Observation basically means watching something and taking note of anything it does. For instance, you might observe a bird flying by watching it closely. The sciences of biology and astronomy have their historical basis in observations by amateurs. There is pleasure in observation, which explains the participation of hobbyists.
In cosmology, the original observation was that we seem to live in a firmament.
The sun seemed to rise and set, travelling on a huge transparent bowl which was set around our world.
Various paradigms which explained our world, came and went, but the universe seemed static. Even Einstein believed this.
In 1965, Arno A. Penzias and Robert W. Wilson announced that microwave radiation was surrounding us in all directions, at a level which was of the order of magnitude predicted by Gamow.
Penzias and Wilson got the Nobel Prize for this discovery.
The creatures that can be observed include humans, e.g. from a sidewalk café. This may be especially interesting in an exotic country, or at a place where exotic people pass. Also one may like to look at sexually attractive people.
There are parallels in those hobbies relating to man-made items. International political events may sometimes generate a gathering of VIPaircraft, and an international football match may cause a sudden influx of charter airliners to the region where the match is played.
There is likely to be a social aspect to such hobbies, since fellow enthusiasts will normally alert a hobbyist to forthcoming (or even current) opportunities to witness unusual items within the scope of the shared pastime. New technologies such as mobile telephones and the Internet have clearly increased the opportunities for passing such information between fellow enthusiasts when it is timely.