A cross-sectional study in which data are collected at one point in time.
A longitudinal study is a study in which data are two or more points in time (at least three points is preferrable).
Both approaches have their place in research and have brought us much knowledge. However, it is important to understand their limits and the limits of research in general.
The bottom line is that it is super hard to know something.
Suppose you go to your local Barnes and Noble and pick a novel off the shelf, open to page 152, and begin reading. It's going to be really hard to capture the gist of the plot. You might get a little more if you pull ten novels written by the same author from the shelf, open to page 152 of each of them, and begin reading.
The obvious flaw in your strategy for learning ht eplot is that you are not beginning at the beginning. Jumping into a story right in the middle of it is the way most social research is conducted.
For example, there are tons of studies on teenagers. Most of these studies are conducted when the teenager is a teenager, but have not considered the decade and half prior to the time of the study. I did one of these studies myself. I will probably do more.
You'd do better starting at the beginning and ending at the end.
But that is really hard to do. There are some studies (major projects and datasets) that have followed people from birth and into several decades, but these are super expensive and take tremendous commitment and patience on the part of the researchers. It also takes great forsight. How do you know which questions to ask at birth which will matter in 10, 30, or 50 years?
Research is an important thing in our world and has lead to some terrific discoveries and creations; however, there should be no hope that it will be the panacea, the solution, the end of the matter. Research is the search for truth - the ever-present, ever-elusive reality that refuses to be tamed. We should research this world we live in and at the same time understand that there is no arrival with research.
Research does not bring about intellectual closure, and anyone hoping for it to do so will become disillusioned. Instead, research reveals options, pathways, directions. Intellectual closure is an impossible mission.
If you want to be a researcher, you had better be prepared to walk into a story that is already in progress and do your best to find your way around in it. You enter the story of the lives of the people you interact with and you enter the larger story of research itself.