Changing Time

Since today will have one extra second added to it, I thought it would be a good time to talk about calendar corrections in history. We'll avoid the modern arguments about what actually constitutes a "year" and go back to a simpler time.

Readers of this blog are probably familiar with the pope's work to "fix" the calendar when he realized that the Julian calendar was "off" by several days. Sure, the Julian took into account that a "year" is actually 365 and a quarter days, but it wasn't exactly a quarter-day off, and so adding a day every four years wasn't sufficient to even things out. Every 131 years, the solstices and equinoxes would be off by a day. This was a real problem for the Church, because over the centuries Easter started shifting to summer, rather than spring.

Pope Paul III (1468-1549) saw the problem and gathered several astronomers to develop reforms.* Then Paul III died, five popes came and went, and in 1572 Pope Gregory XIII was elected and found himself faced with a growing problem and several possible solutions. He mulled it over for a few years, and then declared to the world:

• The Leap Day would move to after February 28 (it had been tucked in before February 25).
• The date of Easter would be calculated differently.
• A Leap Year that is divisible by 400 will not have a Leap Day.
• Ten days needed to be omitted from the calendar, to bring it back into alignment with the solstices. Accordingly, in 1582, the day after Thursday, October the 4th, was Friday the 15th.

...and then everything was fine. Except when it isn't, like when we have to add an extra second here or there. (And can you imagine trying to explain to someone 1000 years ago that the year gained about 45 seconds every 10 millenia because the Earth's rotation was slowing?)

The 1582 change gave rise to a slight oddity for historians, because technically, every date prior to October 4th, 1582 is a Julian date, but also has a corresponding Gregorian date that is several days different. How you count it depends on whether you consider the date to be just a label given by those who were there at the time, or a certain number of days prior to the present day. Or its important if you want to declare the anniversary of an event.

Anyway, enjoy your extra second; it will be added just before midnight, so feel free to stay up so you can say you remember the time when a minute was 61 seconds long!

*Copernicus had dedicated his work De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres) to Pope Paul III, hoping it would save him from potential hostility for his radical thoughts. Had Paul called, Copernicus probably would have come running to join the team of calendar reformers; alas, he had died in 1542.

Rosemary in England

The fragrant and tasty rosemary (from ros marinus, "dew of the sea") grows natively in the Mediterranean. Perform an Internet search for "rosemary + England + herb" and you'll likely be told that it was brought to England by Queen Philippa of Hainault in 1327, when she married Edward III of England. (Examples are here, here, and here; but don't bother.)

The origin of this story seems to be with the translator of a manuscript found in the library of Trinity College, Cambridge. The manuscript is a poetic treatise sent by Queen Philippa's mother, the Countess of Hainault, about rosemary's many uses. Presumably it was sent with a rosemary plant, and thus the legend of the introduction of rosemary to England was born. The treatise has such useful advice as:

mighteth the boones and causeth goode and gladeth and lighteth all men that use it. The leves layde under the heade whanne a man slepes, it doth away evell spirites and suffereth not to dreeme fowle dremes ne to be afeade. But he must be out of deedely synne for it is an holy tree.

Strengthens the bones and causes good, and gladdens and lightens all men that use it. The leaves, laid under the head when a man sleeps, do away with evil spirits, and will not allow one to dream foul dreams or be afraid. But he must not be in a state of sin, for it is a holy tree.

So was it known in England before the 14th century?

The British Library has a manuscript (Royal 12, D xvii) known colloquially as "Bald's Leechbook,"* a ninth-century medical text written in Old English. Rosemary is mentioned in a number of remedies, for fever, horses leprosy, and toothache (always mixed with multiple other substances). It is highly likely that the Saxons found rosemary growing all over England because of its introduction by the Romans centuries earlier.

*Bald's Leechbook deserves its own entry or three. [Update: here it is.]

Nicholas of Cusa: Ecumenist

After the Fall of Constantinople in 1453 to the Turks, Nicholas of Cusa (c.1400-1464) wrote De pace fidei (On the Peace of Faith), in which he envisioned a conference in Heaven of representatives of all religions, including Islam and Hussites (the followers of Jan Hus were one of the first Christian Protestant movements, over a century before Luther).

The imaginary conference concluded that it was possible to have a single unified religion, even if it manifests in many different versions with separate practices. After all, the Roman Catholic Church had co-existed with the Eastern Church for centuries; the Eastern Church may not have recognized the authority of the Pope over the East, but the East and West did consider themselves two parts of the same faith.

Nicholas clearly prefers and exalts Christianity, but is willing to find accord with others. He had written Cribratio Alchorani (Sifting the Koran), which acknowledges that Islam and Judaism still possess seeds of the truth. Let me be clear: Nicholas doesn't treat Islam or Judaism as equals with Christianity—in 1451 he had used his authority as a bishop to require Jews in Arnhem to wear badges, and he had imposed other restrictions on Jews that were later lifted by Pope Nicholas V. The fact, however, that a well-known Catholic prelate and respected theologian could write on and publish such tolerant ideas was remarkable for the time.

Reasoning Wrong

Nicholas of Cusa (c.1400-1464) believed in using reason to determine how the universe worked. He did not exactly take a "scientific" approach: he argued from his understanding of metaphysics and, in some cases, numerology. His guesses, however, were better than some early scholars' observations.

Because he could not accept that God was finite, and since God is not separate from the entirety of the universe, he argued that the universe must by necessity be infinite. Also, because God must provide the center for His own totality, the Earth cannot be at the center of the universe—that would mean Earth was the center of God. Not being at the center of the universe, the Earth cannot be immovable, and it, along with the Sun, must be in motion just like every other observable heavenly body. This idea influenced Giordano Bruno.

Again, denying perfection for anything but God, he would not accept planetary orbits as perfectly circular, paving the way for Kepler (who referred to Nicholas as "divinely inspired") to design elliptical orbits in his planetary theory.

Cusa's thoughts on what we now call infinitesimals in his De Circuli Quadratura (On Squaring the Circle) helped Kepler out when trying to calculate the area of a circle, by picturing it as an infinite series of triangles. Cusa's reasoning for this was that the circle encompassed all other forms. Cusa's and Kepler's work was later important to  Leibniz' Law of Continuity.

Tomorrow: his views on bringing religions together.

Nicholas of Cusa

American History has a charming anecdote that the head of the US Patent Office once declared there was nothing left to invent. The background for this fable is explained here. Someone in 2010 said that we create as much data in 2 days as in all of 2003. Our ability to find and learn new things looks like it will never match the things there are to learn.

With the amount of knowledge being gained rapidly accelerating, why don't we know everything yet? Will we know everything?

Nicholas of Cusa (c.1400-1464) had an opinion on this. He outlined the problem in the opening remarks of his De Docta Ignorantia (On Learned Ignorance):

It so far surpasses human reason, however, to know the precision of the combinations in material things and how exactly the known has to be adapted to the unknown that Socrates thought he knew nothing save his own ignorance, whilst Solomon, the Wise, affirmed that in all things there are difficulties which beggar explanation in words;

He argued, however, that every living creature is

endowed with suitable faculties and activities; ... there is in them a discernment that is natural and in keeping with the purpose of their knowledge, which ensures their natural inclination serving its purpose and being able to reach its fulfillment.

If that is true, he says, then limits to our ability to know things must be built-in by our Creator for a purpose. And that purpose is that, at the end of our reasoning, when reason fails, we must turn to faith for the ultimate answers.

For this, Nicholas has been declared an anti-intellectual by at least one prominent modern scholar. But this misses his point: until we reach the limits of what we can figure out, we must travel as far as we can with reason. Just how far Nicholas of Cusa let his reason take him (and in ways that were appreciated by men like Bruno, Copernicus, Galileo and Kepler) I will explore this week.

Gervase and the Moon

Yesterday I mentioned Gervase of Canterbury and his fairly unremarkable life. What follows is the reason he makes an appearance in footnotes in astronomical texts.

Among his other writing duties and pleasures, Gervase also kept the monastery's chronicles, and in 1178 he recorded something interesting:

This year on the 18th of June,[*] when the Moon, a slim crescent, first became visible, a marvelous phenomenon was seen by several men who were watching it. Suddenly, the upper horn of the crescent was split in two. From the mid point of the division, a flaming torch sprang up, spewing out over a considerable distance fire, hot coals and sparks. The body of the Moon which was below, writhed like a wounded snake. This happened a dozen times or more, and when the Moon returned to normal, the whole crescent took on a blackish appearance.

Scholars and astronomers have puzzled over this. Astronomer Jack Hartung in the September 1976 issue of the journal Meteoritics proposed that they had witnessed a meteor strike that created the lunar crater "Giordano Bruno." Recent photos by Apollo made it, in his opinion, a candidate for a young crater only 800 years old. Hartung had his critics, but without new data on the crater, the verdict could rightly claim to be "up in the air." Until years later a student named Paul Withers in Weekly Scientist pointed out an obvious flaw. An impact great enough to create the 14-mile wide Giordano Bruno would have thrown "10 million tons" of debris off the lunar surface; the ejecta in short order would have created unbelievably spectacular meteor showers in Earth's atmosphere. These meteor showers were not noted by monks in England or any other of the sky-gazing cultures that dotted the globe at the time.

So what did the monks see? His guess is that they were looking in the right place at the right time to observe a meteor entering Earth's upper atmosphere between their line of sight and the horns of the crescent moon.

For a modern astrophysicist's view on Gervase' chronicle, see Brian Koberlein's Google + post for today, June 25th, called "Mining for Science."


*N.B. The Proleptic Gregorian Calendar (used for dates prior to 1582 because of a necessary correction) turns this date into June 25th, meaning today is the 834th anniversary of the monks' observation.

Gervase of Canterbury

Gervase of Canterbury (c.1141-c.1210) was a monk of Christ Church in Canterbury. He was ordained on February 16, 1163 by Archbishop of Canterbury Thomas Becket (who would be assassinated in 1170, canonized by Alexander III a few years later, and become the focus of the most famous shrine in England).

Gervase had a fairly undistinguished career as a monk, and his "importance" for us is in his connection to other people and events. He was involved in the dispute between the monks of Christ Church and Archbishop of Canterbury Baldwin of Forde. Baldwin disapproved of the luxury in which the monks of Christ Church lived, derived from the donations made by pilgrims to the shrine of St. Thomas Becket. At one point, Baldwin had the entire monastery confined under house arrest for a year and a half. Gervase was one of the monks sent to inform Baldwin that they were making an appeal to the pope. Ultimately, the monks were given their freedom and normalcy restored (except that Baldwin appointed Roger Norris, a pretty ineffective administrator, as their prior).

For a brief time Gervase held the position of sacristan (the person who takes care of the church and its contents), working for the Archbishop Hubert Walter.

...and that's about it for Gervase, except for one little thing that has intrigued historians and astronomers for centuries. Gervase kept records and wrote books, such as a history of the Archbishops of Canterbury and his Mappa Mundi, which lists bishoprics and ecclesiastical houses in England, Wales and Scotland. He was also the chronicler of his monastery, and on the night of June 18th, 1178,* he recorded something curious and previously unseen.

...but what the event was, and how June the 25th is actually the anniversary of the event on the 18th, I'll save for tomorrow.

*The same year this was happening in Italy, as mentioned on the June 20th post.

Mappa Mundi

Approximately 1100 mappae mundi (maps of the world) have survived from the Middle Ages; about 200 are separate maps; the rest are in books.

Maps of the world were obviously limited by knowledge of geography. This did not deter medieval cartographers, however, since maps existed for along time not as guides for travelers, but as diagrams of the layout of God's creation. Minutiae weren't as important as portraying the overall plan, or showing a particular feature. For instance, Gervase of Canterbury's Mappa Mundi was drawn up specifically to show the bishopric s and ecllesiastical foundations of England, Scotland and Wales.

One of the most common of the map designs was the T-O design, called that by modern scholars because it resembled those letters. The "T" was the division of the major landmasses with the enormous Asia topping Europe and Africa; "O" was the encircling Ocean.

Note that Asia is on top. When choosing proper positioning for the arrangements of the continents, the direction of the rising sun seemed to be a logical place to begin. East is therefore placed at the top of the maps, and the arrangement of things in their proper place on a map therefore was called "orienting."*

Cicero's Dream of Scipio (in which a vision of the world is viewed in detail) was very popular in the Middle Ages for what it had to say about the world and the divine. Macrobius' (5th century CE) Commentary on it was how many readers became familiar with it. Many copies of the Commentary include mappa mundi of the type therefore called Macrobian. The Dream includes a description of the various zones, cold to temperate to hot to temperate to cold. Only the temperate zones were considered habitable.

Fourteen manuscripts of Beatus of Liébana's (c.730-800) popular Commentary on the Apocalypse of St. John include the quadripartite-style map that squeezes the Antipodes into the extreme south.

The most famous and most detailed maps provide the most variety and data and start to approach the realism and usefulness of modern cartography. The symbolic value of the mappae mundi began to be replaced by the need for accurate information to aid in travel and, especially, navigation on the seas. The new "Portolan Charts" became far more valuable to have and reproduce, and the number of mappa mundi were produced less.

If you would like to see some maps from across the ages, a good start is the Imaginary Museum.

*An extra tidbit: "orient" comes to Middle English from Latin via French and the verb oriri, "to rise"; once in English it starts being used for the direction itself in which the sun rises.

The Antipodes

As the unknown becomes close and familiar, imagination must seek new realms further away. Modern fiction doesn't write about alien life on the Moon or Mars or Venus, since we now "know" those spheres. In the Middle Ages, it was assumed that fantastical lands and creatures existed "over there." As far lands such as Africa, for instance, began to be explored and found mundane, the fantastical was mentally placed in a more distant spot.

The Antipodes (literally "opposite feet": the place on the globe directly opposite to where you are standing)* fascinated the Middle Ages for both philosophical and geographical reasons. On the one hand, the idea had come to them from the Classical World, and so was difficult to dismiss. This was the place that held creatures out of legend: Cyclopes, dog-headed men, Sciopods whose single large foot could be used as an umbrella when they lay on their backs, Blemmyae with faces on their torsos. On the other hand, the Medieval idea of symmetry demanded that there be lands to the far south, balancing the known lands in the north. Especially as the idea of earth's roundness took shape, Africa wasn't enough to satisfactorily balance Europe and Asia.

The Antipodes were also a natural extension of the theory of "zones." Since it became very cold up north, and hotter as you went south, it was assumed that the temperate zone enjoyed by most of Europe had a counterpart below the equator which was equally inhabitable. Below that, of course would be the area that is opposite the Arctic: the Anti-Arctic, or Antarctic.

The term was first used by Plato in his Timaeus. It was first used in English in 1398 in John of Trevisa's translation of Bartholomaeus Anglicus' De Proprietatibus Rerum (On the Order of Things):

Yonde in Ethiopia ben the Antipodes, men that haue theyr fete ayenst our fete.
(Yonder in Ethiopia are the Antipodes, men that have their fete against our feet.)

Some in the Middle Ages used the image of upside-down men as a reason to reject the Earth as a globe and opt for a flat Earth. Plato had addressed this himself, however, in an assumption of orientation that he does not call gravity but that works the same way:

For if there were any solid body in equipoise at the centre of the universe, there would be nothing to draw it to this extreme rather than to that, for they are all perfectly similar; and if a person were to go round the world in a circle, he would often, when standing at the antipodes of his former position, speak of the same point as above and below; for, as I was saying just now, to speak of the whole which is in the form of a globe as having one part above and another below is not like a sensible man.

But sensible men apparently did not read the Timaeus, or never grasped the concept of "up" and "down" being relative to where you were standing. A universally accepted map of the world was unknown; maps of the world typically took one of four forms, which we will look at tomorrow.

*Have fun finding your antipodal point here.

Summer Solstice

The first full day of summer seems to be the appropriate place for this:

Sumer is icumen in,
Lhude sing cuccu!
Groweþ sed and bloweþ med
And springþ þe wde nu,
Sing cuccu!
Awe bleteþ after lomb,
Lhouþ after calue cu.
Bulluc sterteþ, bucke uerteþ,
Murie sing cuccu!
Cuccu, cuccu, wel singes þu cuccu;
Ne swik þu nauer nu.
Sing cuccu nu. Sing cuccu.
Sing cuccu. Sing cuccu nu!

Summer has come in;
Loudly sing, cuckoo!
Groweth seed and bloweth meadow
And the wood springeth anew,
Sing, cuckoo!
Ewe bleateth after lamb;
Cow loweth after calf.
The bullock starts, the buck farts,*
Merrily sing, cuckoo!
Cuckoo, cuckoo, sing well cuckoo;
Nor never now stop.
Sing cuckoo now. Sing cuckoo.
Sing cuckoo. Sing cuckoo now!

*There is disagreement on the meaning of "uerteth"; is it for "verteth" meaning "turns" (the buck is looking around for a doe?) or does the "v" sound indicate the fricative "f" for "ferteth" meaning "farts" (Middle English ferten = "to fart"; indicating...the richer diet of spring means life is being generated?). I have chosen "farts" simply because it rhymes with "starts" and gives me a chance to include this footnote. Happy Summer!

The Pisan Mistakes

According to legend, a rich widow named Berta di Bernardo left 60 coins in her will for the construction of a campanile, a bell tower, for St. Mary's Cathedral in Pisa. A 55-meter tower was planned, belvedere style so that people could walk up the outside and enjoy the view. Ground was broken August 9, 1173.

The architect/builder is unknown. Either he was simply undistinguished, or he was careful to keep his name out of the public eye once the project was begun in order to avoid the inevitable ridicule. Pisa was built at the junction of two rivers, the Arno and the (now dried and gone) Auser; when the builder dug a 3-meter deep hole for the foundation, he found only river clay and sand. This lack of bedrock did not deter him. He proceeded to build what was designed to be a 14,000-ton structure on top of this soft foundation; by the time he got to the third level in 1178, it had already started to list to one side.

They had no way to change the ground on which it stood, and they didn't want to give up the project, so they built the next floors with one wall higher than the other, so that the upper floors wouldn't slant.

A series of financial problems and wars with neighboring city-states (mostly Florence, but Pisa also lost a major conflict with Genoa) meant the campanile wasn't finished until 1319. Nothing they did could fix the increasing list. But then, nothing they did would have. Adding the bells? More leaning. Deciding to add an impressive 3.5 ton additional bell? More leaning. Even so, the tower was only leaning by five degrees in 1837, when Alessandro della Gherardesca decided to turn an embarrassment into a tourist attraction by exposing the famous flawed foundation. He dug around the base...below the water table. The space flooded, the clay and sand softened more, and over the course of a few days, the tower leaned another meter!

In 1934, the idea arose to drill holes in the foundation and force grout into them. It looked good on paper; in reality, it increased the tilt. Tinkering in 1966 and 1985 just made it worse, and the tower was closed to the public in 1990.

Years later, the combination of a steel corset around the base, concrete, and counterweights were supposed to be the ultimate fix. They weren't, although they did almost cause the whole tower to break apart from stress. Finally, someone decided to go to the root of the problem. They dug away at the foundation on the other side of the tower, hoping it would lean back the way it had come. A little over 800 years of tinkering since the problem first appeared, and not only was the tilt reduced, but the tower was stabilized and re-opened to the public. Once again, tourists can climb the tower and take photographs looking out over Pisa, rather than just ground-based pictures of them appearing to hold up the tower with their hands.

Medieval Tablets

Herman of Tournai (1095–1147), an abbot of St. Martin of Tournai (in Belgium) wrote an account called The restoration of the Monastery of Saint Martin of Tournai. In the prologue, he comments "I even wrote down a certain amount on tablets." He would have been referring to a wax tablet, which were used since the Roman Empire and became increasingly common in the Middle Ages.


A shallow wooden or ivory frame filled with soft wax could be written on with a stylus; the other end of the stylus was a flattened blade shape that could be used to smooth out the wax for re-use. The beeswax would be mixed with plant oils and/or carbon to create a more legible surface. Tablets were often not simply one piece; They could be turned into leather-laced "booklets" where the outer tablets held wax on the inside, and inner "pages" would have wax on both sides.


Wax tablets were great for business records. A hospital in Enns, Austria used multiple-page wax tablets (that still exist) to track some financial records. Payables due would be printed on parchment and stuck to the left side; money received would be written on the right side in wax and added up. The impression would store indefinitely—so long as you didn't let it get hot.


Tablets were also ideal for transactions that did not need to be kept long-term. The impression could last for a week or month or season, and then warmed up and smoothed over for the next set of transactions off the boat.


Here's an extensive directory of links to pictures of tablets from across the centuries.

The Church & Marriage

On April 19, 1213, Pope Innocent III declared via papal bull that a great council would be held; the Fourth Lateran Council started November 11, 1215. One of the reasons Innocent felt the need for reform was the poor performance of the Crusades: the Third had been disappointing in its result and the Fourth a downright disaster in its execution. Organizing the Fifth Crusade was one of the purposes of the Council; Innocent produced 70 additional decrees which were generally accepted with little argument.

One change created by the Fourth Lateran was in marriage. Prior to 1215, marriage was simply a legal contract between two parties, not a sacrament.* Innocent made it a sacrament, making marriage a much more important bond and presided over by the Church.

He next declared that marriages could not be clandestine: they must be proclaimed publicly ahead of time so that anyone who knows a legitimate impediment against the marriage has a chance to make it known. He warns, however, that lying about such impediments will bring the wrath of the church down on you.

Was this a power grab? An attempt to assert the church's control over political unions? Innocent knew there was bound to be some opposition to "re-defining marriage"; he prefaced his marriage decrees with this:

It should not be judged reprehensible if human decrees are sometimes changed according to changing circumstances, especially when urgent necessity or evident advantage demands it, since God himself changed in the new Testament some of the things which he had commanded in the old Testament.

Innocent was pope from January 1198-July 1216, during which he made many changes, some of which are still with us.

*Pre-1215, there were only 5 sacraments: Baptism, Communion, Confirmation, Holy Orders, Last Rites; 1215 established Marriage and the practice of individual Penance at least once per year.

God as Mother

The Leadership Conference of Women Religious is making news, and my desire to make this blog not just interesting and varied but also relevant prompts me to talk about Julian of Norwich.

Mystics—people who attain knowledge of the divine not by rational study but by a direct connection or intuition, often during a state of ecstasy—are known in all faiths and all eras.

Julian of Norwich was an anchoress (a female anchorite, a hermit; she lived a life of religious seclusion) who had a series of mystic visions of Jesus in 1373 (she was about 31 years old) while on what was thought to be her deathbed. She recovered on May 13, and wrote down a short version of the visions. In about 1393 she wrote a much longer version, Sixteen Revelations of Divine Love, possibly the first book written in English by a woman.

One of her most controversial habits is to refer to God and Christ as Mother as well as Father. One such passage:

And thus I saw that God rejoiceth that He is our Father, and God rejoiceth that He is our Mother, and God rejoiceth that He is our Very Spouse and our soul is His loved Wife.
...
God is Very Father and Very Mother of Nature...

Church authorities at the time did not challenge her. This cannot be because she wrote in obscurity: there are plenty of contemporary references to her, and she was operating in Norwich, the second most populated city in England. Either the church did not consider her ideas likely to become influential, or they were not shocked by them. After all, she did not say God was solely mother; she simply allowed for feminine qualities as well as masculine. Perhaps this all-inclusive approach was sufficiently non-threatening to be accepted as non-heretical. Perhaps the Middle Ages was willing to embrace the importance of the feminine along with the masculine.

Tide Goes In, Tide Goes Out

The Classical World and the Middle Ages wrestled with the cause of the tides for centuries. Although one early scholar (Alpetragius, who flourished in the late 1100s) felt it was caused by some general motion of the world/celestial spheres that ran from east to west, most others (such as Bede and Gerald of Wales) felt it had a stronger connection to the movement of the Moon.
Alpetragius died in 1204, and his theory on the motion that caused the tides was translated into Latin by Michael Scot. This brought it to the attention of Robert Grosseteste (c.1175-1235), who had an explanation for the tides that relied on his theories of light. (The following is from the Questio de fluxu et refluxu maris, attributed to Grosseteste, although that attribution is disputed.)

Remember that there was no working theory of gravity yet; just a feeling that substances could be heavier or lighter depending upon their composition and gravitate (see? in this enlightened age, the concept of gravity pervades even our language) toward like substances: solids fall to earth; liquid (containing more of the element of water) flows to a lower spot to find its kind; fire yearns upward through air, because fire is even "lighter" than air.

For Grosseteste, light imparted force. Rays of light could carry with them the power to generate heat, for instance (see his theory on the sun). He postulated that, when the Moon rose above the horizon, its rays impressed against the waters and pushed them ahead of it, toward the west. This was not as simple and direct as a physical object pushing against water, and so water didn't rush to the shore as soon as the Moon rose. The rays of the Moon started pushing against the sea closest to it, pushing that water toward the observer. When the Moon was overhead, its rays had pushed as much water as it could at that time. Once the Moon passed the zenith and was over land, then the waters started to recede. The Moon then passes west and under the earth, at that point causing (somehow) the tides again.

Grosseteste admits that we don't know everything about this process, and my summary is a radical simplification of his detailed analysis. He notes the changes in tides as the Moon changes its declination, and theorizes that the Sun also "helps" the Moon in some manner.

For more detail, find the Question on the flow and re-flow of the sea (available in Isis, Vol. 57, No. 4 (Winter, 1966), pp. 455-474 in an article by Richard C. Dales) and enjoy.