Observation is foundational to the human race. We learn, we change, we are who we are because of what we learn from our observation both insitu observation, when we live an experience and passive observation, when we learn from others experience. In the last few weeks I have been reading the book "Building Earth Observation Cameras" by George Joseph. In the discussion of how light is captured and recorded for observation he considered briefly aberration of light, as it enters the camera equipment, which gave me pause to think about aberration of observation especially in mapping, more deeply.

**aberration of fixed stars** SEE aberration, secular.

**aberration of light** **1.** *n.,* *Astro.*, SEE aberration, astronomic. **2. ***n.,* *Optics*, SEE aberration, optical.

**aberration, annual** *n.,* That apparent change in direction of a star, planet, or other celestial body which is caused principally by the Earth's revolution about the Sun. That part of stellar aberration which has an annual period. (The Sun's barycentric motion combines with the Earth's revolution to cause a very small aberration which is also covered by the term.) During the year, a star in the plane of the Earth's orbit (the ecliptic) appears to move from one end of a straight line to the other and back again. The length of this line is twice the constant of aberration. A star 90° from the ecliptic appears to move in a circle which has a diameter twice the constant of aberration. Stars at intermediate angles above or below the ecliptic appear to move in ellipses with major diameters twice the constant of aberration and with minor axes getting longer with increasing angular distance of the star from the ecliptic. Because the Earth moves in an elliptical rather than a circular orbit, the apparent paths of stars not on the ecliptic are not quite elliptical. SEE ALSO aberration, stellar.

**aberration, astigmatic** SEE astigmatism.

**aberration, astronomic** *n.,* The deviation of the observed direction of a source of radiation from the true direction (the direction of a straight line drawn, at the instant of observation, from observer to source), when the source and observer are moving with respect to each other. It is caused by the finite velocity of light in combination with the motions of the observer and the observed object. Also called astronomical aberration, aberration of light and aberration. Astronomic aberration is zero if the source and observer are moving directly towards each other. It is greatest when the relative motion is entirely transverse to the line joining source and observer. If the motions are considered relative to an independent coordinate system, the aberration can be considered caused by the separate motions of the source and observer in that system. The aberration caused by the observer's motion alone is then called stellar aberration or aberration of light; that caused by the source's motion alone is called light-time aberration or planetary aberration. SEE ALSO aberration, annual; aberration, differential; aberration, light-time; aberration, mensual; aberration, planetary; aberration, relativistic; aberration, secular; and aberration, stellar.

**aberration, astronomical** SEE aberration, astronomic.

**aberration, axial chromatic** SEE aberration, chromatic.

**aberration, chromatic** *n.,* The separation, by an optical or other system, of a single ray of polychromatic light in object-space into distinct monochromatic rays in image space, without recombining them. In the absence of other kinds of optical aberration, chromatic aberration causes white light from a point in object-space to focus into a continuous sequence of differently colored points in image-space. Two kinds of chromatic aberration are distinguished: longitudinal chromatic aberration (also called axial chromatic aberration) and transverse chromatic aberration also called lateral chromatic aberration or off-axis chromatic aberration). Longitudinal chromatic aberration produces a sequence of differently colored points along a line parallel to the optical axis of the system. Transverse chromatic aberration produces a similar sequence of points along a line perpendicular to the optical axis. Chromatic aberration can be overcome over a moderate range of wavelengths by using a lens system composed of optical elements having different dispersive powers. (An optical system made achromatic for visual observation is not strictly achromatic for photographic work.)

**aberration, comatic** SEE coma.

**aberration, constant of** SEE constant of aberration.

**aberration, constant of diurnal** SEE constant of diurnal aberration.

**aberration, differential** *n.,* The difference in apparent directions of two sources of radiation moving with respect to the observer and having the same true direction, i.e., lie on the same straight line with the observer. Also called parallactic refraction. It is a form of light-time aberration and is sometimes referred to as differential planetary aberration. It appears when, for example, artificial satellites are photographed against a stellar background. SEE ALSO refraction, parallactic.

**aberration, differential planetary** SEE aberration, differential.

**aberration, distortional** SEE distortion (optical system).

**aberration, diurnal** *n.,* The apparent change in direction of a star or other celestial body because of the observer's angular velocity about the Earth's center, combined with the finite velocity of light. Diurnal aberration is taken into account in determining astronomical azimuth and longitude for first-order surveys. It is not considered in determining astronomical latitude because an observer has practically no motion in latitude. SEE ALSO aberration, stellar.

**aberration, elliptic** SEE E-terms (of aberration of light).

**aberration, lateral chromatic** SEE aberration, chromatic.

**aberration, light-time** **1.** *n.,* Displacement of an astronomical direction from the true direction of a source, at the instant of observation, to the direction of the source at the time the observed radiation was emitted. Also called light-time correction. It is the angle through which the source of radiation has moved during the time it took for the radiation to reach the observer. **2.** *n.,* The sum of the annual aberration and the light-time aberration as defined in (**1.**). When the source of radiation is an artificial satellite of the Earth, light-time aberration is called differential aberration by some geodesists.

**aberration, longitudinal chromatic** SEE aberration, chromatic.

**aberration, mensual** *n.,* A component of stellar aberration caused by the revolution of the Earth and Moon about their common center of mass. It is approximately equal to 0".011 cos H_{M}, where H_{M} is the hour-angle of the Moon. This is a small quantity, taken into account when determining the deflection of the vertical from astronomical observations.

**aberration, off-axis** *n.,* Optical aberration at considerable angles to the optical axis. Do not confuse this with off-axis chromatic aberration. Also called extra-axial curvature and off-axis curve.

**aberration, off-axis chromatic** SEE aberration, chromatic. Do not confuse this with off-axis aberration.

**aberration, optical** *n.,* Any failure of an optical system to image a point in object-space as a single point in image space or to preserve a uniform scale over the image. Also referred to simply as aberration if the meaning is unambiguous, or as lens aberration if a lens system is being discussed. An optical aberration causes either a blurring or a distortion of the image. Seven basic varieties are recognized. The five (Seidel aberrations) that affect monochromatic radiation are: spherical aberration, coma, astigmatism, curvature of field, and distortion. Radiation over a range of wavelengths is affected by two kinds of chromatic aberration: longitudinal (axial) and transverse (off-axis or lateral) aberrations. The first three cause points to be imaged as blurs. The next two image points as points but distort the shapes of lines because a point in object-space and the corresponding point in image-space do not both lie on a straight line through the center of perspective. The two chromatic aberrations cause an object seen in white light to appear as a white image with colored edges. Aberrations occur in systems other than optical e.g. radio systems and infrared systems. The above terminology applies also to aberrations in such systems. SEE ALSO aberration, chromatic; aberration, off-axis; aberration, spherical; aberration, zonal; aperture aberration; astigmatism; coma; distortion; Seidel aberration.

**aberration, planetary** *n.,* The apparent angular displacement of the observed direction of a celestial body as caused by the observer's motion (stellar aberration) and the actual motion of the observed object (light-time aberration). SEE ALSO aberration, light-time.

**aberration, relativistic** *n.,* The tendency of a beam of charged particles moving at relativistic speeds to stay narrow and not spread. As the speed of the particles increases, the electrostatic field accompanying the particles diminishes and the tendency to spread therefore diminishes also.

**aberration, secular** *n.,* That difference between the apparent direction of a source of radiation and the true direction resulting from the essentially uniform and rectilinear motion of the center of mass of the Solar System with respect to the source. Also called aberration of fixed stars. SEE ALSO aberration,stellar.

**aberration, spherical** *n.,* The focusing of rays from a point-source on the optical axis into points whose distance from the focal plane varies as the angle at which the rays entered the optical system, with rays entering at a small angle to the optical axis focusing closer to the focal plane than rays entering at a larger angle. The envelope of the converging rays is called the caustic of the optical system. The amount of spherical aberration varies with the location of the source and for any particular source is approximately proportional to the square of the distance of the outermost rays from the optical axis. SEE ALSO Seidel aberration.

**aberration, stellar** *n.,* The difference between the direction of a radiating source as it would be seen by a stationary observer at a specified point, and the direction in which the source is actually seen by a moving observer at that point. In the case of an observer on the Earth, stellar aberration has three principal components: diurnal aberration, resulting from the diurnal rotation of the Earth; annual aberration, resulting from the annual motion of the Earth about the Sun; and secular aberration, resulting from the motion of the Solar System's center of mass. SEE ALSO aberration, annual; aberration, diurnal; aberration, secular.

**aberration, transverse chromatic** SEE aberration, chromatic.

**aberration, zonal** *n.,* That defect of a lens system in which the focal point shifts when the aperture is decreased. It is the result of the combined effects of a number of optical aberrations. SEE ALSO aberration, optical.

As I continue to think about how light is collected, recorded and shared with others, I hope we can all see from these few words that without aberration that change and time and the light itself is harder to observe, record, or document.

In this blog we will post terms and themes of related terms and their definitions that come from the The Glossary of Mapping Sciences with commentary. In the hope that by discussing the meaning of words we come to deeper understanding and more meaningful usage.

0 comments

210 views