Which astronomer financed the first publication of Newton's Principia?

The astronomer who financed the first publication of Newton’s Principia was Edmond Halley.

Halley’s role went well beyond supplying money. In 1684, he visited Isaac Newton after discussions with Christopher Wren and Robert Hooke about planetary motion and the inverse-square law. Newton had already made major advances but had not published them. Halley’s questions and encouragement helped prompt the work that became Philosophiæ Naturalis Principia Mathematica.

Newton’s manuscript expanded dramatically during composition, and disputes over Robert Hooke’s priority claims threatened to delay or derail publication. Halley persuaded Newton to continue, edited and prepared the work for the press, and acted as its publisher. When the Royal Society could not pay for the printing because its funds had been exhausted by an earlier book on fish, Halley covered the publication cost personally.

The Principia appeared in 1687 and established Newton’s laws of motion and universal gravitation. Halley later used Newton’s gravitational theory to calculate the orbit and periodic return of the comet now bearing his name. John Flamsteed contributed astronomical observations, but he did not finance the Principia’s first edition.