Why do We See only One Side of the Moon?

Relative to the Earth, the Moon makes one rotation every 29.5 days. That happens to also be the time it takes for the Moon to complete one revolution around the Earth. This might seem like a coincidence, but it's not.

In the past, the Moon used to rotate much faster than it does now. But over millions of years, the effect of the Earth's gravity has slowed down the Moon's rotation until it became gravitationally locked to the Earth. This is why we always see the same side of the Moon. 

It would seem logical to say that at any one time we can see 50% of the Moon's face. If the Moon were flat, that would be correct, however we know the Moon is a sphere. And the spherical shape of the Moon hides the area close to the perimeter and we can, at any one time, see only 41% of the Moon's face.

Even though the same side of the Moon's faces us, we do see a bit more than half of the Moon's face. Over time, because of librations, we can see up to 59% of the Moon's surface. 

Librations are irregular motions of the Moon in its elliptical orbit around the Earth. They are measured using longitudinal and latitudinal coordinates. Both are computed from a central point that is at a fixed geographical location on the lunar surface. This point is in the Sinus Medii, a small plain just below and to the right of he large rayed crater Copernicus. Two meridians emanate from this point: the Central Meridian runs north and south, and the Lunar Equator runs east and west.

Different librations affect different sides of the Moon and each contribute added surface area. The added viewable surface area changes from day to day and month to month. Some librations overlap, but the maximum amount of lunar surface that we can see over time is 59%. 

The longitudinal libration is produced by the elliptical orbit of the Moon. Because of the nature of the elliptical orbit, the speed of the Moon changes depending on which part of the orbit it is in. When moving from its fastest point (closest to Earth) to its slowest point (farthest from Earth,) the Moon's speed is slowing down. But, because the Moon's rotational speed stays the same, for a period of time the Moon's face is not pointed directly at us, and this "lag" effect allows observers to see an extra bit of the lunar surface. In effect, we are "peeking" around the edge of the Moon! When the Moon is one quarter of the way around the Earth, it is 97 degrees through its rotation. 

This libration is called longitudinal because the extra surface areas exposed are along the lines of longitude (perpendicular to the equator.) The total extra surface area we can see from longitudinal librations is about eight degrees.

There is also a libration that is latitudinal. The plane of the Moon's orbit is titled 5 degrees to the ecliptic. For half the lunar cycle, the Moon is above the ecliptic and for the other half it is below the ecliptic. Each of these half cycles expose an extra bit of the Moon. The extra surface area shown during these librations is at the top of the northern hemisphere or the bottom of the southern hemisphere. The total extra surface area we can see from latitudinal librations is almost 7 degrees.

There is one more librations called Diurnal Librations. This librations occurs every day. Observers can "see" over the top of the Moon as it is rising. Likewise they can see under the bottom when it is setting. Diurnal librations are caused because the radius of the Earth adds an extra 4000 miles of height for looking "over" or "under" the Moon when it is on the horizon. This daily libration gives us an extra one percent of lunar surface area for viewing.

The Phases of the Moon

The revolution of the Moon around the Earth makes the Moon appear as if it is changing shape in the sky. From Earth we see the Moon grow from a thin crescent to a full disk (or full moon) and then shrink back to a thin crescent again before vanishing for a few days.

The Moon phases are produced by the alignment of the Moon and the Sun in the sky. 

The lit part of the Moon always points the way to the Sun.

 The changing shape of the bright part of the Moon that we see is called its phase.


The moon is illuminated because it reflects the light from the sun. The part of the moon facing the sun is lit up. The part facing away from the sun is in darkness.

The phases of the Moon depend on its position in relation to the Sun and Earth. As the Moon makes its way around the Earth, we see the bright parts of the Moon's surface at different angles. These are called "phases" of the Moon.  

The phases of the moon work in a cycle starting with the new moon.

There are eight phases of the moon

The phases are named after how much of the moon we can see, and whether the amount visible is increasing, or decreasing each day.

Phases of the moon as seen in the Northern Hemisphere

 

 

Phases of the moon as seen in the Southern Hemisphere

 

It takes our Moon about 29.5 days to completely cycle through all eight phases. This is known as a Lunar month

The Moon orbits near the equator of the Earth. In the northern hemisphere, we're standing on the opposite side of the globe from countries in the southern hemisphere i.e. we are "upside down" from each other! We therefore see the Moon from a completely different vantage point from each other.

One of the biggest misconceptions about the Moon is that its phases are caused by the Earth's shadow. The Earth's shadow causes eclipses, but it has nothing to do with phases. Rather, the phases of the Moon are produced by the alignment of the Moon and the Sun in the sky.

The orientation of Sun and Moon determines the phase of the Moon. At new moon, the angular distance of the sun and moon is small, less than a few degrees. At first quarter, when the moon is half full, the Moon lies 90 degrees east of the Sun. As an example, if you were to point to the setting sun with one arm and to the moon with the other, the angle between your arms would be 90 degrees. At full, the moon is 180 degrees from the sun and at last quarter, it is 90 degrees west of the sun.

The new moon is essentially invisible because it is between the earth and sun and therefore lost in the glare of the sun. Even on day 2 or 3 it is tough to spot the moon because it is just the tiniest sliver and still very close to the sun. The waxing crescent moon grows till it is about half full in the phase we call first quarter. From there it grows to full moon and then begins to shrink back to third quarter, waning crescent and finally back to new moon. 

The first and last quarter moons mark the halfway points between the new moon and full moon. The first quarter moon is illuminated on the right hand side. The last quarter moon is illuminated on the left hand side. It seems conflicting to call a half full moon first of last quarter, but the quarter refers to the position of the moon in the sky, not it's phase. A quarter moon is one quarter of a full circle (90 degrees) away from the sun.

The lighted part of the Moon always points the way to the Sun. This means that a waxing crescent moon in the western sky at sunset has the lighted part on the right which is the direction of the Sun. The sequence of the lunar phases always proceeds with the lighted part of the Moon growing from right to left until the moon reaches full. After full moon, the light recedes from right to left until new moon.

The line boundary between dark and light on the Moon is called the terminator. It is rarely an even line because the surface of the Moon is not smooth. The terminator is closest to a straight line at first and last quarter. Librations affect exactly what portion of the face of the Moon is facing the Earth, during first and last quarter phase, the terminator's exact location may vary. This is illustrated when the terminator does not line up with the Moon's meridian at first and last quarter. In other words, you'd think that during first and last quarter when the Moon is half full, that the meridian (the imaginary line which runs from north to south on the Moon,) would be right on the terminator, but it isn't so. In fact the terminator can be almost 8 degrees off to either side of the meridian.

WORKSHEET 

The diagram below shows the Earth and Moon as viewed from space far above the north pole of the Earth.Write the name of the phrases.

 

Label the Moon Phases Diagram Read the definitions, then label the moon phases diagram below.

Definitions

Crescent Moon - when we can see only a sliver of the moon's disk (the side of the moon facing us) Full Moon - when the moon's disk is light because the Earth is between the sun and the moon Gibbous Moon - when we can see roughly three-quarters of the moon's disk Half Moon - (also called quarter moon) when we can see one half of the moon's disk (this is one-quarter of the entire moon's surface)

New Moon - when the moon's disk is dark (and invisible to us) because the moon is between the sun and the Earth Quarter Moon - (also called half moon) when we can see one half of the moon's disk (this is one-quarter of the entire moon's surface) Waning Moon - when the moon seems to be getting smaller, going from full to gibbous to half to crescent to new Waxing Moon - when the moon seems to be getting bigger, going from new to crescent to half to gibbous to full

黄道12星座

星座的英文名为constellation，星座是指天上一群群的恒星组合。
实际上同一个星座内的恒星相互间没有实际的关系，不过其在天球上投影的位置相近。自古以来，人对于恒星的排列和形状很感兴趣，并且很自然地把一些位 置相近的星联系起来，组成星座。星座分为北天星座、南天星座和黄道星座。当前的全天分为88星座。星座一般以仪器或希腊神话人物命名。

我们通常所说的的星座多指的是占星学中的星座。在占星学上，黄道12星座是宇宙方位的代名词， 代表了12种基本性格原型，一个人出生时，各星体落入黄道上的位置，正是说明著一个人的先天性格及天赋。黄道12星座象征心理层面，反映出一个人行为的表 现的方式。于是将黄道分成12个星座，称为黄道12星座。依次为白羊座、金牛座、双子座、巨蟹座、狮子座、处女座、天秤座、天蝎座、射手座、摩羯座、水瓶 座、双鱼座。

星座 CONSTELLATION

星座是指天上一群群的恒星组 合。自从古代以来，人类便把三五成群的恒星与他们神话中的人物或器具联系起来，称之为“星座”。星座几乎是所有文明中确定天空方位的手段，在航海领域应用 颇广。对星座的划分完全是人为的，不同的文明对于其划分和命名都不尽相同。星座一直没有统一规定的精确边界，直到1930年，国际天文学联合会为了统一繁杂的星座划分，用精确的边界把天空分为八十八个正式的星座，使天空每一颗恒星都属于某一特定星座。这些正式的星座大多都以中世纪传下来的古希腊传统星座为基础。与此相对地，有一些广泛流传但是没有被认可为正式星座的星星的组合叫做星群，例如北斗七星（参见恒星统称列表）。
在三维的宇宙中，这些恒星其实相互间不一定有实际的关系，不过其在天球这一个球壳面上的位置相近，而其实它们之间可能相距很远。如果我们身处银河中另一太阳系，我们看到的星空将会完全不同。自古以来，人们对于恒星的排列和形状很感兴趣，并很自然地把一些位置相近的星联系起来组成星座。

Constellation

In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms (which themselves are generally referred to in non-technical language as "constellations"), which are patterns formed by prominent stars within apparent proximity to one another on Earth's night sky.

There are also numerous historical constellations not recognized by the IAU, or constellations recognized in regional traditions of astronomy or astrology, such as Chinese, Hindu and Australian Aboriginal.