, including storms that can be as big as continents – swirling clouds on a cold, dark planet.

Despite the lack of sunlight, Saturn looks bright in our sky because it’s huge – about nine times the diameter of Earth. It’s easy to see how bright the next couple of early mornings because it’s close to the Moon. Tomorrow, it’s to the lower left of the Moon at dawn. It’ll be a little farther to the right of the Moon on Wednesday.

Script by Damond Benningfield

Earth will reach its farthest point from the Sun for the entire year around midday tomorrow. We’ll be about 3.1 million miles farther than we were at closet approach, in early January.

That far point is known as aphelion or ap-helion. The term comes from a mash-up of Latin and Greek. “Ap” is from a Latin word that means “far away,” while “helion” is a Greek word for the Sun.

The changing distance is a result of the shape of Earth’s orbit. Instead of a nice, round circle, it’s an ellipse – like a lopsided circle.

Over tens of thousands of years, the shape changes – the result of the gravitational influence of the Sun, Moon, and planets. The orbit seesaws between being a little more circular and a little more lopsided. Right now, we’re toward the more-circular end of the seesaw.

Today, aphelion always comes about two weeks after the summer solstice. But that won’t always be the case. Thanks to a slow wobble in Earth’s rotation, aphelion shifts an average of one day later every 58 years. So it occurred on the solstice about 900 years ago. And about 4600 years from now, it’ll occur on the fall equinox, in September.

Incidentally, the lopsided orbit has a big impact on the length of the seasons. Our planet moves slowest when it’s farthest from the Sun. That makes summer in the northern hemisphere almost five days longer than winter.

Script by Damond Benningfield

Bellatrix, Spica, and Omega Herculis have something in common. They’re all good “semiquincentennial” stars. That has nothing to do with the stars themselves – only their distance. All three of them are roughly 250 light-years away. In other words, we see them as they looked roughly 250 years ago – around the time of the American Declaration of Independence.

One light-year is the distance light travels in a year – almost six trillion miles. So each of the anniversary stars is about 1.5 quadrillion miles away – the number 15 followed by 14 zeroes.

The distances were measured with a technique known as parallax. Astronomers look at a star when Earth is on opposite sides of the Sun. That causes a tiny shift in the star’s position against the background of more-distant objects. The size of that shift reveals the star’s distance. But there’s a slight margin of error, so the distances might not be exact.

The best measurements say that Bellatrix is a little more than 250 light-years away. Omega Herc is a little less. Spica is in the middle – almost exactly 250 light-years from Earth. So we see it as it looked around the year 1776.

Spica is the brightest member of that trio. It’s low in the southwest as night falls. Earth won’t see the light it emits tonight until the American quincentennial – 250 years from now.

Script by Damond Benningfield

The next big meteor shower is weeks away – at least, here on Earth. But some predictions say that a big shower could take place this weekend on the planet Venus – a result of the long-ago break-up of an asteroid.

Regardless of where it takes place, a meteor shower happens when a planet flies through the orbital path of a comet or asteroid. Bits of rock and dust shed by the small body ram into the planet’s atmosphere at high speed. They heat up and vaporize, forming the glowing streaks known as meteors.

In this case, the source of the particles could be two asteroids. They have a similar composition, and they follow similar paths around the Sun.

A team of European astronomers recently plotted the orbits of the two bodies a hundred-thousand years into the past. The study found that, about 20,000 years ago, both asteroids passed especially close to the Sun many times. And the asteroids were so close to each other that they could have been one larger asteroid.

Warmed by the Sun, the surface of the asteroid could have cracked and splintered. The way the asteroid heated up could have caused it to spin faster. The weakened asteroid then split apart, spewing debris into space. Venus periodically slips through this trail of debris – perhaps triggering a meteor shower.

From Earth, only the brightest of those meteors might be visible through a telescope – streaking through Venusian skies.

Script by Damond Benningfield

The planets Mars and Uranus will stage an especially close encounter the next couple of mornings. Uranus is quite faint, but its proximity to Mars can help you pick it out.

Uranus is a giant – about four times the diameter of Earth. But it’s so far away that it’s faint. Under especially dark skies, people with good eyesight can just make it out. Moonlight or light pollution mask it from view, so you need binoculars to find it.

And even then, it looks like a meager star, perhaps with a hint of blue-green. Methane in its upper atmosphere absorbs red light, so only the blue and green can reach us.

Mars is much smaller – only a bit more than half of Earth’s diameter. But it’s also much closer, which makes it easier to see.

Even with the eye alone, it’s no problem to make out the planet’s orange color. That’s produced by iron oxide in the rocks and the tiny dust grains that coat much of the surface. That color will become easier to make out over the coming months, as Earth and Mars get closer and closer.

For now, look for Mars low in the east-northeast beginning a little before dawn. It’s quite easy to make out. Tomorrow, Uranus will stand a little to the lower left of Mars, so both of them will fit in a binocular field of view. And Uranus will be even closer above Mars on Saturday – a faint giant appearing to almost touch the Red Planet.

We’ll talk about a much brighter planet tomorrow.

Script by Damond Benningfield