Is there a difference between astronomy, astrophysics, space science (and navigation)?By G. O. Okeng'o

A friend of mine recently asked me if there is any difference between the three disciplines: astronomy, astrophysics and navigation. My instincts told me that by mentioning the latter he probably mean't space science, so I chose to hold my cards close to my chest and provide a full answer to his (intended and asked) question....

The termastronomycomes from two Greek words astronmeaning starand nomosmeaning law and therefore 'literary' means "the laws of stars". It is anatural science(a science that seeks to explain the natural world using laws of science) that deals with the study of celestial objects or celestial bodies such as planets, asteroids, comets, stars and galaxies,and all phenomena that originate from beyond (or outside) the Earth's atmosphere (also called 'space') such as the cosmic microwave background (or CMB in short).

Astrophysicsis a branch of astronomy that deals with the physics, chemistry, meteorology, motions and interactions between celestial objects, their physical properties such as temperature, chemical composition and structure, and the physics of the space between stars (interstellar medium) and that between galaxies (intergalactic medium).

The differences above are, however, only historical; and were based on astronomy being considered an observational science charged with observing and storing data about positions and properties of celestial bodies (cataloguing the heavens), while astrophysics was deemed a theoretical science concerned mainly with computing models and formulating theories to be tested against the observations.

In the present era of modern astronomy, the two disciplines refer to one but the same subject since both astronomers and astrophysicists today use a combination of observational and theoretical tools such as telescopes and various analytical techniques to study the universe.

Space science- can be defined as the study of 'issues' related to outer space (and voila! Astrophysics becomes one branch) and may include, but not limited to: Planetary science- study of planets other than the Earth Solar astronomy- study of the Sun Stellar astronomy- study of stars Galactic astronomy- study of the Milky Way galaxy (our galaxy) Astronautics- the science and engineering spacefaring and spaceflight, a branch of aerospace engineering (which includes atmospheric flight) space food, space medicine, astrobiology etcNavigation-, on the other hand, is a field of science that focuses on the monitoring and controlling of the movement of an aircraft, spacecraft or vehicle from one place to another. In includes sub-fields such as: land navigation, marine navigation, aeronautic navigation, and space navigation, which refer to monitoring motion through land, sea, air and space respectively. In the context of astronomy and astrophysics, the relevant type of navigation (such as the one used by NASA to land theCuriosityrover on Mars, yesterday) is space navigation. And space navigation requires good knowledge about space and hence astrophysics.