🌍 1. Sidereal Period (Actual orbit time around the Sun) • This is the true orbital period of Neptune: 164.8 Earth years • Based on this, the first full orbit since its discovery in 1846 was completed in late 2011, and the next would be in 2176, as we calculated before. ⸻ 👁️ 2. Observation-Based Completion (Return to discovery position in Earth’s sky) • Some astronomical sources define “completing one orbit” as Neptune returning to the exact position in Earth’s sky (relative to the background stars), which includes accounting for: • Earth’s motion • Orbital perturbations (tiny shifts from gravity of other planets) • Discovery point in the sky, not just orbital mechanics This can shift the apparent “one orbit” time to about 165.9 years instead of 164.8 years. 1846 + 165.9 = 2011 + 0.9 = ~2181 So, 2011 vs. 2181 depends on: • 2011 → Sidereal orbit (pure orbital mechanics) • 2181 → Apparent return to original position from Earth (including sky position) ⸻ ✅ Bottom line: • Scientifically, Neptune completed one true orbit in 2011. • Visually/observationally, Neptune will return to its discovery sky position around 2181. Both are correct — just based on different definitions of “completing an orbit.”