On the 1st of December the sun sets at 4 h. 13 m. p.m., Standard Time (Eastern). A trail of planets follows after him in a regular crescendo of brilliancy. Mars, setting at about six o'clock, will, from its lowness in the haze, be difficult to see, except with a telescope. Saturn, about forty minutes further east, is followed by Jupiter, only a minute behind, and less than half a degree south, so that both will be in the same telescopic field with a power of thirty, and will continue so for at least a week.

Venus will have passed the two and will be fifty minutes further east, being near her greatest eastern elongation, which occurs on the 4th of the month. So that for a short time after sunset the twilight sky will be graced by the three principal planets in a row, each brighter than the one next west of it; a beautiful sight. Unfortunately they will be too low in the south for good definition in the telescope, though the contrast in the brightness and color of Jupiter and Saturn will be very evident, as long as they remain in the same field.

On the morning of the 1st, Mercury will be a little more than an hour east of the sun, and five degrees north of it, so that though it will probably be invisible to the naked eye, any one having a clear horizon and circles to his telescope may be able to pick it up between six o'clock and sunrise.

The sky at eight o'clock in the evening on the 1st of December will be very nearly the same as described for midnight of the 1st of November, with the exception that all the constellations will be about an hour further east, so that the two "Dogs will not have risen. No planets will be visible, all having set, and the moon, being near her third quarter, will not rise for nearly three hours.

The moon will be absent from the evening sky until the 11th or 12th of the month, as she will be twenty days old on the 1st. She passes her last quarter on the 2d, and is new on the 10th ; is at the first quarter on the 18th, and fulls on Christmas Day.

The early evenings of the month, therefore, will afford excellent opportunities for the study of the winter constellations. Orion, Gemini, Taurus and Aries are all up in the east, the former two stretching along horizontally from southeast to northeast, Taurus above them, and Aries above Taurus.

In the east, and to the north of Taurus, are Auriga and Perseus. The constellations above enumerated contain a great many stars of the first three magnitudes, in beautiful and striking configurations, and make the eastern sky very brilliant.

There will be eight stars of the first magnitude above the horizon ; namely, Betelgeuse (Orion), Rigel (Orion), Aldebaran (Taurus), Capella (Auriga), Deneb (Cygnus), Vega (Lyra), Altair (Aquila), Fomalhaut (Pisces Austrinus) : an hour or so later these will be reenforced by the arrival of Sirius and Procyon in the east.

Here.the reader may very naturally ask, "What is a magnitude ? " a question which it is not quite a simple matter to answer to his comprehension.

To an astronomer or physicist it would be comprehensively answered by the statement that a magnitude is that ratio between the light of two stars which is expressed by the number whose logarithm is 0.4. For the lay reader the most satisfactory way of explaining the question is to go a little into the history of the subject.

The early uranographers divided the stars visible to them into six classes or magnitudes, rather roughly defined, from the first, which included about a dozen of the very brightest, to the sixth, which were the faintest that a good eye could certainly distinguish on a clear night. Each magnitude was approximately half as bright as the next above it. While the stars were only roughly mapped and studied with the naked eye, it was sufficient.

But with the advent of the telescope the extension of the scale downward at once began to be a matter of difficulty. Men began to estimate half-magnitudes : the absence in the small field of the telescope of a number of standards of comparison, such as were available in naked-eye classification, made the estimation a matter of memory and judgment, and also greatly increased the difficulty of maintaining a constant value for the ratio expressed by the word magnitude. Struve, the Russian observer, and after him others, began to estimate tenths of magnitudes. Each observer used a scale of his own, without much reference, apparently, to those of others.

From this resulted a great confusion of magnitudes in the catalogues which were brought out during the early half of the last century. The first to begin to reduce this chaos to something like order was Argelander of Bonn, in his great survey of the northern heavens, which included practically all the stars down to the ninth magnitude, and a great many yet fainter, and extended from the north pole to one degree south of the equator. Each star was observed three times, and as the catalogue when complete included 110,985 stars, it will be seen what an enormous labor it. was. Each star when observed for position had an estimate of its magnitude recorded at the same time, and each magnitude in the catalogue represented the mean of three estimates, made by men experienced in this particular work. A great point was made of keeping to a constant ratio in the magnitudes, this ratio to be as nearly as possible that of the old naked-eye scale. The magnitudes in this catalogue (called the Bonn Durch-musterung) were at once accepted by American astronomers, and have been the standard to which all later attempts to reform the scale of magnitudes have been referred. ' Webb (" Celestial Objects," page 207, 4th ed.) gives a very interesting table, comparing the telescopic scales of several eminent observers with this " DM " scale, as it is called, which it would repay the reader to look up.

While Argelander was at work on this catalogue, the early experiments in stellar photometry were being made by Zollner and others. In the seventies and thereafter, with the introduction of serious photometric work on the stars, in Pritchard's labors with the "wedge " photometer at Oxford and Pickering's with the "meridian " photometer at Cambridge, an attempt was made by the astronomers and physicists interested in the question to unite upon a definite standard ratio, on which all photometric work should be based. The ratio finally adopted, as representing the scale hereto-, fore used among the naked-eye stars, was the one first stated; viz., that number whose logarithm is 0.4, which is 2.52 ; so that a star is very nearly twice and a half brighter than those of the next magnitude below it.

As this ratio is a geometric one, it follows that a first-magnitude star is twice and a half brighter than a second, six times brighter than a third, sixteen times brighter than a fourth, forty times brighter than a fifth, and so on, until at the ninth it is four thousand times brighter.

Deneb in the Swan, and Altair in the Eagle, are good types of first-magnitude stars. Such stars as Sirius, Capella and Vega are much brighter than this, and their magnitudes are expressed in the photometric scale by decimals less than one; or if the difference is more than a full magnitude, by negative values; e.g., Capella, 0.24; Vega, 0.10 ; Sirius, - 1.72.

In the six ordinary naked-eye magnitudes, Heis enumerates 1,380 stars as being visible in these latitudes; the numbers in the several grades are : first to second, 13; second to third, 48 ; third to fourth, 152 ; fourth to fifth, 313; fifth to sixth, Vega.