StarDate PodcastAuthor: McDonald Observatory
25 Sep 2017

StarDate Podcast

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StarDate, the longest-running national radio science feature in the U.S., tells listeners what to look for in the night sky.

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    A small, faint “shield” of stars scoots across the southwestern sky on these early autumn nights. It represents the coat of arms on the shield of John Sobieski, a 17th-century king of Poland and one of the country’s great national heroes.

    The shield is the constellation Scutum. Johannes Hevelius first drew it around 1687, using a few stars from a relatively barren region of the sky. He originally named it Scutum Sobiescianum — the shield of Sobieski. But in more recent times, the last part of the name has been dropped.

    Sobieski became king of Poland in 1674. He built alliances with several European neighbors, and fought the Ottoman empire when it tried to expand westward. In 1683, he earned fame across all of Europe by defending Vienna against the invading Ottoman army.

    To honor his native country, Hevelius included the new constellation in a beautiful star atlas, which was published in 1690. He depicted several faint stars as the cross on Sobieski’s shield. Scutum Sobiescianum has kept its place in the stars ever since then.

    Unfortunately, the stellar shield isn’t all that much to look at. You need fairly dark skies to see any of its stars. Right now, look for it in the southwest as night falls. It’s above the teapot-shaped constellation Sagittarius. The shield of Sobieski drops down the southwestern sky during the evening, and sets after not long after midnight.

    Script by Damond Benningfield


  • Posted on 24 Sep 2017

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    When spring and autumn arrived in the ancient city of Cahokia, the city’s chief reaffirmed his connection to the cosmos. On the equinoxes, priests watched the sunrise from inside a ring of tall poles that served as a calendar. They saw the Sun climb into view behind Monks Mound, a great pile of earth topped by the chief’s home and court. The Sun was reborn — and so was the chief.

    Cahokia stands across the Mississippi River from present-day Saint Louis. The city reached its peak around eight centuries ago. It covered about six square miles, incorporated more than a hundred earthen mounds, and its population swelled to perhaps 20,000 — larger than London.

    The focal point for Cahokia was Monks Mound — the largest earthen structure in the early Americas. It covered 14 acres and rose a hundred feet into the sky. Most of it is still standing.

    To the west of the mound, inhabitants built a calendar circle. Archaeologists named it Woodhenge because of its resemblance to Stonehenge. The Cahokians actually built five Woodhenge circles. Each consisted of a central Sun-watching station, surrounded by a ring of cedar poles. The Sun aligned with these poles on the equinoxes, solstices, and other important dates.

    Archaeologists have rebuilt one of the rings, which spans more than 400 feet. Visitors watch the equinox sunrise from Woodhenge even now — observing an act of nature that was a potent symbol for a long-ago culture.

    Script by Damond Benningfield


  • Posted on 23 Sep 2017

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    It’s been a scorching summer for much of the country. And while we can’t say whether the scorching is over, we can say that summer is. Autumn arrives in the northern hemisphere today at 3:02 p.m. Central Time — the autumnal equinox.

    The season changes when the Sun crosses the equator from north to south. It will continue its southward trek until the winter solstice, in December. As it does so, the days will get ever shorter in the northern hemisphere, while they get longer in the southern hemisphere.

    For a few days around the equinox, day and night are about the same length. In fact, that’s where the name “equinox” comes from. It’s a Latin word that means “equal nights.” So at the equinoxes, the interval between sunrise and sunset is roughly 12 hours. And that’s the case across the whole planet.

    On the equinox itself, though, the interval is actually a few minutes longer. There are several reasons for that. One is that Earth’s atmosphere acts like a lens, “bending” the Sun’s rays back over the horizon. So when you see the Sun sitting on the horizon, it’s an illusion. The Sun itself is below the horizon, but an image of it is projected back into view.

    With the Sun in view for less time each day, and the sunshine coming in at a lower angle, temperatures north of the equator will fall. That will bring more comfortable days after a long, hot summer — and provide more time for watching the stars under the cool nights of autumn.

    Script by Damond Benningfield


  • Posted on 22 Sep 2017

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    Moon and Jupiter

    A giant lake sits on the surface of Io, one of the big moons of Jupiter. It’s about 125 miles wide, and it covers an area bigger than Lake Ontario. But that’s where the comparisons to earthly lakes end. That’s because Loki Patera is filled not with water, but with lava.

    Io is the most volcanically active body in the solar system. The gravity of Jupiter and its other big moons pull at Io, stretching and squeezing its interior. That melts rock below the surface. Some of the sulfur-rich molten rock punches its way to the surface, forming volcanoes, along with pools of super-hot lava.

    There could be more than 400 active volcanic features on Io. And Loki Patera is the biggest of them all. It’s been filled with molten rock since at least 1979, when it was discovered in pictures by Voyager 1. But it doesn’t always look the same. Every year or two, it flares up.

    That’s probably because the top of the lake gets cooler, so it forms a solid crust. The solid rock is denser, so it sinks, allowing fresh molten rock to bubble to the surface. This process happens in waves, which take several weeks to ripple across Loki Patera — the largest active volcano in the solar system.

    We can’t see Io right now, but we can see Jupiter — barely. The giant planet is close to the left of the crescent Moon shortly after sunset. Although it’s quite low in the sky, if you have a clear horizon it should be fairly easy to spot through the fading twilight.


    Script by Damond Benningfield

  • Posted on 21 Sep 2017

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    Beta Pegasi

    With summer just about ready to give way to fall, one of the signature star patterns of the new season is climbing into prominence in the evening sky. The Great Square of Pegasus is in the east as night falls. It’s tilted a bit as it rises, so it looks a bit more like a diamond than a square.

    The jewel at the top of the diamond is known as Beta Pegasi. It’s a red giant — an old, bloated star that shines reddish orange. That color isn’t visibile to the unaided eye because the star is relatively faint — mainly because it’s about 200 light-years away. But the tint should be easy to see through binoculars.

    A star’s color is the result of its surface temperature. Hot stars are blue-white, while cool stars are orange or red. Beta Pegasi is thousands of degrees cooler than the Sun. That’s because the star is nearing the end of its life. Changes in its core have caused its outer layers to puff outward, so the star is almost a hundred times the Sun’s diameter. That caused the gas in those layers to get cooler — and redder.

    The change in temperature also produces a change in the type of energy the star radiates into space. Stars like the Sun produce most of their energy at visible wavelengths. But most of the light from Beta Pegasi is in the infrared — wavelengths that are too long for human eyes to see. When you add all the wavelengths together, Beta Pegasi shines about 1500 times brighter than the Sun — a red jewel sparkling across the galaxy.

    Script by Damond Benningfield


  • Posted on 20 Sep 2017


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