StarDate

Day and night will be just about equal the next few days for the entire world. That’s because fall arrives in the northern hemisphere tomorrow. It’s the September equinox – the moment the Sun crosses the equator from north to south.

At that moment, the Sun stands directly above the equator. So no matter where you are, the Sun rises due east and sets due west.

Equinox means “equal nights.” That tells us that day and night should be the same length. But that’s not quite the case. In the northern hemisphere, daytime – the interval from sunrise to sunset – lasts a few minutes longer than nighttime. They won’t balance out until a few days after the equinox.

One reason for the difference is the way we figure the moments of sunrise and sunset. For day and night to be equal, we’d have to think of them as the time the Sun is bisected by the horizon – when half is in view, and half is hidden.

Instead, of course, we consider sunrise as the moment the top of the Sun first peeks into view. And sunset is the moment when the Sun fully disappears. That adds a minute or so to the “daytime” side of the equation.

And we don’t actually see the Sun rise and set – at least not live. Earth’s atmosphere bends the Sun’s rays around the planet. So by the time you see the setting Sun touch the horizon, it’s actually already set. You’re seeing an extended version of things – stretching the daylight for a few extra minutes.

Script by Damond Benningfield

In Shakespeare’s play “Julius Caesar,” Caesar makes a bold proclamation: But I am constant as the northern star, / Of whose true-fix’d and resting quality / There is no fellow in the firmament.

Caesar turned out to be not so constant, of course. And neither is the northern star. In fact, Earth sees a cycle of North Stars – a cycle that lasts for 26,000 years.

The current North Star is Polaris. It stands almost due north. So from the northern hemisphere, all the other stars appear to wheel around it as Earth turns on its axis. But Polaris will slide away from that honored spot over the coming centuries. And about 1200 years from now, it’ll be replaced by Errai, in the constellation Cepheus the king.

We go through a sequence of north stars because of a slow wobble in Earth’s axis – the result of the gravitational pull of the Sun and Moon. When you combine a star’s brightness and its proximity to true north, Polaris may be the best of them all. Errai appears only about a third as bright as Polaris, and it won’t get quite as close to due north. Even so, it will reign as the North Star for about 2,000 years, before passing the crown to another star in Cepheus.

The king is high in the north at nightfall, to the upper right of Polaris. It looks like a child’s drawing of a house, although it’s upside down during the evening hours. Errai is the peak of the house’s roof – a future inconstant North Star.

Script by Damond Benningfield

Two well-known star patterns highlight the northern sky this evening. The Big Dipper is low in the north-northwest at nightfall, and in the northeast at first light tomorrow. And W-shaped Cassiopeia is just the opposite – in the northeast at nightfall, and the north-northwest at dawn.

As that sequence tells us, both star patterns make a big circle around the sky during the night. They circle the North Star, Polaris – the hub of the sky. All the stars in the northern sky appear to move around Polaris – the result of Earth turning on its axis.

For much of the United States, the stars of the Big Dipper and Cassiopeia never set – they’re close enough to Polaris that they never drop below the horizon. So they’re in the sky every day and night of the year, endlessly circling the North Star.

Such stars are called circumpolar. The number of such stars from any given location depends on your latitude. From 30 degrees north, anything within 30 degrees of Polaris always remains above the horizon.

From 50 degrees north, it’s anything within 50 degrees of Polaris. So as you go farther north, more stars are circumpolar.

And if you go all the way to the north pole, all the stars are circumpolar – nothing ever rises or sets. Each star follows the same path across the sky night after night – circling Polaris, the hub of the northern sky.

Polaris won’t keep that position; we’ll talk about its successor tomorrow.

Script by Damond Benningfield

A thousand spacecraft could head for our closest neighboring planetary system in just a few decades. Don’t book your ticket just yet, though – each craft would weigh about as much as a penny. But working together, they could provide a few sharp pictures of the system, and even look for signs of life.

A team of scientists and engineers published the idea earlier this year. The team proposed sending the probes to Proxima Centauri. It’s the closest star outside the solar system – four-and-a-quarter light-years away. And it has two confirmed planets. One of them is about the size and mass of Earth, and it’s in the region that’s most comfortable for life.

The probes would be equipped with tough but thin “sails” 13 feet wide. A powerful laser would fire at each probe for eight minutes. The pressure of the light would boost the probes to 20 percent of the speed of light. It would take them 21 years to reach their target.

The probes would use lasers to stay in touch with each other, and with Earth. About 300 probes could survive the trip. Their instruments could hunt for evidence of life in the planets’ atmospheres.

There’s a lot of work to make it happen – advances in materials, lasers, computers, and even our knowledge of the distance to Proxima Centauri. But the researchers said the current rate of advancement should make such a trip feasible in the decades ahead.

Script by Damond Benningfield

The winds on a giant planet more than 200 light-years from Earth are like the Big Bad Wolf: they’ll huff and puff and blow your house down. And if that’s not enough of a problem, the temperature can jump by 500 degrees in just a few hours.

The planet orbits the star HD 80606. The star is a lot like the Sun. And it has a distant companion star that’s a near twin. They’re in Ursa Major, the great bear. At dawn, the system stands to the upper right of the Big Dipper, although you need a telescope to see it.

The planet is HD 80606 b. It’s about four times the mass of Jupiter, the giant of our own solar system. But it’s about the same size as Jupiter, so it’s much denser.

Its orbit is one of the most elongated of any known planet – its distance ranges from just three million miles to 85 million. As the planet approaches the star, it’s like being popped into an oven: The amount of energy it receives when it’s closest to the star is 800 times greater than when it’s farthest. So the planet heats up in a hurry.

The approach also stirs up the winds. They blow outward from the point that’s in the middle of the hemisphere that faces the star. They may top out at 11,000 miles per hour – huffin’ and puffin’ all the way around this turbulent planet.

Script by Damond Benningfield