DIURNAL

, something relating to the day; in opposition to nocturnal, relating to the night.

Diurnal Arch, is the arch described by the sun, moon, or stars, between their rising and setting.

Diurnal Circle, is the apparent circle described by the sun, moon, or stars, in consequence of the rotation of the earth.

Diurnal Motion of a Planet, is so many degrees and minutes &c as any planet moves in 24 hours.

The Diurnal Motion of the Earth, is its rotation round its axis, the duration of which constitutes the natural day.—The reality of the diurnal rotation of the earth is now past all dispute.

Diurnal Parallax. See Parallax.

Diurnal is also used in speaking of what belongs to the nycthemeron, or natural day of 24 hours: in which sense it is opposed to annual, menstrual, &c.

The diurnal phenomena of the heavenly bodies are solved from the diurnal revolution of the earth; that is, from the rotation of the earth round its own axis in 24 hours. This rotation is equable, and from west to east, about an axis whose inclination to the ecliptic is now 66° 32′. Since the earth is an opaque body, that small part of its surface which comes at the same time under the confined view of the spectator, though really spherical, will seem to be extended like a plane: and the eye, taking a view of the heavens all around, defines a concave spherical superficies, concentric with the earth, or rather with the eye, which the said plane of the earth's supersicies will divide into two equal parts, the one of which is visible, but the other, because of the earth's opacity, hid from the view.

And as the earth revolves about its axis, the spectator, standing upon it, together with the said plane he stands upon, called his horizon, dividing the visible from the invisible hemisphere of the heavens, is carried round the same way, viz, towards the east. From hence it is, that the sun and stars, placed towards the east, being before hid, now become visible, the horizon as it were sinking below them; and the stars &c towards the west are covered or hid, and become invisible, the horizon being elevated above them. So that the former stars, to the spectator, who reckons the place he stands on as immoveable, will appear to ascend above the horizon, or rise; and the latter to descend below the horizon, or set.

Since the earth, with the horizon of the spectator fixed to it, continues to move always towards the same parts, and about the same axis equally; all bodies, and all phenomena, that do not partake of the said motion, (that is, all such things as are entirely separate from the earth) will seem to move in the same time uniformly, but towards the opposite parts, or from east to west: and every one of these objects, according to sense, will describe the circumference of a circle, whose plane is perpendicular to the axis of the earth. And because all these circles, together with the visible objects describing them, appear to be in the concave spherical superficies of the heavens, every visible object will seem to describe a greater or less circle, according to its greater or less distance from the poles, or extremities of the earth's axis produced; the middle circle between these poles, called the equator, is consequently the greatest.

It may farther be observed, that whereas, by the diurnal revolution of the earth, all the several luminaries seem to move in the heavens from east to west, hence this seeming diurnal motion of the celestial lights is called their common motion, as being common to all of them. Besides which, all the luminaries, except the sun, have a proper motion; from which arise their proper phenomena: as for the proper phenomena of the sun, they likewise seem to arise from the proper motion of the sun; though they are really produced by another motion, which the earth has, and by which it moves round the sun once every year, and thence called the annual motion of the earth.