Question: When was the field of professional astronomy divided into observational and theoretical branches?
About the article "Astronomy", section "Abstract".

Candidate passages:
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. 
The competing part of IAO consists of three rounds: a theoretical, an observational, and a practical.
Problems of the theoretical round involve classical problems in branches of astronomy, astrophysics, space and planetary physics, and maybe hypothetical situations.
The observational round involves recognizing stars, constellations, estimating star magnitude and angular distance, working with telescopes or other observational technique.
The practical round consists of problems based on data results of observations, solutions propose analysis of these data.
The chairman is Michael Gavrilov. 
The Moscow branch () is itself divided into approximatedly eight divisions.
These branches conduct research in Theoretical physics, the thermal history of the universe, various properties of extragalactic objects, and design and development of space and astronomy research equipment The research of the Moscow Branch enters other related areas of research.