StarDate PodcastAuthor: McDonald Observatory
15 Aug 2018

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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    More Moon and Venus

    Until Nicolaus Copernicus demonstrated otherwise, most people thought Earth was the center of the universe. And that wasn’t an unreasonable idea. The Sun, Moon, and stars all appear to circle around us, so it looks like we’re right in the middle of things.

    The only problem was the planets. Some of them periodically reverse direction across the sky. And Venus and Mercury don’t go all the way across the sky. They climb part of the way into the morning sky, then double back into the evening sky. That’s hard to explain if everything is circling around Earth.

    The explanation, of course, is that they don’t circle around Earth — and neither does anything else except the Moon. The planets all orbit the Sun. Some of them are outside Earth’s orbit, so they swing all the way across the sky. But Mercury and Venus lie inside Earth’s orbit, so they have a limited motion across the sky.

    And in fact, Venus is at the limit of that motion over the next few nights. It’s farthest from the Sun for its current “evening star” appearance — about 45 degrees. By the end of the week, it’ll start sliding toward the Sun.

    Because of the angle at which Venus sets at this time of year, though, it doesn’t look very far from the Sun. It’s quite low in the west as night falls; tonight, it’s below the crescent Moon. The planet doesn’t drop straight down toward the horizon. Instead, it slides along the horizon as it descends — a world that orbits the Sun, not Earth.

    Script by Damond Benningfield

  • Posted on 14 Aug 2018

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    Moon and Venus

    The Moon slides past a trio of bright lights the next few evenings: two planets and a star. The planets are especially bright, so you won’t have any trouble picking them out, even from a light-polluted city.

    The first of the Moon’s companions is also the brightest. It’s Venus, the brilliant “evening star.” It’s off to the left of the Moon tonight, and will be even closer below the Moon tomorrow night.

    As it leaves Venus behind, the Moon will cozy up to the star Spica, which is well to the upper left of Venus. It’s not nearly as bright as the two planets. Even so, it’s one of the brighter stars in the night sky. And its proximity to the planets, and to the Moon, will make it pretty easy to pick out.

    Companion number three will be the planet Jupiter, to the upper left of Spica.

    Jupiter usually is the third-brightest object in the night sky, after the Moon and Venus. Right now, though, it has slipped to fourth position. In part, that’s because Jupiter itself is near the bottom of its range in brightness. But it’s also because Mars is shining especially bright right now, so it’s temporarily moved ahead of Jupiter. In fact, you can see Mars low in the southeast, looking like an orange star. But the king of the planets will reclaim its usual spot in a few weeks.

    So keep your eye on the southwestern quadrant of the sky the next few evenings, as the Moon passes by Venus, Spica, and Jupiter.

    More about the Moon and Venus tomorrow.

    Script by Damond Benningfield

  • Posted on 13 Aug 2018

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    NGC 6397

    NGC 6397 seems like a pretty ordinary globular star cluster. It’s not especially big or bright, and nothing about its stars is going to strike anyone as odd. Yet it’s been an important astronomical laboratory. Among other things, it’s helped astronomers determine the age of the galaxy, and the minimum “weight” of a star.

    NGC 6397 is in the constellation Ara, the altar. It’s below the stinger of Scorpius, the scorpion. That puts it so far south that it’s not visible from most of the United States. But it’s in great view for Hubble Space Telescope, which has monitored the cluster for decades.

    NGC 6397 is a globular cluster — several hundred thousand stars packed into a ball only a few dozen light-years across. The Milky Way contains about 150 of these clusters, which are the galaxy’s oldest inhabitants.

    Over the decades, Hubble has carefully monitored many of the cluster’s stars. One set of observations helped astronomers lock down the minimum mass required for an object to shine as a true star — about eight percent the mass of the Sun.

    And a more recent set of observations helped measure the cluster’s distance with unprecedented accuracy — 7800 light-years, one of the closest of all globulars. That number is important for determining the cluster’s age — 13.4 billion years. That means that NGC 6397 — and its home galaxy, the Milky Way — took shape when the universe was just about 400 million years old.

    Script by Damond Benningfield

  • Posted on 12 Aug 2018

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    The gods of ancient Greece weren’t especially good parents. One of the ancient gods known as the Titans, for example, swallowed his newborn children. Eventually, though, the kiddos escaped and overthrew him. Their victory is commemorated in the stars, as the altar where they swore allegiance to each other. It’s the constellation Ara, and it’s below the tail of the scorpion.

    In the story, Cronus was the king of the Titans. He’d come to power by defeating his own father. A prophecy said that one of his children would, in turn, defeat him. To prevent that, he swallowed each of his first five children.

    Eventually, though, his wife, Rhea, got tired of that plan. When the last child, Zeus, was born, she hustled him away to an island and gave Cronus a rock wrapped in a blanket.

    When Zeus grew up, he returned home and forced his dad to cough up his brothers and sisters, who were fully grown gods and goddesses. They pledged to defeat the Titans, and won a 10-year war. Zeus became the king of the new gods and ruler of the sky. To celebrate their victory, he placed the altar among the stars.

    The altar is so far south, though, that most of it is visible only from far-southern parts of the United States — places like Florida, south Texas, and especially Hawaii.

    Ara is one of the smaller constellations, with only a couple of moderately bright stars. But it does have some good deep-sky objects, and we’ll talk about one of them tomorrow.

    Script by Damond Benningfield

  • Posted on 11 Aug 2018

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    Solar Eclipse

    Solar eclipses roll around like clockwork — astronomers can predict them thousands of years in advance. But the clock is like something from the Middle Ages, with lots of complicated moving parts. So it took a long time to figure out how the parts all work together.

    A solar eclipse occurs when the Moon passes directly between Earth and the Sun, blocking some of the Sun’s light. If the alignment is just right, the Moon completely covers the Sun, creating a total eclipse. If the alignment is off, though, then the Moon covers only part of the Sun.

    Calculating an eclipse requires detailed knowledge of Earth’s orbit around the Sun, and the Moon’s orbit around Earth. Both orbits are slightly lopsided, so the distance between Sun and Earth, and between Earth and Moon, changes from day to day. And orbital speed changes depending on the distance.

    The angle of the Moon’s orbit is important, too, because it tells us where on Earth that shadow will appear. So is Earth’s rotation on its axis, because it’s not steady. Thanks to the tides — caused by the Moon — the rate at which Earth spins can vary — one of the most complicated factors in calculating a solar eclipse.

    And astronomers have calculated that there will be a partial eclipse early tomorrow. It’ll be visible from the Arctic Ocean down through Greenland, Scandinavia, and parts of Asia. The U.S. misses out on this one, though — skipped over by the clockwork motion of the solar system.

    Script by Damond Benningfield

  • Posted on 10 Aug 2018


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