SUMMER. [WtYrEn, &e.] SUN (Latin, sal; Greek, 5Aws, Relies). In the present article we confine ourselves to the astronomical characters of the sun's orbit, and to what we •know of its physical constitution. For more particulars on the measurement of time, see TIME ; on chronology as dependent on this body, and on the more common characters of its motion, see YEAR. See also MOON ; SEASONS ; ASTRONOMY ; ZONA° (on mythology); ZODIACAL LIGHT; TWILIORT ; &e.
It is needless to say that if the utility of the subject of an article were to determine its length, the one we are now commencing ought in justice to occupy several volumes of the work : were we, however, seriously to mete out the importance of the sun in columns of a Cyclopedia, our panegyric would not be more quaint than that of Sir John Bill, who says that this luminary is "enough to stamp a value on the science to which the study of it belongs." In relation to astronomy this is particularly true ; for it would be possible to preserve life on the earth, and to keep order, without any knowledge of the moon, planets, or stars; but to do this without any acquaintance with the sun's motions would be absolutely impossible. The source of light and heat, and through them of the alternations of the vegetable world, is, in the highest secondary sense, the giver and sustainer of life ; but this very importance ensures names to so many results of solar phenomena, that the present article is stripped of details, by their entering more appropriately into others.
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The motions of the run are in fact those of the earth, written In the heavens. if the diurnal motion of the earth were stoma!, the sun would appear to move slowly among the stars, from west to east, at the rate of about twice its own diameter in twenty-four hours by the dock. This [\lottos] is the consequence of the orbital motion of the earth, which is communicated in appearance to the sun. If the eartlis orbital motion were stopped, the diurnal motion continuing as usual, the sun would appear to move round daily, from cast to west, as at present ; but since there would then be no motion of that body among the stars, those stars which are at any one time hidden by tho daylight would always be hidden, and the face of the heavens at any given hour of night would be the same at all times of the year. The effect of the orbital motion of the earth combined with the diurnal motion is that the solar day, or the interval between two meridian passages of the sun, is a little longer than the sidereal day (about four minutes), or than the actual revolution of the earth; so that all the stars have their turn, and every star in the course of the year comes on the meridian at every period of the natural or solar day. [Srsome; TIME.]
The great phenomena of day and night are attended with very different circumstances in different parts of the globe. We are not speaking now of the high polar regions, north and south, in which the sun never sets for days together, but of those harts of the earth its which there is actual appearance and disappearance of the luminary, or real day and night. Let its take the day of the vernal equinox as a specimen, when the sun is in the equator (we presume in our reader a knowledge of the terms and notions In SPHERE, Dourniss OF TUE). If we take one fixed line to represent the horizon of different places, as B A c, the sun will rise to a place on the equator so as to move along the circle D A E, and to come directly up from the horizon; while at a place near the pole it will move, relatively to the horizon (still B A c), along the circle r Ac. Now the first evidence which the sun gives of its approach is this (the diagram, though of very distorted dimensions, may be of use) : before it has risen above the horizon of a place, eo as to be visible, it can throw its rays Into the atmosphere above the place, which atmosphere ,reflects something both of light and heat to the place itself. This period is called the twilight, and it is said that there is more or less of twilight as long as the sun is not more than 15° below the horizon ; though certainly the twilight which eaves candle light does not last so long. But be the number of degrees which are allowed to twilight, more or leas, it is obvious that at the equator, where the whole of the sun's way is made directly to or from the horizon, the intermediate period of twilight must be much shorter than at a place near the pole, where the motion towards the horizon is very oblique, instead of being all ascent, as before rising, or descent, as after setting. The consequence Is well known : iu the tropics, the warning is short, and soon after the light begins to break the sun makes its appearance, and it is broad and hot day ; while nfter the setting the light as soon disappears, and it is dark night. With us, on the coutrary, and still more in higher northern latitudes, there is a long warning of the approach of the luminary before the sunrise, and a long remembrance of it after sunset. In all climates the-transition from day to night is broken by the two circumstances mentioned in SEASONS. In the same article it is pointed out that the heat received during the winter and summer halves of the year is the same over the whole earth.