Lifting our heads to the sky has always been one of the favorite activities of the human race. Among the stars, there are countless phenomena that intrigue us, and that over time we have studied to better understand the laws of the universe. Among these, one of the most fascinating is impossible not to mention the lunar eclipse, which has always been able to excite even those who are not exactly an expert on celestial bodies. Just as it happens for the solar eclipse, also the one involving our satellite is an optical-astronomical effect during which the shadow of the Earth completely or partially obscures the Moon when the latter is illuminated by the Sun and intersects the nodal axis, in full moon phase.
As a consequence, when there is a lunar eclipse, the Moon itself is in "full" phase with an alignment that provides, in order, Sun, Earth, Moon. During this long-awaited event, their mutual distances go to create a shadow of conical shape much larger than the Moon, which is accompanied by a wider cone, the so-called cone of penumbra, in which only part of the Sun's rays are intercepted by the Earth. These two cones are particularly important because they determine the type of eclipse: penumbral, partial or total.
Moon eclipse: how it happens
For any phenomenon of lunar eclipse to occur, certain basic conditions are required. These are three single events that together give life to one of the most beautiful spectacles that the hemispheric vault can give us: Sun, Earth and Moon must be aligned, the Moon must be full and must cross a portion of the projected shadow of the Earth.
As we said before, when this happens, the respective distances between the three celestial bodies involved allow the creation of a cone-shaped shadow that is introduced by the interposition of the Earth. The amplitude of the cone, greater than that of the entire Moon, together with that of the cone of penumbra, causes part of the Sun's rays to be intercepted by our planet. You will then understand how the various types of eclipses of the Moon depend totally on its movement, which then goes to determine the appearance of a total eclipse, a partial eclipse or a penumbral lunar eclipse. in case the Moon is totally or partially in the cone of penumbra.
We will then discover the distinctive features of the three astronomical events, as well as their effects and their duration. Before proceeding, however, it is well to point out that those under consideration are not frequent optical phenomena, since the orbit of lunar revolution has a different inclination from that of the Earth, about 5° 9′, and, if they matched, each lunar month we would witness the darkening of the Sun when the Moon is in new moon, known as the New Moon, and that of the Moon when the latter is in the full Moon phase.
Total Lunar Eclipse
In scientific literature, a total lunar eclipse is when the Moon transits completely through the Earth's shadow. This means that the satellite first goes through the penumbra, then continues with the shadow and finally, after leaving it, goes to involve the penumbral belt again. In particular, it is during this eclipse that the phenomenon of the Red Moon can occur. The celestial body, so loved by the Hellenic poet Sappho, in fact, takes on a characteristic reddish coloration, due to the partial darkening before entering the shadow and during the exit, and to the refraction of the solar rays that pass through the Earth's atmosphere.
The same rays that in part reflect on the surface of our planet, and that the Earth sends back to the Moon. According to sources and astronomical evidence, we are facing the most observed lunar eclipse in history, and which showed peaks of absolute beauty in 2015. The eclipse occurred on September 28 of that year was not coincidentally called "the Blood Moon", because of the red color so intense.
Not only that, we know that the last total lunar eclipse occurred on January 21, 2019 at 05:12 and that the next one will be on May 16, 2022 at 04:11 (maximum darkness). The most imminent totally visible in Italy will be the one on December 31, 2028; moreover, it is known that the lunar eclipse of July 27, 2018 was the longest of the century, having reached a record-breaking maximum duration of one hour and 43 minutes. As for the duration of total lunar eclipses, these can in fact reach an average of 100 minutes, and they are total for all affected places - except for transition points.
To explain it, we must think that our satellite crosses the cone of shadow with a speed, that of the motion of lunar revolution, decidedly lower than that of the Earth's rotation, which determines precisely the duration of solar eclipses. In this period of time, we will see the Moon begin to eclipse immediately with a black color, actually the coloration is red, but it is impossible to perceive with the naked eye because the part still illuminated reflects the light towards us: the glow prevents us from seeing directly the reddish coloration, which will begin to show progressively as the glow disappears for the darkening. The same principle applies to electronic devices, considering that cameras are sensitive to light and tend to sharpen the brightest image.
Partial Lunar Eclipse
A partial lunar eclipse, unlike a total lunar eclipse, can occur in our sky when the Moon is not close enough to the ecliptic to pass through the entire Earth's shadow. In this case it is only partially occulted, showing a typical hawkish profile. For this reason, a partial Moon eclipse is of less scientific interest than a total one, especially for astronomers, i.e. non-professional astronomers who usually lack the qualifications of the astronomy profession, but who nevertheless enjoy studying and observing astronomical phenomena.
More followed by the amateurs is instead the already mentioned lunar eclipse with red Moon, which gives the possibility to make photographic or video camera shots of a Moon with appearance and colors obviously out of the ordinary. Among the most known partial lunar eclipses, visible also here in Italy, we remember that of September 7, 2006, during which it was possible to observe only the exit from the shadow after the Moon had risen. Then there is the one, always visible from Italy, which occurred during the night between 16 and 17 August 2008. More recent is then the one on December 31, 2009, which was almost like a total penumbral eclipse with only a brief phase of partial shadow eclipse.
Penumbral Lunar Eclipse
To close, we have the penumbral lunar eclipse. It occurs when the Moon transits only and exclusively through the Earth's penumbra, without being occulted by the shadow. This particular phenomenon, from the scientific point of view and exquisitely visual, is considerably less conspicuous than the previous ones, although with its own charm. In some cases, only a tiny portion of the shadow may be observable, provided, however, that the Moon is completely within the penumbra: this is called a partial penumbral eclipse.
As for eclipses of shadow, in these conditions there is also a magnitude, or magnitude, with penumbral declination. The last total penumbral eclipse was recorded on February 9, 2009, and is currently the last total penumbral eclipse in recent years. From this date on, in fact, all subsequent ones will be partial until February 20, 2027, when there will be another total penumbral eclipse, also visible from Italy. The last partial penumbral eclipse was instead on July 7, 2009, but it was not possible to observe it from our country.
Duration and effects of lunar eclipses
Even those who are not astronomy experts, will know that the planets periodically return to occupy the same positions. Having said that, it is inevitable that lunar eclipses, in the same way as solar eclipses, also repeat cyclically. A periodic cycle, this one, already discovered and theorized by the astronomers of Mesopotamia 2.500 years ago, known as Saros cycle. It has a duration of 6585 days, which correspond more or less to 18 years, at the end of which the same lunar and solar eclipses are repeated.
According to studies, during a Saros occur 29 lunar eclipses and 41 solar eclipses. Not only that, considering that the Earth rotates about 120 degrees in 8 hours, the same eclipse will be repeated in a different location at the end of each cycle. During total eclipses, then, the Moon does not completely stop receiving reflected light. The solar rays that pass through the Earth's atmosphere are actually deflected by refraction and reach our satellite giving it a coloration that has been observed to change during the course of the same eclipse: this can range from dark copper red to orange red, passing through other shades including brown, blue and dark green.
The colors reflected by the Moon, on some occasions, are also due to the particular area of the Earth that reflects the light, perhaps because an area rich in water, such as oceans and forests, which give a unique chromatism to the celestial body "conquered" by man in 1969.