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The Astronomy of Antiquity

c. 3050 BCE

The Egyptian calendar already has 365 days, based on the observation of stars (including Sirius, the brightest in the sky) and the flood of the Nile. There already seems to be a religious calendar (lunar, so 354 or 383 days) and a civil calendar (365 days). The Egyptian year is divided into three seasons: ꜣḫt (akhet, “flood”), prt (peret, “growth,” winter), and šmw (shemu, “harvest,” summer). According to Parker [1950; p. 9], Egyptian months began on the first day that the crescent of the (old) Moon was no longer visible. Months have no name.

The year does not begin with the heliacal rising of Sirius, as is often said, because of a shift of one day every four years in the 365-day calendar with respect to the Earth’s true orbit around the Sun. There was a so-called “sothiac cycle” of 1460 solar years (1461 Egyptian years), after which the heliacal rising of Sirius actually occurred on the first day of the year. By calculating the difference between these two moments, one can even trace the year of an event in the Egyptian calendar. According to Wikipedia:

For example, a 12th-dynasty text mentions a heliacal rising on the 16th of the 8th month of the 7th year of the reign of Sesostris III. So the difference until the next coincidence is found by adding the remaining 14 days of the 8th month, the 120 days of the remaining 4 months, the 5 epagomenal days, and the “apocatastasis” day, for a total of 140 days. At the rate of one day every four years, we find that year 7 of the reign of Sesostris III was 560 years (140 × 4) from the next coincidence, that of −1320; in other words, the year −1880, in perfect agreement with other estimates of the dates of this reign.

At about the same time, the Sumerian calendar counted twelve months of 29 or 30 days each, according to the observation of the first crescent Moon at sunset (the phases of the Moon repeat every 29.5 days). The names of these months vary from one Sumerian town to another.

c. 2650 BCE

The Sumerians name the years after a high-ranking officer, for example: “the eighth day of the office of governor Nômērèš.” The year length is deduced from agricultural cycles, so it is not regular, but as early as around 2400 BCE, Sumerian scribes use a “regular” year of 12 months of 30 days each, for a total of 360 days.

Days begin at sunset. The year has 12 lunar months (araḫ, “month”), for a total of 354 days:

Nisānu
Āru
Simanu
Dumuzu
Abu
Ulūlu
Tišritum
Samna
Kislimu
Ṭebētum
Šabaṭu
Addāru

An extra month, Makaruša Addari (or Ve-Adār) (“Second Addaru” [?]), of 30 days, must sometimes be added to align the year with the solar calendar; priests decide when to add them. Hammurabi reforms the system around 1750 BCE, but it’s only much later (see below, c. 750 BCE) that intercalation started following a regular pattern.

Hebrew months are named after Babylonian months: Nīsān, Iyyār, Sīwān, Tammūz, Av (or Āb), Elūl, Tišrī, Marḥešwān (or Cheshvan), Kislēw, Ṭēbēt, Šebāṭ, and Adēr.

c. 2055 BCE

During the Middle Kingdom of Egypt (c. 2055 BCE–c. 1650 BCE), Egyptian calendar months finally got a name. The following table shows the month names for the Middle Kingdom (first line) and for the New Kingdom (second line, 15th–11th centuries BCE), as well as their Greek and Latin transcriptions (third and fourth lines):

ꜣḫt:
Tekh
Dhwt
Θώθ
Thoth
Menhet
Pa-n-ip.t
Φαωφί/Φαῶφι
Phaophi
Ḥwt-ḥwr
Ḥwt-ḥwr
Ἀθύρ
Athyr
Ka-ḥr-ka
Ka-ḥr-ka
Χοιάκ/Χοίακ
Choiak
prt :
Sf-bdt
Ta-'b
Τυβί/Τῦβι
Tybi
Rekh wer
Mḫyr
Μεχίρ/Μεχείρ
Mechir
Rekh neds
Pa-n-amn-htp.w
Φαμενώθ
Phamenoth
Renwet
Pa-n-rnn.t
Φαρμουθί/Φαρμοῦθι
Pharmouthi
šmw :
Hnsw
Pa-n-ḫns.w
Παχών
Pachons
Hnt-htj
Pa-n-in.t
Παϋνί/Παῦνι
Payni
Ipt-hmt
Ipip
Ἐπιφί/Ἐπείφ
Epiphi
Wep-renpet
Msw-r'
Μεσορή
Mesore

c. 17th century BCE

Sumerian/Akkadian/Babylonian calendar measures years according to the reigns of kings. Each year begins on the first day of the month of Nisānu; the days between the beginning of a new reign and the beginning of Nisānu are noted “from the beginning of the reign.”

From c. 1650–c. 1570 BCE

Venus Tablet of Ammi-ṣaduqa

Venus Tablet of Ammi-ṣaduqaAmmi-ṣaduqa is a Babylonian king. Observations of planet Venus (called “Ištar” in Babylonian, “Inanna” in Sumerian) are recorded for 21 years: the first and last visibility above the horizon before or after sunrise and sunset, etc. Dates are lunar. It is the sixty-third tablet of a series called Enuma Anu Enlil (“When [the gods] Anu and Enlil”), which mostly have to do more with astrology than astronomy…

The dates are according to the Short Chronology.

c. 1370 BCE

Mul Apin

It’s a map of the sky and of constellations on two (or three?) clay tablets. The duration of the day is measured with a water clock, according to the weight of water that flows out of it.

To define the length of a 'night watch' at the summer solstice, one had to pour two mana of water into a cylindrical clepsydra; its emptying indicated the end of the watch. One-sixth of a mana had to be added each succeeding half-month. At equinox, three mana had to be emptied in order to correspond to one watch, and four mana were emptied for each watch of the winter solstitial night.
Neugebauer, Otto. “Studies in Ancient Astronomy. VIII. The Water Clock in Babylonian Astronomy.” Isis. Vol. 37, No. 1–2 (1947): 37–43. * The mana is a Greek weight unit corresponding to about 450 g (1 lb).

𒀯𒀳

The date is according to Bradley Schaefer.
The stars “Mul.Apin” have been identified to the Triangle and to γ And.

c. 1140 BCE

Table of HoursNight hours are counting by observing the transit of specific stars through an imaginary vertical line extending upwards from the head of the assistant astrologer sitting in front of the master astrologer, consulting an “hour table.”

c. 750 BCE

A cycle of 19 solar years or 235 lunar months is noticed: it will be used to determine intercalation years, namely the third, sixth, eighth, eleventh, fourteenth, seventeenth, and nineteenth. Addaru 2 (30 days) is added at the end of each of these years, except for the seventeenth, where a second Ulūlu (of 29 days) is added midyear, to keep the calendar in check with the Sun. The king still announces intercalary months, but is advised by an astronomer.

Intercalation reports are very complete from 623 BCE, and the 19-year cycle is strictly observed (after a period of irregularity, probably due to internal conflicts) from 424 BCE.

This cycle is now called the Meton cycle, from the Greek astronomer who introduced it in the West, but it dates from much earlier… Who knows who its real discoverer is?

Intercalation makes it possible to make sure the beginning of the year (Nisānu 1) is within a few days of the observation of the first new moon after the spring equinox (between March 20 and April 17 according to our modern calendar). It turns out that the month of Nisānu begins the earlier on the 17th year of the cycle.

Over the 19-year cycle, the calendar is precise to one day in 219 years compared to the Sun.

At some point, Babylonians understand that the movements of the Sun and of the Moon on the ecliptic are not uniform, but they do not know why (only in 1610 will Kepler discover that orbits are ellipses). The saros, an 18-year cycle after which eclipses repeat, is also known.

c. 725 BCE

In the Ilyad and the Odyssey, Homer mentions stars now identified to Bootes, the Hyades, Orion, the Pleiades, Sirius, and Ursa Major. About 100 years later, Hesiod adds Arcturus, a star in Bootes, to this list.

c. 550 BCE

Anaximander

AnaximanderAnaximander claims that the Universe is eternal and unchanging. To him, the Earth is a cylinder that is three times as large as it is thick; it floats in the void in the center of the Universe. One of its flat sides is the inhabited world. Strangely, it seems like Anaximander knew that sailors see new stars appearing while traveling south…

Ἀναξίμανδρος
c. 610 BCE–c. 546 BCE

c. 500 BCE

Pythagoras

Pythagoras claims that the Earth revolves around the Sun and is spherical. He figures out that planets Hesperus and Phosphorus are one and the same (our Venus).

Πυθαγόρας
c. 570 BCE–v. 495 BCE

c. 470 BCE

Anaxagoras

The Moon reflects the Sun’s light. The shadow of the Earth causes lunar eclipses. (It was probably suspected already by Pythagoras.)

Ἀναξαγόρας
c. 510 BCE–c. 428 BCE

c. 425 BCE

Orphic poems mention that the Earth is round and that it rotates on its axis in one day, that it has three climate zones, and that the Sun “magnetizes” stars and planets.

Orphism is a religious and philosophical school of thought.

c. 330 BCE

Heraclides

Suggests that Mercury and Venus revolve around the Sun which, like other planets and the Moon, revolves around the Earth. The latter turns on itself in 24 hours.

Ἡρακλείδης, c. 390 BCE–c. 310 BCE

c. 330 BCE

Aristotle

Four elements—fire (hot and dry), water (cold and wet), earth (cold and dry), and air (hot and humid)—make up everything, a theory already proposed by Empedocles (c. 490 BCE–c. 430 BCE). They always move in a straight line. Our “lower world” is always changing and imperfect. All that is “earthly” falls towards the earth; water tends to gather in a sphere around the center; air is found in a sphere around all this. Fire, meanwhile, tends to return to the lunar sphere to which it belongs.

A fifth element—the ether—makes up stars and planets and is never found on the Earth. Its movement is always circular. Except for planets, that move around, the celestial sphere does not change: the heavens are immutable.

This worldview will persist for about 1800 years, and anyone who claims differently is persecuted by religious and state leaders.

Aristotle evaluates, without saying how, the circumference of the Earth to be about 400,000 stadia, which is far off.

Ἀριστοτέλης, 384 BCE–322 BCE

c. 250 BCE

Aristarchus

Believes the Earth rotates around the Sun, but can’t prove it.

Measures the Moon’s diameter and distance. His reasoning is logical, but his measures are inaccurate, which gives false results. Measuring inaccuracy won’t be fixed for another thousand years or so.

Schematic diagram of Aristarchus’ experimentIn order to do so, Aristarchus measures the angle between the Sun and the Moon when the latter is half-lit; he finds a value of 87° (the actual value is about 89.85°). Through Euclidean geometric analysis, Aristarchus determines that the ratio of distances to the Sun and the Moon is between 18 and 20 times—which he rounds to 19 times.

Aristarchus' ExperimentBy analyzing lunar eclipses, Aristarchus determines the Earth’s diameter to be three times greater than the Moon’s, and that the Moon is 20 Earth radii from us. By combining this result with that of the preceding paragraph, he figures that the Sun is 380 Earth radii away, and 6.7 Earth radii wide

Later, with similar methods, Hipparchus obtains 67 Earth radii, and Ptolemy 59, for the Moon’s distance. In fact, the Sun is 109 times larger than the Earth, which is 3.5 times bigger than the Moon. The Moon is 60.32 Earth radii of us, and the Sun 23,500 Earth radii away—nearly 390 times farther than the Moon…

Ἀρίσταρχος, c. 310 BCE–c. 230 BCE

c. 250 BCE

Eratosthenes

Eratosthenes’ ExperienceMeasures the circumference of the Earth, from the angle formed by the shadow of a gnomon planted at Alexandria at noon on summer solstice, while the Sun’s light reaches the bottom of a well at Syene (Aswan). He measures an angle of 150 of a circle (7.2°). The distance between the two cities was measured as 5000 stadia, which yields a circumference of 250,000 stadia (5000 × 50) for the Earth.

Eratosthenes’ ExperienceThere are four possible ways to convert this into modern units: the Olympian stadion (176.4 m); the Ptolemaic/Attic stadion (184.8 m); the Phoenician/Egyptian stadion (210 m); or the itinerary stadion (157.5 m). The first would mean a circumference of 44,100 km (10% more than the actual 40,008 km polar circumference); with the second, we get 46,100 km (15% over); with the third one, the error is huge: we get 52,500 km. However, with the itinerary stadion, we get 39,375 km, which is about 1.6% lower than the modern value.

Even though it’s pretty good for such a rudimentary method, we now know that Alexandria and Syene are not directly on the same meridian, and that the Earth is not perfectly spherical…

Oddly, Eratosthenes later rounds his result to 700 stadia per degree, for a circumference of 252,000 stadia, probably to ease calculations, as the latter is more easily divisible by 60 (= 4200); this gives a modern equivalent of 39 690 km.

A stadium is equal to 600 Greek feet… the length of which depends on the date and the place!

Ἐρατοσθένης, c. 276 BCE–c. 194 BCE

127 BCE

Hipparchus

Builds up his own catalog of stellar positions through his own observations of at least 850 stars with an armillary sphere. Compares his data with those of Timocharis (Τιμόχαρις, c. 320 BCE–c. 260 BCE) and Aristyllus (ίρίστυλλος, c. 260 BCE). Notes that stars seem to have moved about 2° since their time.

Also notes that the Sun returns to the same equinox (tropical year, 365,24217 solar days [2000]) in a different time than it returns next to the same star (sidereal year; 365,25636 solar days [2000]). Concludes that the equinoxes “precess” along the zodiac, a movement he considers to be less than 1° per century (the real value is 1° in about 72 years).

Ἵππαρχος, c. 190 BCE–c. 120 BCE

c. 100 BCE

Posidonius

Measuring the height of the star Canopus (the second brightest in the sky) above the horizon in Alexandria (7½°) and taking into account the fact that it skims the horizon from Rhodes, which Posidonius believes to be 5000 stadia exactly north, he calculates the Earth’s circumference to be 240,000 stadia (maybe 39,000 km). In fact, Posidonius was mistaken because the Rhodes–Alexandria distance is really 596 km (and Alexandria is SSE, not south, of Rhodes); if the angle between the two was really 7½°, the Earth’s circumference would be (360 ÷ 7½) × 596 = 28,608 km…

Strabon (Στράβων; 64 BCE–c. 24 CE) eventually notes this error too: Rhodes is 3750 stadia from Alexandria, yielding a circumference of 180,000 stadia (about 29,000 km).

Claudius Ptolemy (Κλαύδιος Πτολεμαῖος; c. 100 CE–c. 170 CE) keeps these values, and the error persists until Columbus in 1492, who believes India to be only 70,000 stadia from Europe (about 11,375 km, while the real distance through the Atlantic is about 27,000 km). Columbus arrived in America after “only” 9000 km or so, close enough to his estimate that he believes himself in India…

Ποσειδώνιος, c. 135 BCE–c. 51 BCE

c. 100 ÈC

Claudius Ptolemy

Develops on previous astronomers’ heliocentric system, with eccentrics, to which he adds epicycles and deferents. Earth is a sphere in the centre of everything.

Publishes a “Mathematical Treatise” (Μαθηματικὴ Σύνταξις, Mathēmatikē Syntaxis), later renamed “The Great Treatise” (Ἡ Μεγάλη Σύνταξις, Hē Megalē Syntaxis), whose superlative form (μεγίστη, megiste, “the greatest”) is converted to Arabic: al‑majisṭī (المجسطي), which became “Almagest.” It is the sum of everything then known about astronomy. It contains a list of longitudes and latitudes for Mediterranean places, so precise that it will be unsurpassed for about 800 years.

Κλαύδιος Πτολεμαῖος, c. 100 CE–c. 170 CE

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