The Unconquered Sun: Winter Solstice Today!

At 11:47 am Central Time on Monday, December 21, the sun is overhead at the Tropic of Capricorn. This is the farthest point south at which the sun can be overhead, indicating that the North Pole is tilted as far away from the sun as possible. At the Tropic of Capricorn and elsewhere in the Southern Hemisphere, the high sun results in the longest day of the year and the beginning of summer.  Up here in the Northern Hemisphere, however, the sun is as low as possible in the sky, and we have our shortest day of the year.  This is the winter solstice for us.

Ancient peoples across America, Europe and Asia noticed that the sun got lower and lower and the daylight shorter and shorter throughout autumn.  When the sun reached its lowest point, this meant that it had stopped going away and would return–a cause for celebration.  One of the many pagan winter solstice festivals was Yule, celebrated in northern Europe.  Another was the festival of Sol Invictus (the Unconquered Sun) celebrated in Rome on Dec. 25.  Keep in mind that in antiquity the 25 was the date of the solstice itself–the sun which had stopped going away and begun to return was ‘unconquered.’  Due to the imprecision of the Julian calendar, the solstice had shifted to Dec. 21 by the year 325 A.D., when the Nicene Council convened. Since Pope Gregory’s reform was calculated to restore the equinoxes and solstices as of the Nicene Council, the winter solstice is now on Dec. 21 (occasionally Dec. 22).

No one in antiquity knew what date Jesus was born.  For one thing, many of the early Christians rejected all birthday celebrations of any kind as a pagan ritual.  Even had folks wanted to observe Jesus’ birth, the lunar calendar used in Israel at the time would complicate the choice of date.  The Chronology of 354 is the oldest document to list Christmas as a festival.  When the church selected Dec. 25 for this festival, it was probably because late December was already a festive time across the Roman Empire.

Sunset over Chicago
Creative Commons License photo credit: kevindooley

Although today is the shortest day of the year, you may have already noticed that sunset is a few minutes later now than at the beginning of the month.  In June, the North Pole was tilted towards the sun as much as possible.  Since then, the North Pole has tilted a little more away from the sun each day.  Days have been getting shorter because each day the sun has taken a slightly lower path across the sky.  Sunrises have been getting earlier and sunsets have been getting later.  By late November the sun had already gotten about as low as it is now.  As the day to day difference in the sun’s height gets smaller, another effect begins to dominate.

Earth’s orbit is not a circle; it is an ellipse.  The orbit is almost a circle, however; the eccentricity (out-of-roundness) is just 0.016, where 0 is a perfect circle and 1 a parabola.  This is enough of a difference to bring Earth slightly closer to the sun in early January and take it slightly farther away in early July.  Therefore, Earth is now beginning to make its closest approach to the sun (called perihelion).  As a result, Earth is speeding up on its orbit.  This causes sunrise, local noon, and sunset to occur just a little later each day.  By the 21, sunset will occur at 5:27 pm, as opposed to 5:22 pm on Dec. 2 (the actual date of the earliest sunset).  Sunrise, however, will have shifted from 7:00 am to 7:13 am.  Thus, that days are still getting shorter even though the sunsets are a little later.

Many people assume that the winter solstice should be the coldest day, but this is usually not true. January is usually colder.  Although days get a little bit longer and the sun a little bit higher beginning Monday, it takes quite awhile for this to add up to an appreciable difference in the Sun’s height in the sky and in the amount of light and heat reaching the arctic.  Frigid air masses continue to form in the arctic and move across the Northern Hemisphere throughout January, February, and often March.  Although the sun is higher in those months than in December, the air can be just as cold if not colder.

Equinox 2

Hopefully, we are getting all of our cloudy, gloomy weather over with , and the solstice will be sunnier. If so, you can join us on the museum sundial at noon on Monday, Dec. 21 to observe the sun!  This is one of the Fun Hundred events celebrating our 100 anniversary here at the Museum.   On top of the gnomon on our sundial is a silver ball with three sets of holes, which allows the sun to shine through pairs of lenses near each solstice or equinox.  To account for cloudy weather, our gnomon’s holes are big enough that the sun aligns with them for a few days before and after the exact equinox or solstice date.  The holes aligned with the winter solstice are so big that you can still project the sun’s image through them deep into January!  If the weather does not cooperate Monday, you can come and observe the sun on our sundial near noon on any day in the next few weeks.

Go Stargazing! February Edition

Venus continues to dominate the western sky on February evenings.  Look west-southwest right as night falls for the brightest thing there except for the Moon, which is nearby at the end of the month.  Venus, on its faster orbit, is coming around to our side of the Sun (and will pass us in March).  Therefore Venus is about as bright as it can be this month. 

Saturn is now a late evening object, rising in the east by 9 p.m. now and by 7 p.m. at month’s end.  Early next month, it will be opposite the Sun in the sky and be visible all night long. 

Mars and Jupiter are lost in the Sun’s glare much of this month.  They form a close conjunction on the morning of February 17, but the pair rises right as the twilight begins to brighten the sky that morning.  Mercury joins them later in the month. 

Dazzling Orion rises in the east, reminding us that winter is here.  His belt points up to Aldebaran, the eye of Taurus, the Bull.  The Dog Stars Sirius and Procyon are below Orion in the east.  Sirius is the brightest star we ever see at night.  To Orion’s left as he rises are two stars of similar brightness less than five degrees apart.  These are Castor and Pollux, marking the heads of Gemini, the Twins. 

Moon
Creative Commons License photo credit: Fingerz


Moon Phases in February 2009:

1st Quarter             February 2, 5:12 pm
Full Moon               February 9, 8:49 pm
Last Quarter           February 16, 3:38 pm
New Moon             February 24, 7:35 pm

The Full Moon of February 9 almost enters the Earth’s shadow.  It does enter a region of space called the penumbra, in which the Earth partially blocks the Sun.  This event is thus called a penumbral eclipse.  However, it begins right as the Moon is about to set here in Houston.  What’s more, a penumbral eclipse is only a slight darkening of the Moon, barely noticeable in the darkest skies.  In the morning twilight of February 9, you won’t see much of any difference in the Moon as it sets.

IMG_0959
Creative Commons License photo credit: S1lvers Family


Punxsutawney Phil saw his shadow on Groundhog Day, meaning six more weeks of winter.  What does this have to do with astronomy?  Well, Groundhog Day occurs about halfway between the winter solstice and the vernal (spring) equinox.  It therefore occurs near one of the cross-quarter days, called Imbolc by the Celts.  The other three are at the beginning of May, August, and November, and they represent points halfway between the quarter days, which are the equinoxes and solstices. 

Since we don’t want Phil to see his shadow, we clearly don’t want sunshine on February 2.  If this seems ‘backwards,’ consider that there is another day when we don’t want sunshine or warmth-Christmas. 

Houstonians still fondly recall our Christmas Eve snowstorm of 2004, while a similar snowfall on January 24 would have been much less romantic.  The French have the saying, “Christmas on the balcony, Easter by the fireplace.”  Early pagans considered the winter solstice and Imbolc symbols of winter itself.  If these days were appropriately wintry, with clouds and cold, then it was a sign that all was in order.  Winter, which was happening on time, would end on time.  However, if these days were not appropriately wintry, then something was wrong, and a ‘remedial’ winter would need to occur in springtime.  The traditions of the winter solstice and Imbolc were transferred to Christmas and Groundhog Day, respectively.  Thus, sunny weather (with shadows) is a bad omen on Groundhog Day, while cloudy weather (no shadows) is a good omen. 

Go Stargazing! December Edition

The ‘main event’ of December evenings occurs in the southwest at dusk, where you can watch Venus pull away from Jupiter.  Look southwest right as night falls for the two brightest things there except for the Moon.  The brighter one low in the southwest is Venus, which outshines everything else in the night sky.  Jupiter is the dimmer of the two, although it still outshines all the stars we ever see at night.  Venus and Jupiter begin the month about 2 degrees apart (your finger at arms length blocks about 1 degree.)  However, Venus will extend that gap quite noticeably each night, until it appears high above Jupiter on December 31. 

Mercury emerges from the Sun’s glare in time to form a nice pair with Jupiter on New Year’s Eve.  As you prepare to ring in 2009, take a moment to look at Mercury just to Jupiter’s left in late twilight.  That same night, the Moon will be near Venus. Saturn can be found high in the south at dawn.  Mars is lost in the Sun’s glare this month, and will remain out of sight into 2009.  It is directly behind the Sun (in conjunction with the Sun) on December 5.

Orion no céu
Creative Commons License photo credit: giumaiolini

The enormous Summer Triangle, consisting of the stars Deneb, Vega, and Altair, sets in the west.  The Great Square of Pegasus is overhead at dusk.  The star in its upper left hand corner is also the head of Andromeda.  Facing north, you’ll see five stars in a distinct ‘M’ like shape—this is Cassiopeia, the Queen.  Her stars are about as bright as those in the Big Dipper, and she is directly across the North Star from the Dipper.  In fall and early winter, while the Dipper is low and out of sight, Cassiopeia rides high.

Dazzling Orion rises in the east, reminding us that winter is on the way.  His belt points up to Aldebaran, the eye of Taurus the Bull.  By 9 pm tonight (7 pm by New Year’s Eve), the Dog Stars Sirius and Procyon will have risen below Orion in the east.  Sirius is the brightest star we ever see at night. 

Moon Phases in December 2008:

1st Quarter         December 5, 3:25 pm
Full                     December 12, 10:38 am
Last Quarter        December 19, 4:30 am
New                    December 27, 6:22 am

At 6:04 am on Sunday, December 21, the Sun is directly overhead at the Tropic of Capricorn, meaning that the North Pole is tilted as much as possible away from the Sun.  This is the winter solstice.  For people in the Northern Hemisphere, December 21 has less daylight and more night than any other day of the year. 

Sunset at Appalachian Trail
Creative Commons License photo credit: Pardesi*

However, the earliest sunsets occur on December 1 and 2.  We are already close enough to the solstice that the Sun’s apparent path across the sky on December 21 is only slightly lower than on any other day this month.  Meanwhile, Earth is about to make its nearest approach to the Sun, called perihelion, in January.  As a result, the Earth is speeding up.  The effect isn’t much (Earth’s orbit is nearly circular), but it’s enough to make both sunrise and sunset a little later each day this month and next.  With the Sun’s apparent height in the sky not changing that much in December and January, the small effect of Earth’s acceleration near perihelion dominates.  Since most of us sleep through sunrise and witness sunset, the days seem be slightly lengthening between the beginning of the month and the 21st, although they are actually getting slightly shorter.

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.