The classical European civilizations of Greece and Rome had had extensive contact with the Middle East, India and, indirectly, China, resulting in a widespread exchange of cultural, philosophical, scientific and religious ideas. In the OTL this contact was lost after the rise of Islam, resulting in the stagnation of European civilization even as the Islamic world flourished. It was not until the First Crusade of the 11th century and then the Mongol conquests of the 13th century, which temporarily reopened the transcontinental trade routes to Europeans, that Europe was able to catch up with the rest of the world.
However, since in this timeline most of Europe was slowly becoming Muslim as well, the disconnection was much less severe. Indian astronomers and mathematicians, Italian engineers, Persian alchemists and Roman medics traveled freely from Ireland in the west to Japan in the east, spreading their ideas across all Eurasia. All civilizations, west and east alike, benefited from this great exchange, and science and technology progressed far faster than they might otherwise have done.
Gunpowder is believed to have been discovered independently by Roman and Chinese scholars at some point in the 9th century, but its first recorded use for military purposes is by the Southern Wu army at the siege of Yuzhang in 904. Firearms were developed beginning in the 11th century and were used with great effect by the Romans against the Bulgars, and the new technology rapidly spread across the Middle East and the Mediterranean. In 1241 a Mongol invasion of Hungary was defeated at the Battle of Mohi using primitive hand cannons - the first recorded engagement in which gunpowder weaponry played a significant role for both sides.
The matchlock musket was invented in the early 14th century and revolutionised warfare forever. For the first time it became feasible to use firearms as the primary infantry weapon, slowly displacing the older longbow and crossbow. Since muskets took a long time to load and the musketeer was vulnerable while doing so, the pike and shot formation was developed to protect against cavalry, and until the invention of the bayonet it was the dominant tactic on battlefields of the era.
Until rifling was introduced towards the end of the 16th century firearms were very inaccurate over long distances, but after that the technology improved rapidly. Due to the added cost of manufacture it did not become commonplace until the 18th century, but it did allow for the possibility of sharpshooting and precision field artillery - tactics which had been lost since the days of bow and arrow. During the 18th century the flintlock, then the percussion cap, and finally the automatic machine gun were invented, thus displacing for good the last remnants of the pre-gunpowder era. Pikes, and indeed just about all other hand-to-hand weapons, were replaced by the bayonet and the rifle butt, and horse-mounted cavalry were gradually phased out of active combat following the annihilation of the Horse Grenadiers of the Aquitanian Imperial Guard at the Battle of Épinal in 1815.
Today, although gunpowder has largely been replaced with other propellants and explosives, firearms remain the staple weapons of every professional military in the world.
Humans have studied the skies for millennia, but the Middle Ages saw a flourishing of the astronomical sciences in the Middle East. Scholars all across the Islamic world, from Spain to India, established observatories to watch the stars and their findings were shared freely with other astronomers of all nationalities. When the telescope was invented in 1021 by ibn al-Haytham, an Arab Persian based in Charax, progress increased exponentially.
Ibn al-Haytham first observed the largest four moons of Jupiter in 1030, a clear contradiction of the accepted geocentric model of the universe, and so instead began to develop an early version of the heliocentric model. The precise movements of the planets, including their elliptical orbits, were described by Bhaskaracharya of Ujjain in the form of three laws, which were later explained by Neapolou's Laws of Motion in his book Mathematikes Arches, published in 1175.
From this, scientists were able to predict the motions not just of the planets, but also of bodies such as comets and distant stars as well. Their masses and semi-major axes could be calculated, and perturbations from the expected orbit could be used as evidence of another nearby massive object - leading to the discoveries of the planets Uranus and Neptune. By the 17th century humanity apparently knew almost everything there was about the universe without ever having had to go anywhere.
Rocketry and Astronautics
The first gunpowder-propelled rockets were invented in China at some point before the 12th century. An important early use was as fireworks, but they could also be used as weapons and were utilised extremely effectively by Chinese and Mongol armies. Rocket technology was spread by the latter across a vast swathe of Eurasia, but was somewhat neglected in Europe and the Middle East in favour of firearms.
As a result, by the time Albion became involved in India in the 18th century, Indian rockets were far more advanced than anything Albion could come up with. However, Albion learned quickly and used rockets to great effect during the First World War from 1789-1815, reigniting interest in the West.
The laws of planetary motion had already been known for centuries, and the principles of astronautics were first being discovered by European scientists while the war was ongoing. Further developments allowed the first man-made object beyond the atmosphere to be launched by Romania in 1852, thus beginning the era of space exploration.
The first man in space was Giorgios Phalakrokoras, an Imperial Air Force pilot from Cappadocia, in 1865. However, after this progress slowed significantly, and no more major milestones were reached for the rest of the century. It was not until 1901, in the midst of the international tension preceding the Second World War, that the Space Race truly began, culminating on the first manned lunar landing in 1913.
The Second World War saw the extensive use of rocket weaponry, including the first use of ICBMs. After the war the victorious Allies continued to compete for any advantage, leading to the first permanent manned space station in 1922, the first manned landing on Mars in 1928, and the creation of the first orbital weapons platform in 1937. The Long Ceasefire of the Third World War saw the creation of permanent bases on the Moon and Mars, which subsequently became involved in the conflict when the war resumed.
After the Third World War various treaties were signed to try and reverse the militarization of space. Today all satellites in Earth orbit are for scientific, communications or intelligence purposes, and the sole surviving lunar colony, Clavius Base, is staffed solely by scientists from all over the world. However, the newfound cooperation between different country's space programs has allowed scientific progress to accelerate exponentially.
Huojian 1, the first probe targeted at another solar system, was launched in 1982. Powered by nuclear pulse propulsion, it is expected to reach Alpha Centauri, the nearest star system after the Sun, in 2107.
Ancient superstition held that disease was the result of a curse from the gods, but the first attempt to explain it was humourism. This held that the human body was regulated by four different bodily fluids, or humours, and that sickness occurred whenever there was an imbalance. Though the theory later proved to be complete nonsense, it did succeed in associating different symptoms with each other, which allowed ancient physicians to predict which herbs and treatments might be of use.
The first scientific explanation based on observations was the miasmatic theory, in which decomposing organic matter would produce a miasma, infiltrating the air and water and infecting any person who came into contact with it. This was of great use as a description of how disease could pass from one person to another, and emphasised good sanitation as a way to prevent it. The theory was heavily developed and refined throughout the early Middle Ages as a result of the devastating Plague of Justinian, and the improvements in public health proved to be sufficient to minimise the death toll when the plague returned in the 14th century.
However physicians and philosophers were unable to agree upon the substance of the miasma, with some holding that it was a poisonous byproduct of decomposition and others believing that tiny creatures, too small to be seen with the naked eye, were to blame. The latter hypothesis was confirmed in the 14th century with the invention of the microscope and formed the basis of the germ theory of disease used today. Over the following centuries the effects of vaccination, antibiotics and sterilisation were all conjectured and successfully tested, resulting in the almost total elimination of infectious disease in the developed world by the 18th century.
Unfortunately it was soon found that bacteria were becoming resistant to antibiotics faster than new ones were being discovered. The late 19th century saw the last known chemical antibiotics becoming useless and a corresponding surge in infections. Attempts to genetically engineer bacterial strains to be harmless proved to be not effective enough, though research in the field continues.