Supermoon

A supermoon is a full moon that takes place when the moon is at its perigee, or the point at which it comes closest to Earth during its elliptic orbit. Depending on the exact distance between Earth and the moon, a supermoon can appear to be up to 14 percent larger and 30 percent brighter than normal. Although the variation is relatively slight, supermoons can also lead to larger-than-normal tides. The strictest definition of the term states that a supermoon is the closest new or full moon of the year. According to this definition, no more than two supermoons will occur in a single calendar year. However, the more commonly accepted definition is that a supermoon is any new or full moon that occurs when the moon is closest to Earth. Going by this description, multiple supermoons may occur each year. In 2020, there were a total of three supermoons, with one occurring monthly in March, April, and May.

Background

The concept of a supermoon is closely tied to the moon’s phases, which refer to the portion of the moon that is lit by the sun and therefore visible from Earth. As the moon orbits Earth and both orbit the sun, the moon’s shape appears to change as it is viewed from Earth. Obviously, the shape of the moon does not physically change. Instead, the perceived changes in the moon’s shape are a direct result of the position of the moon in relation to Earth and the position of both these bodies in relation to the sun. It is also important to note that because the moon revolves around Earth at about the same rate at which it revolves on its axis—nearly a month in Earth time—the same portion of the lunar surface is always visible to observers on Earth.

The moon has eight phases: the new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, and waning crescent. The cycle of lunar phases begins with the new moon. When a new moon occurs, the portion of the moon facing Earth is not lit by the sun and therefore not visible from Earth. The moon starts to become visible during the waxing crescent phase. During this phase, a crescent-shaped portion of the moon facing the sun is visible from Earth. As the moon continues through its orbit, it enters the first quarter phase. At this point, half the moon is visible for a portion of the evening. The next phase is the waxing gibbous phase. During this phase, most of the moon’s sunlit surface is visible, and only a small portion remains darkened. This leads to the full moon phase, which is a period when the entire sunlit surface of the moon can be seen from Earth. Under the right circumstances, it is during the full moon phase that a supermoon can occur. The full moon phase is followed by the waning gibbous phase. At this stage, the amount of the lunar surface that is lit by the sun begins to shrink. In the last quarter phase, only half the moon’s sunlit surface is visible again. Finally, the visible sunlit lunar surface is once again reduced to a crescent-shaped sliver as the moon cycles back to the new moon phase.

Overview

Although supermoons have been occurring for as long as the moon has been orbiting Earth, the term “supermoon” has only existed since 1979. It was first coined by astrologer Richard Nolle in an article for Dell Horoscope magazine. At the time, Nolle defined a supermoon as “a new or full moon which occurs with the moon at or near (within 90 percent of) its closest approach to Earth in a given orbit.” Given its astrological origins, the word supermoon is not considered to be an official astronomical term. In fact, it was rarely used until around 2011. In the years that followed, the term came to enjoy more widespread use, especially in North America and Southeast Asia.

In astronomy, supermoon is more accurately viewed as a lay term for what astronomers typically refer to as a perigean full moon, or a perigee syzygy. These terms are used because a supermoon happens when a full moon occurs at or near the time in its orbit when the moon is closest to Earth. This point is referred to as the moon’s perigee. Conversely, the point in the moon’s orbit when it is farthest from Earth is called its apogee.

Supermoons occur because the moon follows an elliptical orbit around Earth. Gravitational forces that pull on the moon prevent it from having a perfectly circular orbit. In addition, orientation of the moon’s orbit shifts over time as Earth orbits the sun. As a result, supermoons are an irregular occurrence and not something that happens every time the moon cycles through its different phases.

A supermoon appears larger and brighter than a normal full moon. While a supermoon can be up to 30 percent brighter than a typical full moon, its increased size is much less dramatic than people think. While a supermoon appears to be a bit larger than a normal full moon, the actual difference in size is often not even detectable to the naked eye. Observers often think that a supermoon looks especially large because of a phenomenon known as the “moon illusion.” When the moon is near the horizon, the human brain naturally perceives it to be larger than it actually is. Experts think that this might be because the brain automatically compares the moon to the earthbound structures surrounding it when it is on the horizon.

Supermoons affect Earth in some ways. They may lead to extreme high and low tides because of the increased gravitational pull the moon exerts across Earth’s diameter when it is so close. While supermoon high tides can contribute to flooding and other issues in some coastal areas, the degree to which a supermoon impacts tidal variation is usually not great enough to be problematic.

There are two special types of supermoons: the super blood moon and super blue moon. A normal blood moon occurs when a lunar eclipse causes the moon to appear red because the sunlight that reaches its surface first passes through Earth’s atmosphere. A super blood moon happens when a lunar eclipse coincides with a supermoon. In some instances, two full moons occur in a single month. When this happens, the second full moon is called a blue moon. When a blue moon coincides with a supermoon, the result is a super blue moon.

Bibliography

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