## Glossary | ||

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- Anti-Aliasing
- A remarkable Acorn
invention to improve display of outline fonts, where the edges are made
*fuzzy*, blending to the background. This process can also be applied to vector graphics, whereby lines and curves are smoothed, rather than having the classic computer*staircase*effect. The logo above is fully anti-aliased.The following images, using the same font, illustrate the poor text display of a Windows™ PC, despite being in a 32-bit screen mode, when compared to the RISC OS display - which matches the printed document as closely as possible: PC: Acorn: - Astronomical Unit (symbol AU)
- The Astronomical Unit
*used*to be defined as the average distance from the Sun to the Earth. It is now*defined*as 1·496 × 10^{8}km. Which is the same thing! - Arc Second (symbol
**"**) - Angular measurement. 1/3600 of a degree.
- Arc Minute (symbol
**'**) - Angular measurement. 1/60 of a degree.
- bar (no symbol)
- A unit of pressure, it is equivalent to 10
^{5}pascal (Pa). S.I. prefxes can be attached; e.g.*mbar = millibar*. - Bestfit
*or*Best Line. - A
__straight__line through a set of points on a graph. From this line, it is possible to predict the outcome of experiments outside the range of data; but to ever-increasing uncertainty. - CSV
*Comma Separated Variable*. The simplest form of file for holding scientific, or other, data. Data is listed in columns in a text file, each value being separated by a comma. Each new line represents a new set of data.- Cubic Spline
- A curve fitted through, or close to, all the points on a graph. It takes points in small groups, and fits a simple cubic equation to fit these, then moves on to the next group. q.v. bestfit.
- Degree (symbol °)
- Angular measurement. A circle is divided uniformly into 360 degrees, so half a circle is 180°, a quarter 90°, etc.
- Density (symbol - Greek letter,
*rho*) - The amount of matter in a given volume of space, derived from mass/volume. Its basic unit is kg/m³, but g/cm³ is often used instead.
- Ecliptic
- The name given to the plane of the solar systm in which the planets approximately lie.
- ESA
*European Space Agency*.- EXP
*Exponential*. Can have two meanings, but here we refer to*exponential curves*where the curve through data can be assymptotical to the axes, or other.- Frequency
- The number of complete oscillations or cycles of a physical quantity in a unit time. It is the reciprocal of period, and the S.I. unit is the hertz (Hz).
- hertz (no cap.; pl. hertz)
- The S.I. unit of frequency, the same as cycles per second: 1 Hz = 1 s
^{-1} - Light Year (abbreviation
*ly*) - The Light Year is defined as the distance travelled by light (or any other electro-magnetic wave) in a year in a vacuum. Since the speed is 3·00×10
^{8}m/s; and 1 year = 365 × 24 × 60 × 60 seconds = 3.15×10^{7}s; it follows that a light year is: 3·00×10^{8}× 3.15×10^{7}= 9.46×10^{15}m. - Log
- Abbreviation and symbol for
*logarithm*. The logarithm to the base*a*of*x*is written: log_{a}*x*. The most common is a = 10, i.e. log_{10}*x*, or lg*x*. - Log-Log
- A graphing method, whereby both the X and Y axes are logarithmic scales.
**See this in action!** - LN
*Natural Log*- a logarithm, where*a = 2.718 282...*(a transcendental number), usually denoted log_{e}*x*, or ln*x*.- mbar
- See bar.
- Origin
- On a graph, the point where the x- and y- axes intersect, i.e. at co-ordinates (0,0).
- Parabola
- A curve of the form
**Y = u + vX + wX²**, where u, v and w are constants, X and Y the variables. The symbiosis between kinetic and gravitational energies in a simple harmonic system is an example of a parabolic relationship. - Parsec (symbol pc)
- A PARallax SECond. The diagram (right) shows how it is defined. The Earth's orbit is shown around the Sun. Over a period of 6 months, we move from A to B. A nearby star (white) will appear to change position against the background of more distant stars (not shown). The angular change in position (parallax) that the star undergoes is
*p*. The parsec is defined as being the distance*d*for which*p*is 1 arc-second. No star is quite this close, but this unit is useful as if one can measure parallax, you can instantly find the distance in parsecs. - Pascal (symbol
**Pa**) - The S.I. unit of pressure. 1 Pa = 1 Nm
^{-2}. Named after Blaise Pascal. - Period
- The duration of one cycle or oscillation of a periodic phenomenon; i.e. the reciprocal of frequency. S.I. unit is the second.
- Polynomial
- A curve of the form
**y = a + bx + cx² + dx³ + ex**, where a, b, c, etc. are constants, x and y variables. Good graphing software typically can fit a polynomial to the sixth power to a set of data.^{4}+ ... - RADAR
- RADAR stands for RAdio Detecting And Ranging.
- Reciprocal
*“One over x is the reciprocal of x.”*i.e. 1/x, or x^{-1}.- Retrograde
- Literally meaning "going backwards", this can refer to the apparent backwards motion that planets appear to take in the sky when the Earth overtakes them. It can also refer to planetary moons orbiting in the opposite direction to the mother planet's rotation.
- Scatter
- A graph where the data is plotted as points only. Normally one would plot a curve through the points to show any trend in the data.
- Semilog
- A useful graphing method, whereby one of the X or Y axes has a logarithmic scale.
- S.I.
- Abbreviation for Système International (d'Unités)
- SID
*Software Independent Data*format. An enhanced version of CSV, that includes information about graphs, electronic sensors, units, etc.- Spectroscopy
- The study of light from stars or planets. A spectrum of light is produced by magnifying light from objects with a telescope, and separating it into colours with a prism or diffraction grating. The spectrum can reveal the chemical composition of the star, or constituents of a planet's atmosphere.
`More`. - Square
*“x × x = x squared.”*i.e. x × x = x².- Square Root
- or n
^{½}, where n is a number. The square root of n, multiplied by itself, is n. - TSV
*Tab Separated Values*. Just like a CSV file, but with variables separated by tabs (mainly used on Apple Macintosh computers).- watt, W
- The unit of Power, that is
. One watt is equal to one joule per second. (**the amount of energy used each second** )**1W = 1J/s**
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