astronomy@iowa

Astronomers believe they may have solved a cosmic mystery: the source of an extremely rare and energetic type of cosmic ray. Dubbed ultra-high energy cosmic rays, UHECR for short, these are not “rays” but are in fact extremely energetic atomic nuclei, typically protons, that are hurtling through space at nearly the speed of light. Because of their extremely high velocity, the kinetic energy of a single UHECR may be 10¹⁸ to 10²⁰ electron-volts.¹ Although nothing more than an infinitesimally small proton, that particle has the energy equivalent of a tennis ball serve or a baseball thrown to a first baseman!

“Ordinary” cosmic rays are also primarily protons, but are produced by more mundane processes such as a solar flare or the interaction of a supernova remnant with surrounding interstellar material and have been known to exist for nearly 50 years. The record-holder in terms of cosmic ray energy belongs to an event recorded over Utah in 1991, however. Irreverently called the “Oh-my-God particle,” this single particle had 100 million times more energy than can be achieved by the world’s most powerful particle accelerator.

These extreme cosmic rays are now being recorded by the Pierre Auger Cosmic Ray Observatory in Argentina and may one day—pending budgetary approval—be observed by a companion observatory in Colorado that would be able to detect UHECRs emanating from more northerly sources. In the November 9, 2007, issue of Science a team of over 370 scientists reported that the arrival directions of these cosmic rays correlate with the distribution of active galactic nuclei (AGNs) within several hundred million light-years of Earth. The determination of the extra-galactic origin of these cosmic rays provides a fundamental piece to the puzzle of their creation and will provide fodder for years as astronomers sort out the complex physics of these extremely violent galactic cores.

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¹ An electron-volt (eV) is simply a unit of energy like a joule but much smaller since it was formulated to describe energies of sub-atomic particle and interactions. Simply, it is the amount of energy an electron gains passing through an electric field of 1 volt.

Source: Schilling, G., “Cosmic Superparticle Mystery Solved?” Sky & Telescope, March 2008.

Photo Credit: NOAO (inset), E. Schreier (STScI) and NASA.

Around 6:00 UT on March 19 a gamma ray burst shattered the existing record and became the most distant object visible to the unaided eye. Theoretically visible, that is, because it’s not known if anyone happened to be looking towards the constellation Boötes at the time the faint glow would have brightened to the naked eye threshold. With a redshift of z=0.96, the estimated distance to the burster is 7.5 billion light-years; halfway across the observable universe! Not exactly a recent event; the explosion—dubbed GRB 080319B—actually happened about 3 billion years before Earth formed.

Classified as “Top Secret” by the U.S. Department of Defense for many years, gamma ray bursts are thought to occur when a supermassive star reaches the end of its life and literally blows itself apart as a supernova (hypernova?) while its inert core collapses into a black hole.

So how would we see a gamma ray burst when our eyes can’t see gamma rays? Gamma rays are produced when the massive star’s core implodes and the outflow of energy is directed into superheated jets. When the jets of energy slam into surrounding matter, the matter is heated and glows in visible wavelengths.

Jai guru deva.

Photo credit: NASA/Swift/Stefan Immler, et al. NASA press release.

In my last post I mentioned the first Full Moon after the vernal equinox is called the Egg Moon in some cultures. I created a table of some other Full Moon names. It’s by no means an exhaustive list and I’m sure you can find more.

A number of times over the past few days people have asked me about Easter and how the date of Easter is determined. The most common question involves the statement that Easter occurs on the first Sunday following the first full moon after the beginning of spring. As with all things astronomical the reality is quite a bit more complicated than that. I’ll try to explain and not bungle things too much in my simplified response.

First we’ll need to define a few terms: the vernal equinox as described in most astronomy textbooks refers to the point on the celestial sphere where the Sun’s apparent motion on the ecliptic crosses the celestial equator moving northward. The precise astronomical definition is the point on the celestial sphere where the Sun’s ecliptic longitude is 0°. The precise time of this occurrence shifts from year to year. The ecclesiastical rules that determine the date of Easter go back the the year 325 CE when the Roman Emperor Constantine convened the First Council of Nicaea. In that year the Vernal Equinox fell on March 20 and the calendar in use was the Julian calender established by Julius Caesar. The first Full Moon after the vernal equinox, called the Paschal Full Moon, was actually defined as occurring 14 days after the ecclesiastical New Moon and is thus not necessarily the actual first Full Moon of northern spring, called the Egg Moon, in some cultures. In fact, the Paschal Full Moon may vary by up to three days from the astronomical full moon. Is it getting fun yet?

So, the date of Easter was set as the first Sunday following the Paschal Full Moon and elaborate tables were developed to set the date of Easter precisely but the early Christian Church didn’t consult Earth with regard to the slow 26,000-year wobble in its rotational axis. This slow precession caused the date of Easter to not coincide with the astronomical beginning of spring. Pope Gregory XIII oversaw a calendar reform in 1582 that removed 10 days from the calendar (October 5 – 14) to reset the date of Easter and established new rules for determining the date of Easter.

Basically, the rules are

• Easter falls on the first Sunday following the Paschal Full Moon
• The Paschal Full Moon occurs 13 days following the ecclesiastical new moon before the vernal equinox
• The vernal equinox is fixed as March 20 regardless of the astronomical occurrance

The result is that Easter can never occur before March 22 or later than April 25. Because we are dealing with an “imaginary” full moon and equinox and not the actual astronomical occurrences, it is possible for the date of Easter to differ from what is predicted by the actual Egg Moon and vernal equinox. For example, in 1962 the astronomical Full Moon occurred on March 21 7hr 55min after the astronomical vernal equinox but according to the ecclesiastical tables the full moon occurred before the vernal equinox. Following the rules, the Paschal Full Moon occurred on April 18 making Sunday, April 22 Easter. According to an email spreading this spring, this Easter (on March 23) is the earliest that it has been since 1913 and won’t occur this early again until 2160 (not 2228 as claimed in the chain email.) The last time Easter fell on the earliest possible date—March 22—was in 1818 and it won’t fall that early until 2285. The latest date Easter can fall (April 25) last occurred in 1943 and will next occur in 2038.

For all the gory details, check out The U.S. Naval Observatory’s The Date of Easter web page and Astronomical Society of South Australia’s Easter Dating Method page.

Note: a couple of typos and a missing sentence were corrected.

With apologies to Rodgers and Hart…At 05:48 UT (or 12:48 a.m. Central Daylight Time) on March 20 the Sun in its northward motion along the ecliptic crossed the celestial equator bringing spring to the northern hemisphere. The Latin origin of the term equinox literally means “equal night” but in reality the length of the day and night are not equal on the day of the equinox. Because the Sun is not a point source of light, its upper limb remains above the horizon as it sets and refraction of sunlight in Earth’s atmosphere makes the Sun visible even after it descends below the horizon resulting in day being a few minutes longer than night, the exact difference varies with latitude.

Here in Eastern Iowa, sunrise occurs at 7:09 a.m. with sunset following 12 hours and 10 minutes later at 7:19 p.m.Complete solar and lunar data for each day can be found at the U.S. Naval Observatory’s Solar and Lunar Daily Data web page.The Naval Observatory is the world’s preeminent authority on time and celestial positioning.

Welcome to the official blog of Brent Studer’s Introduction to Astronomy class at Kirkwood Community College.