Reasons for the Seasons

If you have been outside lately, you may have noticed a slight increase in sweaty people around you, and potentially an increased amount of personal perspiration. You may long for the chill of the lukewarm Decembers Mother Nature promises you as a Houstonian. I have found myself, on occasion, cursing the sun and its inexorable inferno. “Why, WHY can’t we all live in Southern California???”

 …But why curse the weather and your resulting ridiculous air conditioning bills when you can have WAY more fun trying to understand the heat source! The seasons, or the regular change of weather, happen because of the planet’s orientation to the sun. There is a common misconception that they occur because of the earth’s elliptical (like an oval) orbit, making it closer to and further from the center of our solar system at different times throughout our year. However, the ellipse that the earth follows is very nearly a circle, so this theory just doesn’t hold water.

GlowGlobe
Creative Commons License photo credit: etohaholic

 The real reason for the seasons is explained by the earth’s axis! Our planet is tilted at an angle of about 23.5 degrees with the perpendicular to Earth’s orbit around the sun. This means that the world is leaning slightly to one side at all times. Also, this would be a good time to note that this tilt stays the same throughout the orbit; it doesn’t swirl and swivel around as it moves along. So, the Northern Hemisphere leans slightly away from the sun in our winter, making the sun’s rays hit the earth at an oblique angle, which, in turn, makes its heat more diffuse over a large area, which equals cooler weather! The seasons are opposite in the Southern Hemisphere for the same reasons; when the North is tilted away from the sun, the South is tilted towards it, and vice versa. This alternating cycle of direct and obtuse solar rays effects other facets of life on earth; it is the reason for the changing lengths of days and the reason why some people get so sunburned in more tropical areas (which are closer to the equator, go figure.)

mars-06-crop
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 We all have first hand knowledge of what the seasons bring to the blue planet, but what about some of the others? Mercury rotates 3 times in 2 of its years and it has some of the most extreme temperature variations in the solar system, with a range of about -297 to 800 degrees F! On Mars, seasons change every 7 months and are much more severe than those on Earth. And although seasons on a gas giant don’t mean what they do on a terrestrial planet, on Jupiter, a change occurs only every seven years! Facts like these make me extremely glad to be an Earthling.

So, instead of pondering what to wear now that heather gray is out of the question, or heading out to buy new bead covers for your flesh-searing vinyl car seats, come to the Houston Museum of Natural Science and explore science in the cool, climate controlled heart of the Museum District.

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?
Creative Commons License photo credit: shioshvili

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.

Great Caesar’s Ghost!

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Creative Commons License photo credit: glacial23

Why is this month called July? The short answer is that July is named for Julius Caesar. The longer answer involves the ancient Romans’ attempt to keep track of the year.

There are two bright lights in the sky which can help us mark the passage of a year: the Sun and the Moon. Since the year is defined by the Earth’s motion around the Sun, it is best to measure it using the Sun’s apparent changing position among the stars. However, this observation is difficult, since we do not see the Sun and the background stars at the same time. The first ancient culture to do this was ancient Egypt, where the rising of Sirius just before the Sun occurred right before the annual flood of the Nile. Egyptians marked the beginning of each new year with this event.

Sirius vanishes for several months each year when Earth’s orbital motion puts the swath of sky containing Sirius on the far side of the Sun, and therefore behind the Sun from our point of view. Eventually, Earth’s continued motion brings that swath of sky from behind the Sun, allowing Sirius to rise just before dawn. To measure years by the reappearance of Sirius, then, is to measure them by the positions of the Earth and the Sun–a solar calendar.

Lune
Creative Commons License photo credit: ComputerHotline

The Moon, by contrast, is much easier to observe each clear night. Most ancient cultures, including the ancient Romans, measured months by tracking the cycle of lunar phases from new to full and back to new. Since a lunar phase cycle takes 29.5 days on average, we might expect the month lengths to alternate between 30 and 29 days. However, the Romans considered even numbers unfortunate, so their months, beginning with March, had the following lengths:

Martius 31, Aprilis 29, Maius 31, Junius 29, Quintilis 31, Sextilis 29, September 29, October 31, November 29, December 29, Januarius 29, Februarius 28

Yes, January and February were tacked on to the end of the year at first (the Romans originally did not count days at all between December and March) and later moved to the beginning. (In 153 BC, Roman consuls began to take office as of January 1, making that the start of the civil year). February was given an even number of days (making it unfortunate) so that the year as a whole would be fortunate, with an odd number of days.

Unfortunately, there is not an even number of lunar phase cycles per solar year. A cycle of 12 lunar months is 354 days long, 11 days shorter than the cycle of seasons which is about 365.25 days long. (The Roman calendar outlined above has 355 days, because the Romans preferred odd numbers). If this is not corrected, each month occurs 11 days earlier each year, compared to the start of the seasons, and months are no longer associated with the seasons. This is exactly what happens in the Muslim calendar. To keep months roughly aligned with the seasons, an extra 13th lunar month must be added to some years, as in the Hebrew calendar. Thus, the Romans periodically added an extra month, called Intercalaris, between February and March. After all, February was originally at the end of the year.

Martes 13
But is it really? Ordinary Romans didn’t
know.
Creative Commons License photo credit: kozumel

However, it was up to the Roman priests to decide which years would have the extra month. Priests often used this power to arbitrarily shorten the terms of political opponents and lengthen the terms of their friends. Also, the extra month was considered unlucky and avoided in times of crisis, such as the Second Punic War against Carthage. By Caesar’s time the calendar had become so chaotic that regular Roman citizens, especially those far from Rome, did not know the date.

Even as he waged civil war against his rivals for power in Rome, Caesar began many popular reforms as dictator. He offered citizenship to many more Romans and enlarged the Senate to provide more representation. He canceled one fourth of all debts. And he reformed the calendar so even the average Roman could know the date.

While in Egypt, he consulted the Greek astronomer Sosigenes, who told him of the Egyptian solar year of 365 days. Caesar thus decided to add ten days to the 355 day Roman calendar. First, in 46 BC, he realigned the months with their traditional seasons by using Intercalaris and by adding two more months between November and December. 46 BC thus became a 445-day year, the ‘last year of confusion.’ Beginning in 45 BC, the new 365 day Julian Calendar was to come into effect. Caesar added 2 days each to January, ‘Sextilis’, and December, and one day each to April, June, September, and November. The original long months (March, May, Quintilis, and October) remained 31 days long. February, the ‘unlucky’ month devoted to religious rituals, also remained unchanged.

The extra month Intercalaris was dropped forever, replaced by an extra leap day every four years. As Romans had added the extra month in late February, Ceasar placed leap day there as well. The year took its modern shape starting in 45 BC:

January 31, February 28 (29), March 31, April 30, May 31, June 30, Quintilis 31, Sextilis 31, September 30, October 31, November 30, December 31

Julius Caesar
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get directly down

On the Ides of March (March 15), 44 BC, Julius Caesar was assassinated. The Roman Senate felt that a fitting tribute, given Caesar’s work with the calendar, would be to name a month of the year after him. Caesar had been born on the 4th day to the Ides of Quintilis (Quintilis 12th). When that month came around in 44 BC, the Senate proclaimed that from that year on, Quintilis shall be known as Julius (or July in English).

Romans at first counted inclusively, i.e. 1, 2, 3, 4/1, 2, 3, 4/1, 2, 3…. This led them to use leap years too often. The first Roman emperor, Caesar Augustus, noticed this and suppressed all leap years between 9 BC and AD 8. In recognition of this, the Senate offered Augustus a month, and he chose Sextilis, the month when his greatest victories had occurred. Thus Sextilis became August.

As it turns out, the year is not exactly 365.25 days long; its closer to 365.2422 days. The Julian year is thus 11.8 minutes too long on average. There is some evidence that Caesar, Sosigenes et al. knew of the error but considered it insignificant. However, those 11.8 minutes add up to 1 day about every 130 years. In 1582, Pope Gregory XIII noticed that the year had gotten off by 10 days since AD 325, when the Nicene council set the rule for calculating the date of Easter. The Pope decreed that October 15 would follow October 4, 1582, and that century years are leap years only if they are divisible by 400 (thus 2000 was a leap year, but 2100, 2200, and 2300 will not be). Our current calendar, then, is the Gregorian calendar, not the Julian.

But consider this: Pope Gregory did not change the length of the year, nor the lengths of the months, nor the fact the the leap day is in February. All of these decisions by Julius Caesar remain in effect. Caesar, then, still deserves his place of honor in the year.

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