Study Map

Astronomy Glossary

The universe is amazing, isn't it? While scientists and organizations like NASA have already discovered so much about the planets and outer space in general, there is still so much to learn.

Here are some common words that you might want to know, along with their definitions. These are all important words to know when you read about or study astronomy, cosmology, or space exploration.

Asteroid: A minor planet. It's a solid body that orbits that sun and is made up of rock and metal. Most asteroids are only a few miles wide, and they can commonly be found between the orbits of Jupiter and Mars. While a few get close enough to cross Earth's orbit, they're too small and far away to be seen with a telescope.

Astronomical Unit: This is the average distance from the Earth to the sun, which is around 93 million miles.

Barlow Lens: This is a lens that's put into the focusing tube to essentially double or triple the focal length of a telescope, so you can see farther.

Comet: Also known as a "dirty snowball," a comet is made up of rocky debris and ice, and comets are usually a few miles wide. They orbit the sun in a long ellipse, or oval. Sometimes, they'll fly by Earth again just a few years after we last saw them, and other times, it can take tens of thousands of years for a comet we see to come back by.

Earthshine: Like sunshine, earthshine is a type of light, but earthshine is the light that comes from the sun, bounces off of Earth, and strikes the moon. This light makes the dark part of the moon faintly glow.

Eclipse: This is an event that happens when a planet or moon's shadow falls on another planet or moon. A solar eclipse happens when the moon's shadow falls over Earth, which we see as the moon blocking the sun. A lunar eclipse is when Earth's shadow falls over the moon.

Equinox: This happens twice a year, near March 20 and Sept. 22, when the sun is directly overhead at noon at the equator. Days and nights are equal lengths on equinox dates.

Galaxy: A galaxy is a huge collection of stars, dust, and gas held together by gravity. The galaxy we live in is called the Milky Way.

Light-year: This is the distance that light can travel in one year: almost 6 trillion miles. Light moves at a speed of about 186,000 miles per second!

Meteor: A ball of rock that enters Earth's atmosphere at a very fast speed (about 20 to 40 miles per second). It's also sometimes called a shooting star. If the meteor is able to survive the trip through the atmosphere and actually land on Earth's surface, it's then called a meteorite.

Milky Way: The galaxy we live in, which can sometimes be seen as a glowing band that stretches across the sky at night, made up of billions of stars from within our galaxy.

Solstice: This occurs twice a year, around June 20th and December 21st, and happens when the sun is the farthest north or south in the sky. When this occurs in the summer, the night is the shortest and the day is the longest of the year on that day. On the winter solstice, the opposite occurs.

Star: A star is a very large ball of gas that creates immense amounts of energy (including light) from the nuclear fusion that's found in the hot and dense core. The sun is actually a star!

Twilight: This takes place either before sunrise or after the sunset, when the sky isn't completely dark yet. An astronomical twilight will typically end following sunset and will begin before the sun rises, when the sun can be found at 18° below the horizon.

Zenith: This is the point in the sky that is directly overhead.