Home » The World’s Largest Digital Camera-“The Big Picture of Knowledge”-PanSci

The World’s Largest Digital Camera-“The Big Picture of Knowledge”-PanSci

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The total lunar eclipse of 2021/05/26, a blood-red moon appears in the east, whether it is watched at home or broadcasted by the planetarium, it has become one of the must-see astronomical sights in May. In the past, when we looked up and admired the moon, we always thought that we only saw one side of the moon, and we would never be able to see the other side on the earth.

We usually think that observers on the earth can only observe the bright side of the moon. Figure/envato elements

But can we really only see half of the moon?

The reason why only half of the moon can be seen-tide lock

The moon is a satellite of the earth. Since the formation of the moon, it has revolved around the earth, because the earth’s rotation produces the moon’s eastward rise and westward fall, and because of the moon’s revolution, we see new moons, first quarters, full moons, and last quarters every month.

From past experience, we know that when we are on the earth, people will always only see half of the moon! It means that the rotation speed of the moon is equal to the revolution speed. The moon revolves around the earth once, and at the same time it rotates once, so the moon will always face the earth on the same side! This phenomenon is also called “Tide lock」。

The reason for the tide lock is that when the moon revolves around the earth, it is also affected by the tidal force of the earth (the earth is also affected by the tidal force of the moon, so our seawater will have the phenomenon of ebb and flow!), but because of the lunar surface There is no ocean itself. The gravity of the earth directly affects the irregular mass distribution inside the moon. The two ends of the moon facing and facing the earth will gradually form a shape similar to an ellipsoid.

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In order to facilitate understanding, we simplified the elliptical moon affected by the earth’s tidal force into a lever. If the direction of the earth’s tidal force is parallel to the lunar lever, it will not affect the moon’s rotation (shown in green in the figure below), otherwise, it will be affected by the tidal force. It changes the rotation motion (shown in red in the figure below), and will eventually return to the green state.

Tide lock diagram, the black circle in the figure is the earth, the big circle is the revolution orbit, the small circle is the rotation trajectory, the green represents the state of the moon not changing its rotation by tidal force, and the red is the state of the moon changing its rotation under the influence of tidal force. Figure/wikipedia

Under the continuous action of the earth’s tidal force, the moon at different positions will all turn into a green state. At this time, the rotation speed of the moon will be equal to the revolution speed, and will always face the earth on the same side, as if the moon is locked by the earth.

In fact, not only is the moon locked in tides, after a longer period of time, the earth will also be affected by the moon and locked by the tides of the moon, but because the mass of the earth itself is much larger than the moon, it may be until the sun continues to evolve. After becoming a red giant star and engulfing the solar system, the earth will not reach the phenomenon of tidal lock.

Although the two celestial bodies of the earth and the moon have not reached the tidal lock to each other, the outer celestial bodies in the solar system: Pluto and its satellite-Karen have reached the phenomenon of tidal lock to each other! So Pluto will always face Karen with the same side, and Karen will always face Pluto with the same side.

The side of Pluto facing Charon (left) and the side facing Charon (right) (June 27, 2015). Figure/Wikipedia

The principle of tidal locking is also applied to the orbit design of artificial satellites. The orbital period of some artificial satellites around the earth is equal to the rotation period of the earth, so that artificial satellites can always observe the same place on the earth. From the perspective of the earth, the satellite They will stay above the same place as if they are not moving. Such satellites are called “geosynchronous satellites.”

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Because geostationary satellites are always above the same place, they can observe the same area for a long time and obtain data, and they can also help us understand the evolution and movement of the weather system in the area. Take East Asia as an example. The geostationary satellite “Sunflower” was launched to obtain information in the atmosphere.

Is there a chance to see a larger surface of the moon?

But in fact, the above explanation only represents a very ideal state. Due to the moon’s orbital motion, the location of the observer, physical conditions and other factors, the moon we observe actually has the phenomenon of shaking up and down, so we can really observe The moon actually occupies about 59% of the surface! And this phenomenon, we call it Libra movement!

Libra movement can be divided into two categories: geometrical balance movement and physical balance movement.

Geometric Libra represents the change in the relationship between the position of the observer and the moon. For example, when the moon revolves around the earth, there are also perigee and apogee. When the moon reaches the perigee, the revolution speed of the moon is faster; when it reaches the apogee, the revolution speed of the moon is slower, but regardless of the speed of the revolution period, the rotation speed of the moon Both are similar. At this time, there is a slight difference between the revolution and rotation speed, and we can observe a little bit more on the east and west sides of the moon.

The observer on the earth will also affect the observable lunar disk. The earth where the observer is located also has a rotation phenomenon. When the earth rotates, the observer is also moving (just because we are standing on the rotating earth So you can’t feel it), then you can observe a little more of the moon on both sides of the east and west. If the observer is at a different latitude, it is possible to observe a little more moon on the north and south sides.

Because the relative position of the moon and the earth keeps changing,
Indirectly, the surface of the moon viewed by the earth is also slightly different. Picture/envato element

The impact of physical libra is relatively small, and physical libra represents the effect of the moon’s real swing! Because the moon itself is not a true sphere, its three axes are not of equal length, so when it rotates, there will actually be slight differences in the rotation speed of each place, but because the impact caused by physical balance is relatively small, we usually don’t consider it. Its impact!

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In addition to more moons, there are moons that seem to be of different sizes!

In addition to the fact that humans can observe 59% of the moon, why do we sometimes think that the moon is so big? One of the reasons is the correlation with the perigee and apogee mentioned above! Assuming that today, when the moon turns to perigee and encounters a full moon, observers of the earth will feel that the moon has become larger, but the size of the moon has not changed from beginning to end!

Since ancient times, the moon has been a celestial body in the night sky that people are curious about and pursued. Through different astronomical scenes and the efforts of scientists, we have a better understanding of the moon. Next time you have a chance, remember to look up and admire this silver celestial body!

Reference

  1. Taipei Astronomical Science Education Center
  2. Youtube-Teacher Li Yongle, why does the moon always face the earth
  3. Lunar balance
  4. Xiao Junjie, the orbit of the artificial satellite
  5. Tide Lock-Wikipedia, the free encyclopedia

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