Astronomy and related astronomical terms

1. A layer that overlaps many of the other layers from 30 to 250 miles (48 to 402 kilometers), as part of the upper mesosphere and the lower thermosphere.

The ionosphere is the part of the atmosphere in which the air is ionized by such factors as the sun's ultraviolet radiation.

Layers within the ionosphere are also responsible for reflecting radio waves and are important to long-distance communication.

2. A region above a planet where the breakup of atmospheric gases by sunlight leads to large concentrations of free electrons and ions.

These are capable of seriously disrupting radio communications on earth.

The mass of a region of gas above which its own gravity can cause it to collapse in spite of any outward thermal pressure.

The NASA launch site on Merritt Island, near Cape Canaveral, Florida, used for Apollo and space-shuttle launches.

The first flight to land on the moon (1969) and Skylab, the first orbiting laboratory (1973), were launched from here.

Three basic laws of planetary motion:

1. The orbit of a planet is an ellipse, with the sun at one focus.
2. The line joining the position of a planet in its orbit to the sun (the radius vector) sweeps out equal areas in equal times.
3. The squares of the orbital periods of the planets are proportional to the cube of their mean distances from the sun.

One of a series of vacancies in the distribution of the orbits of the minor planets, marked by the absence of minor planets whose orbits have periods that are simple fractions of the orbital period of Jupiter.

The reason for this activity is that in such positions any minor planet would be repeatedly perturbed by Jupiter's gravitational field until it was forced out of the "forbidden" orbit.

The gaps are an example of gravitational resonance.

1. A ring of small astronomical objects orbiting through the outer solar system, beyond the farthest planets, Neptune and Pluto.

It is believed that the Kuiper belt is a source of comets.

2. A disk-shaped swarm of 200 million comets and comet fragments located from just beyond the orbit of Neptune and extending past Pluto.

Named after Gerard Peter Kuiper (1905-1973), a Dutch-born U.S. astronomer who made extensive studies of the solar system. His discoveries included the atmosphere of the planet Mars and that of Titan, the largest moon of the planet Saturn.

His spectroscopic studies of Uranus and Neptune led to the discovery of features subsequently named "Kuiper bands", which indicate the presence of methane.

He was an adviser to many NASA exploratory missions, and pioneered the use of telescopes on high-flying aircraft.

1. One of a set of five positions at which a small object can maintain a stable orbit under the influence of two more massive objects.
2. A point in space at which a small body, under the gravitational influence of two large ones, will remain approximately at rest relative to them.

The existence of such points was deduced by the French mathematician and astronomer Joseph-Louis Lagrange in 1772.

In 1906, the first examples were discovered. These were minor planets moving in Jupiter’s orbit, under the influence of Jupiter and the sun.

The angular distance of a point from the earth's equator, measured upon the earth's surface.

The zodiacal constellation in the northern hemisphere, represented as a lion.

The sun passes through Leo from mid-August to mid-September. Its brightest star is the first-magnitude Regulus at the base of a pattern of stars called the Sickle.

A magnitude represents the brightness of an astronomical object as a numerical measure of the apparent brightness, on a scale in which a lower number represents a greater brightness.

A plot of the change in brightness, expressed in apparent magnitudes, against time of variable stars.

The distance traveled by a beam of light in a vacuum in one year, approximately 9.46 trillion (million million) kilometers or 5.99 trillion miles.

An an average speed of 186,291 miles or 299,792 kilometers, per second; which equals approximately 5.88 trillion miles or 9.4607 trillion kilometers, or 63,246 astronomical units.

The light-year is also divided into light-minutes and light-seconds; for example, the moon is 1.3 light-seconds from the earth; the sun is 8.3 light-minutes away from the earth.

Although a light-year is a measurement of distance and not time, it does imply time; such as, the light from a star that is ten light-years from the earth takes ten years to reach the earth; so, an observer on earth is seeing the star as it appeared ten years ago.

The edge of a planet or other celestial body as seen from far away.

Electromagnetic radiation emitted or absorbed at discrete frequencies or wavelengths.

The upper region of the body of a planet consisting of the crust and top layers of the mantle.

This layer is often broken up into tectonic plates.

A cluster of about thirty galaxies that includes our own, the Milky Way.

Like other groups of galaxies, the Local Group is held together by the gravitational attraction among its members, and does not expand with the expanding universe.

Its two largest galaxies are the Milky Way and the Andromeda galaxy; most of the others are small and faint.