Archive for December, 2009


Note: If you have not read the earlier posts about the Star of Bethlehem, you might want to begin with the December 15 post, His Star in the Easts.

I think you can see how astrology could have played a role in the coming of the magi. I hope you can also see that it’s not a way to guide your life. The stars and planets move on their courses according to laws (of gravity) which are not the same as what governs free persons. How people interpret the cosmic meaning of these motions must often be called lawless.

To give you a flavor of the whole matter and then some further interesting details, here is a list of the relevant (and irrelevant) celestial events of the years 7-1 BC.

Beginning with 7-4 BC:

  1. In 7 BC, a triple conjunction (three meetings) of Jupiter and Saturn. This conjunction was discovered (by research) by Kepler in 1614. All three conjunctions took place in the constellation Pisces, one in May, one in September, one in December. The stars were at least one degree (one Moon-width) apart, an asterism, not a “star.”
  2. In February of 6 B.C. Jupiter, Mars Saturn came within 8° of each other, also in Pisces, and also an asterism.
  3. On April 17, 6 BC, the Sun, the Moon, Venus, Jupiter all gathered in constellation Aries; a gathering, not a conjunction. One meticulous researcher claims that Aries, not Pisces, is the constellation of Judaism, and this is the “star.”
  4. A supernova in Capricorn, in 5 BC, was recorded by the Chinese and would have been visible to all sky-watchers. It was visible for 70 days, but 40° off the ecliptic, so not the first thing you would see looking up, and not necessarily long enough to be visible through an entire journey from Persia to Jerusalem…
  5. There were comet sightings in 5 and in 4 B.C. but these not likely candidates for “the star” because comets are generally seen (by the superstitious) as negative.

From 3-1 BC

  1. March of 3 BC is the probable birth time of St. John the Baptist; no celestial marker has been suggested for this.
  2. May 19 of 3 BC, Saturn and Mercury came close only 40’ apart (40’ means 40 minutes of arc, which means 40/60 of a degree or 2/3 of a degree.)
  3. June 12 year 3 BC, Saturn and Venus were only 7.2’ apart (7/60) of a degree. This might have been a visual union, though Saturn is not very bright.
  4. August 12 of the year 3 BC, there was a conjunction of Jupiter and Venus in Leo, 10’ apart. They were visually indistinguishable as both are bright planets, so it must have been very conspicuous and beautiful. Note also that Jupiter was close to Regulus, only 19.4’ or 1/3 of degree away. And this took place in the early morning, so “in the easts.” The magi – or any astronomer/astrologer — would have noticed it.
  5. September of year 3, Jesus was born.  Ernest Martin argues for the Feast of Trumpets, Rosh Hashanah, which is regarded as a commemoration of the first day of Creation, and which is the day of the “last trumpet.” He says that this was the New Moon and that the celestial arrangement of that day is found in the book of Revelation 12:1-3. The New Moon of that month appeared on the 11th. (In case you’re wondering, the second edition of Martin’s book was 1991. I think the first was 1976, but I’m not sure. I have read parts, but not the whole book.)
  6. On the 14th of September in 3 BC, Jupiter had a conjunction (19.8’) with Regulus; the following February 17th, they had a second conjunction, 51’ apart; on May 8 of year 2, they had a third conjunction, 43.2’ apart
  7. June 17 of 2 BC, Venus and Jupiter met in Leo 6” apart. For some, this is an irresistible date for Jesus’ birth. (6” is six arc seconds, one arc second being one 60th of a minute while an arc minute, remember, is 1/60th of a degree. Give thanks for the decimal system.) This conjunction was visible in the early evening, therefore seen as “west” — at precisely 6:11 p.m. – a time therefore suggested as Jesus’ birth hour by some. (But not compatible with the consideration of Zacharias’ service.) These two planets could not possibly have been distinguished from each other, naked eye, at their maximum conjunction. Jupiter signifies kingship; Venus motherhood, and Leo is considered (by some) to be the head of the zodiac. It is the royal constellation, whose brightest star is Regulus = king. Irresistible!
  8. August 27, 2 BC, there was massing of Jupiter, Mars, Venus, and Mercury in Leo. This was also the 750th anniversary of Rome’s putative founding
  9. Then Jupiter stopped its wandering motions on December 25, 2 BC. This is the normal stop preceding a retrograde motion. It stopped in Virgo for six days. Since this was the time of the equinox, the Sun was also “standing still” so that if the magi turned south from Jerusalem on this day, then Jupiter lay before them in the southern sky — in the middle of Virgo (the Virgin) — shining directly over Bethlehem.

Cool beans.

So how did they find his house?

I don’t know. We draw pictures of the star shining right over the stable, but everyone agrees that the magi came some time after Jesus’ birth. He was in a house, presumably in town; no natural star or planet could have shown where. So the angels might have told them, or some friend of Zacharias and Elizabeth or some protégé of Simeon might have learned of their quest and helped them. God did not allow them to cross the desert for 3 months in search of the Messiah and then drop them. He’s a loving father!

Finally, as you know, Herod had all the Bethlehem boys under 2 years of age killed, not just all infants, so he must have thought that Jesus might be over a year old already when the magi came; surely also he thought he was safely under two years old.

Note: The eclipse of Jan 10th of the year 1 BC makes sense as the one for Herod’s death because he murdered several rabbis just before he died, and then, when there was an eclipse of the Moon, it was said that the Moon was red with the blood of the murdered rabbis. The eclipsed Moon does not disappear from view because earthshine keeps it visible, but it takes on a strikingly somber red color. This is one reason for thinking the eclipse at Herod’s death was total, not partial as in 4 BC.


And that closes our discussion of the Star of Bethlehem. What a strange mix of history, scripture, astronomy, and astrology! But the threads can be teased out and woven into a harmonious and intriguing account which does not, I think, offend theology.

He came. Emmanuel!

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December 25

If you have not been following these posts about the Star of Bethlehem, you might want to begin with His Star in the Easts and then His Birthday.

From 7 to 4 BC

The years between 7 and 4 BC were unusually crammed with events of astrological import. Let me give you an idea of the material that astrologers work with:

In 7 BC, there was a triple conjunction between Jupiter and Saturn. That is, these planets moved into such positions, partly due to retrograde motion, that they came fairly close to each other, not once, but three times, in May, September, and December. Kepler first discovered this and, believing that the constellation Pisces was special to the Jewish people, concluded that the triple conjunction was the birth star of Jesus, one for his birth perhaps, and the other two to “guide” the magi. Actually, they were not close enough to appear as a single star, but they would have been a pretty asterism, so it was an interesting idea.

In 6 BC, Jupiter, Mars, and Saturn lined up in Pisces. Interesting, unusual, but again, not really close enough to look like a single bright star.

Also in 6 BC, the Sun, the Moon, Venus, and Jupiter all gathered in the constellation Aries, which, according to one researcher is “really” the constellation of the Jewish people, and that makes this the right astrology. But if the Sun and the Moon are that close to each other, the Moon can only be just a sliver on the horizon, and the planets cannot have been at their most spectacular either — except to the eyes of an astrologer.

The Chinese record a supernova in 5 BC. It would have been visible worldwide; would that have been noticed by the magi? Maybe. Actually it was only visible for 70 days, so it would not likely have been visible through to the end of a journey from Persia.

There were some comets in 5 and 4 BC; most say that comets had a negative connotation, so they shouldn’t be the answer.

And anyway, we already said we wanted 3 BC, right? But I wanted to say that astrology, left to itself, meanders.

From 3-2 BC

So what was up in 3 and 2 BC?

Well, Saturn and Mercury came within half a degree in May of 3 BC. Half a degree is about half the width of the Moon; it would be pretty, but not the appearance of a single star and Saturn and Mercury are not bright. Only an astrologer could note.

And then Saturn had a much closer approach to Venus in June; this time it was a tenth of a degree, much more impressive, visually a single star; Saturn is not very bright, but Venus is.


But on August 12 of 3 BC, we hit the jackpot. Maybe.

Jupiter, king of planets, meets Venus the morning star, the fertility and the “mother” star, — in the constellation Leo the Lion, as in: the Lion of the House of Judah. Yes, the Jews really did see this constellation as their very own lion. Jupiter and Venus are each bright seen alone, and this conjunction was within a sixth of a degree, so their light would merge; furthermore they were attractively close to Regulus, the bright “king star” in Leo. (Regulus means king.) On September 14, in fact, Jupiter moved on to a “conjunction” with Regulus. The star and the planet were about a third of a degree apart, so not really one star, but the story does not end there. After leaving Regulus for a few months, Jupiter started its annual retrograde motion, backed up and returned for a second conjunction in February of 2 BC and then, after completing its retrograde, turned around for a third conjunction in May. None of these were conjunctions of merged light, but they were striking asterisms and it might have been thought that the king planet was circling and “crowning” the king star in the constellation of the Lion of Judah.

Cool, eh?

And after that, there was an even closer conjunction between Jupiter and Venus on June 17 of 2 BC and, this time, they were too close to be distinguished even by the best of human vision. Six seconds of arc. One second of arc is one 60th of one 60th of one degree. Go figure; you can’t see it.

Before you leap to any conclusions, however, it’s worth knowing that this year was the 750th year since the founding of Rome, and the 25th anniversary of the reign of Caesar Augustus, and the year he was proclaimed Pater Patriae. Given that Venus was supposed to be the mother of his family, can you be in any doubt as to how his court astrologers interpreted all this king-crowning stuff? And Leo was supposed to be the protector of Rome. The astrologers were on a roll. And on August 12, when Jupiter, Mars, Venus, and Mercury met in Leo, they had a great bash in Rome, certain that kingship, war, fertility, and whatever Mercury stands for were all united in affirming their Augustus Caesar.


The magi would not have known about this competing interpretation, however, nor would they have cared. Daniel had prophesied the birth of Jesus sometime about this time, and Zoroaster, founder of the Persian religion, had prophesied the coming of a king who would raise the dead and bring a reign of peace – and that he would come from the family of Abraham. The magi had every reason to believe that the time was ripe; it would have been natural for them to set out for Judaea to find the newborn king.

If they set out in late August, perhaps on the propitious morning of August 27, when Venus, Jupiter, Mars, and Mercury gathered in the constellation Leo (though not as a single star), they would have arrived in Jerusalem a few months later, in late November or in December. After discussing matters with Herod, who “inquired closely,” — you better believe that Roman procurator questioned very closely! – they learned of specific prophecies about Bethlehem and set out again, south towards Bethlehem.

And lo! What did they see? No conjunction, but the king planet, bright Jupiter, the character from all the other kingly conjunctions, now burned in the southern sky in the middle of the constellation Virgo. Virgo the Virgin. It was preparing for another retrograde, and, as it does in retrograde, it stopped its wandering for a few days. In fact, it stopped for six days, starting in the dawn of December 25.

“And lo, the star that they had seen went before them and “stopped” over the place where Jesus lay.”

How do you like that?

We know they came. We know they came partly in response to considerations of astrology. Maybe they really got there December 25 of 2 BC when Jesus was, by our earlier calculations, just over 15  months old.

One more post coming on this topic — Emmanuel! God with us.

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When Jesus was born

Before we say any more about the stars, let me share with you a very interesting argument for the birthdate of Jesus. This is based on an article by Susan Carroll, which you can easily find online, and which is, in turn, based on a book by Ernest Martin – whose entire book is posted online. I share it because my intent in discussing the star of Bethlehem is to put it into in a right context with theology and astronomy, and I don’t want the arguments from astrology to overwhelm the topic.

Scripture says nothing about Jesus birthdate, but it does say that he was conceived six months after John the Baptist, and about St. John’s date of conception, we have the clue of his father’s priestly duties. These duties arose twice a year in a cycle which began with the springtime month of Nisan, which floated a bit (the Jewish calendar was not exactly a math exercise) but was something like the end of March; then Zachariah’s turn came eight weeks into the year (because he was the division of Abijah — Luke 1:5); therefore in June. Nine months later, John was born in March and then Jesus was born six months later. Or possibly John was conceived during Zachariah’s second season of duty, putting the birth of Jesus six months later…?

Which time was it, and what year? Can we know?

Well, these questions are related, because the one clue we do have about the birth of Jesus is that there was a census “of the whole world” [i.e. the whole Roman world] going on when he was born. And we do know when Rome held its census: every five years, including 8 BC and 3 BC. The first is too early. The second has long been left out of consideration because it was thought that Herod had died in 4 BC and that would make 3 BC too late for him to be making trouble about the visit of the magi. So everything was unmanageably tangled and there seemed to be no way to sort things out consistently.

The death of Herod is given by the famous historian Josephus, however, and while most copies of his work record Herod’s death as taking place in 4 BC, at least some older copies record it as taking place in 1 BC. It may be that this is a simple copying error and that Herod’s death was in 1 BC. Interestingly, one of the principal clues for Herod’s death date is itself astronomical because the same Jewish historian (Josephus) says there was a lunar eclipse right after his death. That would be memorable, and we can easily calculate the dates of eclipses: there was a total eclipse on January 10 of 1 BC. There was also a partial eclipse in 4 BC, but it does not satisfy various other elements of Josephus’ description.

So it is reasonable, after all, to look at 3 BC, the year of the census, for the birth of Jesus, and now it’s easy. John was born in March, as I said, and Jesus in September. John was not born at the later time of the year (based on Zacharias’ second tour of duty) because that would put Jesus’ birth into March of 2 BC, which is not the year of the census.

Furthermore, we conclude that the birth of Jesus would have been early or middle September, because in late September, there was the big feast of Tabernacles, and that would have brought new crowds to the environs of Jerusalem, including Bethlehem. Since scripture says that Bethlehem was crowded because of the census, not because of the feast, the birth must have been at a different time from the week-long feast that started September 26 and ended October 3.

Notice that this is not an astrological argument. It is scriptural and historical. It does not give a specific date, but it yields a month and a year with some clarity. Other sources have come to more specific dates, but these are astrological in nature, and, left to themselves, the astrologers have been quite divided in their conclusions.

Under all the Stars

Our Father made all the stars in the sky, and it wheels around every 24 hours so that all the stars lie on the meridian above us during some part of each day. We (including Jesus) live and die under all of them. Should Jesus care under which ones he was born? I think not.

Yet it does seem that God used the stars to bring one group of men to the home of his early childhood. And while the Jews were firm in their monotheism and never worshipped things in the sky, Genesis 1 does say that the stars were given “as signs to mark seasons and days and years,” so that some things might be marked in this way. What is essential is that we believe in our freedom, in the freedom of God Himself, (He is not governed by the stars), and in the freedom of Mary who agreed to her vocation. She could have said no. Our destiny is not in our stars but in the meeting between our hearts and the heart of God, under all the beautiful stars that shine.

That said, would you find it charming to discover that December 25 had some significance in this whole story?

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In the Easts

“For we have seen his star in the easts and are come to worship him.” So said the wise men to Herod close to the end of their journey. What star was this? And why is this phrase, which used to be translated “in the east,” now being translated “at its rising”?

The scriptural phrase is actually “in the easts” the plural of east being an idiomatic reference to the morning sky, the sky of almost all astronomy’s risings. Evidently whatever display the magi “followed,” it appeared in the eastern or morning sky, and that alone must make it clear that they weren’t literally “following” the star because in that case they would only have gone east. A quick glance at any map or globe will show you that there are very few places from which anyone could travel “east” to Bethlehem, and none were centers of knowledge at that point in history, though I suppose England already had its Stonehenge in ruins, and Ireland had similar evidence of celestial researches.

But seriously, no, the magi were not from west of Bethlehem.

Note that any persistent astronomical event (anything but a comet or a shooting star) first becomes visible in the east, simply because of the turning and orbit of the earth. On a given night, of course, things appear all over the sky as it becomes dark, but wherever the stars and planets are this evening, they were farther east last night, still farther last week, and farther yet last month; for a given cycle of viewing, astronomical displays begin in the eastern sky.

Exceptions would be planets in retrograde motion and conjunctions of planets, which are so short-lived  – aaah! Precocious readers are instructed to be patient and reasonable. In any case, this star was seen in an eastern rise.

Why they came

Now, bear in mind that the magi belonged to a time before astronomy and astrology were separated into a natural science on the one hand and a body of fortune-telling superstitions on the other. And whatever they saw, star or planet, the magi chose to travel west to Judea because their study of astrology pointed that way. As they understood it, this “star” had to do with a child born among the Jewish peoples who lived west of them, a newborn King, with a capital ‘K’. Though traditionally pictured as one Chinese, one African, and one white or Mid-Easterner, thus representing the revelation of Jesus to all nations, undoubtedly the magi came from Persia or Babylon, where the Jewish people had lived in captivity long enough to leave a permanent mark and to be remembered as a nation with a significant relationship to the destiny of mankind. The prophet Daniel in particular had been “a wise man in the east,” even a magus, and his prophecies of the Messiah, though not perfectly specific, were clear enough to raise an expectation of his coming at just this point in time.

Such interesting reflections naturally lead to two questions:

  1. What did they see, these magi, these educated astronomers and astrologers? Astronomers can certainly tell us the layout of the heavens for any day so recent as 2,000 years ago! Could we not calculate precisely when the magi came by comparing astronomy as now known with astrology as traditionally received?
  2. But then, what is the Christian scripture doing promoting astrology? I thought that this was a superstition and therefore opposed to the First Commandment: “I am the Lord thy God; thou shalt not have strange gods before Me.”

What star did they see?

Let me begin with the first question, not because it is most important, but because that works for my train of thought.

It is not certain, of course, what anyone was looking at 2000 years ago, but it is true that, in the years between 7 B.C. and the end of 2 BC, the planets had some unusual conjunctions: Jupiter and Saturn met several times, even Venus and Mars got into part of the act; and these are the types of display that astrologers have strong, though not always  orderly, opinions about. Anyone who looked at the sky — and dwellers in warm climates without air conditioning look at the sky much more than we do — anyone might have noticed these things, and anyone who believed in astrology could have thought something noteworthy must be going on.

Nevertheless, Jerusalem was not flocked with inquirers; just a small company came, though with rich gifts. If they created a stir, particularly in the paranoid household of Herod, still, they didn’t turn the city upside down or bring a mob to Bethlehem. Their quest was personal and rather quiet. Soldiers came later, alas, following Herod’s directives about homage to other kings; at that point, Mary and Joseph needed to be warned to leave town. But for the start, we must conclude that whatever was in the sky was publicly visible but not perfectly obvious even at the time, or Herod would have sent his soldiers at once.

You might think: that was then; this is now. Any computer in the world can tell us the exact time of every kind of planetary lineup, and a clever astrologer should be able to calculate the day of Jesus’ birth.

You would be wrong, for several reasons. This was one of those times when the retrograde motions of the planets caused one conjunction after another for several years and astrology is not such a science that we can choose between them to find the birthdate of Jesus, even supposing it to have been astrologically marked. If you research this question, you will find several detailed astrological/historical arguments for various different dates. Astrology is too disorderly to be useful.

Thus, we know only that those men of long ago sought the Messiah prophesied by Daniel, and God found a way to reveal this to them in a language that they understood. He found a way to invite them to the Messianic birth they awaited, and, like Elizabeth and Simeon, they shared their faith with Mary and were uplifted by hers; then they went home. They never returned to inform Herod, because they were warned that his intentions were evil, but they didn’t stick around to watch his career either; likely they would have been too old to see it. It’s that simple.

God and astrology

And that just about answers the second question: the scriptures are not affirming astrology. God speaks to those who seek him, and he does so humbly, from the manger straw or sometimes from odd books that are not, in many ways, even very good books — because He is still their Father and He loves them. He reveals himself by whatever means are at His disposal within a seeking man’s actual life. This doesn’t mean that Christians of today would be excused for astrological superstition and it certainly doesn’t mean we are invited to it. We have the scriptures and we have the Church with all its philosophers and all its saints and even all its philosopher-saints, and we would have no excuse for seeking God in the meander of lesser resources. The pure in heart will always find him; but they will never seek him by means which they know to be disorderly.

Having said that, there is interesting evidence outside astrology for the birthdate of Jesus, perhaps in September of 2 BC, and – a distinct subject – evidence for the date the magi arrived. If this sounds intriguing to you, stay tuned.

Continue with His Birthday.

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Take another look at the ice age record, “stages of glaciation” on bottom of the chart. Although it is similar to the solar forcing, it is not the same, and that means there must have been other influences. Not surprising, but it’s worth asking what they might have been. Look, for example, at the low spot — the ice age maximum — at about 640 kyr.

Ice age records compared to solar forcing from Milankovitch cycles.

Ice age records compared to solar forcing from Milankovitch cycles. But there must be other influences...

It’s one of the two lowest spots on the graph. much lower than most of the lows, though there’s a similar low at 440kyr. It’s not just the Milankovitch cycles; something else is going on. Also it’s interesting that the temperature climbs so rapidly out of it. Well, all the temperature climbs are fairly rapid; ice melting seems to go fast when it goes. In any case, the Milankovitch cycles are not the only large-scale influence on our climates.

Yellowstone is another. Any volcano can be important, but Yellowstone is (or was; we hope it’s more settled today) a supervolcano. The difference, though still under debate in its details, is fairly clear: volcanoes build mountains; supervolcanoes erase them. Yellowstone’s largest eruption was 3.1 million years ago, and is not on this graph; it spewed 600 cubic miles of stuff, enough to cover California 20 feet deep, had it been there to cover. But the eruption of 640,000 years ago spewed 240 cubic miles of material, a thousand times more than Mount St. Helen’s, and still enough to cover California 7-8 feet deep. Of course it was spread over most of the western United States and indeed some of the ash went round the world. It has to have diminished the sunlight for quite a while, must have caused a cooling, and thus we can be sure that we are looking at one of the records record of this event.

It’s easy to find maps of the Yellowstone caldera. Here’s an image of part of one edge. The total caldera is 50 miles long and 30 miles wide. It’s so large, and so completely overgrown, that it takes a special eye to see it as a caldera, but there is no doubt of it.

Yellowstone caldera, just a small piece of edge.

The Yellowstone supervolcano gave us a caldera 50 miles long and 30 miles wide. It also contributed substantially to the ice age that was due about that time.

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I don’t know why it’s called “forcing” but I suppose that the sun does, indeed, force us to get warm. But how much solar heat do we really get? It varies by season, and also, as you now know, by eccentricity, obliquity, and precession in the Earth’s basic motions. Furthermore, of course, it differs at each latitude.

So how does it really add up?

Someone has made calculations for solar forcing at latitude 65° north for the last million years (1000 kyr is a thousand kilo-years, which is 1,000,000 years – one million years.) Here’s a graphic that calculates solar forcing due to the Milankovitch cycles. (Milankovitch = Milankoviç).  Precession in red, obliquity in green, and, in blue, eccentricity in a lovely sweeping and swelling motion like Gregorian chant. Finally, in yellow, the three sources of heat are summed up to show how hard the sun was trying to make the world warm in those upper latitudes.

Milankovitch cycles govern solar forcing, yielding warm periods, ice ages, and everything between.

Milankovitch cycles govern solar forcing, yielding warm periods, ice ages, and everything between

Below the yellow graph of solar forcing is the record of ice ages, with tall gray bars marking the warm periods. It’s a good match with the yellow, isn’t it? It’s not perfect, but it’s good.

Notice that the graph is made with time running from right to left, instead of left to right. I don’t know why. There on the left is the warm period when men began to farm, and close by, on the right, is the deep ice age of 20,000 years ago. Look how short the warm periods are! After every warm period, the weather gradually slides down into an ice age, bumps around for a while and doesn’t get really warm again for something like, 100,000 years.

Notice especially what happened between 250,000 and 225,000 years ago. The climate was heading for its best warm period and suddenly it dropped off pretty cold. Then it climbed back up again to finish its warm cycle, though without getting as high, and then it dropped down again.

What happened? Well, for one thing, the coldest phases of two of the three Milankovitch cycles coincided perfectly, and it was just impossible to warm properly. I don’t know if anything else happened; perhaps there was a serious volcano, but maybe nothing else had to happen. After that, with all three cycles in their warming phases, the climate warmed right up.

How would we know about ice ages when nobody was there? How can anybody know these things from so long ago?!

Well, there are several ways to detect ancient ice ages; that’s a question of geology, another topic. But see how the two topics work together?

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I never see the term precession without thinking of “How the Rhinoceros Got His Skin,” one of Kipling’s Just So Stories — because it starts right out on an island that is located near “the promontories of the larger equinox,” an entirely absurd description, but an echo of the phrase “precession of the equinoxes” and so like the way that small children experiment with big words that sound nice and juicy to them.

The equinox, of course, has no promontories, merely being the day of the year that night and day are the same length for everyone. Nevertheless, the stars that are highest in the sky at the time of the equinox change over hundreds of years, and because this change is due to a wobble called precession, in the movement of Earth’s axis, it is called the precession of the equinoxes. To be more precise, precession is the change in the axis of the earth so that it does not always point north to our good friend Polaris, but, after a while, to other stars in the Little Dipper, even to Vega and on to other stars before returning to Polaris again.

Imagine a spinning top. It may begin with a spin so perfect it seems to be standing perfectly still. Then it may begin to lean, and when it leans, the leaning axis slowly revolves, pointing slightly east, then north, west, and south and back to the east in succession. All these changes may or may not include a change in how much it leans (may or may not include a change in its obliquity). Just the change in direction, however, means a change in where the axis points, and if stars were painted on the ceiling, you would see various stars take turns being the “north star” to which the axis points.

So how does that change the weather? Should the Polaris turn around and blow snow upon us when we wander away?

I think not.

But something else happens.

As you know, the winter of the Northern hemisphere comes when Earth is nearest the sun, and the winter of the Southern hemisphere comes when it is farthest. That could be expected to make the southern winters much more intense than the northern ones, because the cold of being tilted away from the sun is added to the cold of being far away from the sun. Yet it does not happen so.

If you look at a globe, you will see why. The southern hemisphere doesn’t have as much land as the northern hemisphere, and this changes how heat moves in the south. Vast ocean currents – rivers within the ocean — carry heat and cold all around the globe, and these moderate the potential extremities of the Southern winter.

Now turn it around.

What happens when the northern, land-rich hemisphere is turned away from the sun during its winter? This happens when the axis points to Vega for its north star. In this case, the cold of being far away is added to the cold of being turned away, and there is no world-circling current of water to moderate it. That could be pretty intense, right? And even though there are world-wide winds which mix around and some of them (from the northern and southern Hadley cells) meet regularly at the equator, they don’t mix that well, and even if they did, air doesn’t change the land heat near as readily as the ocean mixes itself. Remember all those things about phase change in water? So those northern lands are much more apt to freeze than water than water!

Indeed, when the northern hemisphere is both far from and pointed away from the sun, its winters are much tougher, and the whole planet cools. So, again, here is a cycle that takes a very long time – 23,000 years – and it has a strong effect on climate.

Actually, it should take 26,000 years, but the whole orbit of Earth is turning so that the precession gets carried along and saves 3,000 years.


Did that last bit leave you a little confused? Join the club. It takes a long time to integrate so many motions into a single, flowing image. Left to yourself, you’d probably never come up with it, would you?

So you might not be a civil engineer, such as Milutin Milankoviç!

(Or you might be one someday, but not yet.)

These three long-term shapers of weather – the eccentricity of the earth’s orbit, and the obliquity and the precession of its axis — are called the Milankoviç cycles. People argue about them; some say they don’t matter. They do matter! They do shape weather! Or rather, they shape that larger thing, climate, in which the little blips called weather take place. But they are so long-term, and they get piled on top of each other in so many ways that only a serious mathematician would even dream of sorting them out. We’ll take a look some time.

Meantime, you learn everything you can about wind and sun, water and pressure, dryness and precipitation, and then there is something immensely important that you didn’t even consider.

There’s always more.

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You know that the tilt of the earth produces our seasons. When the northern hemisphere is tilted towards the sun, we have summer in the US, and when it is tilted away, we have winter.

Did you know that the tilt, presently 23.5°, varies over time? When there is less tilt, there is less difference between the seasons – winters are warmer and summers are cooler. What do you suppose that does to climate?

You might think it would all cancel out, and with mild seasons, it would be Florida everywhere – or at least the Riviera.

You would be mistaken.

What seems to happen is that the warmer winters have more snow, and the snow reflects more sunlight (thereby resisting heat) and the same snow conserves more cold so that polar ice sheets grow larger and larger. Cooler summers don’t help; they don’t give a thorough warming and get all that new ice melted. So the times of lesser axial tilt (tilt is called obliquity) are colder times, times of potential ice ages; the times of more tilt (greater obliquity) are warmer times.

Our obliquity moves gradually from 22.1° down to 24.5° and then swings back up again. We’re on the upswing, but don’t hold your breath for the ice age; it takes about 41,000 years to complete this cycle. That’s such a long time we can hardly imagine it. How would you ever even know such a thing? But remember that in geology we have the record of our past, and we can see much farther back than 41,000 years. Of course, this cycle has to get mixed up with the cycle of eccentricity, so it gets messy, but it is still there.

Who would have thought of it?

Milutin Milanković, Serbian mathematician and civil engineer, that’s who.

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There are several long-term influences on the weather of earth – and I mean really long-term.

Let’s begin with the shape of Earth’s orbit. I am sure you know that the orbit of the Earth around the sun is round, but did you know that it’s not quite fully round? It is slightly elliptical, and this varies; sometimes our orbit is as much as 5% off-round; sometimes only half of 1% off. Only a sharp eye would see even the 5% ellipse drawn on a piece of paper, but the Earth feels it even if you can’t see it.

What does the Earth feel, then?

When its orbit is more elliptical, the Earth spends a little less time closer to the sun, and a slightly longer time farther from the sun. You would think it would balance out. From a certain mathematical point of view, the balance is perfect, but in terms of heating, it just doesn’t work that way. Winters get about 4 times colder compared to summers when the earth is at 5% ellipticity, and it’s hard to warm back up. As a result, climates all over the earth get colder during the more elliptical parts of the cycle.

What throws off the orbit?

The tug of Jupiter, the tug of Saturn, the tug of both at once, and how close Earth is to the Sun when they tug. It’s very complicated, but in any case, the off-roundness (called eccentricity) gets larger and smaller by turns, sometimes much larger, sometimes only a little larger. It takes about 413,000 years for the orbit of the earth to complete its full cycle of off-round variation. (Four hundred thirteen thousand years! How can anyone even know such a thing?)

However they know, it’s not the only long-term factor, but it’s a big one.

Who would have thought of it?

Milutin Milanković, Serbian mathematician and civil engineer, that’s who.

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