a period of twelve years, from 1890 to 1902, and undoubtedly furnish the most accurate results yet obtained. The measures of Schur give 1920".14, and those of Ambronn 1919".80 as the apparent diameter of the sun, and these results may be accepted as very close to the truth.

Combining these measures with the distance of the sun as given in the last section, the actual diameter of the sun is found to be 864,750 miles, or nearly one hundred and nine times that of the earth. In the chapter on the earth that body was represented for illustration by an ordinary library globe, two feet in diameter: on this same scale the sun would be represented by a globe two hundred and eighteen (218.2) feet in diameter and distant about four and one-half (4.44) miles. The moon would be a little ball six and a half (6.5) inches in diameter and distant from the earth only one hundred and twenty (120) feet; that is, if the globe representing the sun be hollowed out, the system, earth and moon, could be placed within the hollow sun, the earth at the centre, and the moon in her orbit about the earth would never be but little more than half way out towards the sun's surface.

The great globe, the sun, is almost exactly spherical, the difference between the polar and equatorial diameters being so small as to be well-nigh impossible of accurate measurement. From his discussion of the heliometer measures, above mentioned, Auwers concluded that the polar diameter exceeds the equa

torial by 0".038, and he explains this apparent anomaly as being due to the tendency on the part of an observer to measure vertical diameters greater than horizontal ones. This evidence is quoted by Newcomb as conclusive that the sun is sensibly a sphere. Ambronn, as a result of his elaborate and thorough discussion of the Göttingen measures, reaches the same conclusion.

Certain evidence (from solar photographs and from the heliometer measures themselves) has been adduced by Poor, however, which seems to throw some doubt upon this conclusion. It is barely possible that the diameters of the sun are variable to a minute extent.

CHAPTER V

THE PHYSICAL CHARACTERISTICS OF THE SUN

THE

HE study of solar physics began with the invention of the telescope. In 1610 when Galileo pointed his crude instrument toward the sun he found its surface covered with dark, irregular spots. The opinions concerning this discovery were many and varied, it being discussed from all points of view. The old ideas of the divineness and the perfectness of the heavens were revived in a new form; the sun could not be otherwise than perfect and these spots could not be due to actual specks and stains on the bright solar disc, but must be either optical illusions, or dark planets passing in front of its brilliant surface. Gradually, however, it was found that these spots really belonged to the hitherto immaculate sun. Galileo and his followers thought them to be clouds floating near the sun's surface and concealing the brightness below; others thought them to be the waste materials from the solar furnace, the burnt-out cinders of an immense fire. And just as a furnace fire burns more brightly after it has been raked, so these students of solar physics imagined the sun to blaze

forth with renewed brilliancy after this cindery refuse had been thrown off in the form of comets. Many years later, Lalande, the great French astronomer and mathematician, confidently upheld the opinion that these spots were caused by the uncovering of mountain peaks by the alternate ebbing and flowing of a great luminous ocean.

These dark spots, that mar the bright surface, were found to be in motion; when watched from day to day were found to travel slowly across the luminnous disc. The early observers saw a spot appear on the eastern edge of the sun, move slowly toward the centre, cross the disc, and disappear at the western limb. Whether the spot passed through the centre, or along a shorter chord above or below the centre, the actual time of crossing the disc was always about the same number of days, and in this apparent motion of the spots Galileo recognised an actual rotation of the sun about an axis in a period of about one month.

The true form of these sun-spots was first clearly shown in 1774 by Wilson of Glasgow, when, by carefully noting the apparent changes in form which they assume as they cross the disc, he proved them to be vast excavations in the sun's substance. This fact, that the spots are hollows or pits in the sun, he clearly established, but unfortunately he tried to form a theory from this one fact alone and was led astray. He thought the sun might consist of two kinds of mat