|
FACTORS AFFECTING APPARENT STELLAR POSITION |
|||
| FACTOR | TYPE | CAUSE | Period |
| 1. Luni-Solar Precession |
Change of Coordinate System |
Change in direction of Earth's axis of rotation due to net torque arising from gravity of moon and sun. Causes vernal equinox to move westward along the ecliptic 50.371" per year. | ~25,725 yr |
| 2. Nutation | Perturbation to precession caused mainly by motion of nodes of lunar orbit. Causes true NCP to move in an ellipse around the mean NCP. This ellipse has major and minor axes of 9.21" and 6.86." Also affects the obliquity of the ecliptic, ε. | 18.6 yr | |
| 3. Planetary Precession | Affects the position of the ecliptic plane. At present is causing the vernal equinox to move eastward 0.125"/yr and is reducing ε by 0.47"/yr. | ~ 40,000 yr | |
| 4. Latitude Variation | Earth slips about its rotational axis, i.e. its pole position wanders. Also known as "polar motion" and "Chandler Wobble." Range about 15 meters (0.5'' in latitude) in about 14 months. Motion not very predictable, it has much randomness. | 14 month (loop), 6.5 yr (spiral) | |
| 5. Parallax |
Motion of Origin of Coordinate System |
Caused by Earth's orbital motion. |
1 yr |
| 6. Aberration | Deflection of Image |
Deflection due to finite speed of light and motion of observer resulting from earth's revolution and rotation. |
1 day, 1 month, 1 yr |
| 7. Gravitational Deflection by Sun |
Deflection due to spacetime curvature resulting from sun's mass. |
1 yr | |
| 8. Refraction | Refractive deflection of image by Earth's atmosphere | 1 day | |
| 9. Galactic
Aberration (not corrected) |
Deflection due to finite speed of light and motion of observer resulting from solar motion and galactic rotation. | 2.5´108 yr | |
| 10. Proper Motion |
Motion of Object |
Actual motion of object through space. | Cumulative |