Carefully and systematically observing the stars.
It has always been used to plot the positions of the heavenly bodies, follow their movements in the sky, and be alert for unexpected events, such as the arrival of a comet.
The main difference between astronomers and most other scientists is that astronomers can only conduct direct experiments in the solar system-by sending spacecraft.
They cannot experiment on stars and galaxies.
Galileo Galilei
Edwin Hubble
Nicolaus Copernicus
Johannes Kepler
Isaac Newton