Question: The field of professional astronomy was eventually divided into 2 branches, observational and theoretical, when did this happen?
About the article "Astronomy", section "Abstract".

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Astronomy is a natural science that is the study of celestial objects (such as moons, planets, stars, nebulae, and galaxies), the physics, chemistry, and evolution of such objects, and phenomena that originate outside the atmosphere of Earth, including supernovae explosions, gamma ray bursts, and cosmic background radiation.
A related but distinct subject, cosmology, is concerned with studying the universe as a whole.
Astronomy is one of the oldest sciences.
Prehistoric cultures have left astronomical artifacts such as the Egyptian monuments and Nubian monuments, and early civilizations such as the Babylonians, Greeks, Chinese, Indians, Iranians and Maya performed methodical observations of the night sky.
However, the invention of the telescope was required before astronomy was able to develop into a modern science.
Historically, astronomy has included disciplines as diverse as astrometry, celestial navigation, observational astronomy, and the making of calendars, but professional astronomy is nowadays often considered to be synonymous with astrophysics.
During the 20th century, the field of professional astronomy split into observational and theoretical branches.
Observational astronomy is focused on acquiring data from observations of astronomical objects, which is then analyzed using basic principles of physics.
Theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena.
The two fields complement each other, with theoretical astronomy seeking to explain the observational results and observations being used to confirm theoretical results.
Astronomy is one of the few sciences where amateurs can still play an active role, especially in the discovery and observation of transient phenomena.
Amateur astronomers have contributed to many important astronomical discoveries.
Astronomy is not to be confused with astrology, the belief system which claims that human affairs are correlated with the positions of celestial objects.
Although the two fields share a common origin they are now entirely distinct.
Planetary science (rarely planetology) is the scientific study of planets (including Earth), moons, and planetary systems, in particular those of the Solar System and the processes that form them.
It studies objects ranging in size from micrometeoroids to gas giants, aiming to determine their composition, dynamics, formation, interrelations and history.
It is a strongly interdisciplinary field, originally growing from astronomy and earth science, but which now incorporates many disciplines, including planetary astronomy, planetary geology (together with geochemistry and geophysics), atmospheric science, oceanography, hydrology, theoretical planetary science, glaciology, and exoplanetology.
Allied disciplines include space physics, when concerned with the effects of the Sun on the bodies of the Solar System, and astrobiology.
There are interrelated observational and theoretical branches of planetary science.
Observational research can involve a combination of space exploration, predominantly with robotic spacecraft missions using remote sensing, and comparative, experimental work in Earth-based laboratories.
The theoretical component involves considerable computer simulation and mathematical modelling.
Planetary scientists are generally located in the astronomy and physics or earth sciences departments of universities or research centres, though there are several purely planetary science institutes worldwide.
There are several major conferences each year, and a wide range of peer-reviewed journals. 
Theoretical X-ray astronomy is a branch of theoretical astronomy that deals with the theoretical astrophysics and theoretical astrochemistry of X-ray generation, emission, and detection as applied to astronomical objects.
Like theoretical astrophysics, theoretical X-ray astronomy uses a wide variety of tools which include analytical models to approximate the behavior of a possible X-ray source and computational numerical simulations to approximate the observational data.
Once potential observational consequences are available they can be compared with experimental observations.
Observers can look for data that refutes a model or helps in choosing between several alternate or conflicting models.
Theorists also try to generate or modify models to take into account new data.
In the case of an inconsistency, the general tendency is to try to make minimal modifications to the model to fit the data.
In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model.
Most of the topics in astrophysics, astrochemistry, astrometry, and other fields that are branches of astronomy studied by theoreticians involve X-rays and X-ray sources.
Many of the beginnings for a theory can be found in an Earth-based laboratory where an X-ray source is built and studied. 
Today, observational cosmology continues to test the predictions of theoretical cosmology and has led to the refinement of cosmological models.
For example, the observational evidence for dark matter has heavily influenced theoretical modeling of structure and galaxy formation.
When trying to calibrate the Hubble diagram with accurate supernova standard candles, observational evidence for dark energy was obtained in the late 1990s.
These observations have been incorporated into a six-parameter framework known as the Lambda-CDM model which explains the evolution of the universe in terms of its constituent material.
This model has subsequently been verified by detailed observations of the cosmic microwave background, especially through the WMAP experiment.
Included here are the modern observational efforts that have directly influenced cosmology. 
The Institute of Astronomy (IoA) is the largest of the three astronomy departments in the University of Cambridge, and one of the largest astronomy sites in the UK.
Around 180 academics, postdocs, visitors and assistant staff work at the department.
Research at the department is made in a number of scientific areas, including stars, star clusters, cosmology, gravitational-wave astronomy, the high-redshift universe, AGN, galaxies and galaxy clusters.
This is a mixture of observational astronomy, over the entire electromagnetic spectrum, computational theoretical astronomy, and analytic theoretical research.
The Kavli Institute for Cosmology is also located on the department site.
This Institute has an emphasis on "The Universe at High Redshifts".
The current plans are to also bring the Cavendish Astrophysics Group onto the site eventually.