Astronomy and related astronomical terms

1. The scientific study of the structure and evolution of the universe as a whole.
2. A field of study that brings together the natural sciences, particularly astronomy and physics, in a joint effort to understand the physical universe as a unified whole.
3. That branch of astronomy concerned with the origin, structure, content, and evolution of the universe.

Creation myths and the Big Bang are theories of cosmology.

Saturn's inner ring, also known as the C-ring, which extends inward to the planet from the brightest ring or the B-ring.

One of the phases of the moon or the inner planets (Venus and Mercury) as seen from Earth, caused by the relative angles of sunlight and the observer's viewpoint.

From spacecraft, crescent phases of the Earth, Mars, Jupiter, and Saturn have also been see.

1. The thin, outermost rocky layer of a moon or terrestrial planet.
2. The topmost layer of the solid body of a planet, extending down to the mantle.

The point when a celestial object reaches its greatest possible altitude above the earth's horizon.

According to Einstein's theory of gravitation, massive objects in space; such as, stars, cause space to curve and light to bend.

Comprising a large portion of the universe including matter that can not be seen but can be perceived through its gravitational effects.

1. The position of a star as located through the combination of two coordinates, east-west (right ascension) and north-south of the celestial equator.
2. The celestial coordinate analogous to latitude, usually measured in degrees, minutes, and seconds of arc north (+) or south (-) of the celestial equator.
3. The angular distance between a celestial object; such as, a star, and the celestial equator.

The symbol for this term is the Greek letter δ or delta.

The large circular orbit around which a planet was thought to orbit, in one or many epicycles.

An epicycle is a circular orbit of a body around a point that is itself in a circular orbit round a parent body.

Such a system was formulated to explain some planetary orbits in the solar system before they were known to be elliptical.

1. Thickness of consistency; impenetrability.
2. Complexity of structure or content.
3. A measure of how tightly mass is packed into a given space.
4. Mass per unit of volume.

In astronomy, the mean relative density compares a planet's density with the density of water, 62.4 pounds per cubic foot or one gram per cubic centimeter.

A theory to account for the spiral structure of galaxies.

Supposing that such a wave could be set up in the first place, the theory suggests that the spiral arms mark the positions of regions of higher than average density, which rotate around the galaxy.

Stars orbiting the center of the galaxy spend a considerable amount of time in the higher density regions before moving out, with the higher density also favoring the formation of young stars by fragmentation within it.

1. A phenomenon seen as a flash of sunlight shines down a lunar valley during a total solar eclipse.
2. An effect created as the total phase of a solar eclipse is about to begin, when the last Baily's bead, a remaining bit of phosphere, glows so intensely by contrast with the sun's faint corona that it looks like the jewel on a ring.

It also refers to the equivalent phase at the end of totality.

The change in an astronomical object's apparent position caused by the change in the observer's position because of the motion or rotation of the earth during a day.

1. Change in frequency of sound or light waves caused by the relative motion of the source and the observer.
2. The shift of spectral lines due to a body's motion toward or away from an observer.

Astronomers can tell by the Doppler effect if a distant star is moving toward or away from us.

3. A perceived change in the frequency of a wave as the distance between the source and the observer changes; for example, the sound of a siren on a moving vehicle appears to change as it approaches and passes an observer.
4. Etymology: named after Christian J. Doppler (1803-1853), an Austrian physicist and mathematician who first described the principle known as the Doppler effect in 1842.

Doppler observed that the frequency of light and sound waves is affected by the relative motion of the source and the detector.

Another example is that of a train which approaches an observer, and a lower pitch after it passes by. The Doppler effect applies to all types of waves, including light.

1. A pair of stars that appear close together in the sky only because they lie in the same direction from the earth, and not because they are physically associated binary stars.
2. A system containing two or more stars.

In a true double, the stars are physically close to each other; in an optical double, they lie in approximately the same direction from the earth and so appear close to each other, but are actually far apart.