astronomy@iowa

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.

At 4:13 a.m. CDT the Full Moon of occurred. Being the Full Moon nearest the autumnal equinox, which occurs at 10:44 a.m. CDT on September 22, this Full Moon is traditionally called the Harvest Moon. So what is so special about this particular Full Moon, which can occur any time from mid-September to early October? Typically the Moon rises about 50 minutes later each day due to its slow eastward (prograde) drift across the star field, but around the time of the autumnal equinox that time is reduced to about 22 minutes.

The reason for this difference is the smaller angle between the ecliptic—or apparent path of the Sun across the sky—at this time of year and the Moon’s orbital path in addition to the narrow angle the Moon’s orbital path makes with the horizon. Given this particular geometry, there are fewer minutes between sunset and moonrise on the successive nights. More hours of bright moonlight on these evenings meant extra time for the farmers working outside to finish the harvest.

References: Some names of the Full Moons.

At 11:59 UT¹ the Sun reached its most northerly point above the equator and marked the beginning of summer in the northern hemisphere. From the Latin name for the Sun (Sol) and sistere (meaning to stand still), solstice refers to the two times during the year when the Sun’s annual apparent motion around Earth’s sky reaches the most northerly and southerly extremes. The summer solstice—sometimes incorrectly referred to as the longest day of the year—is not any longer than other days of the year. The fact that the Sun reaches its most northerly point at the summer solstice simply means that today is the date that has the most daylight hours and the shortest night. And while we use this event to mark the beginning of summer in the northern hemisphere today, that has not always been the case.

Once referred to as midsummer (see “A Midsummer Night’s Dream“), the summer solstice once marked the middle of summer while the winter solstice marked the middle of winter and the two equinoces (“equal nights”) marked the middle of spring and autumn respectively. In some old European (such as ancient Irish) and East Asian cultures, the actual seasonal changes were related more to what are today called cross-quarter days. Based on modern calendars, these pagan celebrations fell at the beginning of November (Samhain), February (Imbolc), May (Beltane) and August (Lugnasadh) and are still observed today. Traditionally the new day began at sunset so many of these festivals span the eve and next morning.

Late October/Early November: Halloween, All Saints Day, All Souls Day
Early February: St. Brigids Day, Groundhog Day, Candlemas
Late April/Early May: Walpurgis Night, May Day
Early August: Lugnasadh, Lammas

The holiday that may be least familiar to those living in North America is Lammas. The Loaf-mass was a celebration of the first wheat harvest of the year and it was traditional to bring a loaf to church that was made from that first-harvest wheat.

While we often associate summer with the warmest season, it is wrong to assume the warmth of summer is due to our proximity to the Sun. Earth’s orbit is slightly eccentric (Kepler’s first law of planetary motion notes that planets orbit the Sun along elliptical paths), but distance is not the contributing factor to summer warmth. In fact, Earth is at its closest point to the Sun (perihelion) in early January and at its farthest point (aphelion) in early July! It is the inclination of Earth’s axis of rotation and the angle of sunlight striking Earth (see figure) that causes both the variation in daylight hours and efficiency of solar heating responsible for seasonal variations.

¹ UT refers to Universal Time and is the astronomical standard of time based on the mean solar time at the Greenwich meridian, which marks 0° longitude. To convert to the local time zone, simply add or subtract (taking daylight savings time into consideration!) the number of time zones between your location and the Greenwich meridian. For example, the Central Time Zone is 6 hours west of Greenwich so Central Standard Time  is UT – 6 hours while Central Daylight Time is UT – 5 hours. Hence, the summer solstice occurred at 11:59 p.m. – 5 hrs or 6:59 p.m. on June 20.

Image in link courtesy of Przemyslaw “Blueshade” Idzkiewicz. This work is licensed under the Creative Commons Attribution-ShareAlike 2.0 License.

Okay, it’s not exactly astronomy-related but he did record an entire album of songs about the Moon. He was also a legendary night owl who probably saw more dawns than most contemporary astronomers so I figure it’s worth noting the 10th anniversary of the passing of The Voice. Besides, it’s hard to miss the spate of movie and television re-broadcasts, the release of a stamp (see above), new DVD releases of movies and several new CD compilations that have come out this month. He also lived in the public spotlight for over 50 years and an incredible amount of ink was spilled reporting on his every alliance, every affair and every feud. Some of it was even true.

In the end, however, it was the monumental body of work that remains and is testament to what he truly loved—making music. A few years ago I looked through one of my books (Sinatra: The Man and the Music—The Recording Artistry of Francis Albert Sinatra 1939 – 1992, by Ed O’Brien and Scott P. Sayers, Jr.) to settle a debate about just how prolific he actually was and found that in one year in the mid-1950s Mr. Sinatra recorded over 70 songs and made 5 movies! That was a busy year, no doubt, but that one year surpasses the total output of many of the celebrities whose work lines the shelves of record stores or fills the shopping carts of iTunes users today. Looking at the recent compilations and boxed sets, I also think it says a lot about us as society when most living entertainers of any musical genre can’t match the current output of someone who’s been dead 10 years and whose vast archive still yields lost or forgotten treasures.

So, what’s my point? It’s fair to say that every serious music lover needs at least a couple Sinatra recordings in his or her collection. If you’re young, you may not understand their significance or relevance yet, but you will. Trust me on this. If you’re old enough to have suffered a few setbacks and broken hearts, you’ve already completed the prerequisites. In no particular order, here are a couple of suggestions:

• “I’ve Got You Under My Skin” (1956)
• “Wave” (1971)
• “What is This Thing Called Love?” (1955)
• “All I Need is the Girl” (1968)
• “Angel Eyes” (1958)

That should give any iPod a respectable representation although it’s by no means a definitive list (for more on that see my web pages starting at brentstuder.com/diversions/sinatra.html).

If you choose to wade in a bit deeper after testing the water, there are several albums—yes, albums where all the songs are thematically linked—that are essential starting points. Happily they also represent extremes on the emotional spectrum. They are Songs for Swingin’ Lovers from 1956 and In the Wee Small Hours from 1955. If you choose to acquire “I’ve Got You Under My Skin” and “What is This Thing Called Love?” you might as well get them with the original albums. Alone, they are incredible performances. In context they are sublime. All the more amazing is that each song was recorded in a single evening along with three other songs before the days of multi-track recording equipment, stereo, and every other electronic technique that moves the musician one step further from the intimacy of performing. These are essentially live performances; removing mistakes was time-consuming and not worth the effort since it involved splicing tapes and not simply moving a cursor and pressing a mouse button to mark a digital file.

Forget everything you’ve heard about Frank Sinatra, just listen.

John Wheeler, the physicist who coined the term “black hole” in 1967 died Sunday at the at of 96. Working with Neils Bohr, the Danish physicist who gave us the model of the atom and was one of the architects of quantum theory, Wheeler co-developed the liquid drop model of the atomic nucleus that lead to an understanding of nuclear fission. Later, as a professor and colleague of Einstein’s at Princeton, Dr. Wheeler established the university as a leading center in the study of general relativity—Einstein’s theory of gravitation.

It was at that time that Wheeler debated one of the more bizarre aspects of general relativity with J. Robert Oppenheimer. Namely, that a stellar core of sufficient mass would collapse into a point of infinite density and wrap the spacetime around it so severely that the resulting object would be cut off from the observable universe. At first resistant to the idea of their existence, he later accepted their mathematical inevitability and called the resulting object a black hole.

Throughout the later years of his life, Professor Wheeler wrestled with the Big Ideas that were famously debated by Einstein and Bohr—the fundamental nature of matter and how reality is revealed through the laws of quantum theory. “Today we demand of physics some understanding of existence itself,” Wheeler once said.

Further reading: Dennis Overbye’s 2002 profile for the The New York Times “Peering Through the Gates of Time”.

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.