Libration (from Latin, verb: librare, meaning "balance in swing", see Libra "scale") is a very slow oscillation in real or visual sense observed from the celestial bodies surrounded by satellites.
Libra motion generally refers to the apparent motion of the moon relative to the earth.Since a point is selected to balance and compare the scale of shaking during observation, the libra movement will vary with the selection of balance point.The orbit of the moon around the earth is synchronous orbit, and the so-called synchronous rotation is not strict.Since the lunar orbit is elliptical, when the moon is at perihelion, its rotation speed will not catch up with the revolution speed, so we can see that the eastern part of the moon reaches 98 degrees east longitude;On the contrary, when the moon is at aphelion, its rotation speed is faster than its revolution speed, so we can see the area of 98 degrees west of the moon.This phenomenon is called libra movement.[1]
Longitude libra moving
Announce
edit
Longitude Libra moves around the moonEarth revolutionThere are a fewEccentricityResults.When the moonPerigeetowardsApogeeWhen moving, the orbital speed gradually slows down, and when moving from apogee to perigee, the speed gradually increases, but the rotation speed of the moon is always the same, so the position of the moon in orbit sometimes leads, sometimes lags behind the rotation speed.Every degree on the lunar equator is 30 kilometers, so the longitude libra can make us observe more about 235 kilometers of the lunar surface on the east and west sides.Although this is not a large area, the point we see facing the earth will change accordingly.
Longitude scale movement: ± 7 ° 45 '
Latitude Libra
Announce
edit
Latitude Libra isLunar rotationAxialnormalOf the moon orbiting the earthTrack planeIt is caused by slight inclination, and its origin is similar toEarth revolutionThe phenomenon of seasonal variation caused by solar time.LunarAutorotation shaftRelative to the white path (it seems that it should be the ecliptic here)Inclination angle1.54 °, yellow and whiteAngle of intersection5.14 °, so the maximum latitude libration is ± 6.7 °, which is equivalent to a distance of about 200 km in the north and south polar direction.
Latitude libration: ± 6 ° 41 '
Sunday Tianweidong
Announce
edit
The libra movement on Sunday has little impact, which is caused by the rotation of the earth. It makes the observers on the ground turn from the west side of the line between the earth and the moon center to the east side, so first see more of the east side of the moon, and then see more of the west side of the moon.The average distance between the earth and the moon is 380000 kilometers,Earth radius6378km, the rotation of the earth will enable observers to see about 30km more area on the east and west sides of the equator at most.
Daily scale movement: ± 1 °
Physical libration
Announce
edit
Unlike the first three, this is the real swing of the moon.Due to the unequal length of the three main inertial axes of the moon and the distance change caused by the elliptical orbit, the average position will be shifted under the action of the earth's gravity.However, the movement of the physical scale is much smaller than that of the geometric scale, and its swing has never been greater than 0.04 °, so it is generally ignored.In addition, the "earthquake" on the moon will also affect the point on the moon facing the earth.
