REVOLUTIONS OF SUN, MOON, AND PLANETS
AS SEEN FROM THE EARTH.
| Days. | Hrs. | Mins. | Secs. | |
Tropical Solar Year: period from one vernal equinox to another, or from one summer solstice to the next, &c. (Our civil year is based on this). | 365 | 5 | 48 | 45·5 |
Sidereal Year: period between two successive returns of the sun to any star on his path | 365 | 6 | 9 | 8·9 |
The difference between these two kinds of year is due to Precession of the Equinoxes, vide infra, [p. 23]. | ||||
| Days. | Hrs. | Mins. | Secs. | |
Solar Day: period between two successive passages of the sun across the meridian (noon) | — | 24 | 0 | 0 |
Sidereal Day: period between two successive passages of a star across the meridian | — | 23 | 56 | 4 |
The difference between these two kinds of day is due to lag of the sun behind the stars, his daily motion westward being slightly retarded by his slow yearly motion eastward. | ||||
| Days. | Hrs. | Mins. | Secs. | |
Mean Synodic Month: period between two full moons | 29 | 12 | 44 | 2·8 |
Mean Sidereal Month: period between two successive returns of moon to any star on her path | 27 | 7 | 43 | 11·5 |
The difference between these two kinds of months is due to the fact that while the moon is making her revolution among the stars, the sun is also moving slowly on in the same direction. | ||||
| Days. | Hrs. | Mins. | Secs. | |
Mean Anomalistic Month: period between perigee and perigee (vide infra) | 27 | 13 | 18 | 37·4 |
Planets:—Mean Synodic Revolution: period between two successive conjunctions with the sun, and mean zodiacal revolution: period of revolution round the zodiac.
| Mean Synodic Revolution. | Mean Zodiacal Revolution. | |||
| Mercury | 116 | Days[4] | 1·0 | Years |
| Venus | 584 | ” | 1·0 | ” |
| Mars | 780 | ” | 1·88 | ” |
| Jupiter | 399 | ” | 11·86 | ” |
| Saturn | 348 | ” | 29·46 | ” |
Precession of Equinoxes: 50·25 seconds of arc in one year; that is, 360 degrees (a revolution among the stars of the zodiac) in 25,800 years nearly.