Astronomy and related astronomical terms

A period of nineteen years after which the phases of the Moon reoccur on the same days of the year: After the span of 19 years, the lunar calendar starts on the same date based on the Metonic cycle.

A series of four or five eclipses also occur on the same dates after this interval.

Particles of cosmic dust: The extraterrestrial micrometeoroid is typically less than 0.1 millimeter in size and a microgram in mass.

The radiation which fills the universe uniformly in all directions: The peak intensity of the microwave background radiation has said to have had about one millimeter wavelength in the microwave region.

It is interpreted in the "big bang theory" as the remnant of the initial explosion.

The faint band of luminescence crossing the whole celestial sphere, and made up of stars lying in the central plane of our galaxy: The Milky Way crosses the night sky and consists of stars in the plane of our galaxy.

The Milky Way passes through the constellations of Cassiopeia, Perseus, Auriga, Orion, Canis Major, Puppis, Vela, Carina, Crux, Centaurus, Norma, Scorpius, Sagittarius, Scutum, Aquila, and Cygnus.

One of a family of stony objects mostly orbiting between the rotations of Mars and Jupiter: The minor planets probably represent planetesimals that failed to form a planet.

A variable star typified by the red giant Mira Ceti: The light curve of the Mira variable is irregular, with a varying range of illumination and of a period between peaks of brightness.

The interface between the Earth's crust and mantle: Andrija Mohorovi?i? was a Yugoslav geophysicist for whom the Mohorovicic discontinuity was named (1857-1936).

Mohorovičić concluded that the earth consists of surface layers above an internal core.

He was the first scientist to establish, based on the evidence of seismic wave behavior, the discontinuity that separates the crust of the planet Earth from the mantle.

According to Mohorovičić, a layered structure would explain the observation of depths where seismic waves change speed and the difference in chemical composition between rocks from the crust and those from the mantle.

Two or more atoms chemically combined: A water molecule consists of two atoms of hydrogen and one atom of oxygen.

A natural satellite orbiting a planet: Mercury and Venus are the only planets in the solar system that do not have moons.

A U.S. government agency for space flight and aeronautical research, founded in 1958 by the National Aeronautics and Space Act: The National Aeronautics and Space Administration headquarters are in Washington, D.C., and its main installation is at the Kennedy Space Center in Florida.

NASA's early planetary and lunar programs included Pioneer spacecraft from 1958, which gathered data for the later crewed missions, the most famous of which took the first people to the moon in Apollo 11 on July 16-24, 1969.

A tide of decreased range that occurs twice a month: A neap tide takes place when the Moon is in quadrature, that is during its first and last quarters, when the Sun and the Moon are at right angles to each other relative to the Earth.

The neap tide takes place on the Earth when the Sun and Moon are in positions forming a right angle at the Earth's center.

The Sun and Moon largely cancel each other out, resulting in an extra low high tide and an extra high low tide.

A cloud-like region of gas or dust in outer space: If a nebula is bright, it shines either by its own light (emission nebula) or by reflected light (reflections nebula), or it simply absorbs light falling onto it if it is a dark nebula.

Even with the unaided eyes, faint hazy patches of light can be seen dotted around the night sky.

Through a telescope these patches are resolved into various kinds of diffuse nebulae, which are clouds of dust and gas that can be sen in the visible part of the spectrum.

Nebulae are classified in three basic types:

1. Emission nebulae; ultraviolet radiation from nearby bright blue stars excites hydrogen atoms in the gas of an emission nebula.
2. Dark nebulae; an absorption, or dark, nebula is seen as a dark patch, sometimes surrounded by a halo of light. The light from stars behind the nebula is either absorbed or scattered by the nebular material.
3. Reflection nebulae; the dust particles in the cloud reflect and scatter the light from stars that are not hot enough to make the nebula itself emit light.

1. An atomic particle given off during nuclear fusion and which apparently has little or no mass and moves at the speed of light.
2. An elementary particle with no charge and a very small, perhaps zero, mass.

Produced in nuclear reactions in stars, the particle has a very weak interaction with matter.

An atomic particle having high mass but no electrical charge.

Neutrons are present in the nuclei of all atoms except hydrogen.