astronomy@iowa

Asteroid 2011 AN52 skimmed past Earth at a distance of only 195,000 miles (that’s a bit closer than the Moon!) on January 17. Don’t bother looking for it unless you’re well-equipped, though. At a visual magnitude of +18, it is beyond the range of binoculars and all but the largest amateur telescopes. In fact, the faintest star your eyes can see from a very dark location is over 60,000 times brighter than 2011 AN52! Zipping through the northern constellations of Draco and Cygnus at over 35,000 mph, it would take quite a bit of skill to actually find and photograph the extremely dim, fast-moving rock.

The asteroid is estimated to be only 25 feet across—about the size of a living room—but even this little object would pack a punch if it hit Earth. The kinetic energy released by even a room-sized asteroid would be roughly equivalent to the energy released by one of the atomic bombs used in World War II. Sleep well.

Today was the latest sunrise of the year for observers at middle latitudes of the northern hemisphere (the earliest sunset occurred on December 7, 2010.) From this point onward, sunrises will be a little bit earlier each day until June 14 when the earliest sunrise of the year will occur. The most common question may be why the latest sunrise and earliest sunset don’t occur on the winter solstice, which is commonly referred to as the shortest day of the year (in fact, all days are 24 hours according to the civil time-keeping system.) The “shortest day” occurred on December 21 when the Sun reached the winter solstice at 5:28 p.m. CST observers and daylight lasted for 9 hr 20 min. The reason the latest sunrise and earliest sunset do not occur on the date of the winter solstice is a combination of factors based on the fact that Earth’s orbit around the Sun is an ellipse rather than a circle and Earth’s rotation axis is tilted 23½° with respect to its orbital plane.

The actual length of the day based on the Sun’s motion across our sky (i.e., “sundial” time or apparent solar time) and one solar day is determined by the length of time between two successive meridian crossings of the Sun. That’s basically one noon to the next noon for an observer. Because the earth’s rotation axis is tilted, however, the Sun’s daily motion is not strictly east-west across our sky. There is a southward component to the Sun’s motion between the summer solstice (solstice is Latin for “the Sun stands still”) and winter solstice and a northward component between the winter solstice and summer solstice. The slight north/south drift results in variations in the true Sun’s motion and causes the the length of the solar day being more or less than 24 hrs depending on the time of year.

The Equation of Time

A second factor involves the elliptical motion of Earth along its orbit. Johannes Kepler determined the orbits of the planets were ellipses in the early 17th century after first discovering that Mars’ speed varied along its orbit. Mars moved fastest when it approached the Sun and slowed as it moved farther away. The speed of Mars was greatest at perihelion and slowest at aphelion (see There Goes the Sun.) Earth passes through perihelion in early January according to the modern calendar and as a result of our greater than average velocity, the Sun appears to move faster across our sky. If all this sounds confusing, it is. To simplify the situation, an average or mean sun was envisioned and it is this mean sun upon which our civil time-keeping system is based. The mean sun is assumed to move directly east to west across Earth’s sky at a constant speed.

On any given day, the apparent solar day can be up to 20 seconds shorter or up to 30 seconds longer than the mean solar day. Over the course of many days, the cumulative effect results in a apparent solar time being greater than mean solar time by 16 minutes in early November and less than mean solar time by 14 minutes in early February. The difference between apparent solar time and mean solar time is known as the equation of time.

Living in a society based on a standardized 24-hour day and extremely precise clocks, most of us have no need to watch the Sun in order to tell time. That was not always the case, however, and knowing the precise time was vital to navigation at sea. Using the stars (sidereal time) was fine at night, but what about during the day? The Sun was the only option and understanding variations in its daily motion was vital to one’s survival when a miscalculation of a single degree at sea might mean a ship is 70 miles off course!

Analemma on ancient globe

Variations in the Sun’s daily motion was known to the Babylonians, but the great cartographer and astronomer Claudius Ptolemy devoted an entire chapter of his book The Almagest around A.D. 140. Up until a generation ago it was easy to find graphical representations of the equation of time in any library or geography classroom. Globes and maps were decorated with a puzzling “bowling pin” called an analemma. Around the edges of the figure are dates of the year while a horizontal bar across the figure gives the number of minutes the clock was “ahead” of the Sun (western half of figure) or the number of minutes a clock would be “behind” the Sun (eastern half of figure.) A vertical bar through the analemma marked in degrees is used to determine the altitude of the Sun on a given date.

The first photographic image of the analemma was constructed during 1978-79 by Dennis de Cicco of Sky & Telescope magazine. Since that time, and the advent of digital photography, a number of these composite images have been created.

At 1 p.m. CST on Monday afternoon, Earth was at perihelion. At a distance of 91,407,282 miles from the Sun, we were about 3.5% closer to the Sun than we will be on July 4, when Earth reaches its farthest point from the Sun (aphelion.) In the early 17th century Johannes Kepler discovered planetary orbits—all naturally occurring orbits, for that matter—are elliptical rather than circular as had been assumed since natural philosophers began contemplating the motion of the “wandering” stars. Hence Earth does not revolve around the Sun at a constant distance as a circular orbit would dictate.

Based on the sidereal period of Earth’s orbit around the Sun, the time required for Earth to complete one orbit using the stars as a reference point (sidereal comes from the Latin sidus meaning “star) is 365.256363004 days. From our perspective on Earth, this is also the time required for the Sun to return to the same position in the sky with respect to the stars. Its actually a bit more complicated than that and harder to determine than it sounds because of a slow “wobbling” of Earth’s rotation axis. The wobble, called precession, is due to the combined gravitational influence of the Sun and Moon on the earth. As early as 150 B.C. the Greek astronomer Hipparchus observed that the sidereal period (the actual length of the year) was longer than the length of the year based on the changing seasons—also based on the apparent motion of the Sun, but called the tropical year.

Throughout history religious observances have been dependent on the ability to precisely measure and mark the passage of time. Maintaining calendars and tables of planetary positions (which included positions of the Sun and Moon) was traditionally one of the most important roles of astronomers.

Note: The aphelion passage of Earth on July 4, is purely coincidental and has nothing to do with the celebration of Independence Day.

Forensic sketch of Copernicus, age 70.

Nicolaus Copernicus, the 16th century Polish astronomer and canon in the Catholic Church, was reburied in a formal ceremony in Frombork, Poland, on May 22, over 460 years after his death in 1543. In that same year, his greatest work, De Revolutionibus Orbium Coelestium (On the Revolutions of the Heavenly Spheres), was published and legend has it that the dying cleric received a copy on his death bed. Were it not for the efforts of Georg Joachim Rheticus, a young scholar from the University of Wittenberg who traveled to Frombork in 1539, Copernicus’ revolutionary theory that the Sun is the center of the universe and that the earth rotates on its axis might never have been published. Sometime in late 1538 or early 1539, Rheticus became aware of the Polish astronomer’s work and made the decision to visit him and learn all he could of this extraordinary theory.

Choosing to live far from the intellectual hotbeds of Europe, Nicolaus Copernicus chose instead to pursue a life of relative ease. After studying both medicine and astronomy in Italy and serving as his uncle’s assistant for a number of years, he resigned that position and accepted the position of canon in little Frombork. This was not the path to greatness that his uncle, the prince-bishop of Warmia, had in mind for his nephew. In this tiny fishing village that even Copernicus described as “the remotest corner of the world,” is nascent astronomical talents emerged and Dr. Nicolaus began outlining his criticism of Claudius Ptolemy’s earth-centered model of the heavens. In a scant six leaves, Copernicus described in non-technical language what he considered “no small difficulties” and “defects” in the model that had dominated astronomy for 1,300 years. A total of seven assumptions, or axioms, were presented that refuted the Ptolemaic model. Some time before 1514, the anonymous, handwritten manuscript was circulated among friends and scholars from Copernicus’ university days. Eventually copies were made and disseminated throughout astronomical circles and the promised “larger work” that included all of the requisite mathematica detail was eagerly awaited. Although people waited decades, the promised treatise never materialized. Back in little Frombork, however, Copernicus was at work writing the book that would help usher in the scientific revolution.

While it is often said that Copernicus withheld the publication of De Revolutionibus because of concerns he would be labelled a heretic, evidence does not support that conclusion. Shortly after the distribution of the little pamphlet that eventually came to be called Commentariolus, Copernicus’ name became well enough known that Pope Leo X invited him to participate in the calendar reform that had been started in 1475 but never finished. We don’t know how Copernicus responded to the request, but regardless the calendar reform never took place. Additionally as late as 1536, by which time Copernicus would have been putting the finishing touches on his book, Cardinal Nicholas Schönberg, who had discussed Copernicus’ sun-centered model of the heavens with Pope Clement VII in 1533, sent Copernicus a letter in which he praised the Polish astronomer and urged that he publish his new theory in which the earth moves and that “the sun occupies the lowest, and thus the central place in the universe.” The cardinal even offered to send one of his assistants to Frombork and arrange for the copying of all of Copernicus’ notes in preparation for publication of this new theory. Copernicus chose to ignore the appeal. He may have been concerned that some of his calculations still needed refinement and were not yet ready for presentation. Perhaps there were other reasons as well. Unlike astronomers who held university positions, Copernicus had a day job—administration of church business and medicine occupied much of his time as did a growing scandal around Copernicus’ living arrangements with a woman although he had taken a first-order vow of celibacy. While not completely uncommon at the time, at a time when the Catholic Church was growing increasingly sensitive to the influence of the Lutheran Church in Poland, this was just the kind of ammunition enemies seeking the position of canon needed. Evidence of sympathy to some Lutheran doctrine didn’t help matters.

Copernicus’ greatest contribution to science might have been left among his belongings waiting a later scholar to uncover and publish were it not for the efforts of the Lutheran professor from Wittenberg. Rheticus carried the manuscript of De Revolutionibus with him from Frombork and delivered it to Andreas Osiander, a colleague charged with overseeing the publication in Nuremberg. Without the consent of Copernicus or Rheticus, Osiander wrote and inserted a preface in which the sun-centered model is stated to be merely a hypothesis that allows for more accurate calculations and not necessarily the truth sought by natural philosophers. Rheticus was justifiable furious upon learning of the unauthorized insertion. For years it was assumed that Copernicus wrote the preface and that he might not actually believe the bold statements made in his book. Rheticus must also have been devastated by the glaring omission of his own name in the acknowledgements written by Copernicus. While many others were mentioned by name, Rheticus was never mentioned and historians have yet to agree on a satisfactory explanation for the omission.

Copernicus died on May 24, 1543, months after suffering a stroke and falling into decline. According to reports of the time, he received a printed copy of De Revolutionibus the day he died.

Although the general location of where Copernicus’ body was buried has been known, the exact location and identification of his remains was not made until 2005 when a team of archaeologists and historians discovered the remains of a man that matched the age of Copernicus. A forensic analysis of the skull provided a sketch (see above) that seemed to match contemporary paintings of Copernicus, but DNA recovered from the body awaited verification from another source. Finally scientists had a lucky break when several hairs were found in a book once owned by Copernicus. The DNA samples from several of the hairs matched the DNA of a tooth in the skeletal remains found in the cathedral at Frombork. Copernicus’ remains now rest in the alter of the Cathedral he presided over and under a 10-foot granite monument depicting his sun-centered model of the heavens.

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For further reading, check out Copernicus’ Secret by Jack Repcheck and The Book Nobody Read: Chasing the Revolutions of Nicolaus Copernicus by Owen Gingerich.

On April 13 a Type II supernova was discovered in the unusual galaxy NGC4088. This particular 90-second image was taken by Cedar Amateur Astronomers charter member Doug Slauson on April 21, using his 9.25-inch Celestron Schmidt-Cassegrain telescope with an attached SBIG STV at f/3.75.

At a distance of 55 million light-years, NGC4088 is a spiral galaxy in the Ursa Major cluster that has some properties of a barred spiral. Designated SN2009dd, the supernova is thought to be a type II supernova—the result of a cataclysmic explosion of a massive supergiant star at the end of its thermonuclear life. After fusing less massive nuclei into more and more massive nuclei, the supergiant star eventually ends up with iron in its core and a dead-end. Since iron has the greatest binding energy of all nuclei, all reactions involving iron are endothermic and rob the star’s core of energy resulting in an inability to support the star against the crush of gravity due to its extreme mass. In a fraction of a second, the massive star’s core implodes triggering a shock wave that eventually rips the star apart is a spectacular explosion.

The energy released in the supernova explosion is almost unimaginable—in a few seconds, the destroyed star gives off more energy than 10,000 sun-like stars will emit over their entire 10 billion year main sequence lifetimes!

At 13:40 UT (that’s 7:40 a.m. CST) on March 2, the near-Earth asteroid 2009 DD45 zipped past Earth at a distance of only 0.00048 A.U. Considering that the average Earth-Sun distance is 1 Astronomical Unit, 0.00048 A.U. works out to be a mere 45,000 miles! That’s a close shave by anyone’s standards. The Moon’s average distance is 240,000 miles and the geosynchronous satellites monitoring our weather and blanketing Earth with a global positioning and communication network orbit the earth at 23,000 miles. Having an Apollo group asteroid careen past the planet at twice the distance of the geosynchronous satellites is certainly something to note.

This kind of celestial fly-by is not entirely unprecedented, however. In 1972 a small asteroid (or meteoroid, if you prefer) skimmed through Earth’s upper atmosphere as if it were a stone skipping off a pond. That object entered the atmosphere over the Northwest United States and exited somewhere over Canada. Estimated to be about the size of an SUV, an impact or airburst of an rock this size would have been very impressive. Not the kind of event that would spell doom for civilization, but enough to get your attention. The most recent event of this type was the Tunguska blast over a remote area of Siberia in 1908 when a loose-aggregate meteoroid, or perhaps a comet fragment, entered the atmosphere and exploded in the upper atmosphere. Trees were stripped of branches and felled for many square miles around the spot directly below the blast. On a bit larger scale, anyone standing in the desert Southwest 50,000 years ago might have been in for a shock when a 100-foot meteoroid slammed into the ground (becoming a meteorite in the process) and created the Barringer meteor crater. At over 3/4 mile in diameter and nearly 600 feet deep, Barringer crater is the best preserved example of what happens when big rocks cross Earth’s orbit.

Thought to be about the same size as the Tunguska event’s object and just  bit smaller than the meteorite responsible for the Barringer crater, 2009 DD45 is over 100 feet in size and could have potentially unleashed many dozens of times the explosive force of the atomic bombs used at the end of World War II. Sleep well.

For further information: 2009 DD45 orbital parameters from NASA’s Jet Propulsion Laboratory (JPL) and a movie on YouTube showing the asteroid’s fly-by.

If all goes well on March 6, the Kepler telescope will blast off from Cape Canaveral’s launch pad 17-B aboard a Delta II rocket and usher in a new era of space exploration. NASA’s Kepler mission will begin an unprecedented 3 1/2 year mission to locate Earth-like extrasolar planets. While over 250 planets outside our solar system have been discovered to date, the vast majority of those planets are massive “gas giants” similar to the outer planets found in our solar system. In some cases, however, these giant worlds are located much closer to their stars—roughly the distance Earth is from the Sun—and thus dubbed “hot jupiters.” Regardless of the distance, though, their large masses make them easier to find than planets of more modest mass so the discovery of rocky, earth-like planets lags far behind the discovery of these hot gas giants. Kepler’s mission is to improve the odds of finding those smaller, terrestrial bodies.

Kepler is the first telescope specifically designed to locate Earth-like planets by measuring the periodic dimming of a star as it is eclipsed by its planets. If the plane of a planet’s orbit is aligned with the telescope, the planet will transit the star’s disk and cause it to decrease in brightness. While astronomers are not sure how many Earth-like planets might exist around other stars, they hope that a survey of 100,000 stars in the Cygnus-Lyra region of the Milky Way will turn up  dozens or hundreds of rocky planets. If any of those planets are located in the “Goldilocks zone” (where temperatures are neither too cold nor too hot, but just right for liquid water to exist), they may be habitable. Kepler’s photometer is so sensitive that it can detect a decrease in a star’s magnitude of only 20 parts in a million. As described by James Fanson, Kepler project manager, if turned on Earth at night, the telescope could detect the dimming of a porch light as a person walks in front of the light! To accomplish this feat of technology, Kepler will have use the most sensitive detector ever launched into space—a 95 megapixel array.

According to Debra Fischer of San Francisco State University, Kepler is essential to our understanding of the kind of planets that form around other stars. Scientists chose to name the mission after the German astronomer Johannes Kepler in honor of his fundamental discoveries in the fields of optics and celestial mechanics. Although he initially studied theology, Kepler became fascinated with the work of Nicolaus Copernicus and while a professor of mathematics, openly embraced Copernicus’ sun-centered model of the heavens in his 1596 book Mysterium Cosmographicum. Throughout 2009, we celebrate the 400th anniversary of the publication of his first two laws of planetary motion. Kepler’s three laws of planetary motion are the cornerstone of celestial mechanics and can be used to describe all closed orbits.

For further information: Kepler Mission home page, highlights of the life of Kepler, and suggestions for further reading.

Spanning nearly 200 light-years, IC 1805—known colloquially as the heart nebula—is a stellar nursery in which we find the star cluster Melotte 15. Compared to our own 5 billion year-old Sun, the stars of Melotte 15 are still in their infancy at a mere 1.5 million years of age. Destined to live short lives and violent deaths, the stars of the cluster are ionizing the hydrogen gas from which they formed and it is the recombining hydrogen atoms that are producing the red-pink emission characteristic of the element’s spectrum. Even at a distance of over 7,000 light-years, the star-forming region is so large that it spans five times the size of the Full Moon.

Ironically, while the nebula’s shape evokes thoughts of Valentine’s Day, it is located in the constellation Cassiopeia—the mythological queen of Ethiopia whose vanity drove her to boast that her daughter Andromeda was more beautiful than the daughters of the god Poseidon. As retribution, Poseidon sent a sea monster (depicted as the constellation Cetus) to ravage the kingdom. King Cepheus, wishing to spare his country from devastation, consulted an oracle and was told he must sacrifice his daughter to the sea monster. Andromeda was chained to a mountain and awaiting her fate when the warrior Perseus—riding the winged horse Pegasus—came upon her and used the severed head of Medusa to turn the sea monster to stone and save the princess from being devoured. In gratitude, Cepheus and Cassiopeia offered Andromeda’s hand in marriage to Perseus.

As punishment for her boastfulness, Cassiopeia was placed in the heavens after her death and is found near the north celestial pole where she spends half of her diurnal motion on her head. The other characters of the legend can also be found as constellations.

The Full Moon of February occurred at 8:49 a.m. CST this morning. Known as the Snow Moon, it is the second Full Moon of the northern hemisphere’s winter. As far as the lunar calendar is concerned, February is unique in that it is the only month in which is it not possible to have two Full Moons and it is that prospect of two Full Moons in a month that brings up an interesting historical oddity referred—incorrectly, as I’ll discuss in a few moments—as a Blue Moon. It is impossible for February to have two Full Moons, incidentally, due to the 29.5 day synodic period of lunar phases. Thus, the interval between Full Moons is over one day longer than the length of a typical February (and one-half day longer than a leap year February.)

But just what is a Blue Moon, anyway? Not what most people believe when you ask them for a definition. The earliest known usage of the phrase “blue moon”, however, appears to be a 1528 pamphlet entitled Rede Me and Be Not Wrothe in which the author wrote “Yf they say the mone is belewe We must beleve that it is true.” The expression seems to refer to something so absurd as to be unbelievable. According to common lore, however, a Blue Moon is usually described as the second Full Moon in a month, but in reality they are not so rare as to warrant the phrase “Once in a Blue Moon.” In fact, there are two Full Moons in a month about once every two years—hardly a rarity. Believe it or not, this particular usage of the phrase appears to stem from a misinterpretation of the original meaning and can be traced to the original Trivial Pursuit game of the 1980s.

The modern usage of Blue Moon appears to have its origin in the Maine Farmer’s Almanac. Editors at Sky & Telescope, with the assistance of several librarians, obtained 40 copies of the Almanac dating back to the early 1800s and found numerous citations that referred to Blue Moons, but not one of them referred to the second Full Moon of the month. In fact, the Blue Moons always occurred on the 20th – 23rd days of February, May, August, or November! These Full Moons always occur about one month before a seasonal change. A further nuance discovered with additional research revealed that the almanac definition relied on the use of the tropical year, which is measured from one winter solstice (Yule) to the next, instead of using the traditional calendar year. Most tropical years have 12 Full Moons—three per season—but occasionally there are 13 Full Moons with one season having four Full Moons.

Why is the third Full Moon significant? Because historically the name of the fourth Full Moon must be in accordance with the seasonal change as the name of that Full Moon relates to the impending equinox or solstice. Thus, the Full Moons of this winter are: the January (Moon After Yule) Full Moon, the February (Snow) Full Moon, and the March (Lenten) Full Moon. Occasionally, however, the first Full Moon of winter occurs just hours after the winter solstice and is therefore in December and not January. In those years, the occurrence of Full Moons will be: the December (Moon After Yule) Full Moon, the January (Snow) Full Moon, the February (Blue) Full Moon, and the March (Lenten) Full Moon. In both cases, the sequence of Full Moons occurs prior to the arrival of the vernal equinox and the start of spring. Thus, Easter won’t occur until after the first Full Moon of spring—referred variously as the Egg, Grass or Hare Full Moon.

A complete description of how Blue Moons are determined is quite convoluted, but is an interesting example of how calendar reform, religious observances (namely Lent and Easter), and astronomical events are joined together.

Read the full Sky & Telescope article for a detailed explanation of the calculation and how the editors traced the history of the Blue Moon definition.

In the early morning hours of January 26, the Moon will slip in front of the Sun producing a solar eclipse visible to observers on the Indian Ocean. This  eclipse is a bit unusual, however, in that the Moon will be near the apogee of its orbit around Earth. As discovered by Johannes Kepler in the 17th century, orbits are not circular as the prevailing wisdom of thousands of years assumed, but are instead elliptical. The Full Moon of two weeks ago occurred when the Moon was at the closest point in its orbit around earth—perigee—and the New Moon of January 26 will occur near the farthest point from Earth (apogee.) Since the Moon will be slightly farther from Earth than usual, it will cover a smaller area of the sky. Add to this the fact that Earth is near the closest point in its orbit around the Sun (perihelion) at this time of year and the circumstances result in the Moon appearing just a bit too small to cover the visible disk of the Sun. The result is a ring of the Sun’s disk remaining visible producing a annular eclipse.

Note: Because the Sun’s visible disk is not completely obscured (the maximum coverage will be about 93%), it is NOT safe to look directly at the eclipse without protective filters. If protective filters are not available, the Sun’s image can be projected so that the projected image can be observed.

Fred Espenak’s “Mr. Eclipse” graphic for the January 26 annular eclipse.

Visit spaceweather.com for animations, videos and photo galleries of the eclipse.