As seen by an observer on Earth, the planets move across the sky at a variable rate. They even reverse their direction of motion occasionally but resume the dominant direction of motion after a while. To describe this variable motion, Ptolemy assumed that the planets revolved around small ...
the universe was far larger than most astronomers had imagined. Hubble continued to find galaxies at greater and greater distances. He developed the system for classifying galaxies by their structure, which is still used today. Hubble also discovered that the distant galaxies are moving away from ...
The first law of planetary motion states that planets move in slightly elliptical orbits — subtle ovals rather than circles. Furthermore, it states that the sun is located at one focus of the ellipse. With a circle, there is a center that is equidistant from all points on that circle. In...
NASA's Kepler space telescope, designed to find Earth-size planets in the habitable zone of sun-like stars, has discovered its first five new exoplanets, or planets beyond our solar system. Kepler's high sensitivity to both small and large planets enabled the discovery of the exoplanets, named...
cool planets—precisely the type of planet where moons are thought to be most likely due to dynamical considerations11,12. Nevertheless, a small sample of long-period planetary candidates was discovered by Kepler13,14,15,16,17—worlds with orbits greater than that of the Earth around the Sun....