1 Edmond Halley: Let's see. Copernicus, Galileo, Kepler... Who made the next great breakthrough in astronomy?
1 Isaac Newton: Ahem.
2 Edmond Halley: Sorry, Mr Newton, I shouldn't be so restrictive. The next great breakthrough in physics?
2 Isaac Newton: Ahem.
3 Edmond Halley: Right, right... Who made the next great breakthrough in all of natural philosophy?
3 Isaac Newton: A-hem.
4 Edmond Halley: Are you hinting at someone, Mr Newton?
4 Isaac Newton: Not hard enough, obviously.

A strong intermediate candidate is Christiaan Huygens, who made several significant contributions to physics and astronomy. He developed a wave theory of light to explain various basic properties of light propagation - a theory that Newton later ignored in favour of one based on light being composed of particles.*

In astronomy, Huygens made important observations of Saturn, discovering its largest moon Titan, and being the first to propose that the previously mysterious shapes seen near Saturn were encircling rings. Fittingly, the first probe craft to land on Titan was named after Huygens.

* Of course, it later turned out that both men were right - light behaves both like waves and particles.