One of the hallmarks of science is the ability to make testable predictions, astronomy is no exception. While we may be reduced in most instances to passively receiving the electromagnetic emanations from beyond our horizons we can nevertheless form conclusions about the way the Universe works, formulate theories to explain those observations and make predictions to confirm (or falsify) those theories. This last step is a main distinction between science an pseudoscience.
The planet Uranus was discovered in 1781 by William Herschel, soon after it was found to be acting in a way contrary to what Newton’s laws predicted. The astronomers at the time attributed the perturbations of Uranus’ orbit to another unknown planet whose gravity was influencing the motions of it’s neighbor. Thus began the search for this mysterious next planet, this involved fiendishly complex calculations accounting for the effects of the other solar system bodies on Uranus and deducing the location of the unknown planet from the residual unexplained motions. Finally in 1846 using the results of these calculations Neptune was discovered.
Embarrassingly for astrologers neither of the new planets had been predicted by their own work, despite regularly making predictions on persons personality based on the alleged influences of just these sorts of heavenly bodies. While they quickly attributed influences to the new planets there were apparently no observed anomalies in their readings that even hinted at their existence prior to discovery by astronomers.
After having so successfully predicted Neptune astronomers moved onto the next unusually moving body in the solar system and attempted to apply the same reasoning. Mercury was also behaving badly as far as Newtonian physics was concerned so another new planet was introduced to explain it’s behavior: Vulcan. Another search was on.
Not to be left in the dust this time astrologers attributed influences to this new planet as well before confirming observations of it’s existence was made by astronomers. You don’t need to be an aficionado of current events to know that no such confirmation ever came, Einstein explained the discrepancies of Mercury’s movements with Relativity in 1915 after which the notion of Vulcan was relegated to the dustbin of science.
Though the was no longer any scientific need to invoke the existence of Vulcan astrologers seem to have held on to it anyway. Which leads to another central concept of science, falsification (alluded to above) the ability of science to discern between competing explanations, find the one that fits best and discard the blind alleys. Sometimes science doesn’t achieve this as well as we would like but pseudoscience seems not to do it at all, no idea is too ridiculous to keep.