A Trick or a Treat?

In less than a week, people all over the country, including right here at our museum, will be celebrating Halloween. Perhaps your workplaces and schools are already festooned with ghosts, skeletons, graveyards, and the like.  If you stop and think about it, you may wonder just how it is that we came to celebrate by trying to disguise ourselves or by trying to frighten people.  Is this a trick or a treat?

Picket fence and yellow trees
Creative Commons License photo credit: joiseyshowaa

The short answer as to why we celebrate this time of year with images of death is that we are in the middle of autumn, the season when nature itself is dying.  To fully understand why we celebrate Halloween when we do, we must fully understand the seasons.

Earth orbits the Sun with its axis pointed at the North Star, Polaris. As a result, its axis is tilted by about 23.5 degrees with respect to its orbital plane.  This tilt, combined with Earth’s revolution around the Sun, causes the seasons.  If the North Pole leans towards the Sun, the Sun is higher in our sky and we get more direct sunlight.  Also, daytime is longer than nighttime.  As the North Pole begins to tilt away fron the Sun, the Sun appears lower and lower across the sky, and daytime gets shorter and shorter.  Eventually, the slanted-in solar rays and short days bring about winter.  Very cold air masses form in the darkened Arctic and begin to move south, some of which can even reach Houston.

Keep in mind that the Earth’s axis does not tilt back and forth; it points at Polaris the whole time.  In June, the North Pole is leaning towards the Sun, but by December, the Earth’s motion has carried it to the other side of the Sun.  The North Pole, still tilting the same way, now leans away from the Sun.

A common misconception is that the Earth is closer to the Sun in summer and more distant in winter, and that is what causes our seasons.  In fact, Earth’s perihelion (closest approach to the Sun) occurs just after the new year (January 1-4), while aphelion (greatest distance from the Sun) occurs around the 4th of July.  Earth’s orbit is an ellipse, but the Earth-Sun distance does not change by enough to affect our seasons.

where are you?
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In the cycle of seasons, there are four points of note.  At the March equinox, neither pole is tilted toward the Sun and the Sun is directly overhead at the equator.  The is the vernal (spring) equinox for us and the autumnal (fall) equinox for folks south of the equator.  At the June solstice, the North Pole is tilted as much as possible towards the Sun, and the Sun is overhead at 23.5 degrees North (the Tropic of Cancer).  This is the summer solstice for us and the winter solstice in the Southern Hemisphere.  At the September equinox, once again neither pole tilts toward the Sun, and the Sun is again overhead at the equator.  This is our fall equinox and their spring equinox.  At the December solstice, the North Pole is tilted as much as possible away from the Sun, and the Sun is overhead at 23.5 degrees South (the Tropic of Capricorn).  This is the winter solstice for us and the summer solstice below the equator. 

We generally think of these points as the beginning of spring, summer, fall, and winter, but it doesn’t have to be that way.  After all, nothing magically happens with our weather on these dates.  We could just as well consider these points the midpoints of each season.  In that case, the seasons would begin and end at points roughly halfway between the equinoxes and solstices, in early February, May, August, and November.  If the equinoxes and solstices are ‘quarter days,’ the points halfway between them become the ‘cross-quarter days.’

The ancient Celts of Europe appear to have divided their year in precisely that way.  Gauls living in what is now France used a calendar of twelve lunar months with a 13th month added every 2.5 years (similar to the Hebrew calendar today).  Their two most significant months were Gamonios (lunar month corresponding to April/May ), which began the summer half of the year, and Samonios (lunar month corresponding to October/November) which began the winter half of the year.  Julius Caesar noted that daytime followed nighttime in Celtic days.  By extension, the dark (winter) half of the Celtic year preceded the light (summer) half, making Samonios the start of their new year.

The Celts in the British Isles (Irish and Scots) also had festivals aligned with the cross-quarter days.  In early February was Imbolc (or St. Brigid’s day).  Weather predicting traditions of this day are preserved in our current Groundhog Day.  Traditional May Day celebrations are similar to those of the Celtic BeltaneLughnasadh, in early August, marked the start of the harvest. 

'' The Sentiment of Light''
Creative Commons License photo credit: jdl_deleon

The most important, though, was Samhain (pronounced ’sah win’, not ‘Sam Hane’, due to rules of Gaelic spelling), in early November.  This three-day festival marked the beginning of the winter half of the year and the start of the whole year, like Gaulish Samonios.  It was the close of the harvest opened at Lughnasagh, and the time for culling excess livestock.  At this time, the veil between the living and the world of the dead was considered thinner than usual, and people looked forward to meeting and communing with ancestors and relatives who had died.  A ‘dumb supper‘ was set aside for departed relatives.  To scare away unwanted spirits, people dressed in frightening garb.  Note that these spirits were considered unpredictable and possibly mischievous because they were not the familiar ancestors–not because they were particularly evil.  Divination was also practiced at this time, as people sought to predict whom they would marry or how many children they would have. 

Doing the math, you’ve probably figured out that Halloween is not quite halfway from the equinox (September 22) to the solstice (December 21).  But remember, the Celts used a lunar calendar.  They celebrated their festivals on a certain phase of the Moon, possibly full moon, occurring nearest the cross-quarter day.  Upon the adoption of the Julian calendar, which was not strictly lunar, the festivals were moved to the beginning of February, May, August, and November, although this meant they were no longer exactly on the cross-quarter days. 

Saint
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The Wandering Angel

In the eighth century AD, Pope Gregory III moved the church’s commemoration of the souls in heaven (All Saints’ Day) from May 13 to November 1.  Another name for All Saints’ Day is All Hallows Day.  (’Hallow’ is an older term for ’sanctify’ or ‘make holy.’  Think of ‘…hallowed be thy name’ from the Lord’s Prayer).  The next day became All Souls’ Day.  The day before All Hallows Day or All Saints’ Day is All Hallows Eve, or Halloween.  The traditions of Samhain, with its similar focus on honoring the dearly departed, were a natural fit for All Hallows Day and All Hallows Eve.

Halloween, then, is ultimately just one expression of the human need to come to terms with death as a natural occurence and to honor those who have gone before.  In the season of the fall of the leaf, with the Sun taking a slightly lower path across the sky each day, the natural world is going through its own ‘death,’ providing a perfect context for our own activities.  We can therefore think of Halloween itself as a treat, not a trick.

I wish everyone a Happy Halloween, with many more treats than tricks.

Happy New Year!

The New Moon of Monday, September 29, is an important one to many people of the world. In the Hebrew Calendar, it marks Rosh Hashanah (literally, ‘head of the year’) which is the beginning of year 5769. On Tuesday night, September 30, Muslims across the world will see the first slender crescent of this lunar cycle. That will mark the end of Ramadan and the beginning of the next month, Shawwal. 1 Shawwal is ‘Eid ul Fitr, one of the greatest holidays in the Islamic calendar. This week, then, is a good time to think about the Moon, why it’s here, how it orbits, and how we have used it to measure time.

Moon and stars
Creative Commons License photo credit: joiseyshowaa

Unlike our months, Hebrew and Islamic months begin with the New Moon. Because twelve lunar months add to only 354 days, less than the 365.25 day solar year, an extra month is occasionally needed to keep the months roughly aligned with the seasons. In a 19 year cycle, years 3, 6, 8, 11, 14, 17, and 19 have the extra month. The year that is ending, 5678, is number 11 in its cycle and was a leap year.

Interestingly, the Jewish year has two ‘beginnings’. Tishrei (the month which begins now) is the first month of the civil calendar, and the month where 5678 becomes 5679. However, it is actually the seventh month of the religious calendar, which begins at Nisan (the month of Passover).

The Islamic calendar functions slightly differently. Its months begin with the first visible crescent low in the west at dusk, which is not with the actual New Moon. Keep in mind that at New Moon, the Moon is in line with the Earth and Sun, and the entire near side of the Moon has nighttime (and is therefore dark). The New Moon is visible, therefore, only if it blocks the Sun during an eclipse.

Since this New Moon occurs early Monday morning, the 29th, we expect it to be visible by Tuesday evening, the 30th. Observant Muslims, then, will continue to fast in daylight hours Monday and Tuesday. Upon seeing the Moon Tuesday night, they will know that Ramadan has become Shawwal, and they may break their fast on Wednesday.

Due to early controversy as to which years would have it, Muhammad outlawed the 13th month that kept Islamic months tied to the seasons. As a result, Ramadan (and each other month in that calendar) begins 11 days earlier each year according to our Gregorian calendar.

The Moon is Earth’s only natural satellite, orbiting our planet once every 27.3 days. However, a cycle of moon phases (say, from New Moon to the next New Moon), takes 29.54 days. This is because the Earth itself is moving during each 27.3 day Moon orbit. Since it is much easier to observe the Moon’s changing phase cycle than to observe the Sun directly, the 29.54 day phase cycle was the basis of many ancient calendars. Words for ‘moon’ and ‘month’ are related in English and are identical in many other languages. There is some evidence that our word ‘moon’ is ultimately related to an Indo-European word for ‘measure.’ Given how long we’ve measured time by the Moon, it is easy to take its presence for granted.

Released to Public: Jupiter Montage (NASA)
Creative Commons License photo credit: pingnews.com

However, our Moon is quite remarkable in several ways. Moons in our solar system are generally much smaller than the planet they orbit. Jupiter and Saturn, for example, are about 25 times bigger across than their biggest moons. Earth, though, is only 3.67 times the diameter of our Moon. Also, moons usually orbit in the same plane as their planet’s equator. Our Moon, though, orbits within about 5 degrees of Earth’s orbital plane, called the ecliptic, which is not the plane of the equator since Earth is tilted 23.5 degrees on its axis.

This leads most astronomers to believe that the Moon did not form with the Earth, but is the result of a collision with with an object roughly the size of Mars. According to this theory, the impactor (sometimes called ‘Theia’) struck a glancing blow on the Earth and was completely destroyed, and the Moon formed from the debris of Theia’s and Earth’s mantles.

This impact is what left Earth with a Moon much larger than what a planet Earth’s size would normally have, and left that Moon near Earth’s orbital plane (where the impact occured). Our relatively big moon has crucial effects not only on our tides, but also on the stability of Earth’s tilt.

Earth’s orbital tilt of about 23 and a half degrees as it goes around the Sun causes the seasons. The axis precesses, describing an aparent circle roughly every 26,000 years, but the amount of tilt (obliquity) stays nearly the same. Because the Moon acts a counterweight, the obliquity varies only between 22.1 degrees and 24.5 degrees over about 41,00 years (we are now at 23.44 degrees and decreasing). Even this orderly variation, called the Milankovitch cycle, is enough to influence our Ice Ages. Imagine the impact on Earth’s climate if there were no Moon, and the obliquity varied chaotically. This is exactly what happens at Mars, where the tiny moons Phobos and Deimos are not massive enough to influence Mars’ tilt.

Public Domain: Apollo 8 Looks at the Moon (NARA/NASA)
Creative Commons License photo credit: pingnews.com

One thing our Moon does have in common with most others is that it orbits the Earth and rotates on its own axis at the same rate. This is called ‘synchronous rotation’ and it occurs because the Moon is not exactly uniform in composition. From the time the Moon formed, the slightly heavier side was attracted to the Earth. Over time, this effect de-spun the Moon until it attained synchronous rotation. The Moon’s gravitational attraction also de-spins the Earth, although much more slowly as the Moon is less massive. As it does so, the Moon moves slightly farther from the Earth (just over 3 cm per year). The Moon is now 1.5 meters farther away that it was when Apollo astronauts went there. Don’t worry, though, by the time the Moon is far enough away to escape, the Sun will have become a red giant and swallowed both Earth and Moon anyway.

What is the shape of the Moon’s trajectory around the Sun? Perhaps not what you’d expect.

So, I encourage every one to watch for the reappearance of the Moon in the evening sky this week, even if you aren’t celebrating a New Year or an ‘Eid. The Earth’s companion gives all of us something to appreciate.