Total Solar Eclipse of August 19, 1887.
Eclipses recur in cycles of 18 years and 10 days (= 6585 days). This period was determined by the ancients, and called the saros. By its means the times and magnitudes of eclipses were roughly computed long before astronomy became an exact science.
A solar eclipse is really an occultation of the Sun by the Moon; for the word eclipse, in its usual reference, denotes the obscuration of one body by its immersion in the shadow of another. During any single year there are never less than two eclipses, nor more than seven. Whenever there are two only, both are solar.
Since the fine solar eclipse of December 22, 1870, no large eclipse of the Sun has been visible in England. It is remarkable that during the thirty years from 1870 to 1900 these phenomena are all of an unimportant, minor character. Within the thirty years following 1891 there will be twelve solar eclipses, for which the Rev. S. J. Johnson has given projections (as shown on p. 98) for the period of greatest obscuration.
Total eclipses are extremely rare as regards their visibility at a given station. Thus between 878 and 1715 not one was observed at London, and during the next 500 years there will be a similar absence of such a phenomenon. The observer of total eclipses must perforce journey to those particular tracts of the earth’s surface over which the band of totality passes. On such occasions photography plays an important part; and the corona, the red flames, the shadow-bands, and numerous other features become the subjects of necessarily hurried observation and record, for totality endures for very few minutes[11].
As regards ordinary partial eclipses, amateurs usually find ample entertainment in noting the serrated aspect of the Moon’s contour projected on the bright Sun. It is also interesting to watch the disappearance and reappearance of the solar spots visible at the time. Rather a low magnifying power, with sufficiently expansive field to include the entire disk, is commonly best for the purpose of these observations.
Periodicity of Spots.—This detail may be said to have been fully investigated. Schwabe and Wolf have accomplished much in this direction. A work of this kind must, by the nature of it, extend over many years and entail many thousands of observations. It is therefore more suited to the professional astronomer than to the amateur, whose attention is more or less irregular owing to other calls. The sun-spot cycle is one of about 11 years, during which there are alternately few and many spots on the Sun. There appear to be some curious fluctuations, disturbing the regular increase and decrease in the number of spots; and these variations are worthy of more attention. The following are the years of observed maxima and minima of sun-spot frequency:—
| Maxima. | Minima. |
| 1828. | 1833. |
| 1837. | 1843. |
| 1848. | 1854. |
| 1860. | 1867. |
| 1870. | 1878. |
| 1883-4. | 1890 (?). |