Why a Year Is 365 Days: Earth's Orbit (3D)

Presets

Orbit Parameters

Orbital radius a 1.00 AU

Star mass 1.00 M☉

Eccentricity e 0.02

Time speed 0.60 yr/s

Click the orbital plane (the dark disc) to drop the planet at that distance from the star. Drag to orbit the camera, scroll to zoom.

Live Readouts

Orbital period T

1.000 yr

In days

365.25 d

vs. Earth's real year

= 1.000× (identical)

Kepler's third law

A planet's orbital period is set entirely by its distance from the star and the star's mass — not by the planet's own mass or size. In astronomical units (AU), years and solar masses (M☉), Kepler's third law reduces to:

T² = a³ / M
T = period in years, a = semi-major axis in AU, M = star's mass in M☉

Earth orbits at a ≈ 1 AU around a 1 M☉ star: T² = 1³/1 = 1 → T = 1 year, 365.25 days. That single relationship is the entire answer to "why is a year 365 days" — it isn't arbitrary, it falls straight out of gravity and distance. Move the orbit farther out and the period grows as a^1.5; make the star heavier and its stronger gravitational pull shortens the period at the same distance. This 3D view renders the true Kepler ellipse in perspective, with the planet visibly speeding up near the star (perihelion) and slowing down far from it (aphelion) for a non-zero eccentricity — orbital angular momentum stays constant, so speed and distance trade off exactly as Kepler's second law requires.

Star Planet Earth's real orbit (ref.)

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