How is a lunar eclipse formed?

On the 15th and 16th days of the lunar calendar, the moon moves in the direction opposite to the sun. At this time, if the centers of the Earth and the Moon are roughly in the same straight line, the Moon will enter the Earth's umbra, resulting in a total lunar eclipse. If only part of the moon enters the Earth's umbra, a partial lunar eclipse occurs. When the moon enters the penumbra of the earth, it should be a penumbral eclipse, but because its brightness weakens very little and is not easy to detect, it is not called a lunar eclipse. Therefore, there are only two types of lunar eclipses: total lunar eclipse and partial lunar eclipse. Lunar eclipses all occur in Wang (full moon), but not every moon has a lunar eclipse. This is the same as a solar eclipse in every moon.

Lunar eclipses

lunar eclips

A phenomenon in which the moon is fully or partially obscured by the earth's shadow. Lunar eclipses usually occur on the lunar calendar, that is, on the 15th or 16th day of each month in the lunar calendar. At this time, the earth moves between the sun and the moon. However, not every lunar eclipse may occur because there is a gap between the ecliptic and the ecliptic. There is an angle of intersection, so observers on the earth can only watch the lunar eclipse when the moon is eclipsed again on consecutive photos of the night and the shadow of the earth is visible near the intersection of the ecliptic and the ecliptic. The number of lunar eclipses that occur each year is generally two. The diameter of the sun is much larger than that of the earth, and the earth's shadow can be divided into the umbra and the penumbra. The diameter of the Earth is about 4 times that of the Moon. At the lunar orbit, the diameter of the Earth's umbra is still equivalent to 2.5 times that of the Moon. A partial lunar eclipse occurs when only part of the Moon is always obscured by the Earth's umbra. A total lunar eclipse can be seen when the moon completely enters the Earth's umbral shadow. If the moon enters the penumbral area, the sun's light can also be blocked. This phenomenon is called a penumbral lunar eclipse in astronomy. However, because the sunlight is still very strong in the penumbral area, penumbral lunar eclipses are not easy in most cases. It can be distinguished with the naked eye, but in fact, penumbral lunar eclipses occur frequently. According to observation data, the percentages of penumbral lunar eclipses, partial lunar eclipses, and total lunar eclipses in each century are about 36.60%, 34.46%, and 28.94%.

■Lunar Eclipse

A total lunar eclipse is a special astronomical phenomenon that occurs when the moon moves into the shadow of the earth. The area will be blocked by the earth due to sunlight, and now a piece of the moon is missing.

In other words, the sun, earth, and moon are exactly (or almost) in the same straight line at this time, so the light from the sun to the moon will be obscured by the earth.

As far as the earth is concerned, when a lunar eclipse occurs, the directions of the sun and the moon will be 180 degrees different, so the lunar eclipse must occur at 'Wang' (that is, around the 15th day of the lunar calendar). It should be noted that since the orbits of the sun and the moon in the sky (called the ecliptic and the ecliptic) are not on the same plane, but have an intersection angle of about 5 degrees, only the sun and the moon are located at the two intersections of the ecliptic and the ecliptic respectively. Only nearby can they have a chance to form a straight line and produce a lunar eclipse.

■What kinds of lunar eclipses are there?

Total lunar eclipse: when the entire moon enters the earth's umbra

Partial lunar eclipse: when only part of the moon enters the earth's umbra

Penumbral lunar eclipse: At this time, the moon only passes through the penumbral area of ??the earth, causing the luminosity of the moon's surface to be slightly weakened, so it is less easy to detect.

■Seven steps of total lunar eclipse

The penumbral eclipse begins after the total lunar eclipse: the moon has just made contact with the penumbral area, and it is not noticeable to the naked eye at this time.

Initial waning: the moon slowly enters the earth's shadow from the east edge, and the moon and the earth's umbra are circumscribed for the first time.

Eclipse: The moon enters the Earth's umbra and incurs the umbra for the first time.

Eclipse: The moment when the center of the moon is closest to the center of the Earth's umbral shadow.

Luminescence: The moon moves within the Earth's umbra and inscribes it for the second time.

Rerounding: The moon gradually leaves the earth's umbra and circumscribes the earth's umbra for the second time.

End of penumbral eclipse: The moon leaves the penumbral shadow, and the entire lunar eclipse process officially ends.

■Did you know?

Since the Earth's umbral shadow is much larger than that of the Moon, this also means that during a total lunar eclipse, the Moon will completely enter the Earth's umbral area, so there will be no lunar annular eclipse.

Lunar eclipses occur up to three times a year, and sometimes not even once a year.

■Lunar eclipses and scientific research

The earliest record of a lunar eclipse was recorded in Mesopotamia in 2283 BC. When China was in the Han Dynasty, Zhang Heng had already discovered the principle of lunar eclipses. Aristotle in the 4th century BC deduced that the Earth was round based on the round shape of the Earth's shadow seen during a lunar eclipse. Ancient Greek astronomers Aristarchus in the 3rd century BC and Hipparchus in the 2nd century BC both proposed using lunar eclipses to determine the size of the sun, earth, and moon.

The principle of lunar eclipse. On the fifteenth and sixteenth days of the lunar calendar, the moon moves in the direction opposite to the sun. At this time, if the centers of the Earth and the Moon are roughly in the same straight line, the Moon will enter the Earth's umbra, resulting in a total lunar eclipse. If only part of the moon enters the Earth's umbra, a partial lunar eclipse occurs. When the moon enters the penumbra of the earth, it should be a penumbral eclipse, but because its brightness weakens very little and is not easy to detect, it is not called a lunar eclipse. Therefore, there are only two types of lunar eclipses: total lunar eclipse and partial lunar eclipse.

Lunar eclipses all occur in Wang (full moon), but not every moon has a lunar eclipse. This is the same as a solar eclipse in every moon. Under normal circumstances, the moon either passes above the Earth's umbral shadow or leaves below it. It rarely passes through or partially passes through the Earth's umbral shadow. Therefore, lunar eclipses generally do not occur. Lunar eclipses occur up to three times a year, and sometimes none occur at all.

Lunar eclipse formation diagram

The process of lunar eclipse

The process of lunar eclipse is divided into five stages: initial waning, full eclipse, full eclipse, light generation, and full circle. .

First waning: The moon just touches the Earth's umbral shadow, marking the beginning of a lunar eclipse.

Eclipse: The western edge of the moon is inscribed with the western edge of the earth's umbra, and the moon just completely enters the earth's umbra.

Eclipse: The center of the Moon is closest to the center of the Earth's umbral shadow.

Light generation: The eastern edge of the moon is inscribed with the eastern edge of the earth's umbra, and the total eclipse phase ends.

Compound circle: The western edge of the moon is tangent to the eastern edge of the earth's umbra. At this time, the entire lunar eclipse ends.

The degree of eclipse of the moon is called "eclipse fraction", which is equal to the ratio of the farthest distance from the edge of the moon wheel deep into the earth's umbra to the apparent longitude of the moon during the eclipse.

In ancient times, people did not understand the scientific reason why lunar eclipses occur. They were as afraid of lunar eclipses as they were of solar eclipses. There are legends in foreign countries that in the early 16th century, Columbus sailed to Jamaica in South America and had conflicts with the local indigenous people. Columbus and his sailors were trapped in a corner with no food or water. The situation was very critical. Columbus, who knew something about astronomy, knew that a total lunar eclipse was going to occur that night, so he shouted to the indigenous people, "If you don't bring food, I won't give you moonlight!" At night, Columbus' words came true, and there was no moonlight. The natives were horrified when they saw this, and quickly turned their hostility into friendship with Columbus.

The lunar eclipse record in Mesopotamia in 2283 BC is the earliest lunar eclipse record in the world, followed by the lunar eclipse record in China in 1136 BC. The phenomenon of lunar eclipses has always promoted the development of human understanding. As early as 1881, Zhang Heng, an astronomer from the Han Dynasty in China, figured out the principle of lunar eclipses. In the 4th century BC, Aristotle concluded that the Earth was spherical from the round shadow he saw during a lunar eclipse. Ancient Greek astronomers Aristarchus in the 3rd century BC and Hipparchus in the 2nd century BC both proposed to determine the relative size of the sun-earth-moon system through lunar eclipses. Hipparchus also proposed observing lunar eclipses in two distant places at the same time to measure geographic longitude. In the 2nd century, Ptolemy used ancient lunar eclipse records to study the movement of the moon, a method that continues to this day. Before the advent of rockets and satellites, scientists had been observing lunar eclipses to explore the structure of Earth's atmosphere.