The discovery of the Milky Way and of other galaxies

Around the 1920s, a great debate was raging about the nature of the nebulae, and the size of the MW. There were two camps. According to one camp, the nebulae were structures within the Milky Way, probably proto-planetary systems or something similar, and the Sun was near the centre of MW. The other camp claimed that many of the nebulae were galaxies outside of the MW, and the Sun was at large distance from the centre of the MW. The reason they couldn't decide was mainly because they didn't know how to determine distances to the nebulae. The reason the first group got it wrong was not because they were stupid or anything like that: their mistake was quite interesting. And so we'll go through this debate, hoping we can learn from the mistakes. Before we do this, let's describe the main observables, and observing techniques.