A medieval astronomy lesson

If you drilled a hole through the centre of the earth, and dropped a stone down it, what would happen?  How big are the Sun, Moon and stars?  And, hardest to answer of all, are there any people on the other side of the globe?  I’ve been reading up on medieval science.  In particular, I’ve been looking at De temporibus anni, attributed to the Anglo-Saxon abbot and homilist Ælfric of Eynsham (c. 955-1010), which is found in nine manuscripts, such as London, British Library Cotton MS Titus D. xxvii and London, British Library Cotton MS Tiberius A. iii.  This text is itself a translation and adaptation of two Latin works by St Bede, De temporum ratione and De natura rerum, which are found in several manuscripts including Cambridge, University Library MS Ff. 1. 27.

Medieval science is a fascinating subject.  Judging from texts such as Bede’s and Ælfric’s, people were fascinated by the natural world and wanted to understand how it worked.  Looking at these three works from a modern perspective, two things are particularly striking.  Firstly, the fact that medieval knowledge-writing did not differentiate between what would now be considered to be theology and that which we would call science (or, for that matter, between astronomy and astrology).  And secondly, it should be remembered that ancient and medieval writers knew some things which were in fact scientifically accurate, while being really very wrong about other things.  Let’s look at some of them …

A round earth

Sometimes, you will hear it said that medieval people believed that the Earth was flat.  A few apparently did think so: a couple of years ago, at the British Museum’s exhibition on Egypt in the post-Pharaonic period, I saw a map made by a late antique/early medieval Christian who believed in a flat earth.  But the ancient world’s knowledge of a spherical earth was inherited by early medieval writers and this knowledge was passed on.  A list of medieval writers who knew of a round earth, compiled by Reinhard Krüger, can be found here.

Earth CULGg1.1.359r
Cambridge, University Library MS Gg.1.1 fol. 359r

Knowing that the north pole was very cold and the south was very hot, and that they lived in the temperate zone between the two, medieval Europeans assumed that a temperate zone and freezing pole existed on the other side of the equator, although they did not expect to get there.  But Ælfric for one supposes that there are no people in this other temperate zone:

Soðlice þære sunnan ormætan hætu wyrcð fif dælas on middanearde, þa we hatað on Leden Quinque Zonas, þæt sind fif gyrdlas. An ðæra dæla is on ælemiddan weallende 7 unwunigendlic for ðære sunnan neawiste, on ðam ne eardað nan eorðlic man for ðam unaberendlicum bryne; þonne beoð on twa healfa þære hætan twegen dælas gmetegode, naðor ne to hate ne to cealde. On ðam norðran dæle wunað eal mancynn under þam bradan circule þe is gehaten zodiacus. Beoð þonne gyt twegen dælas on twa healfa þam gemetegodum dælum on suðeweardan 7 norðeweardan þises ymbhwyrftes cealde 7 unwunigendlice, forðan ðe seo sunne ne cymð him næfre to ac, ætstent on ægðre healfe æt ðam sunstedum.

The heat of the immense sun creates five zones on the earth which we call in Latin quinque zonas, that is five belts.  One of these areas is in the very middle, boiling and uninhabitable because of the nearness of the sun; no human lives there because of the unbearable burning.  Then on either side of this heat are two temperate areas, neither hot nor too cold.  In the more northerly area lives all of humanity under the broad circle which is called zodiacus.  Then there are two further areas on either side of the temperate areas at the south and north of this world, cold and uninhabitable because the sun never comes to them, but stands still on either side of the tropics.

Martin Blake, ed. and trans., Ælfric ‘s De Temporibus Anni, pp. 88-9


Writing in the fourteenth century, Dante Alighieri imagined a rather direct way of getting to the people on the other side of the earth.  In the Divine Comedy, he imagines Hell as a kind of conical shape stretching down into the earth, where an enormous Satan is lodged.  Having climbed down Satan’s torso to his waist – the very centre of the earth – Dante and his companion Virgil must then climb upwards, up the devil’s legs, in order get to the other side of the planet, where Purgatory is found.  Puzzled by the fact that they had somehow changed direction, Virgil explains that the centre of the earth is the centre of all gravity.  So even though they continued travelling in the same direction, they had to switch from going downwards to going upwards.  C. S. Lewis once called this the first science-fiction effect in all literature.

Dante wasn’t the only medieval writer who contemplated the effects of gravity.  Suppose you drilled a hole through the centre of the earth and out to the other side, and dropped a stone down it.  What would happen?  Realistically, if the hole were filled with air, it would burn up due to friction; the medieval answer to the question was that it would fall until it reached the centre of the earth, and hang there, suspended: clearly, the effects of gravity in a round earth were well understood.  (And at this point I must admit that I am repeating this from memory, as I don’t have any relevant book to hand; I will update when I find a reference.)  I wonder if that is what is going on in the Anglo-Norman treatise L’image du monde, preserved in this fourteenth-century manuscript?

Gravity CULGg1.1.359r
Cambridge, University Library MS Gg.1.1, fol. 359r

Nevertheless, the earth was incorrectly believed to be at the centre of the universe, and orbited by the sun and ‘wandering stars’ (that is, the known planets) in concentric spheres around the earth; Dante, again in the Divine Comedy, must rise through each of these before he reaches his final vision of God.

Spheres RoyCi50r.JPG
The earth, and the spheres in which the planets and fixed stars move.  London, British Library Royal MS 19 C. i, fol. 50r.

The sizes of the heavenly bodies

In De temporibus anni, Ælfric remarks that the stars are not, in fact, the tiny twinkling things that they appear to be.

Eac swilce ða steorran ðe us lytle ðincað sind swiðe brade, ac for ðam micclum fæce þe us betweonan is hi sind geðuhte urum gesihðum swiðe gehwæde.

In the same way, the stars which seem small to us are very wide, but because of the great distance which is between us they appear tiny to our sight.

Martin Blake, ed. and trans., Ælfric ‘s De Temporibus Anni, pp. 78-9

He also knew that the Sun is not, in fact, a small thing …

Seo sunne is swiðe micel; eal swa brad heo is þæs ðe bec secgað swa eal eorðan ymbhwyrft, ac heo ðincð us swiðe unbrad forðan ðe heo is swiðe feor fram urum gesihðum.

The sun is extremely big; it is as wide, so books tell us, as the whole extent of the earth, though it seems very small to us because it is very far from our sight.

Martin Blake, ed. and trans., Ælfric ‘s De Temporibus Anni, pp. 78-9

See, the Sun is so large, it’s as big as the earth.  Well, he got some things right …

Phases of the Moon

The Moon was of particular interest for medieval writers, because, as I have said, its phases were used to calculate religious feasts.  A number of texts in manuscripts such as Ælfwine’s Prayerbook are concerned with computus or with other applications of the Moon’s changes.  The effect of the Moon upon the tides of the sea were also written about.

Sea and moon TitDxxvii56v
London, British Library Cotton MS Titus D. xxvii, fol. 56v

Here is the order of the motion of the moon and the tide of the sea.  The sea’s tide wanes on a moon of three nights old, until the moon is eleven or twelve nights old.  The sea tide waxes from a moon of eleven nights old, until a moon of eighteen nights old.  From  a moon of eighteen nights old, the sea tide wanes until a moon of twenty-six nights old.  From a moon of twenty-six nights old, the sea tide waxes, until the moon is three nights old again.

Moon CULGg1.1.377v.JPG
Phases of the Moon. Cambridge, University Library MS Gg.1.1, 377v.

The different stages in the Moon’s cycle were also believed to have an effect upon which things should and should not be done on a particular day of the lunar month, and upon dreams, the outcome of illnesses, the safety of bloodletting, and upon the fates of babies born:

Birth lunarium TitDxxvi7v.JPG
London, British Library Cotton MS Titus D. xxvii, fol. 7v

Here begins St Daniel’s lunarium of birth.  1st [day of the] moon: he who has been born will be healthy.  2nd moon: he will be average …

These lunaria appear to have been popular in Anglo-Saxon monastic manuscripts, and are often copied alongside prayers and liturgical texts.  However, Ælfric wasn’t so keen on them:

Ne sceal nan cristenman nan ðing be ðam monan wiglian; gif he hit deð, his geleafa ne bið naht.

No Christian must divine any thing according to the moon; if he does so, his faith is nothing.

Martin Blake, ed. and trans., Ælfric ‘s De Temporibus Anni, pp. 90-1.

The Moon and its light

Ælfric knew that the Moon’s light originated from the Sun:

Se mona næfð nan leoht buton of ðære sunnan leoman, 7 he is ealra tungla nyðemest.

The moon has no light except from the sun’s brightness, and of all the celestial bodies it is the lowest.

Martin Blake, ed. and trans., Ælfric ‘s De Temporibus Anni, pp. 80-1.

But as well as bringing the sun’s light to the earth at night, it could also prevent it from reaching the earth by day, if a solar eclipse took place.

Hit getimað hwiltidum þonne se mona beyrnð on ðam ylcan strican þe seo sunne yrnð, þæt his trendel underscyt ðære sunnan on ðan swiðe þæt heo eal aðeostrað, 7 steorran æteowiað swylce on nihte. Ðis gelimpð seldon, 7 næfre buton on niwum monan.

It happens sometimes, when the moon is running on the same path as the sun, that its orb passes beneath the sun’s in such a way that it [the sun] is completely eclipsed, and the stars are revealed just as at night.  This only happens occasionally, and only ever at new moon.

Martin Blake, ed. and trans., Ælfric ‘s De Temporibus Anni, pp. 80-1.

Eclipse CULGg.1.1.378v
And when the Sun is eclipsed, it gets annoyed.  Cambridge, University Library MS Gg.1.1, fol. 378v

In scientific texts from the early Middle Ages onwards, we can see something of how writers, usually monks, were keen to pass on their knowledge, explain it in terms that were relevant to people’s life experience, and put it to use.  Sometimes they had a great deal of understanding of the world around them, sometimes not; but at all times they had curiosity and enthusiasm about the earth and the heavens.

Arief R. Sandan (Ezagren) [CC BY 1.0 (http://creativecommons.org/licenses/by/1.0)], via Wikimedia Commons

Works cited:

Martin Blake, ed. and trans., Ælfric ‘s De Temporibus Anni (Cambridge: D. S. Brewer, 2006).


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