What’s the difference between Astronomy and Astrology

I have always thought that the word Astrology should be the one we use instead of Astronomy. Unfortunately, history decided otherwise and made the wrong choice way back when the science was given a name by the Greeks.

After the Greeks, the Romans used the word which they integrated in their language: Latin. The word was kept by the Gauls who become the French (Astronomie et Astrologie). Since Guillaume the Conqueror won his conquest of the English Throne in 1066 (he was crowned on Christmas Day of that year,) these words also passed to the English language.


The two words are composed of two parts: astro-, -nomie, and astro-, -logy.


The word Astron comes from Greek, it was then used in Latin, French, and finally in English.

In Greek, it primarily means Star, just like in English. But it also meant Planet and Constellation.

The use of the word in French (un astre—the word aster was also used with that meaning in English, but is now considered obsolete, it is still used for the flowers, though,) clearly has the broader meaning of any celestial body in the universe, so it can represent a star, a planet, an asteroid, a moon, a galaxy, etc. This is definitely the meaning used in the word Astronomy now a day.

Of Note: The English word Astral, which means Star from Latin “Astrum” also comes from Astron in Greek. Not to mix-up with the word Astral in French which is an adjective meaning any type of celestial body (i.e. it is what we call a false friend in linguistic.)


Again, this word comes from Greek. The word Nomos in Greek means arranging. Although today the word Astronomy means various things such as indexing, passion for stargazing, or learning about the Universe.

Just like with Astro-, the word became Latin (Nomia), then French (-nomie) and finally English (-nomy).


Once more, this word comes from Ancient Greek. The word -logía was used as a suffix meaning “the study of” or “treating of”.

So Astrology means: the study of the stars.

The Greek word was created after the known science from Egypt. That’s around the time the Horoscope was born (first references to Horoscopes were found in Egypt around 332 BC.)

Actually, until the mid 2nd or 1st century BC, the meaning of the word was not linked to the study of the effect of stars and other celestial bodies directly on humans, animals, or objects. It was the study of the location of celestial bodies at a given time, which helped in trying to figure out what was happening in the skies and whether that had an effect to what was happening on the Earth.

The science had two parts: (1) really scientific that defined the season based on the location and what we could see in the skies; and (2) the divine signs: what the Gods are deciding for us, what your future will hold (fate), etc.

In English, you actually have a way to distinguish between both:

Natural Astrology—the older meaning (4,000 years), which was to look at what effects the stars, planets, and constellations had on nature; used to create precise calendars of the change of season and lunar cycles, including lunar eclipses.

Judicial Astrology—the newer meaning: which is the reading of divine signs based on the position of planets and other celestial bodies; divine signs started some 3,800 years ago, and 2,340 years ago,  puny humans invented the Horoscope to help them determine their fate.

Somehow we do not make that distinction in French, even though the English word comes from the French word.

Astrology History

Lunar Cycles (c. 25,000 BC)

We have discovered drawings in caves and on bones. This basic knowledge helped farmer greatly since knowing about seasons allows you to plant your seeds at the right time of the year. The could also notice the correlation between the lunar cycle and the tides or river flows (and thus floods.)

Temples Raised with Celestial Knowledge (c. 3,000 BC)

There are temples that were built in alignment to the heliacal rising of specific stars. This gives a way for the people to know the date with great precision.

This kind of event is what allowed the Greeks and Romans to create calendars that included a leap year every 4 years. They noticed that after a few years, the heliacal event would happen one day late. Then after another 4 years, another day late… A simple fix was to use a leap year. Of course, it took much longer for us to notice that was not quite correct. But you have to agree that an event that takes place only once every 4 years would not be easy to find out.

Calendar Usage (c. 2,000 BC)

So the first reason for using Astrology was the study the position of the stars, planets, and other celestial bodies. It was used in keeping abreast of exactly when the next season was going to be, which was more and more important as more and more people turned to agriculture.

For more advanced cultures, it was used to predicting celestial events such as the next lunar eclipse.

Today, for all of that, we now have atomic clocks, which give us a little more precise in time keeping. So much that we even invented leap seconds (necessary for satellite navigation.)

From writings, we can tell that the study of stars to track what will happen in nature, such as the seasons, goes as far back as 2,000 BC, Mesopotamia Egypt. We think that older records were most certainly used the same way but we do not have proof of such.

The Divine in the Skies (c. 1,800 BC)

Of course, there was only one step to go from being able to predict seasons to seeing signs of divine communications in the skies. This is certainly where Astrology started taking the wrong turn. Actually, at some point, the meaning of the word Astrology was viewed as being “star-divination”.

The Mayan Calendar (500 BC)

In parallel, the Maya culture also developed a very precise calendar (more precises than the Gregorian Calendar!) and had various valid predictions such as eclipses, although it does not look like they invented something that could be as clearly defined as Astrology, they also incorporated divine entities in their calendar.

The Mayan Calendar had an incredible precision for the time.

Some Guatemalans and Mexicans still make use of the Mayan calendar to this day.

The Mayan codices show that they knew of Venus, Saturn, Jupiter, and Mars. Venus was an important part of their mythology, representing the patron of war. They also had records of the Milky Way.

The Discovery of the Horoscope (332 BC)

Later in Alexandria (so still in Egypt but at the time of Alexander the Great,) came the invention of the Horoscope around 332 BC. The definition of an Horoscope is the visual representations of the celestial bodies on a given day. Depending on those positions various people, called astronomers, would start making divine predictions of what would happen to someone, a creature, an object, or even an outcome to an event such as an election. The date used to make these predictions was the birth date of the person in question.

In other words, these were maps of the heavens at work!

This is the birth of what we know today as modern astrology.

The Spread of Astrology

Note that many cultures have record relating to astrology. It all started in Egypt and was incorporated in the Greek culture when Egypt was taken over.

Alexander the Great (c. 334 BC)

The chronology shows that the science of Astrology was transferred to India and China as Alexander the Great battled there.

The spread of Astrology throughout China was much less violent than the introduction by Alexander the Great!

Conquest of Alexandria by Arabs (641 AD)

Later, the Arab world took account of astrology after the conquest of Alexandria by their leader: Umar the Caliph. They translated everything about Astrology to Arabic and Persian.

Why would Romans not spread Astrology to Europe? (c. 50 BC to c. 400 AD)

The Romans also knew the science of Astrology through the Greeks, but it did not spread to all of Europe at that point in time. We waited until the 12th century (see below) before the science got reintroducing from the Arabic text and finally spread to all of Western Europe.

This is probably due to the fact that some people argued that Astrology was not a true science.

Carneade (213 or 214 BC – 128 or 129 BC) argues that Astrology means your fate is already known and fate denies freewill and morality. Like Empiricus later, that people born at quite different times could all die at the same time in an accident or a battle. He also can’t but notice that with Astrology everything looks like it would follow a perfect pattern, meaning that all humans should be alike, live very similar lives, quite untrue as every known tribe and culture, even at the time, were quite different.

Cicero (106 BC – 43 BC) and Saint Augustine (354 – 430) showed that Astrology could not be a true science. Cicero pointed out that many times twins never have a similar life and yet both are born on the exact same day, just a few minutes or an hour or two apart, so their horoscope had to be nearly the same, if not exactly, and yet they clearly never were. Further he argued that distant planets could not have much of an effect on a person, maybe the moon which is very close; surely, parenting had a strong influence on people already in Roman time, and medicine even if still precarious was already doing wonders; and even just the effect of the weather on people would change their fate, not distant planets.

Favorinus (c. 80 – c. 160), of Gaulish ancestry, argued that it was absurd to think that celestial bodies could affect humans like they affect the tides or the fact that very small motions of distant celestial objects would cause big changes in a person’s fate.

The first proof that humans thought that there are possible side effects of the Moon cycle on humans dates back to the Assyrians/Babylonians. However, so far no scientific evidence has been able to prove that something does indeed happen. Note that the tidal pull of the moon works on the ocean because the ocean is enormous. On us, it has an infinitesimal effect, no more pull than what a mosquito biting you would do to your blood stream. That’s because we’re tiny compared to the ocean. Nothing that would cause diseases, hallucinations, etc. As for moonlight, same thing. It has no special effects. It’s much less powerful than sunlight during the day, by the way. Those rays can burn you if you don’t properly protect your skin!

Sextus Empiricus (160 – 210) argued  that Astrology was a myth. He had many empiric arguments as we call them now (yes, word that was derived from his name.) All of which are still in use today. Some of these were:

  • Sympathy does not exist anywhere;
  • If fate existed then all things are destined and that’s the only way Astrology would work, but forecasting sure events is useless since they cannot be avoided;
  • Forecasting unsure events is useless too since they can be avoided (think of Star Trek and time travel…);
  • Why is birth important instead of the date of conception?
  • Birth time is uncertain as birth is a long process: is it when contractions start, when the water breaks, when we start seeing the head, once the baby is fully out, what happens when we do a cesarean? did we just change the baby’s fate? (Note that cesareans were already performed at the time if the mother died still with child—the oldest recorded cesarean would have been in India, giving birth to Bindusara c. 320 BC, who became the second Emperor of India; another, but fairly unsure, could have happened in China c. 1030 BC, Jilian who founded the house of Mi);
  • The sky cannot be observed during daytime or on a cloudy night (I suppose this is one of the weakest arguments since we now have satellites and could observe at any time of day and night, cloudy or not. Also in his time we could calculate the position with enough accuracy that we would know);
  • Many people are born at the same time but there was only one Plato and one Alexander the Great (and luckily one Hitler);
  • Many people without the same birth date still meet the same end (all killed by a volcano, in a war, by a Tsunami…)
  • Astrology analogies, such as Leo are brave like lions, are just plain absurd;
  • Astrology is meaningless unless the same planetary position produce the same results every single time.

One interesting point about the date of birth (and time) is that we now can now tell time with incredible precision, but we still cannot really tell when a baby was born. However, in regard to the date, the calendar they had at the time was wrong. In other words, Astrologers would always end up using the wrong date of birth anyway since the calendar itself was broken.

Plotinus (204 or 205 AD – 270 AD) also wrote about the problem of causal Astrology. He argues that someone’s fate cannot be influence by a star in a perfect universe. As for the moon, he explains, quite rightly, that when it wanes it is still receiving sunlight on 50% of its surface and therefore that state cannot have any specific effect as describe in the science of Astrology. He also claims that stars and planets are ensouled.

Well… Another possibility for the lack of Astrology in other parts of Europe could be that many Romans and Gauls still used Gallic Soothsayers as their source for prophetic oracles.

Arabic Texts Enter Europe (12th century)

In the 12th century, the Arabic texts made it back to Europe where it was translated to Latin and used as science.

Note that well known people such as Tycho Brahe, Johannes Kepler, and Galileo Galilei, were astronomers in their own right. At the time this was considered a science by most and remained that way for about 400 years.

For that while astrology was part of the scholarly tradition and it was common in most academic circles. It was viewed at the same level as astronomy, alchemy, meteorology, and medicine. Of course, today, alchemy,  which was replaced by chemistry, is pretty much in the same boat as astrology.

The Enlightenment Period (c. 1690 AD)

By the end of the 17th century, the greater majority of scientists agreed about the fact that Astrology was no science and it falls in discourse. This is in part due to Isaac Newton three laws of motion. It is also the time when Heliocentrism is finally the accepted model by the scientific community (i.e. the Sun is the center of our solar system.) This changes how one can perceive Astrology which cannot view stars as objects moving around the Earth. Instead, stars are quite stationary in this new model, at least as far as we’re concerned.

Astrology wasn’t completely forgotten but the elite stopped using it. Many wrote about how inconsistent and untrustworthy it was. Others continued to support it without much traction.

Popular Astrology (c. 1800 AD)


More people learn how to read.

The press has daily newspapers going out.

Newspapers need content that their readers read. They introduce the Horoscope page. By popular demand, Astronomy makes a strong come back.

Even the elite comes back to Astrology.

Astrology: No Scientific Evidence (20th Century)

In the 20th Century, we finally had what we call the scientific method.

This is a way to use math, physics, and experiments to prove that something is true or not true.

Astrology did not pass the test.

Challenged on theoretical and experimental grounds, Astrology passed neither test.

At that point Astrology lost all of its academic standing. And even popular belief in it has drastically declined. Instead, it has earned the status of pseudoscience.

Japan: Astrology Visible Effect (1966)

In Japan, popular belief that giving birth to a daughter on the hinoeuma—Fire Horse—year leads to girls who are impossible to marry. The number of birth in the year of 1966 dropped by more than 25% as parents were trying to avoid giving birth to a daughter on the wrong year.

There are also been years with increase abortions due to similar believes.

India: It is a science (2011)

In 2001, India introduced Astrology as a science class in their universities.

In 2011, a court ruled that Astrology is a science, reinforcing the fact of the matter that universities can teach Astrology as such.

Astronomy History

The term Astronomy is Greek. The science as we know it today started in the antiquity. However, much older objects have been found clearly depicting celestial objects.

Oldest Known Records of Celestial Objects

Since 1990, we found objects from the Neolithic and Bronze Age that depict knowledge of Celestial Objects.

Carved Bones (c 35,000 BC)

The oldest artifacts we’ve found so far are bone sticks. These were carved showing the moon’s phases.

Warren Field Calendar (c 8,000 BC until c 2,000 BC)

The oldest known calendar is the Warren Field Calendar found in the Dee River valley in Scotland. It was first created in the Mesolithic period (between the Paleolithic and the Neolithic periods.)

The calendar was used for a period of some 6,000 years. This is a very long period for such an early calendar and it looks like it was updated from time to time to match the position of the stars changing over the years.

The calendar is composed of 12 pits, which we think helped in tracking the phases of the moon each one representing a month. The monument is aligned with the winter solstice thus coordinating the solar and lunar cycles.

Gosek Ring (c 4,900 BC to c 4,700 BC)

Located in Germany, the Gosek Ring was a wood structure, a tall palisades built  in the Neolithic. The doors of the structure were built to match sunrise and sunset on the summer and winter solstices.

We now found hundreds of sites using the same type of ring and door positions.

Nebra Sky Disk (c 1,600 BC)

The Nebra Sky Disk is made of bronze. It dates from the Bronze period. It was found in Germany, not for from the Gosek Ring.

The disk weights 2.2 kg (4.9 lbs) and is around 30 cm (11.8 in) in diameter.

Our understanding of the object is that it was created to represent the sky and used for about 400 years before it was buried. However, by the time it was buried, it was probably not understood for what it was.

The disk depicts a lunisolar calendar. A way to track the solar periods by using the moon and getting a year duration of about 365.3 to 365.5 days.

This information would have given the users the knowledge of how to add a leap month to their calendar. This is the oldest known artifact presenting this knowledge before any other known method.

Kokino (c 1,900 BC)

Discovered by Jovica Stankovski in 2001, the site is an ancient astronomical observatory. It was used by the community that lived nearby for 1,200 years.

The central space was used to observe the rising of the sun and the full moon.

Markings were used to detect the summer and winter solstices. Other marks matched the minimum and maximum declination of the full moon. They could measure the length of the lunar months.

Used together, this would allow the observers to reconcile the cycles for 235 lunations that occur in 19 solar years. In other words, one could track all the yearly events as required.

The site also included another part with thrones, one of which would receive light around July 31st (mid-summer) which apparently was a time of a ceremony and also the date when crops stop growing.

Golden Hats (c 1,400 BC to 800 BC)

Figure: The Golden hat found in Avanton, France in 1844 (this is near Poitier). From the Bronze age, around 1,000 BC to 900 BC. We only found four such hats in Western Europe.  The hat is probably made of some organic materials that were gold plated with thin gold sheets. Picture by Calame.

Linked to areas now known as France, Germany, and Switzerland, Golden Hats are decorated with motifs of the Sun and the Moon.

We have one such hat that was well preserved and the systematic sequence of numbers and ornaments makes us think that these were lunisolar calendars. The object can be used to determine dates in lunar or solar calendars.

The Berlin Golden Hat is the last one we’ve found so far. It is believed that it was found  in South German (Swabia) or Switzerland. Museum für Vor- und Frühgeschichte purchased it in 1996. Unfortunately, the dig was made by rogues and not scientists so we do not have very much information about the source of this item.

Astronomy With Written Records

So far I talked about objects that we found in various places and that clearly present astronomical records in some way.. Now I will talk about written records that gives Astronomy its name (i.e. Astro- Celestrial Object and -nomy Indexing/Recording).

The first astronomical records appear around 1,200 BC in what is now the Middle East.

Egyptians (c 3,000 BC)

Although we do not have any writing that are as old as 3,000 BC, even from the Egyptians, that would describe how astronomy was used by the Egyptians, it is clear that they knew quite a bit about the celestial objects since they built pyramids and temples that would be perfectly aligned with the Pole Star.

Note that at the time, the Pole Star was Thuban. Today it is Polaris, name that comes from New Latin (18th century) and meaning “pole star”. Polaris is often called the North Star since it indicates the North with enough precision to navigate boats. It’s particularity is that its position stays fixed in the night sky. There is no such star in the Southern Hemisphere. The closest star for Australians to watch for is Sigma Octantis. It is just not as precise as Polaris.

We found several books that show that the Egyptians took notes of the positions and phases of the Sun, Moon, and Stars.

One specific star, Sirius, was clearly describe as the start of the inundations of the Nile River. A very important even in Ancient Egypt.

Egyptians were certainly more Astrologers than Astronomers, but many of their records of stars remain. Such records are clearly Astronomy. For example, Rameses VI and Rameses IX tombs had tables of stars on their ceilings. These stars were used to tell time at night. Seated looking at the Pole Star straight ahead and with another person seated in front of you, you would be able to see various stars from the top of the other person’s head, from his left shoulder, from his right shoulder, … and by determining the name of those stars, that person would be able to tell time.

In order to have a better line of sight and thus higher precision, Egyptians used an apparatus made of a plumb line and a sighting instrument, probably a rudimentary dioptra, a telescope without magnifying lenses that has gradations that gives the angle of the light of sight (Egyptians had lenses that they used on their statues as eyes, but those were not transparent, so they could not have had telescopes with lenses.) Without the magnifying glass, you would still have a good visor, increasing precision, instead of just using your naked eye at random.

Mesopotamian Astronomy (c 1,200 BC)

The Babylonians had star catalogs dating from about 1,200 BC written in cuneiform, a way of writing that Sumerians invented between 3,500 BC and 3,000 BC.

Although the oldest written document we found so far dates from the Babylonian period (circa 1,200 BC),  they wrote about those stars using Sumerian god names. We are thinking there is a strong chance that indexing the stars started as early as 3,500 BC to 3,000 BC.

Astral theology started with the Sumerians. They gave constellation names and the Mesopotamian mythology follow suit with those same names.

Something else stuck around: their numerical system, the a sexagesimal base. The 360° in a circle comes from their system. The sky was divided in 6 x 60 quadrants. Their base of 60 was used specifically for describing the sky and give position to the degree.

Although the Sumerians and Babylonians were recording star names and positions, they were really doing Astrology. In other words, they were already linking stars to the divine.

The Babylonians were the first to describe the fact that periodic events and using mathematics to calculate the duration of a day all year long. This took years of recording. We actually have lists of stars from a period spanning several centuries. In that regard, the most important find were the tablets known as the Enūma Any Enlil.

The oldest quite significant work is a tablet that shows the time of Venus raising over a period of about 21 years.

The Babylonian records actually includes not just records of stars but also formulae used to predict (a.k.a. calculate) the position of stars. They could determine the duration of daylight.

During the reign of Nabonassar (747 BC to 733 BC), there were significant advances in Babylonian astronomy. Their recording methods improved increasing the quality of their index. From that data they found that lunar eclipses had a cycle of 18 years.

Some 300 years later, with all the recording of the past, many calculations could be used to predict the position of planets. Seleucus of Seleucia, an astronomer at the time, was the first recorded proponent of the Heliocentric system (i.e. the system where the Sun is at the center and planets turn around it. Circa 400 BC)

Chinese Astronomers (c 550 BC)

Chinese records of astronomical observations started around 550 BC.

They were able to predict solar eclipses.

Early on, the main purposes of astronomy was to keep track of time. Astronomers were in charge of the calendar. Since they had a lunisolar calendar, it was required that astronomers update the calendar constantly.

Astrological divination was also part of the work and was viewed as an important part of it. Alexander the Great also introduced a modern concept of Astrology with their Horoscope.

Contrary to their European counter part, Chinese astronomers also recorded anomalies, what they called Guest Stars. This is how they ended up recording the very first supernova in 185 AD and the Crab Nebula supernova in 1054 AD.

The oldest known star catalogue was made by Gan De, a Chinese Astronomer who liver in the 4th century BC.

Ancient Greeks (Starting around 350 BC)

Ancient Greeks developed astronomy a great deal treating that science as a branch of mathematics (just like Astrophysics today can be viewed as a branch of Physics!)

The first advanced 3d model was created by Eudoxus of Cnidus and Callippus of Cyzicus. They model (unfortunately?) was homocentric with the Earth as the center.

Heraclides Ponticus (390 BC – 310 BC) proposed that the Earth rotates around its axis.

It is also around that time that Aristotle wrote about the Eudoxus model and reinforced the idea of an homocentric system with the Earth in its center. This is how Astronomy was struck by bad luck for the following 1,800 years… even though many others, such as Aristarchus of Samos, proposed the heliocentric system.

Around 240 BC, Eratosthenes using shadows created at various locations, sufficiently distant from each others, actually estimated the circumference of the Earth with great accuracy (note that this means many people at the time knew that the Earth was not flat.) About 15 years earlier, Eratosthenes invented the Armillary Sphere, a way to model the cosmos using rings. This is different from a smooth sphere that would just represent the constellations. The Ancient Chinese separately invented the Armillary Sphere.

Hipparchus of Nicea (c 190 BC – c 120 BC) created the first star catalog that included the apparent magnitude of each object which we still use today. The brightest of two stars of the same type and that reside at the same distance from Earth is the biggest star. However, were the biggest star further away, it could appear to have the same brightness as the smaller star.

Antikithera Mechanism (between 250 BC and 70 BC)
Antikythera model by Mogi Vicentini (2007)
This is a model by Mogi Vicentini (2007) reproducing what the Antikythera would have been capable of doing some 2,000 years ago. We can see the crank at the top-right and three panels showing the Earth, Sun, and Moon.

The Antikythera mechanism was found on May 17, 1902 by  archeologist Valerios Stais. It is thought that it was aboard a boat that was going to Rome to present the relic to the first Roman Emperor, Augustus.

The device may have been built around 200 years earlier.

We used various types of modern technologies, such as X-Rays, in an attempt to detect as many details as possible about the device. Things such as inscriptions and how many teeth are found on the gears to help us determine how the device worked.

We think the mechanism was more of an experiment, although it could be used to show (calculate) the position of the Sun and the Moon by rotating the 37 gear system. This means they could just turn a crank and see when the next lunar eclipse would occur. The experiment idea comes from two facts: (1) the device is mentioned in literature but not described; (2) there was only one built (that we know of at least) whereas, had it been viewed as very useful, people at the time would have created replica.

Further, mechanical engineer Michael Wright demonstrated that the moon included an additional mechanism that would show the lunar phase. The mechanism needed a differential gear system (one gear that rotates two others at different speeds using shafts) which was thought to have been discovered only in the 16th century.

This device is often referenced as the first astronomical computer. In complexity, it compares to clocks of the 18th century.

Middle East (Medieval)

The Arabic and Persian world under Islam was prosperous and as side effect allowed for a set of people to become highly cultured in many areas, including astronomy.

A lot of the astronomical knowledge was drawn from Greece, Persia, and India. The works of others were translated to Arabic and copied many times to fill libraries all around the Arab World of the time.

In the 10th century, Abd al-Rahman al-Sufi created a star catalog with each star position, magnitude, and color. The catalog includes the draws of constellations and the description of a small cloud. We now know that this cloud is the Andromeda Galaxy. It is likely that others had recorded Andromeda earlier.

Ali Ibn Ridwan observed a temporary star in 1006. It is now known as SN 1006, the brightest supernovae ever recorded.

At the same time, Abu-Mahmud al-Khujandi had an observatory build not too far of Tehran, Iran. He observed series of meridian transits of the Sun. This allowed al-Khujandi to calculate the tilt of the Earth with higher precision than his predecessors.

In the 11th century, Omar Khayyàm used many records that were available to him to offer a calendar reform which was close to the Gregorian calendar.

Ja’far Muhammad ibn Mūsā ibn Shākir was the first to hypothesize that all celestial bodies were submitted to the same physical laws as the Earth.

The Arabs also introduced the exacting of empirical observation and experimental technique. Certainly a big step for science as we know it today.

In the 11th and following centuries, astronomy was separated from the Natural Philosophy as imposed by Aristotle over 1,000 years earlier. Arabs started Astronomical Physics, or what is now known as Astrophysics.

Medieval Western Europe

We have clear records of astronomy in Roman times which stopped around 4th century and it has been thought that did not change until 12th century when Arabic books were discovered and brought to Europe.

One of the problems was that most of the astronomy documents available at the time were written in Ancient Greek and that language was lost by that time. With time, those texts were translated to Latin which was what intellectuals at the time could read. At first, the most useful texts were translated, such as determining the day of the year and the time at night.

By the 9th century, astronomers were interested again in calculating the position of the planets. This interest was in part due to the financing offered by the Emperor Charlemagne to research old libraries about Romans Astronomy.

Interestingly enough, the astronomer of the 9th century found quite a few mistakes in the equations but still spread the word because these calculations were much better than nothing.

Astrolabe quadrant from England 1388 by PHGCOM -- https://en.wikipedia.org/wiki/File:Astrolabe_quadrant_England_1388.jpg
Astrolabe quadrant from England 1388

In the 10th century, Gerbert of Aurillac went to Spain and Sicilly, which at the time were controlled by the Arabs. He learned of similar documents used to keep track of the calendar and time during the day and night. This is around the time the Astrolabe that the Arabs had for a while made its entry in Western Europe. Now we use computers which are much better, faster, way more accurate, but back then using an Astrolabe was way faster than doing all the math all over again (an Astrolabe was like an advanced ruler.)

This generated a large number of documents on how to build an Astrolabe and ways to validate when eclipses will occur.

With two century, most of the Arabic knowledge was translated in Latin and entering these new places called Universities.

The 14th and 15th centuries had various thinkers, such as Nicole Oresme and Nicholas of Cusa, that offered to move away from Aristotle thinking that the Earth was the center of the Universe. That only the heavens (stars) were fixed.

This was a little before Gallileo who came about at the time of the Renaissance.

Renaissance Period

Nicolaus Copernicus is the first who reintroduced the heliocentric system but this time it was done with a large book, De Revolutionibus, which described the planetary system mathematically. This was based on work from before Ptolemy and used newer advancement in mathematics of the day.

Later Galileo Galilei and Johannes Kepler not only defended Nicolaus Copernicus work, but they also expended upon it.

Galileo was one of the first who used telescopes to observe our solar system. He actually had a 20× refractor telescope that he built himself (i.e. a refractor telescope has a magnifying lens at the top and a small mirror at the bottom that reflects the image of celestial objects to the observer.) This powerful device (for the time) allowed him to discover:

  • The four largest moons of Jupiter in 1610. This was the very first time someone found a natural satellite orbiting another planet and he found four of them!
  • The rings around Saturn, which he could not explain as his telescope was not powerful enough to really see what those could possibly be.
  • The craters on the surface of our Moon.
  • The phases of Venus.

The phases of Venus were one of the reasons Galileo insisted on the fact that the solar system was heliocentric and not centered around the Earth. The only way that Venus could have phases as observed by Galileo was if Venus and the Earth were both turning around the Sun.

Although Galileo insisted that he was the one who discovered sunspots, he merely rediscovered them. Sunspots were first mentioned in I Ching, c. 1,000 BC—clear Chinese records appear later in other documents around 364 BC. Back in Europe Theophrastus also mentioned sunspots around the same time as the Chinese. In the 9th Century, Arabs and Europeans made notes of those too… so Galileo did not discover sunspots even if some history documents still say he did. That being said, Galileo was the one who was right when he write that the sunspots where part of the Sun and not natural satellites. Especially, when looking at the spots on the edges of the Sun, they would be very thin, whereas a satellite would keep the same circumference. He also used geometry to show that it makes sense that the spots would accelerate and slow down or grow and shrink if they are on the surface of the Sun. Although, of course, he did not know why/what those spots were (nuclear physics did not exist back then!)

This is also where Galileo went further and said that these sunspots were yet another reason to doubt the orthodox Aristotelian celestial physics, which posited that celestial objects were of unchanging perfection. We now know that at some point stars explode on their own weight creating supernovae and later a white giant, a neutron star, or in case of really large stars, a black hole.

As the renaissance started to wane, Johannes Kepler worked on ways to mathematically explains the movements of planets. We already had qualitative movements figured out since Aristotle (in his Metaphysics and On the Heavens treatises), but those calculations were determine from observations, not properties of the concerned celestial objects.

This is when Kepler found the three laws of planetary motion, which now carry his name. Isaac Newton developed further Kepler’s work with his discovery of the universal gravitation. Isaac realized that the force holding the Moon in place was the same as the one pulling an apple to the ground of the Earth. This work appears in Newton’s Philosophiae Naturalis Principia Mathematica.

Kepler set the first stone and Newton’s work lay the foundation to the modern Physics and Astrophysics sciences.

Newton’s law not Accepted?

For a while, other astronomers did not accept Newton’s law.

First of all, they knew better (hmmm). Not only that, they had their own philosophy on the matter.

Not to mention that the math offered by Newton did not work quite right. Jupiter and Saturn would get off their track all the time (unexplained perturbations.)

In 1748, the French Academy of Sciences offered a reward to whomever would be able to explain why the two large planets wobbled. Euler and Lagrange eventually resolved that problem: Uranus, which was observed in 1781. Uranus also had some perturbations which led to the discovery of Neptune.

The gap between Mars and Jupiter was found to include asteroids. We found Ceres and Pallas in 1801.

This is around the time that the new astronomy concepts reached the Americas (1659). Until then, Aristotle was still very much what was used on the other side of the Atlantic.

Modern Time (19th-20th centuries)

In the 19th century, we finally understood the prism of colors and started decomposing light from the Sun and found out that some colors were missing. Compared with experiences on Earth, we found that certain gases here would block the same light as the Sun. These gases were hydrogen and helium.

It is only once we discovered quantum physics that we got an explanation for light decomposition. We have since used color decomposing to find that space included a lot of different matter including sugars.

On the 19th and 20th century, we also started seeing women enter the Astronomy field. Although most (all?) were not recognized for their work until much later. Their work included:

  • Measuring distances outside of the solar system (cepheid variable by Henrietta Swan Leavitt),
  • Stellar spectra according to stellar temperature (Annie Jump Cannon),
  • Discovered a comet (Maria Mitchell),
  • Found many new stars never catalogued before (Annie Jump Cannon).

Most of our existing cosmological knowledge was discovered in the 20th century. With the help of very large ground and space telescopes reading all sorts of waves in many ranges and all directions, we got an incredible opportunity in finding an enormous amount of astronomical data way beyond listing a few planets and stars as done so far.

These observations led to the discovery of a staggering number of galaxies and the fact that our own Sun is hovering in the Milky Way, at the end of one arm, going up and down our galaxy plane every 60 million years. That means we are never at the same location in the Universe. We are in constant movement and never coming back to where we were before (Even if you’re still on your chair reading this page!)

This is when Astrophysics (at times called physical cosmology) made huge advances. One of the main questions was the origin of the Universe which is now strongly thought to be the big bang. Although still debated (and probably always will be) we have a plethora of reasons to think that this is the likely source of our Universe.

Redshift of distant galaxies (1842)

Christian Doppler first explain what we now call the Doppler effect. The fact that a wave seems to be long as an object emitting a sound wave goes away from you, and is shorten as the object comes toward you.

When that sound effect is translated to light, it is called Redshift. Although a celestial object coming closer to you will emit a Blueshift, we reference the phenomena as Redshift because most of the celestial objects move away from us.

Hippolyte Fyzeau is the first physicist who noticed the Redshift in 1848.

William Huggins was the first to determine the speed of a star moving away from the Earth in 1868. The Redshift and distance between the start and the Earth can be used for that purpose.

We were able to confirm the Redshift phenomena with our Sun in 1871 . Our Sun rotates so part of the light will be moving toward us and part will be moving away from us (on the edges of the Sun.) We detected a shift of 0.1 Å in the red.

In 1887, two astronomers, Vogel & Scheiner, discovered the Annual Doppler Effect. The speed at which the Earth moves has an effect on the evaluation of the Redshift of stars near the ecliptic. This applies to stars in our Galaxy. The movement of our Sun also has an effect on stars further away.

Aristarkh Belopolsky found a way in 1901 to run a laboratory experiment that would prove the Redshift effect with a system of moving mirrors.

In 1912 Vesto Slipher started observations of nebulae (a.k.a. other Galaxies) and noticed a strong Redshift in pretty much all of them. Andromeda having a negative Redshift of -300 km/s, meaning that it is approaching us. In fact, the current estimates (2012) are that Andromeda is coming toward us at 100 km/s and would collide with the Milky Way in about 3.5 to 4 billion years. Note that the collision would not destroy stars in the arms where we are (not like we’d still exist in that many billion of years anyway…) The lack of precision comes from the possible error in the data we collected with the Hubble Space Telescope (HST). Our current estimates place the collision closer to 4 billion years. Although of course such a collision will probably take billions of years to complete. Note that there are other smaller galaxies that are pretty much now colliding with the Milky Way.

Drawing from Vesto’s work, Edwin Hubble discovered an approximate relationship between the Redshift of Galaxies and the distance between Earth and these Galaxies. We call the results Hubble’s Law.

Alexander Friedmann came up with equations in 1922/1924 drawing from Einstein’s work on field equations of gravitation as well as general relativity that govern the expansion of space in homogeneous and isotropic models of the Universe. These equations support the theory that the Universe is in constant expansion and of the Big Bang.

Cosmic Microwave Background Radiation (1964)

The cosmic microwave background (CMB) radiation forms a repeated source of data all around us, which points to a grows of our Universe from a single point. (“point”, well the Universe was not a point anymore, but it was a time before we had stars and galaxies.)

The source of that data is not a star or a galaxy, so it has to come from an event that occurred prior to star and galaxy formation.

The CMB would have occurred when the Universe was about 379,000 years old (so 379,000 years after the Big Bang.) At that point the Universe cooled down to a mere 3,000 Kelvin. This allowed electrons to combine with protons forming hydrogen atoms: matter! (until then the Universe was a plasma.) Since photons did not interact with those neutral atoms, these atoms started traveling away freely. This decoupling of matter generated a radiation which is what we pick up today as the CMB radiation.

Cosmological Abundance of Elements

An interesting aspect of the Universe is the fact that each type of element is clearly limited. Not only that, but of all the elements we know of today, the only ones created in the very early Universe were:

Name Atom Radioactive Quantity1
Hydrogene-1 1H 75%
Helium-4 4He 25%
Deuterium 2H 0.01%
Isotope Helium-3 3He
Isotope Lithium-7 7Li 10-10%
Isotope Tritium 3H Decayed to 3He
Isotope Baryllium-7 7Be Decayed to 7Li

1The Hydrogen and Helium quantities represent 100% because the other quantities are so small that they are negligible. Obviously, there will be a bit less of both of the other elements to satisfy the Deuterium and Isotopes quantities.

So only very light elements existed in the early Universe. The heavy elements we know of today were created later when star formed and exploded.

This was inferred from our observations and these values match with the Big Bang theory. That is, without these elements in those quantity, we would find ourselves with a different theory of the birth of the Universe.

I’ll probably need another page to explain this in more detail. An important aspect of the first atoms is the fact that we have had a constant Neutron/Proton ratio of 1/1 until the Universe got too cold (about 1 second after the explosion started) and then more Protons were created…

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