Can I pursue Masters in a field different to the one in which I got my Bachelors degree? I will be graduating from BS Aeronautical Engineering in about 6 months. But over the last couple of years, I realized that I'm more passionate about Geopolitics, international relations and diplomacy. So, I'm thinking of changing fields in Masters. I intend to opt a degree in International relations, public policy and economics. 

Now, as mentioned in the question, how difficult is changing majors like that? What is the attitude shown by universities toward applicants like me? And how do I go about convincing professors to take me in?
It is not so rare to see people opting a masters programme that is far different from the undergraduate one. So, there is no need to feel out of the crowd. Such people who go after their passion do succeed, but not without the initial struggle to adapt to the new subjects. This would mean you would have to do more homework yourself than others in the same programme. 

The masters you choose should depend on the career you are aiming at. If you are highly confident that you will be working in that field then you may choose the field that highly deviate from your undergraduate degree. But be wary of the fact that you might not be able to switch back. This act might belittle your undergraduate degree, so seeking for a job relating to it might have you at a disadvantage in most communities. That is, unless you choose do masters for that degree too.