By engaging with all the videos within this series, you will effectively complete a full undergraduate course in astronomy, equipping yourself with the knowledge and skills necessary to navigate the night sky with confidence, learning all the basics and many advanced topics! So far, we’ve covered the historical journey from pre-science through the works of Kepler and Galileo. Now, we turn our attention to the monumental contributions of Isaac Newton and his Principia. By the 1660s, Kepler’s laws were widely accepted, but the precise mechanisms behind celestial motions remained unclear. A pivotal moment occurred in 1684 when Edmond Halley, Christopher Wren, and Robert Hooke were discussing planetary motions. Hooke claimed to have derived an inverse square law but never produced his findings. Intrigued, Halley visited Isaac Newton, who revealed he had already worked out the mathematics but had not published it. Encouraged by Halley, Newton dedicated himself to formalizing his discoveries, resulting in his three-volume masterwork, the Principia, published in 1687. Funded personally by Halley, the Principia detailed Newton’s three laws of motion and his law of universal gravitation: 1. An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by a force. 2. The acceleration of an object is proportional to the net force acting upon it and inversely proportional to its mass (F = ma). 3. For every action, there is an equal and opposite reaction. Newton also formulated the law of gravitation, stating that the force between two masses is proportional to the product of their masses and inversely proportional to the square of the distance between them (F = G(m₁m₂)/r²). Despite these advancements, by the end of Newton’s life, phenomena like the Coriolis effect and stellar parallax had not yet been observed, leaving some questions about Earth’s motion unresolved. However, Newton’s laws provided a robust framework for understanding the cosmos, setting the stage for future discoveries. Newton's Laws: Calculus Made Easy: Newton's Derivation of Kepler's Laws: A more detailed derivation: Earth-Moon Barycenter:.