Many scientific discoveries are made by chance. While studying one phenomenon, a scientist may discover a different one. Such was the case when William Herschel discovered the planet Uranus in 1781. Herschel was charting faint stars in the sky, when he discovered something that was not a star. How can astronomers tell the difference between a star and a planet
One simple explanation is that planets move, while stars remained fixed in the sky. You can observe a planet's orbit, but a star will stay in the same position.
