Altitude:
The angular elevation of a celestial object above the horizon. (See Observations Using a Gnomon and Modeling the Earth-Sun System )
Axis:
The imaginary line passing through the center of a sphere around which is rotates . (See Modeling the Earth-Sun System )
Celestrial Equator:
The great circle on the celestrial sphere midway between the celestial poles. (See Modeling the Earth-Sun System )
Celestrial Pole:
One of two points on a celestial sphere around which the daily rotation of stars appear to take place. (See Construction and Use of a Sundial and Modeling the Earth-Sun System )
Celestrial Sphere:
An imaginary sphere with an infinite radius on which the celestial bodies appear to be projected, forming the apparent dome of the visible sky.
Chromosphere:
This middle layer of the Sun's atmosphere has a complicated temperature structure which ranges from 4000 K ( Kelvin ) to about 6500 K. Before this century, observations of the chromosphere were possible only during a total solar eclipse, when the chromosphere could be seen as a thin red ring circling the outer edge of the Moon at mid-eclipse. Today the chromosphere may be observed daily with special Hydrogen alpha (H-alpha) filters and with special devices (coronagraphs) which blot out the bright photosphere of the Sun.
Circle:
A locus (an infinite number) of points that are an equal distance from their center. Segments and angles within the circle are measured in degrees, where there are a total of 360 in the circle. (See Pinhole Camera Observatory)
Circumference:
The outermost edge of the circle and is made from an infinite number of point that are of equal distance from the center of the circle. It is found by using the formula 2pr, where p is approximately 3.1416 and r is the radius. (See Pinhole Camera Observatory)
Corona:
The topmost layer of the solar atmosphere, it is characterized by extremely high temperatures (1 to 2 million Kelvin ) and extremely low densities (10 9 particles per cubic centimeter).
Diameter:
The distance of one side of the circle, through its center, and to its other side. The diameter is exactly one half the length of the radius, and there are an infinite number of diameters although they are all the same size because they all must pass through the circle's center. (See Pinhole Camera Observatory)
Declination:
The angular distance north and south from the celestial equator measured along a great circle passing through the celestial poles. (See Observations Using a Gnomon and The Construction and Use of a Sundial)
Ecliptic:
The plane of the earth's orbit around the sun which is about that same as the plane of the entire solar system . The sun's aparrent path across the sky is the projection of the ecliptic onto the celestial sphere . (See Modeling the Earth-Sun System )
Equator:
A great circle that goes completely around a sphere directly in between the sphere's poles . (See Modeling the Earth-Sun System )
First-surface mirrors:
A piece of glass or other material coated on the topmost surface with a reflective substance. This means that the light does not have to pass through the backing material to reach the reflective surface, thus creating a highly reflective mirror with little distortion. (See Pinhole Camera Observatory)
Gnomon:
(1) A vertical pole of known length embedded into a perfectly level plot of ground or slab of concrete. This device was used by the ancient Egyptians as a method of timekeeping, but its disadvantages are that the hour marks, unlike those of a sundial, are not fixed and move throughout the year. (See Obervations Using the Gnomon)
(2)The triangular plane, or style, that stands vertically on the surface of a sundial. (See Construction and Use of a Sundial)
Great Circle:
A circle formed on the surface of a sphere by the intersection of a plane that passes through the center of the sphere.
Hemisphere:
One half of a sphere . (See Modeling the Earth-Sun System )
Hypotenuse:
The side of a right triangle opposite the right angle of or the side opposite the largest angle in the triangle, therefore being the longest side of the triangle.
Kelvin:
A measure of temperature, it is related to the Fahrenheit and Celsius scales as follows:
to convert from Fahrenheit to Centigrade,
Latitude:
The angular distance north and south from the earth's equator measured through 90° from side to side across the globe. (See Observations Using a Gnomon and The Construction and Use of a Sundial)
Light-Year:
The distance that light travels in the time period of one year. Since
light travels at 300,000 km/s (186,000 mi./s) in a vacuum, this distance is equal to approximately 9 trillion kilometers or 6 trillion miles. (See A Scale Model of the Solar System)
Longitude:
The angular distance of the intersection of the zero meridian with the reference circle to the similar intersection of the meridian passing through the object, measured on a reference great circle. (See Observations Using a Gnomon)
Meridian:
A great circle of the celestial sphere passing through its poles and the zenith of a given place. It also refers to the hour of noon, or miday, and to a great circle on the surface of the earth passing through the poles and any given place (i.e. the longitude). (See Observations Using a Gnomon and Modeling the Earth-Sun System )
Obelisk:
An upright, four-sided pillar made of one solid rock. It gradually tapers upward until it ends in a pyramid. (See Observations Using a Gnomon)
Obliquity of the Ecliptic:
The difference between the plane of the earth's equator and the plane of her orbit. These have a mean value of 23°26'40".16 in 1960 and diminishes 0".48 per year. (See Observations Using the Gnomon)
Photosphere:
The visible surface and lowest layer of the atmosphere of the Sun, it has a temperature of 5800 K ( Kelvin ). The peak intensity of the light emitted from this surface is at 5000 Å (1 Å equals 10 -10 meters). The density of particles in the lower photosphere is about 10 17 particles per cubic centimeter (the density of the Earth's atmosphere is about 10 19 particles per cubic centimeter, at sea level).
Plane:
An imaginary, inifinitly large surface extending in all directions. Modeling the Earth-Sun System )
Polarization:
A term which refers to the way a wave of light moves. For instance, a linearly polarized beam of light may be thought of as "vibrating" up and down in a vertical, horizontal, or other tranverse direction. A beam of light which is not polarized is made up of vibrations in all directions.
Pole:
The topmost (northern) or bottomost (southern) points of a sphere . (See Modeling the Earth-Sun System )
Radius:
The distance of one side of the circle, to its center. The radius is exactly half the length of the diameter, and there are an infinite number of radii (plural of radius) although they are all the same size because they all must touch the circle's center and an edge of the circle. (See Pinhole Camera Observatory)
Revolution:
The amount of time that it takes an object to go around, or orbit, another object. For the Earth to revolve around the Sun it takes 365 days (except on a leap year) and is commonly referred to as a year. (See Pinhole Camera Observatory)
Rotation:
The amount of time that it takes to turn completely around. For the Earth this takes 24 hours and is commonly referred to as a day. (See Pinhole Camera Observatory and Modeling the Earth-Sun System )
Solar System:
The name of the moving group of objects that orbit our Sun. These include the nine planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto), their moons, asteriods, comets, and other space debris. (See A Scale Model of the Solar System).
Sphere:
A three dimensional object, the shape of a ball, whose outer surface is an infinite number of points that are of an equal distance from the sphere's center.
Sunspots:
Areas of concentrated magnetic feilds on the surface of the sun near its equator. These areas are cooler and, thus appear as darker regions as seen from Earth. The number of these storms fluctuate and reach a peak every 11 years. These variations cause an increase of the solar wind, an array of particles ejected from the Sun, which interfere with radio wave transmissions. (See Pinhole Camera Observatory)
Trigonometric Functions:
The ratios between sides of a triangle: sine {sin} (the side opposite the angle over the hypotenuse), cosine {cos} (the side adjacent to the angle over the hypotenuse), and tangent {tan} (opposite over adjacent); and their inverses: secant {sec} (hypotenuse over opposite), cosecant {cosec} (hypotenuse over adjacent), and cotangent {cot} (adjacent over opposite). (See Construction and Use of a Sundial and Observations Using a Gnomon)
Vacuum:
A portion of space that has no or nearly no matter within it. Outer space is a near vacuum, with only a few atoms of gas per cubic meter.
Vector:
Vectors are quantities which are specified by both magnitude and direction. For instance, velocity is a vector quantity. A runner who moves along a road towards the north at 10 km/hr (or 6.2 mi/hr), has a velocity; the direction of her velocity vector is north and its magnitude is 10 km/hr. Vector quantities include electric field, magnetic field, and velocity. A vector may be expressed in terms of its magnitude and its component vectors, the parallel and perpendicular. For sunspot vector magnetic fields, the parallel component is often referred to as the "line-of-sight" component (B_l or B_z), the perpendicular as the "transverse" (B_t). These components may also be tied to an "x-y-z" coordinate system; the x component of the magnetic vector (B_x) is equivalent to the horizontal or transverse component of the field, the y component (B_y) is its direction, and the z component (B_z) is the vertical or line-of-sight component.
Zenith:
The point of the celestial sphere that is directly opposite the nadir (a point vertically downward from the observer) and is vertically above the observer.
Some information adapated from Webster's Seventh New Collegiate Dictionary and The Dynamic Universe: An Introduction to Astronomy by Theodore P. Snow, third edition.
Authors: Clifford Schlecht, a SHARP student from Hazel Green High School
and
Mitzi Adams, MSFC/NASA, mitzi.adams@msfc.nasa.gov, (256) 544-3026