Volume XIX No. 6 February 2008 ggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggg JOHN TEBBUTT By Mike Luciuk The Ultimate Amateur Astronomer J ohn Tebbutt was a major astronomical force in Australia during the nineteenth century. His discovery of the Great Comet of 1881, C/1881 K1, coincided with advances in photography, telescope drives and spectroscopy, and will be highlighted in this article. This paper heavily paraphrases papers by W. Orchis-ton and S. Yousef, as listed in the reference. Introduction John Tebbutt's interest in astronomy knew no bounds. Although this Australian's fame was based on comets, Tebbutt made his mark in observations of variable stars, the planets, and eclipses, as well as on lunar occultations and transits of Mercury and Venus. His first major discovery was the Great Comet of 1861. He plotted over 700 asteroid and comet positions in his career. He also discovered Nova V728 Scorpii, maintained a local time service, a meteoro-logical station and carried out studies on tides and floods. He actively popularized astronomy, published two books and was the author of over 300 scientific papers. His accomplishments led to offers for important professional positions, which he declined. Tebbutt valued the freedom his amateur status gave him to pursue astronomical interests of his own choos-ing. Tebbutt was very precise in his observational meth-ods and record keeping. He was the first Australian to systematically monitor variable stars on a long-term basis. His observations of eta Argus (Carina) ex-tended over four decades documenting its magnitude variations. R Carina was covered over 22 cycles, per-mitting the accurate determinations of this Mira vari-able's period, maxima and minima. Tebbutt was educated in several church schools, with emphasis on the classic languages and Euclidean geometry. He inherited his family's Windsor, New South Wales farm, and built his observatories there. Over a period of time, it consisted of four separate buildings. As with the buildings themselves, the instruments housed became more expensive and elaborate as Tebbutt gained the means to improve them. Beginning with two telescopes, two timepieces, a sextant and an artificial horizon, Tebbutt furbished the observatories with larger aperture instruments, culminating in an eight inch equatorially mounted refractor by Grubb of Dublin - the observatory's largest main instrument. Comet C/1881 K1, the Great Comet of 1881 In Tebbutt's astronomical memoirs, he notes his discovery of Comet C/1881 K1: On going out this evening [1881 May 22] at 6h 15m as I had done several times lately to scan the sky for comets with the naked eye I detected what appeared to be a nebulus [sic] object in the south west a few degrees above the place where I discovered the great comet of 1861. I saw at a glance that it was a new object as I am very familiar with that part of the heavens. Subsequently, Tebbutt observed Comet C/1881 K1 through to June 13, making 68 positional measurements with a filar micrometer, on 13 different dates. A filar micrometer accurately measures the angular separation and relative orientation of two neighboring astronomical objects. He also recorded useful information about the tail and the head (but particularly the changing nature of the nucleus), and determined the apparent visual magnitude of the comet on a number of occasions. He published his first report on the comet for the international astronomical fraternity in Astronomische Nachrichten, and followed this up with further positions. Tebbutt then devoted the next three months to calculate the comet's orbital elements. This difficult task took 40 manuscript pages to complete. As the Great Comet began to appear in the Northern Hemisphere, it drew a great deal of attention. By the end of June, its tail exceeded 25 degrees in length and its coma was magnitude +1. By this time improvements in dry gelatin plates and telescope drives made it possible, for the first time, to photograph a comet including its tail. C/1991 K1's photograph was taken by Janssen on June 30, 1881. Also, Comet C/1881 K1 was the first comet whose spectrum was successfully photographed. All prior cometary spectra were obtained visually. On June 24, Sir William Huggins made a one hour exposure of the Great Comet's spectrum, the success of which was also due to the film and telescope drive improvements. Huggins described the spectrum as follows: The spectrum of the comet consists of a pair of bright lines in the ultra-violet region, and a continuous spectrum which can be traced from about F [486 nm] to some dis-tance beyond H. The bright lines, a little distance beyond H, with an approximate wave-length from 3870 to 3890, appear to belong to the spectrum of carbon (in some form, possibly in combination with Hydrogen), which I observed in the spectra of the telescopic comets of 1866 and 1868. In the continuous spectrum shown in the pho-tograph, the dark lines of Fraunhofer can be seen. Note that the K, H, h and G Fraunhofer lines refer to wavelengths of 393 nm, 397 nm, 410 nm and 431 nm respectively. The bright lines near 388 nm and 420 nm are likely the molecular spectral lines of cyanogen, (CN)2. Tebbutt's comet was characterized by astronomers as structurally complex with a rapidly changing head, and an intricate tail. It was noted that: At one time ... a separation in the nucleus led astronomers to think that it would follow the example of Biela's Comet, but the following night the smaller part had disappeared. Very frequent changes were observed by the telescope, in the shape of the jets and envelopes around the nucleus. Many drawings were made of it. As for C/1881 K1's tail: ... a curious dorsal spine of strong illumination formed the axis of the tail, which extended in clear skies over an arc of 20°. It belonged to the same "type" as Donati's great plume ... the appendage was, for a few nights, and by two observ-ers, perceived to be double. Tempel, on June 27, and Lewis Boss, at Albany (N.Y.), June 26 and 28, saw the long straight ray [and] ... the curved train ... It had vanished by July 1, but made a temporary reappearance July 22. All in all, Tebbutt's comet was indeed, one of the Great Comets of the nineteenth century. (Continued next page) Final Points The Great Comet of 1881 was discovered by John Tebbutt almost twenty years to the day after he detected the Great Comet of 1861. Both comets were spectacular objects and contributed to our understanding of cometary structure and behavior, but be-cause of the emergent role of photography and spec- troscopy in astronomy, the 1881 comet was to play a more notable part in the development of cometary astronomy. In 1973 Tebbutt's contributions in astronomy were recognized when a crater on the Moon was renamed "Tebbutt." In 1986 the Australian government issued their $100 note with John Tebbutt's image overlaying his observatories. These were fitting tributes to this consummate amateur astronomer. Stewart's Skybox by Stewart Meyers A s usual, February is a boring month. About the only major astronomical event is the lunar eclipse on the night of the 20th, but the topic of lunar eclipses has already been covered. However, some recent news on the Space Daily website (http://www.spacedaily.com) has suggested a topic. Last year, I polled the membership about what topic they wanted in one of the columns. Alex Flynn had a suggestion about computer-controlled telescopes and won. But there was one suggestion that was quite good as well and that will be the topic of this column, especially in light of some recent findings. The suggestion is from our own multi-talented Clif Ashcraft. He wanted to hear about jets. No, not those sardine cans with wings that fly all over and used to ruin many a long exposure astrophoto (back when people used film to photograph the night sky). Rather, I am talking about the cosmic variety. Starting Big In the early 20th century, improvements in photographic technology allowed astronomers to take images of relatively faint objects through their telescopes. In 1918, Heber Curtis took a picture of M87, an elliptical galaxy in Virgo, that is well known to many amateurs. When the plate was developed, he noticed what he called "a curious straight ray" stick-ing out of the nucleus of the galaxy. Little progress was made toward explaining this phenomenon until the 1950s. As a result of the pioneering insights of Karl Janssky and Grote Reber in the 1930s being combined with technology derived from radar research in World War II, radio astronomy entered the astronomical mainstream. In those days, astronomers were merely detecting sources of radio emission and establishing their approximate locations. One of the brightest sources was in the constellation of Virgo and was named "Virgo A". As the resolution of radio telescopes improved, the source of the emission was narrowed down to M87, then eventually to the "curious straight ray". With the improved radio and optical data, this "ray" was shown to be a jet of gas streaming away from the nucleus at very high speeds, as is very clearly shown in the Hubble Space Telescope image from the Space Telescope Science Institute in Figure 1. Astronomers were puzzled and could not figure out how a confined jet of material could be shot away from a galactic nucleus. Over the years, more of these jets were found in other galaxies. As knowledge of galactic jets was gradually improving, a clue was provided by the study of qua-sars. Discovered in the early days of radio astron-omy, these objects were considered very mysterious, though by the 1960s, it had been found that they were in the centers of very distant galaxies. However, news of this was slow to spread outside the astronomical community. In fact, in the original "Star Trek" series episode "The Galileo 7", it was main-tained that they were mysterious objects in our own galaxy. In defense of "Star Trek", revisionist as-tronomer Halton Arp also believed that quasars were much smaller and closer than other astronomers thought. While studying quasars, some astronomers noticed evidence of gas moving at very high speeds away from a few quasars. In fact, the speeds were too high - faster than light. It was soon realized that this superluminal speed was just an illusion caused by relativistic effects when the galactic jet from the quasar is almost pointed at Earth and is moving at a substantial fraction of the speed of light. For some time, it was thought that jets were found only within active galactic nuclei. But that would change. SS433 and SNL In 1969, it was found that some X-ray sources in our galaxy were highly variable. But there was a surprise in store. Bruce Margon, in 1978, studied the spectra of an X-ray source in a supernova remnant known as W50 in Aquila. He noticed that the Doppler shift of the signal changed over time. It was red shifted, normal, then blue shifted. This cycle would repeat. Further investigation found that the source was a binary system with a 13-day orbital period which was emitting jets of gas and particles and also that the jets had a precession cycle of 160 days. As usually happens with astronomical discoveries, when word got out to the mainstream media, they misinterpreted the information and thought that there was a star that was both coming and going at the same time. In what was a first in the history of astronomy, this X-ray source, now known as SS433, was featured in a brief segment on the "Weekend Update" skit on "Saturday Night Live" where Don Novello (as his famous character Father Guido Sarducci) claimed that study by the astronomers at the Vatican Observatory revealed that everything in that star system went forwards, then backwards, including the aging process. When studied by real astronomers, it was determined that SS433 was what is known as an X-ray binary. That is a normal star, usually in the red giant phase, in close association with a compact object. In the case of SS433, the object is a neutron star. The neutron star pulls gases from the outer atmosphere of its companion, and forms an accretion disk. Somehow jets of material shoot out from the system. Around this time, astronomers were starting to accept that very massive black holes exist in the cen-ters of galaxies. It was also realized that young gal-axies have large quantities of gas in their central regions, which would eventually fall into the central black hole and form an accretion disk. So SS433 offered a link between the jets emitted by galaxies and quasars and those emitted by neutron stars in X-ray binaries and black holes. White Dwarf It has been recently discovered that white dwarf stars, the end result for stars like our Sun and smaller, can, in some cases, get into the cosmic jet act just like neutron stars and black holes. Suzaku, an X-ray satellite operated by the Japanese Aerospace Exploration Agency (JAXA) and NASA's Goddard Space Flight Center, observed AE Aquarii in 2005 and 2006. AE Aquarii was known to have a very strong magnetic field, and it is in a binary sys-tem with a main sequence star. Yukikatsu Terada of the Institute of Physical and Chemical Research (RIKEN) in Wako, Japan used Suzaku to observe AE Aquarii in an effort to determine that white dwarf stars with strong magnetic fields could generate cosmic ray particles. To the surprise of the scientists, AE Aquarii acted like a pulsar, emitting a jet of from the accretion disk of material that the white dwarf's gravity pulled from its main sequence companion. Small Scale Version Back in 1890, S.W. Burnham discovered a small nebula in Taurus, near a variable star that is now known as T Tauri. At the time, it was known as Burnham's Nebula because it was considered to be a small example of a reflection nebula. Then, in 1946, two astronomers, George Herbig and the late Guillermo Haro, were independently studying NGC 1999 and Burnham's Nebula when they noticed some bright objects in their images. These bright objects were soon known as Herbig-Haro objects and were considered somehow connected to the process of star birth. Based on the evidence of early spectroscopic work, it was thought for a time that these objects contained newborn stars, being flung out of the nebula where they were born. But the actual truth was much different. With improved resolution, the Herbig-Haro objects were revealed to be the results of jets shooting out of the nebula where stars were forming and hitting a dense outer part of the nebula. The resulting shock causes the gases to ionize and glow. At first, as-tronomers were not sure why newborn stars should be emitting jets, since the prevailing theories indi- cated that star birth was a gradual and relatively sedate process. However, it has been learned that is not the case. The disk surrounding newborn stars is not a simple, peacefully rotating object that gradually accretes onto the star. But rather it is a place where gas and dust are in constant motion, with some of the material spiraling in toward the star. Bringing It All Together Seeing jets shoot out of active galaxies is one thing, but when jets are seen associated with other kinds of astronomical objects, that is significant, and it can tell us something about how the universe works. It seems that all cosmic jets have a few things in common. First, they all involve objects (black holes, neutron stars, white dwarf stars, or protostars) that are surrounded by much larger disks of material. Secondly, the material is falling towards the object. Finally, the central object has a magnetic field. These common elements are the crucial clues to how jets form. The current view is that matter is spiraling in towards the central object. As a result, it forms a flattened disk and heats up due to internal friction, becoming hottest close to the central object where the accretion disk is rotating most rapidly. Some of this heated, rapidly moving matter becomes ionized and interacts with the central object's magnetic field and is carried away from the disk in a magnetically contained jet. Recently, this role of magnetic fields in cosmic jets received major confirmation from work done by a team lead by Rodrigo Nemmen of Penn State University. They studied Chandra observations of nine galaxies known to harbor supermassive black holes emitting jets. When factoring in relativistic effects with new information concerning rotation and the magnetic properties of black holes, they found that the combination resulted in an extremely power- ful magnetic field that was able to eject a large frac-tion of the matter falling towards the black hole be-fore it could get close to the event horizon. This ma-terial is confined in jets shooting out from the magnetic poles. The same process - on a much smaller and non- relativistic scale - is thought be re-sponsible for other cosmic jets as well. This confirms that magnetic fields are absolutely required for the creation of cosmic jets. Close But Not Quite In recent years, a new jet-like phenomenon has been discovered. When red giants approach the end of their lives, they shed the outer layers of their at-mosphere, which then forms what we call planetary nebulae. It was thought that this process was spherical. Shapes of planetary nebulae such as M57 (the Ring Nebula) were presented as evidence of this. See Figure 2. But, when telescopes like the Hubble Space Telescope took images of planetary nebulae, some had shapes like butterflies, others had shapes suggesting two lobes, such as seen in the HST image of Hubble 5 in Figure 3. While this phenomenon is not totally understood, it is thought that some of the stellar wind is somehow channeled so that much of it goes out perpendicular to its equatorial plane. While not a true jet like the others mentioned in the article, it is another case where gas flow is directed along an axis. One Process Fits All This amazing process works in a wide variety of environments, from the large scale of active galactic nuclei with multi-million solar mass black holes all the way down to protostars at the beginning of their lives, and, to a limited extent, even to dying red giant stars. Apparently, nature likes to use the same rules over and over again. The Hubble Age By Dr. Lew Thomas T he farther away a galaxy is from us, the greater is the red shift. Hubble and Humason reported this by constructing a graph which plotted red shifts, or recessional velocities, against distances to each galaxy measured. See Figure 1. This work began back in 1924, when Hubble began to use the newly fabricated 100-inch telescope at Mt. Wilson in California. Humason was then driving a mule train to haul equipment and astronomers up a dirt road leading to the summit. Humason later became what amounted to be a janitor at the observatory. His curiosity earned him a position as assistant to the nighttime astronomers, helping them point the huge telescope to collect their data. Humason absorbed information so quickly and learned skills so rapidly that he finally became a full fledged astronomer at Mt. Wilson. This is a success story that shows what can be accomplished when one is highly motivated. These two astronomers concentrated on galaxies in clusters to which the distances could be estimated by assuming that each cluster was made up of about the same variety of galactic types. The more galaxies in each group, therefore, the brighter was the assemblage. By the inverse square law, the distance could then be estimated independently of the red shift. When the red shift vs. distance plot was made, it was observed that the more distant a galaxy was, the greater was its red shift. The slope of the velocity (red shift number) vs. distance curve is the Hubble constant (H). The curve was assumed to be a straight line since the expansion rate was believed to be linear. The Hubble constant is usually expressed as km/sec per megaparsec. If the slope is really constant over eons of time, the Hubble constant gives a measure of the age of the universe. Let this age be To and so, To = 1/H (1) Now a megaparsec equals 3.086 x 1019 kilometers and there are 3.15576x107 seconds in a year, so that for: H = 1 km/s/megapc 1 km/s/mpc = (1 km/sec)/3.086x1019 km = 3.240x10-20 /sec = 3.240x10-20 x 3.15576x107 = 1.0268x10-12 /years = 1.0268x10-12/ 109 = .001027 /billion years (2) If H = 80 km/s/megapc, from (1) and (2) we obtain To = 1/(80 x .001027) =12.1 billion years (3) This is called the Hubble Age and, as we have said, is based upon a constantly expanding universe. We can simplify the arithmetic by revising (3) to express 1/H = 973.7 billion years per 1 km/sec/megaparsec (4) and so, To = 973.7/H ; 1000/H billion years (5) where H = number of km/sec/megaparsecs. Since the expansion of the universe is accelerating, then the universe must be older than the Hubble Age. We hypothesize that dark energy, a repulsive force, is pulling the universe apart. If this energy be constant over time, then as the universe expands, the attractive force of gravity will be reduced in accordance with the inverse square law, and so the acceleration itself must increase without limit. To date, it has been determined that the acceleration itself has two measures of acceleration (two derivatives). It awaits more data to determine if there are any more derivatives. If there are not, then dark energy can not be constant, but must decrease slowly with time. Origin And Use Of The Julian Day Number by Dr. Lew Thomas Julian Day Numbers are the consecutive count of days from noon at the Greenwich meridian on January 1, 4713 BC. There are algorithms to convert any date to a JDN and vice versa. This is particularly important in astronomical work since it avoids months, years, and leap years in our modern calendar. By agreement all dates prior to October 5, 1582 are calculated on the basis of the Julian Calendar containing 365.25 days and having a leap year every 4 years. Dates following October 5, 1582 are calculated on the basis of the Gregorian Calendar having leap years every 4 years with the exception of century years (those ending in 00) in which case a leap year occurs if the date is evenly divisible by 400. Due to the calendar conversion from the Julian to Gregorian calendar the dates of October 6 through October 15, 1582 do not exist. The count of days by Julian Day Numbers was invented by Joseph Scaliger in the 15th century and published in 1583. The word "Julian" in the JDN comes from the fact that 365.25 days in the Julian Day system is considered a year as in the Julian calendar. It must be noted, however, that the Julian Day Number is simply a count of days from noon on January 1, 4713 BC. In doing his work, Scaliger consulted over 50 calendars of his day in order to convert their dates to JDNs. Now the question is why did he choose as the staring point for the count the year 4713 BC? He considered three cycles which were commonly employed in reckoning dates in Scaliger's time, namely 1) the 28 year solar cycle - this is the shortest period in which the days of the week return to the same calendar day in the Julian calendar. 2) the 19 year lunar cycle - this is the Metonic cycle in which the same phase of the moon returns to the same Julian Calendar date 3) the 15 year cycle of the Roman Indiction - a political cycle. When one combines these three cycles one obtains a grand cycle of 28x19x15 = 7,980 years. Now comes the religious part of the puzzle. Scaliger inferred that the birth of Jesus was in 1 BC (which most modern scholars deem as incorrect). Each of the above cycles began at a different epoch and those near 1 BC were 1) 1 BC was the 9th year of the solar cycle 2) 1 BC was the 1st year of the lunar cycle 3) 1 BC was the 3rd year of the Roman Indiction Now Salinger sought a day count which would always be positive, so he needed a time that preceded all recorded history. (Continued page 14 Julian Day Number) Happy Leap Year! By Jeremy Carlo As you may already know, February 2008 will have 29 days. To most people the primary signifi-cance of this fact, like many others, is monetary: if you're paid by the hour, that's an extra day for which you get paid, but you work an extra day for free if you get paid monthly (yet you also get one free day on your monthly rent). For the approximately 0.1% of people who were born on February 29th (like The Asterism editor Ray Shapp's grandfather), it means many less "official" birthdays. But what is the astronomical significance of the leap day, and what is the reasoning behind the laby-rinthine rules governing whether any particular Feb-ruary will have a 29th day? For example, 2008 will have a leap day, as did 2004, 2000, 1996 and every fourth year back to 1904. But 1900 did not (at least in places where the current calendar was in use), although every divisible-by-four year from 1986 back to 1804 did, and neither did 1800 or 1700 (yet all the other intervening divisible-by-four years did have 29 days in February). 1600, on the other hand, was a leap year, unlike the three succeeding century years. 2100, 2200 and 2300 will not be leap years, but 2400 will be. Are the calendar designers trying to torture us? The fundamental need for the leap day arises from the fact that the year does not correspond to an integer (whole-number) multiple of days. If one defines the year as simply being 365 days, the Earth will not yet have returned to the same point in its orbit after one year. This effect will add up over time, and even-tually the seasons will be completely out of phase with the calendar! Happy snowy Fourth of July! The immediately obvious solution, of course, is to add a fractional day to the end of every year, giving the Earth extra time to "catch up." The problem is that the insertion of whatever number of extra hours between 11:59pm December 31st and 12:00am January 1st will cause the Sun's position to be "out of phase" with the clock on the wall (a year of Sunrises at 1am, or 7pm, or 11:45am…), an even more immediately distressing problem! So this is not an acceptable solution; to keep our clocks and the Sun in phase, any "catch-up" time must be added in multiples of a whole day. What we must do is come up with a scheme to have extra days on certain years and not on others, so that one "average" calendar year closely approximates the true year, and the long-term drift of seasons is cancelled out. But first we should agree what it means to talk about "days" and "years." We all learned in school that a "day" is the amount of time it takes the Earth to rotate about its axis and a "year" is the amount of time it takes the Earth to go around the Sun. (Ac-cording to a recent survey, nearly half of Harvard graduates are aware of these facts.) But this simple description glosses over some important points. The Earth's rotation is indeed quite regular; this is enforced by the conservation of angular momentum, although perturbative effects, chiefly due to the Moon, cause the Earth's rotation axis to slowly drift ("precess") and its rotation rate to gradually decrease. When the Ancient Egyptians built the pyramids, the "North Star" was Thuban ( Draconis), and several hundred million years ago, the length of the day was closer to 18 hours than 24 (less time for Jack Bauer to save the world). At the present time, it takes the Earth 23 hours, 56 minutes and 4.1 seconds to rotate once about its axis, relative to a stationary reference frame in space. This is known as a sidereal day, and is useful to astronomers because the sidereal time tells us the Earth's absolute rotational position and thus, for example, whether or not Orion is well-placed for viewing, or when M31 will be rising at a given location. Some hard-core astronomers even have sidereal clocks in their observatories. But for farmers and fishermen and other practical types, the Sun's location is far more important than that of the Big Dipper or the Orion Nebula. Sidereal clocks won't tell you that, and here's why. As the Earth rotates, it also revolves about the Sun. When the Earth completes one rotation in the sidereal sense, it has moved about 1/365th of an orbit, and thus the Sun has not, from our vantage point, yet returned to quite the same place in the sky. So we have to add a small amount of additional time (about 4 minutes, which is about 1/365th of a day), to allow the Sun to "make it all the way around;" this is what normal people call a "day," with 24 hours of 60 minutes each, of 60 seconds each, for 24x60x60 = 86,400 seconds, and we astronomers call a solar day. (As a side note, the amount of "extra" time re-quired to get the Sun to return to the same point in the sky varies due to the Earth's elliptical orbit; sometimes the Earth moves faster than at other times, and thus the "catch-up" time required is not constant. 24 hours (the sidereal day plus the 3 min-utes and 55.9 seconds) represents an average of the Sun's actual motion through the sky, and is referred to as a mean solar day. Put the Earth's elliptical orbit together with the inclination of the Earth's rotation axis, and if you take a photograph of the Sun at the same mean solar time (say, noon) each day, over the course of a year the Sun's position will trace out the beautiful analemma, as explained by Alberto Guzman in last month's Asterism.) Now that we agree that a "day" is actually a mean solar day of exactly 86,400 seconds, what about the year? (There is another complete story about what exactly a "second" is, giving rise to UT1, UTC and a whole alphabet of time systems, and necessitating the occasional insertion of "leap seconds," which I'll save for a future article.) The year, it turns out, is even more complex, as the Earth, strictly speaking, never returns to the ex-act same point it occupied a year ago since its orbit is elliptical and inclined with respect to the Sun's equator. If we neglect this and just consider the time taken for the Earth to move 360 degrees around the Sun with respect to a fixed reference frame, this is a sidereal year (current value: 365.256363 days, to six decimal places). But, the perihelion (the location where the Earth is closest to the Sun) drifts very slowly, so the Earth will not be the same distance from the Sun after one sidereal year; the time for the Earth to go from one perihelion to the next is called the anomalistic year (currently 365.259635 days). Perihelion precession is itself a complex phenomenon, and is partly due to effects of general relativity (Mercury's anomalously large perihelion precession was an important early piece of evidence for Ein-stein's theory of General Relativity), also a topic for a separate article. But, what has the most importance for practical people is the interaction between the Earth's rota- tional inclination of 23.5 degrees and the Earth's or-bit; summer occurs when your hemisphere is pointed towards the Sun, and winter occurs when your hemisphere is pointed away. (This also explains why the seasons in the southern hemisphere are reversed from ours.) Surprisingly, this inclination effect outweighs that of the Sun-Earth distance in determining surface temperatures; perihelion occurs in January, as the Northern Hemisphere is in its winter chill, and aphelion (farthest distance from the Sun) is in July. The vernal and autumnal equinoxes heralding the northern hemisphere's spring and fall, respectively, occur when neither hemisphere points more toward the Sun than the other, while the summer and winter solstices occur when your hemisphere is tilted maximally toward or away from the Sun. The time from one vernal equinox to another (more correctly, the average of the time from any one such point to the next) is known as the tropical year, and is currently 365.242190 days. It may seem like this discussion is overly pedantic, but it's important to note that the sidereal year and the tropical year differ by about 20.4 minutes. This is due to the same rotational precession that causes the north celestial pole to wander. Therefore, the position of the Earth around the Sun at the vernal equinox will slowly move around with respect to the stars; this is known as the precession of the equinoxes. (A somewhat whimsical consequence is the fact that your "star sign" as published in horoscopes is most likely wrong; the Sun was probably in a con-stellation that neighbors your "sign" at the time of your birth due to the accumulated effects of equinoctal precession since the "charts" were first made.) The most useful calendar is one that tells us that the weather is hot and the Sun beams down from high in July and the weather is cold and the Sun hangs low in December, and tells us when to harvest the crops, prepare for monsoons, or to put on the snow tires. We should, therefore, match our calendar as closely as possible to the tropical year, with every year having an integer number of days. A simple calendar with 365 days every year will "gain" about one day every 4 years, probably not enough to be readily noticeable over an individual lifetime, but in-exorable on the timescales of history's long arc. In-deed in a little over a millennium the calendar will have completely "lapped over" a seasonal cycle. The first workable solution was developed in 46-45 BC, during the time of Julius Caesar. Since the little bit extra above 365 is almost equal to a quarter (0.25), why not add one extra day every fourth year? This gave rise to the Julian calendar, which has three years of 365 days to every one year of 366 days, averaging exactly 365.25000 days per Julian year: every year whose number is divisible by 4 shall have a leap day added to the end of February, whereas those years not divisible by 4 shall have 28 days in February. However, the tropical year is 0.007810 days less than the mean Julian year. This discrepancy will accumulate as well, albeit more slowly: the Julian calendar "loses" one day with respect to the seasons in about 130 years. What ultimately brought down the Julian calendar was Easter, which is tied to Passover, and thus is based on lunar cycles rather than the solar calendar. While the Roman Catholic Church and the Church of Alexandria (today the Or-thodox Churches) used slightly different standards, both were dependent on the date of the vernal equi-nox and a preset "window" of dates on the calendar. As the equinox drifted relative to the window of al-lowable dates, conflicts arose as to when Easter should be held. A solution was proposed and then decreed by Pope Gregory XIII in 1582, when the accumulated drift of the vernal equinox added up to about ten days; the Gregorian calendar was gradually adopted by civil authorities and remains in use today. See Figure 1. In switching from the Julian calendar to the Gregorian one, calendars were advanced the appro-priate number of days to negate the accumulated effects of precession under the Julian calendar. Spain, Portugal and Italy went directly from October 4 to 15, 1582 when the switch occurred; France skipped from December 9 to 20 of the same year. Most of Protestant Europe (some fearing the Gregor-ian calendar as a tool of Catholic imperialism) did not switch over until the 18th century, necessitating an 11-day skip, and Russia held out until the October Revolution of 1917, skipping 13 days from January 31 to February 14, 1918. Most of what eventually became the United States switched during the early colonial period, except for Alaska, which switched in 1867 when it was purchased from Russia. The first correction the Gregorian calendar imposed was to decree that century years (those ending in 00) are not to be leap years; this reduces the length of the Gregorian year from the Julian value of 365.250000 days to 365.240000 days, only differing from the tropical year by 0.002190 days, about ¼ of the Julian calendar's deviation. The second correction was to stipulate that every fourth century would have a leap day anyway, raising the long-term average to 365.242500 days per Gre-gorian year, leaving a deviation of only 0.000310 days per year, meaning the Gregorian calendar will "lose" one day per 3000 years. It was felt at the time that this would not cause any trouble over the timescales the calendar was expected to be used. However, it is an interesting exercise to determine what the next "correction" would be, assuming of course we still used the Gregorian calendar after the millennia, or if indeed there still is a "we" to speak of. The Gregorian year differs from the tropical year by 0.000310 days, or equivalently by one day in 3226 years. One could propose, although this has not been done, to add a leap day every 3000 years, say in the years 3000, 6000, 9000 and so on. Of course some of these are leap years already, mean-ing there would be one year every 6000 years with a February 30th! (Think how few birthdays someone born on that day would have!) This could easily be prevented by simply moving the extra leap years to off-cycle years, such as 3001, 6001 and so on. How-ever, I think the most likely solution, should we ever feel the need for one, would be to propose extra leap days on an ad hoc basis, about one every 3000 years. This is essentially the way we in recent years have imposed necessary leap seconds into the clock. Finally, precession is in no way eliminated by the Gregorian calendar or even by any conceivable improvement to it. The equinoxes, and the perihelion, still precess around the Sun due to the mismatch between the sidereal and the tropical, or the sidereal and the anomalistic, years. The equinox makes a complete orbit of the Sun in approximately 26,000 years, and the North and South celestial poles make circles 23.5 degrees in radius, forming fleeting (on the time scale of millennia) alliances with stars they happen to sweep nearby. The perihelion similarly orbits the Sun every 112,000 years. (It is also proposed that precession of the perihelion and the equinoxes, as well as periodic shifts in both the eccentricity of the Earth's orbit and in its rotational inclination may play a role in long-term climate variation, although the details are far from clear at this point.) Since the seasons (and our modern calendar) are determined by the Earth's position relative to the equinoxes and not the equinoxes' absolute positions around the Sun, over time different constellations will be associated with different seasons. We in the Northern Hemisphere associate Orion with the winter and Scorpius with the summer, but in about 13,000 years, not only will Vega be the north polar star, the constellations will appear in reverse seasons since the summer and winter solstices will have traded places around the Sun. The gift offered by the astronomers to the more "practical" sorts is a calendar that keeps phase with the seasons; in the year 15,000 the Fourth of July will not be celebrated in the dead of winter or some other random season, but rather as always in the heat of summer (possibly greatly enhanced by the greenhouse effect, a phenomenon whose existence was first inferred by studies of the planet Venus). Our sacrifice will be that, providing that light pollution has not already obliterated all memories of the night sky, the sweltering evening's fireworks will be set against a backdrop of Orion, Taurus, the Pleiades, and the other stellar formations we of the present era have come to associate with colder times. Julian Day Number (continued from page 10) by Dr. Lew Thomas He needed to find an ancient date upon which all the cycles would start together and would produce the offsets noted on 1 BC. That date became 4713 since 4713/28 = 158 with a remainder of 9 years 4713/19 = 248 with a remainder of 1 year 4713/15 = 314 with a remainder of 3 years Starting the day count and the cycles on this date would cause the three cycles to separate by the above amounts by 1 BC while at the same time produce JDNs which were all positive. For example, the solar cycle is at 9 years at 1 BC. 9 years plus 158 cycles arrives at 4713 BC. (9 + 158x 28 = 4713). And this is the reason that the JDNs start at noon on the Greenwich meridian on January 1, 4713BC. Of course one has to pick a spot on the Earth from which to reckon time . The Greenwich meridian was agreed upon somewhat after our Civil War. GENERAL MEETING February 15, 2008 " A Ray of Light in a Sea of Dark (Matter)" - Dr. Charles Keeton, Rutgers University Dr. Charles Keeton makes use of a sort of natural "telescope" - gravitational lensing - to study mys-terious "dark matter". Dr. Keeton attributes his interest in space to the success of the Voyager missions and the Space Shuttle program in the late 1970s and early 1980s. After earning a B.A. from Cornell University and Ph.D. from Harvard University, Dr. Keeton did research at the University of Arizona and the University of Chicago before joining the faculty of Rutgers University in 2004. Dr. Keeton has observed with the Hubble Space Telescope as well as observatories in Arizona and Chile. His research has recently been featured by National Public Radio, MSNBC.com and the New Scientist magazine. 8PM IN THE ROY SMITH THEATER MEMBERSHIP DUES Regular Membership: $21 Sustaining Membership: $31 Sponsoring Membership: $46 Family Membership: $5 First Time Application Fee: $3 Sky & Telescope: $32.95 Astronomy subscription: $34 (Subscription renewals to S&T can be done directly. See "Membrship- Dues" on website for details.) AAI Dues can be paid in person to Membership Chair or Treas-urer, or by mail to: AAI, PO Box 111, Garwood, NJ 07027-0111 DR. LEW'S SEMINARS See Dr. Lew Thomas for possible upcoming seminar topics. (Choice of topic at Dr. Lew's seminars is determined by participants' interest) EMAIL CONTACTS president@asterism.org President of AAI editor@asterism.org Editor of The Asterism Ray Shapp, Editor Deadline for submissions to each month's newsletter is the first Friday of that month. membership@asterism.org AAI Membership Chair trustees@asterism.org All three Trustees of AAI ray@asterism.org Ray Shapp for the website exec@asterism.org Executive Committee plus Trustees QOs@asterism.org All Qualified Observers Info@asterism.org AAI president, corresp. secretary, and computer services chair DOME DUTY SCHEDULE February 22 Team E February 29 Team A March 7 Team B March 14 Team C FRIDAYS AT SPERRY February 22, 2008 Ask the Astronomers Staff February 29, 2008 The History of the Tele-scope Part 1 (Part of the 400th anniversary of the telescope) Al Witzgall March 7, 2008 TBA All schedules above were accurate at time of publication. Please check www.asterism.org for latest informa-tion (click on "Club Activities") March 2008 is a quiet month for planet-watchers. A few slow, lovely ballets, but nothing really spectacular. The leading couple would be Venus and Mercury. Every year the two inner planets pass each other half a dozen times or so. Usually these conjunctions are quick affairs. A few cloudy days and we would miss the whole thing. Not this month. These morning "stars" will be less than three degrees apart for the entire month, although fairly close to the Sun. This event really began around February 27th as Mercury, racing away from the rising Sun, passed about one degree above Venus, which has been slowly falling toward the Sun since the end of last October. But then, on the 3rd of this month, Mercury slams on the brakes and also starts slowly falling toward the Sun. Now the two planets fly in close formation until Mercury catches up with Venus on the 24th. By that time the planets are probably too low in morning twilight to be observed. But if you should find yourself up and about before sunrise on any clear morning before the last week of March, look to the southeast for Venus. If you spot this brilliant planet, grab your binoculars and scan a degree or two to the right for Mercury, which is near zero mag-nitude or brighter all month. Around the beginning of this event, invisible Nep-tune passes less than one degree above the two inner planets. The thin crescent Moon joins the party on the 5th. Later that day the Moon passes di-rectly over Venus and then Neptune. The first occul-tation occurs over the United States in mid-afternoon, the second over Australia. Two weeks later, Uranus also flies by. Very early risers can enjoy Jupiter, far to the upper right of all the sunrise events. The southernmost Moon of the month passes below the Giant Planet near the beginning of the month, and then again near the end. Prime time planet-watchers will have to content themselves with Mars and Saturn. The Red Planet is still high and bright, but no longer negative magnitude. Look off to the left of Mars for tiny Mebsuta, in Gemini. Mars slowly moves toward the star all month, passing a quarter of a degree below it on the 30th. Saturn is up most of the night setting just before morn-ing twilight. All month the Ringed Planet moves to the right toward Regulus, the heart of Leo, the Lion. Stunning Beauties of Our Solar System Ken Kremer New Territory from MESSENGER Mercury Flyby NASA's MESSENGER spacecraft successfully flew past the planet Mercury on 14 January 2008. The closest approach was 124 miles altitude. After two more flybys in October 2008 and September 2009, the probe will finally orbit the planet in 2011 after a journey of 5 billion miles through space. The orbit during the planned one year science mission will be highly elliptical ranging from 125 miles to 9,420 miles. MESSENGER will orbit Mercury twice every 24 hours. The probe was launched on a Delta II Heavy booster rocket on August 3, 2004 from Cape Canaveral Air Force Station. This false-color mosaic view was taken at an average distance of about 9,000 miles and it shows a portion of the previously unseen and unknown side of Mercury. The large circular light-colored area in the upper right of the image is the interior of the Caloris basin (see close-up next page). Only about 45 percent of Mercury's surface had been previously mapped by Mariner 10, 33 years ago. MESSENGER was designed and built by John Hopkins University's Applied Physics Laboratory (JHU/APL). Image link: http://messenger.jhuapl.edu/gallery/sciencePhotos/pics/Prockter07.jpg Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington Learn more at the MESSENGER Homepage: http://messenger.jhuapl.edu/index.php Science Outreach and Update by Ken Kremer Image link: http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?page =2&gallery_id=2&image_id=144 Science Outreach at Riverside Elementary School: Princeton, NJ, Dec 6. At this annual astronomy night, all of the 3rd grade students and their families enjoyed my presentations and displays on "Twin Robots Exploring Mars", shortly before Mars at Opposition. This year we were relocated to the more comfortable envi-rons of the library with a newly installed projector. Thanks to rare clear weather, outdoor telescopes enabled viewing of nebulas and even a few meteor streaks. Left: Princeton area families "On Mars in 3-D" as Spirit and Opportunity celebrate 4 years of roving on the Red planet. Right: Kids and kin gawk at Martian Craters and Saturn's moons in 3-D at Riverside Elementary School on Dec 6 Please contact me for further information or science outreach presentations. My upcoming talks include: Mill Lake Elementary School: Monroe Twp, NJ, Wed, Feb 13, 6:30 PM. "Exploring Mars at Astronomy Night". Website: http://monroenj.schoolwires.com/8602011112104530/site/default.asp Note: This event will also feature AAI sponsored telescope viewing Rittenhouse Astronomical Society (RAS) at the Franklin Institute: Philadelphia, PA, Wed, Feb 20, 8 PM. "Lunar, Solar and Martian Eclipses". http://www.rittenhouseastronomicalsociety.org Raritan Valley Community College Planetarium: Somerville, NJ, Wed, Apr 2, 7:30 PM. "Launching DAWN (and Phoenix): From Behind the Scenes at Kennedy Space Center". http://www.raritanval.edu/planetarium Washington Crossing Nature Center: Titusville, NJ, April 12, 1 PM. "Mars, Saturn, Asteroids and Beyond" Dr. Ken Kremer Email: kremerken@yahoo.com NASA JPL Solar System Ambassador