In co-ordinate geometry, the abscissa is the x-coordinate of a point (the horizontal distance of that point from the vertical or y-axis). For example, a point with the coordinates (9, 6) has an abscissa of 9. Research Abscissa
A day is either the interval of time during which the sun is continuously above the horizon, or the time occupied by a revolution of the earth on its axis, embracing this interval (the period of light) as well as the interval of darkness. The day in the latter sense may be measured in more than one way. If we measure it by the apparent movement of the stars, caused by the rotation of the earth on its axis, we must call day the period between the time when a star is on the meridian and when it again returns to the meridian: this is a sidereal day. It is uniformly equal to 23 hours, 56 minutes, 4.098 seconds. But more important than this is the solar day, or the interval between two passages of the sun across the meridian of any place. The latter is about 4 minutes longer than the former, owing to the revolution of the earth round the sun, and it is not of uniform length, owing to the varying speed at which the earth moves in its orbit and to the obliquity of the ecliptic. For convenience an average of the solar day is taken, and this gives us the mean solar or civil day of 24 hours, the difference between which the actual solar day at any time is the equation of time.
The length of the days and nights at any place varies with the latitude and season of the year, owing to the inclination of the earth's axis. In the first place, the days and nights are equal (twelve hours each) all over the world on the 21st of March and the 21st of September, which dates are called the vernal (spring) and autumnal equinoxes. Again, the days and nights are always of equal length at the equator, which, for this reason, is sometimes called the equinoctial line. With these exceptions, we find the difference between the duration of the day and the night varying more and more as we recede from the equator, and at the poles the year consists of one day of six months' duration, and one night of the same.
The Babylonians began the day at sun-rising; theJews at sun-setting; the Egyptians and Romans at midnight, as do most modern peoples. The civil day in most countries is divided into two portions of twelve hours each. The abbreviations PM. and AM. (the first signifying post meridiem, Latin for afternoon; the latter ante meridiem, forenoon) are requisite, in consequence of this division of the day. The Italians in some places reckon the day from sunset to sunset, and enumerate the hours up to twenty-four; the Chinese divide it into twelve parts of two hours each.
For astronomical purposes the day is divided into twenty-four hours instead of two parts of twelve hours. Formerly it began at noon, but since the 1st of January 1885, the day of twenty-four hours begins at midnight at Greenwich Observatory; and this reckoning is now generally adopted for astronomical purposes elsewhere than at Greenwich. The Greenwich day practically determines the date for all the world. At mid-day at Greenwich the date (day of the week and month) is everywhere the same, though there are all possible differences in naming the hour of the day. But mid-day at Greenwich is the only instant at which we ever have the same date all over the world. The meridian of midnight, which is then at 180 degrees east or west, goes on revolving, gradually bringing a new date to every place to the west of that line, but obviously not bringing that new date to the places immediately to the east of that line until twenty-four hours after. From this it follows that whereas places on the one side of the globe never have a different date except when midnight lies between them, places on the opposite side of the globe, and on different sides of the meridian of 180 degrees east Or west never have the same date except when midnight lies between them. The actual difference of time between Wellington in New Zealand and Honolulu in Hawaii is only about two hours; yet a person at Wellington may date a letter 9 o'clock AM 26th June, while another writing at the same instant at Honolulu dates his 11 o'clock AM 25th June.
A globe is a sphere, a round solid body, which may be conceived to be generated by the revolution of a semicircle about its diameter. An artificial globe, in geography and astronomy, is a globe of metal, plaster, paper, pasteboard, plastic etc, on the surface of which is drawn a map, or representation of either the earth or the heavens, with the several circles which are conceived upon them, the former being called the terrestrial globe, and the latter the celestial globe. In the terrestrial globe the wire on which it turns represents the earth's axis, the extremities of it representing the poles. The brazenmeridian is a vertical circle in which the
artificial globe turns, divided into 360 degrees, each degree being divided into minutes and seconds. The brassmeridian receives the ends of the axis on which the globe revolves. At right angles to this, and consequently horizontal, is a broad ring of wood or brass representing the horizon; that is, the true horizon of the earth which lies in a plane containing the earth's centre. The horizon and brassmeridian are connected with the stand on which the whole is supported. On the surface of the globe, as on other maps, are marked parallels of latitude, meridians, etc. On a globe of some size the meridians are drawn through every 15" of the equator, each answering to an hour's difference of time between two places. Hence they are called the hour circles. A number of problems or questions, many of them more curious than useful, may be solved by means of a terrestrial globe. Among the most important are such as to find the latitude and longitude of a place, the difference of time between two places, the time of the sun's rising and setting for a given day at a given place, etc.
A grindstone is a cylindrical stone, on which sharpening, cutting, and abrasion are effected by the convex surface while the stone is revolving on its axis. They are made of sandstone, or sandstone grit of various degrees of fineness. Good stones are obtained in various parts of England, especially from the coal districts of Northumberland, Newcastle grindntones being especially famous. The Sheffield grindstone, traditionally used for grinding files and the like, is obtained from Hardsley, about 14 miles north of Sheffield. Artificial grindstones have also been successfully used. Research Grindstone
A hyperbola is a plane curve, one of the conic sections, formed by a plane that cuts both nappes of a right circular cone but does not pass through the vertex of the cone. A hyperbola has two U- shaped non-intersecting branches, identical in form, with the open parts facing in opposite directions; the arms of each branch separate as they recede. A hyperbola is also defined as the locus of all points, such that the difference between the distances from any point on the hyperbola to two fixed points, called the foci, is equal to a constant. Each branch contains one focus in its interior area; the line joining the foci intersects each branch in a point called a vertex. This line, or the segment between the vertices, is called the transverse axis. The line perpendicular to the transverse axis and passing through the point midway between the vertices, midway between the foci, is the conjugate axis. The two axes meet at the centre of the hyperbola, which is symmetric with respect to each axis and the centre.
A hyperbola has two asymptotes passing through the centre; an asymptote of a curve is a straight line with the property that the distance between it and the curve approaches zero as the curve recedes to infinity. A rectangular or equilateral hyperbola has asymptotes that are perpendicular to each other. The hyperbola has useful and important properties. In particular, the angle formed at a point on the hyperbola by the lines joining the point to the foci is bisected by the tangent to the hyperbola at that point. In astronomy, some orbits are hyperbolic in shape. The modern navigational device the loran also uses
hyperbolas. Research Hyperbola
The North Star (polestar) is a conspicuous star in the northern hemisphere, located closest to the point toward which the axis of the earth is directed, thus roughly marking the location of the north celestial pole. A polestar has been used by navigators throughout recorded history for charting navigation routes and is still used for determining true azimuth and astronomic latitude. The positions of the celestial poles change as the earth's axis moves with the earth's processional motion, and as the north celestial pole assumes different positions relative to the constellations, different stars become the North Star. During the past 5000 years the line of direction of the North Pole has moved from the star Thuban, or Alpha Draconis, in the constellationDraco, to within one degree of the bright starPolaris in the constellation Ursa Minor, which is now the North Star. Research North Star
In co-ordinate geometry, the ordinate is the y-coordinate of a point (the vertical distance of the point from the horizontal or x-axis). For example, a point with the coordinates (9,6) has an ordinate of 6. Research Ordinate
A sundial or dial is an instrument for showing the hour of the day from the shadow thrown while the sun is shining by a stile or gnomon upon a graduated surface. The sundial has been known from the earliest times amongst Egyptians, Chaldeans, and Hebrews. From those eastern nations it came to the Greeks. It was introduced into Rome during the first Punic war.
Sundials are of various construction, horizontal, inclined, or upright, the principle in every case being to show the sun's distance from the meridian by means of the shadow cast by the stile or gnomon. The stile is made parallel with the earth's axis,and may be considered as coinciding with the axis of the diurnal rotation. Consequently as the sun moves westwards the shadow of the stile moves round in the opposite direction, falling on the meridian lines so marked as to represent the hours of the day. The sundial of course gives solar time, which, except on four days of the year, is slightly different from that of a well-regulated clock. Since at least 1900 sundials have been rather articles of curiosity or ornament than of use. Research Sundial
In mining, a tippler is a device for emptying coal tubs. The coaltub after being weighed is run into a cage, the tippler, which turns on an axis so that during the first half of the rotation the tub is emptied, and during the second half it is returned to its original position ready to be moved to the pit head. Research Tippler
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Abscissa
