astronomy@iowa

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.