08

TABLE

Showing the Equation of Time within a minute, being calculated

for the second year after leap year.

Those columns that are marked +, show that the clock or watch is faster than the sun; and those marked, that it is slower.

ART. 4. Of the Harvest Moon. 125. If the moon revolved round the earth in 24 days, it is manifest that its mean daily motion would be (360+24) 15°, corresponding exactly to one hour of time, consequently the mean daily difference in the time of the moon's rising would be one hour. But the moon is 29 days in passing from change to change; consequently her mean daily motion is, (360 - 291) 12° 12' 12'', and of course the mean difference in the times of her rising is something less than an hour. It is about 49 minutes. But it was noticed by the husbandman long before astronomers could account for it, that for 6 or 8 nights, near the full moons of September and October, the moon rose nearly when the sun set, and afforded convenient light to continue his occupation. From the peculiar advantages derived from these full moons, the first was called the harvest moon, the second the hunters moon.

126. In illustrating these phenomena, for the present let us suppose the moon's orbit to lie in the plane of the ecliptic. Let Pl. VI, fig. 3, represent a cominon globe rectified for Boston ; that is, having Boston exactly at the top, and the circle on which is the word EAST, in the horizon. By turning the globe on its axis N'S, the equator is always at the same angle with the horizon, and equal portions of it come above the horizon in the east in equal times. But not so of the ecliptic. For when the point Aries is in the horizon in the east, the preceding sign Pisces lies very obliquely to the horizon, and forms but a small angle with it. But when the point Libra is in the horizon in the east, the preceding sign Virgo is nearly perpendicular to the horizon. From these different angles formed with the horizon by different parts of the ecliptic, it is manifest, that a greater portion of the ecliptic comes above the horizon in a.

given time (as 1 hour) when Aries is in the east, than when Libra is in the east. Suppose while the moon is moving in Pisces near Aries, it passes from 1 to 2, from 2 to 3, &c. daily. Suppose while moving near Libra, it passes

from a to b, from b to c, &c. daily. By turning the globe, the points 1, 2, 3, &c. come above the horizon very nearly at once; whereas the points a, b, c, &c. come above the horizon in succession at considerable intervals. Hence when the moon is on successive days in the points 1, 2, 3, &c. the difference in the times of her rising is very small; but while successively in the points a, b, c, &c. the difference in the times of rising is very great.

This subject may be illustrated much more clearly by a globe than by any representation on paper. By pasting small black patches on the points 1, 2, 3, &c. and on a, b, c, &c. at the distance of 12° 12' from each other, and by then turning the globe, a clear illustration will be effected,

127. Although the differences in the time of the moon's rising are always great when she is in or near Libra, and always small when in or near Aries, that is in every moon, yet we do not notice those variations except in autumn. (In fact we seldom notice the moon's rising at all unless it be when she rises near sunset, or in the evening.) The reason is that the moon can be full in or near Aries, where the difference in the times of her rising is least, only when the sun is in or near Libra; that is, at or near the time of the autumnal equinox.

128. It is plain from the figure, that as latitude increases northward, the difference in the times of the moon's rising in or near Aries, decreases. For the part of the ecliptic, Pisces, &c. makes a less angle with the horizon. Beyond the polar circle, the moon is above the horizon during half its revolution, as the sun is during half the year. And here is obviously a wonderful

accommodation to the wants of the inhabitants. For when the sun is above the horizon, the moon, being in the opposite part of the ecliptic, fulls below the horizon. And when the sun is below the horizon, and the moon's light most needed, the moon fulls above the horizon ; and at the winter solstice, the moon is visible during her second and third quarters when her light is greatest, and is below the horizon only when she reflects but little light.

129. All these appearances take place in south latitude as well as in north, only at a different season. The difference in the times of the moon's rising is there least when the moon is in or near Libra; hence their harvest moon comes when the sun is in or near Aries, that is, in our spring. But our spring is their autumn; so that they derive the same advantages from them, and in the same season, that we do.

130. The effects, as we have stated, take place on the supposition that the moon's orbit lies in the ecliptic. But it does not, but varies from it 5° 20'. This variation sometimes augments and sometimes diminishes the effects, of which we have spoken. When the moon's ascending node is in or near Aries, the effects are increased, and the harvest moons are most beneficial; but when the moon's descending node is in or near Aries, the effects are diminished and the harvest moons are least beneficial.

The following Table shows in what years the harvest moons are most or least beneficial, from the year

1817 to 1861. The columns of years under M are those in which the harvest moon is most beneficial; those under L are the years when it is least beneficial.

Of Phenomena arising from the Earth's Atmosphere.

131. It is found by experiment, that when a ray of light passes obliquely from one medium into another of different density, as from air into water, or from water into air, it is bent out of a straight course, and it is said. to be refracted. For example, (Pl. VI, fig. 4,) if a ray from the sun through the air fall obliquely upon water, or any transparent Auid, at F, instead of continuing in that direction to o, it will be bent downwards to Q; so that if a diver should place his eye at Q, he would see the sun at s instead of S. The degree of refraction, that is, the distance between s and S, is greater as the fluid is more dense; and also as the ray falls upon it more obliquely.