AstronomyTerms2.txt

The addition of matter to a body. Examples are gas falling onto a star and asteroids colliding and sticking together

4.6 Billion years

A region between the orbits of Mars and Jupiter in which most of the Solar System's asteroids are located

The light emitted by atoms and molecules in the upper atomosphere. This light is a result of magnetic disturbances caused by the solar wind. Often called the northern or southern lights

A small body in orbit around the Sun, consisting of a tiny, icy core and a tail of gas and dust. The tail forms only when the comet is near the Sun.

Conversion of free gas atoms or molecules into a liquid or solid. A snowflake forms in our atmosphere when water vapor condenses to ice.

The rising and sinking motions in a liquid or gas that carry heat upward through the material. Convection is easily seen in a pan of heated soup on a stove.

A circular pit, generally with a raised rim and sometimes with a central peak. Crater diameters on the Moon range from centimeters to several hundred kilometers. Most craters on bodies such as the Moon are formed by the impact of solid bodies, such as asteroids.

The mass of a body or region divided by its volume.

The separation of previously mixed materials inside a planet or other object. This is the same separation that occurs when a dense material, such as iron, settles to the planet's core while low-density material floats to the surface.

A planet not orbiting our Sun.

The four moons of Jupiter discovered by Galileo: lo, Europa, Ganymede and Callisto.

The trapping of heat by a planet's atmosphere, making the planet warmer than would otherwise be expected. Generally the greenhouse effect operates if visible sunlight passes freely through a planet's atmosphere but the infrared radiation produced by the warm surface cannot escape readily into space, because of gases such as carbon dioxide or water vapor.

A cloud of gas and dust inbetween the stars. Such clouds may bemany light-years is diameter.

Microscopic solid dust particles in interstellar space. These grains absorb starlight, making distant stars appear dimmer and redder than they truly are.

A region from which some comets come. The region appears to extend from the orbit of Neptune, past Pluto, out to approximately 55 AU.

An event in which many meteors occur in a short space of time, all from the same general direction in the sky. The most famous shower is the Perseids in mid-August.

The technical name for the small, solid bodies moving within the Solar System. When a meteoroid enters our atmosphere and heats up, the trail of luminous gas it leaves is called a meteor. When the body lands on the ground, it is called a meteorite.

The bright trail of light created by small solid particles entering the Earth's atmosphere and burning up. A "shooting star".

The solid remains of a meteor that falls to the Earth.

A planet whose orbit lies in the outer part of the Solar System. Jupiter, Saturn, Uranus and Neptune are outer planets.

A form of oxygen consisting of three oxygen atoms bonded together. Its chemical symbol is 0 . The ozone in our atmosphere absorbs and thus shields us from, the Sun's harmful ultraviolet radiation.

One of the numerous small, solid bodies that, when accreted together, form a planet.

The idea that the crust of the Earth (or some other planet) is divided into large regions (plates) that move very slowly over the planet's surface. Interaction between plates at their boundaries creates mountains and activity such as earthquakes.

The slow change in direction of the pole (rotation axis) of a spinning body or of the orientation of an orbit.

The force exerted by radiation on matter.

The breakdown of an atomic nucleus by the emission of subatomic particles.

An element that under goes radioactive decay and breaks down into a lighter element.

The surface rubble of broken rock on the Moon or other solid body.

A condition in which the repetitive motion of one body interacts with the repetitive motion of another so as to reinforce the motion. Sliding back and forth in a bathtub to make a big splash is an example.

The breaking apart of a continental plate.

The sinking of one crustal plate where it encounters another.

The distance from an astronomical body at which its gravitational force can pull apart another astronomical body.

Waves generated in the Earth's interior by earthquakes. Similar waves occur in other bodies. Two of the more important varieties are S and P waves. The former can travel only through solid material; the latter can travel through either solid or liquid material.

One or more satellites that by their gravitational attractions prevent particles in a planet's rings from spreading out and dispersing. Saturn's F-ring is held together by shepherding satellites.

The length of time from the rising of a star until it next rises. The length of the Earth's sidereal day is 23 hours 56 minutes.

The passage of the Moon between the Earth and the Sun so that our view of the Sun is partially or totally blocked.

The passage of the Earth between the Sun and the Moon so that the Earth's shadow falls on the Moon.

The time interval from one sunrise to the next sunrise or from one noon to the next noon. That time interval is not always exactly 24 hours but varies throughout the year. For that reason, we use the mean solar day (which, by definition, is 24 hours) to keep time.

The rotating disk of gas and dust from which the Sun and planets formed.

The outflow of low-density, hot gas from the Sun's upper atmosphere. It is partially this wind that creates the tail of a comet, by blowing gas away from the comet's immediate surroundings.

The condition that a body's rotation period is the same as its orbital period. The Moon rotates synchronously as it orbits the Earth.

The rise and fall of the Earth's oceans created by the gravitational attraction of the Moon. Tides also occur in the solid crust of a body and its atmosphere.

The slowing of one body's rotation as a result of gravitational forces exerted onit by another body.

Numerous small, icy objects orbiting in the outer part of the Solar System beyond the orbit of Neptune.

Doughnut-shaped regions surrounding the Earth containing charged particles trapped by the Earth's magnetic field.