Everything in the solar system moves: planets orbit the Sun, moons orbit planets, and every body spins on its axis. Understanding these motions was one of the first great successes of science, and it is still an active field. The laws of motion Precise orbit calculations led to the discovery of Neptune in 1846, predicted … Read more
The three words are often used interchangeably, and the boundaries really are blurry. Still, each has a distinct emphasis. Astronomy: observing the sky Astronomy is the oldest of the natural sciences. It studies everything beyond Earth’s atmosphere: planets, stars, galaxies and more. Historically, it focused on measuring and observing: positions, motions, brightness and distances. Ancient … Read more
The difference comes down to one criterion: whether the object has cleared its orbital neighborhood. This definition was adopted by the International Astronomical Union (IAU) in 2006, and it is why Pluto is no longer counted among the planets. The IAU definition of a planet A planet in our solar system is a body that: … Read more
Polaris, the North Star, is not the brightest star in the sky. It ranks only around 50th. Its importance comes from its position: it lies almost exactly above Earth’s North Pole, so it barely moves while the rest of the sky turns around it. Why it seems fixed Earth rotates around an axis. Extend that … Read more
A constellation is a pattern of stars that people have linked together and named. For astronomers today, it is also an official region of the sky, like a country on a map, so that every star and galaxy belongs to exactly one constellation. Stars in a constellation are not neighbors The stars of a constellation … Read more
Astronomy is probably the oldest science. People watched the sky to keep time, plan harvests, navigate and make sense of their place in the world long before writing existed. Ancient skywatchers Greek models Greek thinkers sought geometric explanations. Eratosthenes estimated Earth’s circumference around 240 BCE. Aristarchus proposed that Earth orbits the Sun, but the Earth-centered … Read more
A meteor shower happens when Earth passes through a stream of debris left by a comet, or occasionally an asteroid. The particles, mostly smaller than grains of sand, hit the atmosphere at tens of kilometers per second and burn up as streaks of light. Because Earth crosses the same streams every year, showers return on … Read more
Comets and asteroids are both leftovers from the formation of the solar system about 4.6 billion years ago. The main difference is what they are made of, which determines how they behave near the Sun. Asteroids: rock and metal Comets: dirty snowballs Famous comet: Halley’s Comet returns about every 76 years and is next expected … Read more
