Why the stretch from March to September lasts a week longer
From the March equinox to the September one takes about 186 days; the return trip takes only about 179. The gap exists because Earth's orbit is a slightly squashed ellipse and our planet speeds up nearer the Sun. Johannes Kepler worked out those rules four centuries ago, while believing the Sun steered planets with magnetic strands.
Kepler's three laws, published between 1609 and 1621, say that each planet follows an ellipse with the Sun at one focus; that a line from Sun to planet sweeps equal areas in equal times, so planets move faster when closer; and that the time to complete one orbit, squared, scales with the cube of the orbit's semi-major axis, so distant planets take much longer to go round. Together they overturned the circles and epicycles that even Copernicus had kept.
Mars forced the breakthrough. Kepler was a committed Copernican and expected circles, but Tycho Brahe's exceptionally precise observations refused to fit one. Mars happens to have the most eccentric orbit of any planet except Mercury, which made the mismatch impossible to ignore. His explanation was physical rather than purely geometric: the Sun, he thought, emitted slightly elastic magnetic fibrils that tugged planets along, allowing non-circular paths. The mechanism was wrong, yet the laws it produced held.
Acceptance was slow. The area law was clumsy for calculating positions, and Nicolaus Mercator disputed it in 1664. What changed minds was the Rudolphine Tables of 1627, built on Brahe's data, which let astronomers test Kepler's formulas against good observations. Newton later showed that the equal-areas rule follows from any force pointed toward the Sun, while the other two laws depend specifically on gravity weakening with the square of distance.
Even calling them laws came late. Voltaire's 1738 book on Newton's philosophy was the first to use that word, and it was Robert Small's account of 1804 that bundled them as a set of three, wrongly claiming they arose purely from induction. Those unequal seasons also let anyone estimate Earth's eccentricity: the two figures give roughly 0.015, close to the true 0.0167.
Source: Kepler's laws of planetary motion
