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Astronomy ch 1 2


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Right Ascension
The angular distance measured eastward around the sky from the vernal equinox.
The angular distance measured north or south from the celestial equator.
Laws of Physics
Approximate descriptions of what is observed in the physical world.
1/360th of the circumfrence of a circle.
1/60th of a degree.
1/60th of an arcminute, or 1/3600th of a degree.
Celestial Equator
The great circle on the celestial sphere, every point of which is exactly 90 degrees from the celestial poles.
Vernal Equinox
A point on the celestial sphere which astronomers use as the origin of the celestial coordinate system.
The study of extraterrestrial bodies and related aspects of the physical universe.
Physical Universe
Everything that can be sensed, measured, or observed.
The Solat System
A gravitationally bound system consisting of the Suna and all its satellites. Radius = 1 LY
The Solar Neighborhood
All of the stars within 100 parsecs of the Sun (or Solar System).
The Milky Way Galaxy
A gigantic, gravitationally bound system consisting of about 200 billion stars, including the Sun and Solar Neighborhood.
The Local Group
A gravitationally bound cluster of about 28 or so galaxies, including the Milky Way Galaxy.
The Supercluster
A cluster of many smaller clusters of galaxies, including our Local Group.
The Universe, or Cosmos
The largest scale of physical reality, consisting of other clusters and superclusters of galaxies, extending in a frothy-like distribution to the limit of visibility.
Meter (m)
One millionth of the distance between the pole and the equator of the Earth.
Kilometer (km)
One thousand meters
Astronomical Unit (AU ot au)
The average or mean distance of the Earth from the Sun.
Light Year
The distance that light travels in the time of one year.
Parsec (pc)
When the Earth is one parsec from the Sun, the Earth's orbital radius subtends an angle of 1 arcsecond.
The motion of a body around and imaginary axis which passes through that body.
The orbital motion of a body around a point called the center of gravity, or barycenter.
The wobbling motion of the axis of rotation.
Terrestrial- Lunar Motion
The Earth and Moon interact gravitationally in such a way that their individual centers revolve around a common center of gravity once every 27.33 days, or one sidereal month.
A type of body that moves in orbit around a star.
A very hot, usually gaseous body that generates, or at one time did generate, energy through thermonuclear fusion of hydrogen into helium.
The amount of matter in a body.
The dimension of a something, such as raduis, diameter, or volume.
The amount of mass in a given volume.
A body which moves in orbit around another more massive body.
An expression of the force of gravity between an object and another body.
Something that causes an acceleration or change in velocity of an object.
The force of attraction that exists between any two physical objects.
The force that dictates the structure and workings of an atom (the arrangement of electrons around the nucleus) and the interaction of radiation with matter.
Nuclear or Strong Force
The force that dictates the structure and workings of the nucleus of an atom, including nuclear reactions
The Weak Interactions
The force that expresses the interaction of subatomic particles including radioactive decay.
Newton's First Law of Motion (the Law of Inertia)
A motion of an object will remain constant until it is acted on by an unbalanced force.
Newton's Second Law of Motion
When an unbalanced force acts on an object, it produces an acceleration. The more massive the object, the smaller the acceleration will be. (a=f/m)
Newton's Third Law of Motion
When any two objects interact, they exert equal but oppositely directed forces on one another.
The Celestial Sphere
An imaginary sphere with the Earth at its center on which every object in the sky has a location

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