Mars is not a fixed distance from Earth. Because both planets move around the Sun on different orbits, the distance between them changes constantly, from about 54.6 million kilometers at the closest favorable approaches to more than 400 million kilometers when they are on opposite sides of the Sun. In round numbers, the average distance…
The Sun is, on average, about 149.6 million kilometers from Earth, or about 93 million miles. Astronomers call this average distance 1 astronomical unit, usually shortened to 1 AU. That number answers the question simply, but the real story is richer: Earth’s distance from the Sun changes slightly over the year, measuring it took centuries…
The Moon is, on average, about 384,400 kilometers (238,855 miles) from Earth. That is the center-to-center distance, but the real distance changes constantly because the Moon follows an elliptical orbit rather than a perfect circle. At its closest point, called perigee, it can be about 363,300 kilometers away, and at its farthest, called apogee, about…
No one can count every star in the universe one by one, but astronomers can make a careful estimate. The best current estimate is that the observable universe contains on the order of 1022 to 1024 stars, often described in plain language as roughly ten sextillion to one septillion stars. That enormous range reflects real…
The short answer is that no one knows the exact number of galaxies in the entire universe. In the observable universe—the part whose light has had time to reach us since the Big Bang—modern estimates suggest there are likely hundreds of billions to perhaps around two trillion galaxies, depending on how faint and small a…
There are eight planets in the Solar System under the current definition used by the International Astronomical Union: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. That answer is simple, but the reason it is eight rather than nine is the result of both astronomical discovery and a formal decision about what counts as…
The Big Bang theory is the leading scientific explanation for how the universe began evolving from an extremely hot, dense early state and then expanded over billions of years into the cosmos we observe today. It does not describe an explosion into empty space. Instead, it describes space itself expanding, carrying galaxies apart on the…
The universe, as far as modern cosmology can tell, was not formed as an explosion into empty space. It began about 13.8 billion years ago in an extremely hot, dense early state, and space itself has been expanding and cooling ever since. Over time, simple particles formed, then atoms, then stars and galaxies, and much…
Astronomy is the science of the universe beyond Earth’s atmosphere. It studies objects such as the Moon, planets, stars, galaxies, gas clouds, black holes, and the large-scale structure of the cosmos, as well as the physical laws that govern them. In practice, astronomy combines careful observation with physics, mathematics, chemistry, and increasingly computing to understand…
Space is the vast physical realm beyond Earth’s atmosphere, but in science it is more than “empty nothingness.” It contains matter, radiation, magnetic fields, and the structure of spacetime itself. Space begins gradually rather than at a perfectly sharp boundary, and on the largest scales it includes everything from the region around Earth to galaxies,…
