| Number | Name. | Discoverer. | Date. |
| 1 | Ceres | Piazzi | 1801 |
| 2 | Pallas | Olbers | 1802 |
| 3 | Juno | Harding | 1804 |
| 4 | Vesta | Olbers | 1807 |
| 5 | Astraea | Hencke | 1845 |
| 6 | Hebe | Hencke | 1847 |
| 7 | Iris | Hind | 1847 |
| 8 | Flora | Hind | 1847 |
| 9 | Metis | Graham | 1848 |
| 10 | Hygeia | De Gasparis | 1849 |
| 11 | Parthenope | De Gasparis | 1850 |
| 12 | Victoria | Hind | 1850 |
| 13 | Egeria | De Gasparis | 1850 |
Here now is a table showing how other bodies were gradually added to this first list of four, but you will see that no addition was made for a long time. Not that the search was immediately abandoned; but being rewarded by no success for some years, it was gradually dropped, and the belief gained ground that the number of the planets was at last complete. The discoverers of Uranus and of these first four minor planets all died before any further addition was made;Hencke’s long search. and it was not until the end of 1845 that Astraea was found by an ex-postmaster of the Prussian town of Driessen, by name Hencke, who, in spite of the general disbelief in the existence of any more planets, set himself diligently to search for them, and toiled for fifteen long years before at length reaping his reward. Others then resumed the search; Hind, the observer of an English amateur astronomer near London, found Iris a few weeks after Hencke had been rewarded by a second discovery in 1847, and in the following year Mr. Graham at Markree in Ireland (who is still living, and has only just retired from active work at the Cambridge Observatory) found Metis; and from that time new discoveries have been added year by year, until the number of planets now known exceeds 500, and is steadily increasing.
By permission of Messrs. Macmillan & Co.
I.—J. C. Adams.
II.—A. Graham.
DISCOVERER OF THE NINTH MINOR PLANET (METIS).
You will see the great variety characterising these discoveries; some of them are the result of deliberate search, others have come accidentally, and some even contrary to expectation. Of the great majority of the earlier ones it may be said that enormous diligence was required for each discovery; to identify a planet it is necessary to have either a good map of the stars or to know them thoroughly, so that the map practically exists in the brain. We need only remember Hencke’s fifteen years of search before success to recognise what vast stores of patience and diligence were required in carrying out the search.The photographic method. But of late years photography has effected a great revolution in this respect. It is no longer necessary to do more than set what Sir Robert Ball has called a “star-trap,” or rather planet-trap. If a photograph be taken of a region of the heavens, by the methods familiar to astronomers, so that each star makes a round dot on the photographic plate, any sufficiently bright object moving relatively to the stars will make a small line or trail, and thus betray its planetary character. In this way most of the recent discoveries have been made, and although diligence is still required in taking the photographs, and again in identifying the objects thus found (which are now very often the images of already known members of the system), the tedious scrutiny with the eye has become a thing of the past.
Table showing the Number of Minor Planets Discovered
in each Decade since 1850.
| 1801 to 1850— | altogether | 13 | discoveries. |
| 1851 to 1860— | " | 49 | " |
| 1861 to 1870— | " | 49 | " |
| 1871 to 1880— | " | 108 | " |
| 1881 to 1890— | " | 83 | " |
| 1891 to 1900— | " | 180 | announcements |
| In 1901 | " | 36 | " |
| " 1902 | " | 50 | " |
| " 1903 | " | 41 | " |
| Total | 609 |
[N.B.—Many of the more recent announcements turned out to refer to old discoveries.]