Solar day

“ Dawn, as we have it on earth, can have no counterpart upon the moon. No atmosphere is there to reflect the solar beams while the luminary is yet out of actual sight, and only the glimmer of the zodiacal light heralds the approach of day. From the black horizon the sun suddenly darts his bright untempered beams upon the mountain tops, crowning them with dazzling brilliance while their flanks and valleys are yet in utter darkness. There is no blending of the night into day. ”

“ As the sun supplies the energy for most of the physical and all the vital processes of the earth’s surface, the rhythm of the solar day is impressed in multitudinous ways on man and his environment, and he makes it his primary or standard unit of time. ”

“ And as the solar day lengthens fastest at spring when the sun crosses the equator from south to north, so the time the moon is above the horizon lengthens most, day by day, when the moon is crossing the equator from south to north. It lengthens, then, from an hour to an hour and 20 minutes in one day, that is, the interval between moon-rise and moon-setting increases from 30 to 40 minutes. ”

“ Days are distinguished as solar, sidereal or lunar, according as the revolution is taken relatively to the sun, the stars or the moon. The solar day is the fundamental unit of time, not only in daily life but in astronomical practice. ”

“ Answer.—Four correct “noon-marks” are made in a year, on the following days: Dec. 24, April 15, June 14, and Sept. 1. Owing to the inclination of the earth’s axis and its unequal movement in its orbit, solar days vary in their length. The average solar day corresponds to the twenty-four hours of our clocks, which keep what is called mean time. ”

“ It may here be stated as a note by the way that the circumstances under which the Sun first shows itself after it has risen above the horizon has some bearing on the probable character of the weather which is at hand. When the first indications of day-light are seen above a bank of clouds it is thought to be a sign of wind; but if the first streaks of light are discovered low down, that is in, or very near the horizon, fair weather may be expected. ”

“ When totality is over and the corona has faded away, the moon's disc creeps little by little from the face of the sun, light and heat returns once more to the earth, and nature recovers gradually from the gloom in which she has been plunged. About an hour after totality, the last remnant of moon draws away from the solar disc, and the eclipse is entirely at an end.
The corona, the chromosphere, and the prominences are the most important of these accompaniments of the sun which a total eclipse reveals to us. ”

“ That a certain amount of solar heat is reflected to us from the moon is shown by the sharp drop in temperature which certain heat-measuring instruments record when the moon becomes obscured in a lunar eclipse. The solar heat which is thus reflected to us by the moon is, however, on the whole extremely small; more light and heat, indeed, reach us direct from the sun in half a minute than we get by reflection from the moon during the entire course of the year. ”

“ Solar Tide The natural period of oscillation of a body of water may accentuate either the solar or the lunar tidal oscillations. Though as a general rule the tides follow the Moon, the relative importance of the solar effect varies in different areas. There are a few places, primarily in the South Pacific and the Indonesian areas, where the solar oscillation is the more important, and at those places the high and low waters occur at about the same time each day. ”

“ On a really excellent day more may be sometimes detected than during several weeks when the atmosphere is only moderately favourable. The observations, being of a critical nature, should not be103 attempted in winter, when the Sun is low. I have frequently secured fine views of the delicate structure of the solar surface between about 8 and 9 A.M. in the summer months; and this is often a convenient time for amateurs to snatch a glimpse, before going to business. ”

“ In a total solar eclipse, the time which elapses from the moment when the moon's disc first begins to impinge upon that of the sun at his western edge until the eclipse becomes total, lasts about an hour. ”

“ This variation in distance, large as it appears in figures, can, however, not be productive of much alteration in the amount of solar heat which we receive, for during the first week in January, when the distance is least, the sun only looks about one-eighteenth broader than at the commencement of July, when the distance is greatest. ”

“ These words need but little explanation, at any rate, as regards those persons who are able to trace the Latin origin of the words. “Equinox” is simply the place occupied by the Sun twice every year (namely about March 20 and September 22) , when day and night are theoretically equal throughout the world, when also the sun rises exactly in the east and sets exactly in the west. The “solstices” represent the standing still of the sun at the given times and places, and are the neutral points where the Sun attains its greatest northern or southern declination. ”

“ The conclusion from this is inevitable, that spots are not bodies independent of the Sun, as satellites would be, but that they are connected with the Sun’s surface, and are affected by its movement of rotation. If we make every day for a few days in succession a drawing of the Sun’s disc with any spots that are visible duly marked thereon, we shall see that their apparent progress is rapid near the centre of the Sun, but slow near either limb. ”

“ The sky here is so pure and clear in our summer that it would be a shame to neglect such an opportunity of making experiments on heat, and accordingly I have been occupied in the December solstice in determining the constant of solar radiation, that is to say, the absolute quantity of heat sent down to the earth’s surface from the sun at noon, or at a vertical incidence. ”

“ By the dispersive powers of this medium, sunlight is converted into daylight; and instead of unbearable, parallel rays illuminating brilliantly, and scorching up with heat those parts upon which they directly fall, leaving all other parts in the darkness of night, we enjoy the blessings of a diffusion of its rays, and experience the beauties of soft shades and slowly-deepening shadows. Without an atmosphere, the sun of the morning would burst upon us with unbearable brilliancy, and leave us suddenly, at the close of day, at once in utter darkness. ”

“ In the Northern Hemisphere, the summer solstice day has the most daylight hours, and the winter solstice has the fewest daylight hours each year. The tilt of the axis allows differential heating of the Earth’s surface, which causes seasonal changes in the global circulation. ”

“ Speaking loosely, it may be estimated that at the sea-level, in fair weather, neither excessively moist nor dry, about thirty per cent, of the solar radiation is absorbed when the sun is at the zenith, and at least seventy-five per cent, at the horizon. ”

“ This is evident when we consider the different natures of the apparent motions of the sun and the moon. The sun passes through a twelfth part of the zodiac each month, and month by month the successive constellations of the zodiac are brought out, each in its own season; each having a period during which it rises at sunset, is visible the whole night, and sets at sunrise. The solar Mazzaroth are therefore emphatically brought out, each "in its season." ”

“ We have here another periodic variation, which runs its complete course in a period of a year, and it is easy to see that this variation must have a marked effect on the warming of the earth, the long days and great altitudes of summer producing the greater warmth of that season, while the shorter days and lower altitudes of December, by diminishing the daily supply of solar heat, bring on the winter's cold. ”

“ NIEMAND. Why, because twenty-seven days is about the synodic period of solar rotation. That is, if you see a large spot at the center of the Sun's disk today, there is a good chance if it survives that you will see it at the same place twenty-seven days later. But that night Middletown produced another chart that showed the connection with the Sun in a way that was even more convincing. ”

“ Actually, the sun seems little by little to creep up the sky during one half of the year, namely, from mid-winter to mid-summer, and then, just as gradually, to slip down it again during the other half, namely, from mid-summer to mid-winter. ”

“ For, in other countries, the scorching heat of the sun in summer renders the greater part of the day unapt either for labour or amusement, and the frequent rains are not less troublesome in the more temperate parts of the year: But, in this happy climate, the sun rarely appears. Not that the heavens have at any time a dark or gloomy aspect; for there is constantly a cheerful gray sky, just sufficient to screen the sun, and to mitigate the violence of its perpendicular rays, without obscuring the air, or tinging the light of day with an unpleasant or melancholy hue. ”

“ As soon as the Sun has disappeared below the horizon all the clouds overhead continue for a few minutes so highly illuminated as to reflect scarcely less light than the direct light of the Sun. As, however, the Sun gradually sinks lower and lower, less and less of the visible atmosphere receives any portion of its light, and consequently less and less is reflected minute by minute to the Earth at the observer’s station until at length the time comes when there is no sunlight to be reflected—and it is night. ”

“ As the sun passes on toward X′ its daily arc continues to increase and the days to grow longer until at X′ it reaches its greatest declination north of the equator and we have the longest day, June 21, the summer solstice. When the sun reaches this point, its rays strike the northern hemisphere more directly than at any other time causing the hot or summer season in this hemisphere. ”

“ We have already remarked upon the colours of the sunset, which are attributable to the vapours in the atmosphere, and we say a red sky foretells fine weather; a yellow sky changing into green means rain, or rain and wind; on the other hand, when the red rays appear we may anticipate fine weather, as the atmosphere is becoming less and less moist.
A “low” dawn is known as a good sign; so when the first rays appear at, or near the horizon, we may anticipate a fine day, as we may when the morning is grey.
“Evening red and morning grey”
are almost unfailing tokens of fine weather. ”

“ Similarly, near the sun’s poles we should find a gradual darkening, as is the case; but from five degrees to thirty degrees latitude, the sun, in its rotation, by reason of the inclination of its axis, passes at every point directly beneath the planets, or within their area of control, and here we find the solar spots in their greatest number, size, and intensity. These sun-spots cross the face of the sun in about fifteen days, and vary in development from year to year, having a cycle of 11.11 years from maximum to maximum. They also have a long cycle of about fifty-six years. ”

“ That the sun’s light has a powerful influence on the living body, every one will readily admit. Knowing the reviving influence of a moderate degree of solar heat, the ancients had terraces on their house-tops, called solaria, in which, to use their own expression, they took a solar air-bath. ”

“ The solar impulse which produces these effects tends to precede the terrestrial exhibit which depends upon the solar impulse by some months, possibly by a year under certain conditions, and this anticipation of course promises an opportunity to develop what may become a rational ground for a seasonal forecast for terrestrial weather. ”

“ An easterly exposure has the full force of the sun all the morning, but after noon, there is either no sunshine, or feeble, slanting rays, so that at the very time when the sunlight is strongest, an easterly position is deprived of its warmth altogether. Of course a westerly exposure is just the reverse of this, and after a morning passed in the shade, the tree is suddenly overwhelmed with sunlight at a time of day when the temperature is the highest. ”

“ In the summer at mid-day the sun rises high in the heavens, in the winter it is always low. Our luminary, therefore, performs an annual movement up and down in the heavens, as well as a diurnal movement of rising and setting. But there is a third species of change in the sun's position, which is not quite so obvious, though it is still capable of being detected by a few careful observations, if combined with a philosophical habit of reflection. The very earliest observers of the stars can hardly have failed to notice that the constellations visible at night varied with the season of the year. ”

“ It is an interesting fact that the summer solstice, or the point which the sun occupies when it attains its greatest northerly declination, on the longest day of the year, is close by this great cluster in Gemini. In the glare of the sunshine those swarming stars are then concealed from our sight, but with the mind's eye we can look past and beyond our sun, across the incomprehensible chasm of space, and behold them still shining, their commingled rays making our great God of Day seem but a lonely wanderer in the expanse of the universe. ”

“ But if we wish to understand how it is that the sun is the cause of one season being so different from another, we must remember that as the earth takes its yearly journey round the sun it changes its place, getting nearer to him or farther away from him. In our summer-time the part of the earth where we live is turned more towards the sun, and so gets more of the light and heat which have their home there, than at any other time. Our winter days are so short, because at that time we are turned from the sun more than at any other. ”