An Etymological Dictionary of Astronomy and Astrophysics
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One of many → imaginary lines whose direction at all → points along its length is that of the electric or → magnetic field at those points. In → electric fields the lines of force are directed toward → negative charges and point away from → positive charges. In magnetic fields the lines of force are directed from the → north pole to the → south pole.

→ line; → force.

Same as → line of force in a magnetic field.

→ line; → induction.

The line created by the intersection of the equatorial plane and the orbital plane.

→ line, → node.

The imaginary straight line connecting the object and the objective lens of the viewing device.

→ line; → sight.

The representation of a spectral line as produced by an observing instrument.

→ line, → profile.

The change in the shape of a → spectral line over time. For instance, in → Wolf-Rayet stars the line profile varying on time-scales of minutes to hours is attributed to → microturbulence.

→ line; → profile; → variability.

Spectrum consisting of discrete lines (emission or absorption), each corresponding to a particular wavelength, as opposed to a continuous spectrum.

→ line, → spectrum.

Same as → line intensity.

→ line, → strength.

Part of the line profile between the continuum level and the half value of the emission or absorption peak. The wings are due to matter traveling at much greater speeds than that providing the main peak. → red wing; → blue wing.

→ line; → wing.

A stellar atmosphere model which includes metals or uses methods to reproduce their effects, → line blanketing.

→ line; → blanket; → model.

Same as → radiation-driven wind.

→ line; → drive; → wind.

Any of a countless number of dark streaks visible on → Europa's surface that crisscross the whole → Galilean satellite. They are up to 1,000 km long, 20 km wide, and 1 km deep, but only hundred of meters high. In many cases, the ridges are double, often with dark outer edges and a central band. Images show that on each side of the lines, the edges have moved relative to each other. According to the most likely hypothesis, lineae result from eruptions of warm water, in a scenario similar to the present day mid- oceanic ridges on Earth.

From L. linea, → line.

Xaš, → streak.

Confined to first-degree algebraic terms in the relevant variables.

Adj. of → line.

The rate of change of the → linear velocity with time. It is defined by the expression Δv/Δt and is equal to the → first derivative of the → linear velocity.

→ linear; → acceleration.

Taking the first term in the Taylor series as an approximation to a mathematical function at a given point. → first approximation.

→ linear; → approximation.

A version of → planispheric astrolabe in which the → celestial sphere and the various circles of altitude and declination are projected on to a line represented by a staff. The staff is equivalent to the meridian line and contains markings to indicate the centers of these circles and their intersections with the meridian. By attaching three ropes to the appropriate points on the staff to act as radii, the circles and their intersections can be reconstructed. One of the ropes was attached to a plumb line. A scale giving chord lengths in the meridian circle extended the linear astrolabe's range of applications. It was invented by the Iranian mathematician and astronomer Sharafeddin Tusi (c1135-1213), but no early example has survived. Same as → Sharafeddin's staff and Tusi's staff.

→ linear; → astrolabe.

A measure of how well data points fit a straight line. When all the points fall on the line it is called a perfect correlation. When the points are scattered all over the graph there is no correlation.

→ linear; → correlation.

The real physical diameter, as opposed to angular diameter.

→ linear; → diameter.

An equation in which the → dependent variable y and all its differential coefficients occur only in the first degree. A linear differential equation of → order  order n has the form:
f_n(x)y⁽ⁿ⁾ + f_n-1(x)y^(n-1) + ... + f₁(x)y^' + f₀(x)y = Q(x),
where f₀(x), f₁(x), ..., f_n(x) and Q(x) are each continuous functions of x defined on a common interval I and f_n(x)≠ 0 in I. A linear differential equation cannot have, for example, terms such as y² or (y^')^1/2. See also: → homogeneous linear differential equation; → nonhomogeneous linear differential equation.

→ linear; → differential; → equation.

A system of three charges +q, -2q, and +q, arranged along a line to form an axial quadrupole. The → electric potential  V due to a linear quadrupole varies as 1/r³, whereas the → electric intensity  E varies as 1/r⁴.

→ linear; → electric; → quadrupole.