Aphelion Distance Research Articles

Our Astronomical Column

ENCKE'S COMET IN 1881.—So far as can be judged without the calculation of the perturbations since 1878 this comet will again arrive at perihelion about November 8 in the present year. In 1848, when the comet passed this point of its orbit on November 26, it was detected with the 15-inch refractor at Cambridge, U.S., on August 27, as “a misty patch of light, faint and without concentration: its light coarsely granulated, so that were it not for its motion it might be mistaken for a group of stars of the 21st magnitude”(Bond). The theoretical intensity of light at this time was 0.21, and we find that, assuming the perihelion passage to occur on November 8, the comet should have this degree of brightness soon after the middle of August next, so that it may be anticipated observations will be practicable with the waning moon about the 20th of that month. The last perihelion passage took place on July 26, 1878, the period of revolution at that time being 1200.58 days according to the late Dr. von Asten. The aphelion distance is 4.0879, the perihelion distance 0.3335, and the minor semi-axis 1.1675 (the earth's mean distance from the sun = 1). The approach to the orbit of the planet Mercury is still very close (0.031) in about 126°.5 heliocentric longitude. The nearest approximation of the two bodies that has occurred since the discovery of the comet's periodicity took place on November 22, 1848, when their distance was only 0.038. It is known that from his investigations on the motion of Kneke's comet, von Asten inferred a much smaller value for the mass of Mercury than had been previously assigned, viz. 1/7636440.

The Temperature of the Sun

PROF. NEWCOMB, in reviewing P. A. Secchi's work on the Sun, shows that if the temperature reached 10,000,000° Cent., as asserted by the author of “Le Soleil,” the earth would speedily be reduced to vapour. In answer to this objection Pére Secchi urges, “that a body may have a very high temperature and yet radiate but veiy little;” contending that “a thermometer dipped inside the solar envelope in contact with the photosphere” would indicate the temperature mentioned. He adds, “This high temperature, besides, is really a virtual temperature, as it is the amount of radiation received from all the transparent strata of the solar envelope, and this body at the outer shell must certainly be at a lower temperature.” What information is intended to be conveyed by the statement that 10,000,000°Cent. “is really a virtual temperature,” on the ground that it is the “amount of radiation received from all the transparent strata” outside of the photosphere; I will not attempt to explain; but I propose to show that a thermometer dipped inside the solar envelope in contact with the photosphere, cannot possibly indicate the enormous temperature of 10,000,000°Cent assumed by Pére Secchi. The assertion that “a body may have a very high temperature and yet radiate but very little,” were it correct with reference to the photosphere, does not affect the question. It is of no consequence whether the sun's photosphere belongs to the class of active or sluggish incandescent radiators imagined by the distinguished savan; the temperature of the radiant surface, not its capacity to radiate more or less copiously, is the problem to be solved. Accordingly the following statement is intended to show that the temperature of the sun's photosphere at the point where the author of “Le Soleil” supposes his thermometer to be applied, cannot much exceed 4,000,000° Fahr. Observations conducted in lat 40° 42″, with an actinometer (a drawing of which has been published in Engineering) have enabled me to ascertain, with desirable accuracy, the intensity of solar radiation for each degree of the sun's zenith distance from 17° to 75° The atmospheric depth at the first mentioned zenith distance being only 0.46 greater than the vertical atmospheric depth, I have demonstrated, by prolonging the curve constructed agreeable to the observations referred to, that the intensity of solar radiation on the ecliptic is 67.20° Fahr. at the time when the earth passes the aphelion. The accompanying table, the result of two years of observations, shows the atmospheric depth and the intensity of solar radiation for each degree from the vertical to 75° zenith distance. The ratio of diminution of intensity of the radiant heat during the passage of the rays through the atmosphere being accurately defined by this table, it has been easy to calculate that the amount of retardation of the radiant heat on the ecliptic is 0.207 or 17.64° Fahr. Adding this loss of energy to the amount of observed radiant heat, it will be found that the intensity of solar radiation at the boundary of our atmosphere when the earth passes the aphelion corresponds with a thermometric interval of 17.64 + 67.20 = 84.84° on the Fahrenheit scale. Now, the aphelion distance of the earth is 218.1 times greater than the radius of the sun's photosphere; hence, basing our calculations on the established truth that the intensities are inversely as the areas over which the rays are dispersed, we prove that the temperature of the photosphere is 218.12 X 84.84° =4,035,584° Fahr. And if we then add the amount of loss of intensity attending the passage of the rays through the solar envelope, we establish, with absolute certainty, the temperature to which a thermometer will be subjected if “dipped inside the solar envelope in contact with the photosphere.”