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.

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.

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.

For Moon Tracking purposes, the waxing gibbous Moon was a few degrees below Regulus (and Saturn, which is not on your star chart) tonight. Denebola and Spica (in Virgo) would be suitable stars from which to measure the Moon’s position.

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.