Glossary and Terms

Welcome to the Glossary and Terms Page of We will be adding new terms often so check here for information.

Asteroid – Asteroids orbit the Sun and are sometimes called minor planets. They are usually 480 miles (775 km) to less than one mile (1.6 km) in diameter. There is an “asteroid belt” between Mars and Jupiter. An asteroid is larger than a meteoroid, however similar in composition (See the definition of meteoroid below).

Comet – A comet is a rather tiny object compared to other matter in our Solar System that orbits our Sun. As a comet nears the Sun a tail becomes noticeable to observers. This tail is comprised of ice, rocks and dust particles. Because Comets are made up of loose-fitting ice particles, rocks, and dust they are referred to as dirty snowballs. When viewing a comet, one will detect that the tail points away from the Sun; this is due to massive solar radiation and solar wind created by the Sun. Comets range in size from a few kilometers or tens of kilometers wide. Comets are generally responsible for Meteor Showers during the year.

Fireball – A fireball is a bigger and brighter than a common meteor. A 4mm meteoroid the size of a tiny pebble hitting the Earth’s atmosphere at an incredible rate of speed will usually produce a fireball, as most meteors that you see can be dust particles or the size of a grain of sand. Some fireballs are also called Bolides.  A Bolide is a fireball that streams across the sky and then detonates or blows up as it comes to an end.  Read this article about really huge Fireball entering the Earth’s atmosphere.

Meteor – The visible light produced by the occurrence of a meteoroid entering the Earth’s atmosphere. A common thought is that friction causes a meteor to shine; it is in reality “ram pressure” that produces high temperature and light when a meteoroid collides with atmospheric elements and atoms at elevated speeds. The meteor is just the observable occurrence not the meteoroid itself.

Meteorite – A Meteorite is an entity (not artificial) from space that has endured a collision with the Earth’s shell. Meteorites are usually separated into three wide-ranging categories, they are stony meteorites (rocks) mainly comprised of silicate minerals, iron-type meteorites composed of metallic elements such as iron/nickel (sometimes magnetic), and stony-iron meteorites that contain both metallic and rocky matter.

Meteoroid – A meteoroid is a tiny, stony or metallic object orbiting around the Sun. Meteoroids are notably smaller than asteroids. They are commonly made up of rock forming minerals such as quartz or ferrous metals like iron and/or nickel. They can be as small as a grain of sand or dust particle and can be the size of large boulders.  It is believed that meteoroids are the original materials of our Solar System and estimated to be 4.6 Billion years old.

Meteor Shower – A group of meteoroids orbiting the Sun that cross the threshold of the Earth’s atmosphere at a related flight path roughly the same time per year. Comets that also orbit our Sun typically fabricate meteor showers. An example is the Perseids Meteor Shower that gets its name from the fact that the streaks of light radiate and trace back to a point near the constellation Perseus, this point is known as the radiant, hence another meteor shower named the Orionids has its radiant near the constellation Orion and ensuing from the famous Halley’s Comet.

Midnight – The middle of the night; twelve o’clock at night. Meteor showers generally peak in the morning hours after midnight so peak dates for meteor showers will be stated like this for example: “The meteor shower peaks Thursday August 12th” which would translate to Thursday before dawn after (midnight) 12:01 AM not Wednesday evening August 11th. See this post – Midnight FAQ

Peak – The peak or maximum date of a meteor shower is when it is most likely strongest and the most meteors will be seen by viewers. Most meteor showers peak in the early morning hours after midnight and before dawn (there are exceptions).

Persistent Train – The lingering glow of plasma due to ionization in the upper atmosphere after a meteor is seen. The intensity and length of the glow depend on how fast a meteoroid enters the atmosphere, its size, and its chemical composition. Extra large fireballs may sporadically and occasionally cause persistent trains that can hang around for several minutes.

Radiant – The radiant or the apparent radiant is a spot where the backward projection of the meteor flight course appears to start from. For example the Perseids Meteor Shower’s point in the sky where meteors appear to come from is in the vicinity of the constellation Perseus. The radiant is fairly dependable because meteoroids proceed upon a parallel pathway when entering the Earth’s atmosphere and are usually associated to the trail of a transient comet.

Universal Time – The local mean time of the prime meridian or the equivalent of Greenwich Mean Time. Universal Time commences at the 0 hour beginning at Greenwich Mean Midnight.

Zenith – This term when used in astronomy is the highest point reached by a celestial object in the sky.

ZHR – The Zenithal Hourly Rate is the quantity of meteors an onlooker would witness in one hour underneath a clear, night sky.  This figure is based on the radiant being inside the zenith, the closer the radiant is to the horizon the less the ZHR.