| Alcyone | 3.0. |
| Electra | 4.5. |
| Atlas | 4.6. |
| Maia | 5.0. |
| Merope | 5.5. |
| Taygeta | 5.8. |
| Pleione | 6.3. |
| Celæno | 6.5. |
| Asterope | 6.8. |
Good eyes distinguish the first six, sharp sight detects the three others.
In the times of the ancient Greeks, seven were accounted of equal brilliancy, and the poets related that the seventh star had fled at the time of the Trojan War. Ovid adds that she was mortified at not being embraced by a god, as were her six sisters. It is probable that only the best sight could then distinguish Pleione, as in our own day. The angular distance from Atlas to Pleione is 5′.
The length of this republic, from Atlas and Pleione to Celæno, is 4′/23″ of time, or 1°6′ of arc; the breadth, from Merope to Asterope, is 36′.[8]
In the quadrilateral, the length from Alcyone to Electra is 36′, and the breadth from Merope to Maia 25′. To us it appears as though, if the Full Moon were placed in front of this group of nine stars, she would cover it entirely, for to the naked eye she appears much larger than all the Pleiades together. But this is not so. She only measures 31′, less than half the distance from Atlas to Celæno; she is hardly broader than the distance from Alcyone to Atlas, and could pass between Merope and Taygeta without touching either of these stars. This is a perennial and very curious optical illusion. When the Moon passes in front of the Pleiades, and occults them successively, it is hard to believe one's eyes. The fact occurred, e.g., on July 23, 1897, during a fine occultation observed at the author's laboratory of Juvisy (Fig. 26).
Fig. 26.—Occultation of the Pleiades by the Moon.
Photography here discovers to us, not 6, 9, 12, 15, or 20 stars, but hundreds and millions.