The moon always faces us with the same side, so why can we see more edges?
Let the lunar surface grid follow the changes of the real balance, distinguishing the viewing geometry from the moon's own swing.
The familiar patterns on the moon don't seem to have changed drastically, but the areas to the side are sometimes more exposed and sometimes hidden. The Moon's synchronized rotation is still accompanied by a movement in the balance: both a change caused by the viewing angle and a small real wobble.
Guess first
Does synchronous rotation mean we only ever see strictly the same half of the moon's surface?
Show answer and explanation
Doesn't mean, the scales are still moving — The average motion of synchronous rotation does not eliminate optical and physical balance motion.
Then connect experience and science
Synchronous rotation does not mean that you will always see only half of the face
The Moon faces the Earth roughly on the same side, but the speed of movement and the plane of its orbit cause us to sometimes see a little more edge. This is called optical balance movement.
Optical longitude and latitude balance movements l′, b′
Then add the small wiggle of the moon itself
Physical balance motion comes from small changes in the moon's true rotation, and its amplitude is smaller than optical balance motion; the two cannot be confused.
l=l′+l″；b=b′+b″
Correct the axis direction
The North Pole axis of the moon has a position angle P relative to the north of the celestial sphere. The lunar latitude and longitude network in the picture is projected according to the calculated l, b, and P, and cannot be replaced by randomly shaking the entire picture.
The visibility of the lunar surface is determined by the angle between the surface point and the viewing direction
The surface location will also change the viewing direction a bit.
You can switch to station center difference correction and input east longitude, latitude and assumed ΔT. Use UT1 to calculate the earth's rotation and TT to calculate the moon's position.
Center balance movement: geocentric value + parallax difference correction
First find the optical balance movements l′ and b′, and then superimpose the physical corrections l″ and b″; in the station-centered mode, the observer’s parallax must also be added. Each longitude and latitude angle and the lunar axis position angle have different uses, and you cannot add just one common correction.
give it a try
Compare the original book date to seven days later, paying attention to the grid and edge marks; then cut to the surface. The full circle is a coordinate diagram and does not represent the full moon on that day.
No need to memorize symbols
- l′ b′
- Optical balance movement
- l″ b″
- Physical balance motion
- l b P
- Total balance movement and lunar axis position angle
Algorithm, accuracy and scope of application
All ρ, σ, and τ terms of Equation 53.1/53.2 in Chapter 53, the optical and physical latitude and longitude balance motion, and the geocentric axis position angle; the station center difference is based on page 375 of the book, and the positive east longitude is input, and the internal H = apparent sidereal time + east longitude − right ascension. UT1=TT− Assuming ΔT, the input difference cannot be called actual measurement. The station-centered method itself is an approximation and does not make predictions about local lunar edge topography or precise occultation. To illustrate the visible surface, the real l/b/P is used as a spherical orthogonal projection; the longitude of the surface point is along the lunar surface in the original book, and the celestial sphere is north-up and east-left, so the drawing P is rotated counterclockwise on the screen.
my understanding check
Main description of optical balance movement?
Watch the geometry change — It's not a deformation of the lunar surface.
Does the physical balance completely ignore kinetic energy and say it never existed?
No, it is a smaller true rotational change — A small physical correction is calculated to correspond to the physical ephemeris of the original book.
In the picture, the entire disk is drawn, does it mean that the moon must be full that day?
No, it's a coordinate chart — This image shows the visible geometric surface without overlaying the true light and dark moon phases.
It is an average rule that the same side faces the earth; when looking closely at the edge of the moon, you must also consider the viewing geometry and the small wobble of the moon.
Jean Meeus · Astronomical Algorithms, 2nd ed. · No. 53 chapter