Eclipses of the Moon occur when the Earth’s shadow prevents sunlight from reaching the Moon.
In the cover image you can see the lunar eclipse of 21 January 2019.
What are Eclipses of the Moon?
Direct sunlight is being blocked by the Earth, and the only light reaching it is sunlight refracted by Earth’s atmosphere, producing a reddish color.
The Moon lacks its own light, and if it shines it is thanks to the light it reflects when illuminated by the Sun.
If the Earth gets in the way, the sunlight does not reach the Moon, and the Moon is in shadow.
Why a lunar eclipse occurs
This apparent equality in size, is due to the fact that the distance that separates us from the Sun is also four hundred times greater than the distance to the Moon.
Sunlight reaching Earth, casts a converging cone of shadow, and a divergent cone of penumbra.
The height of the convergent shadow cone, is estimated to be 1,300,000 km four times greater than the 384,000 km from Earth to the Moon.
Due to this circumstance, the Moon sometimes enters the cone of shadow, and we can see a lunar eclipse.
A lunar eclipse is visible to the inhabitants of an entire earth’s hemisphere.
It is always impressive to watch the progress of the eclipse, as the Moon enters the twilight zone, and then the total shadow zone.
Orbits of the Earth and the Moon
If the orbit of the Moon were parallel to the ecliptic, there would be an eclipse every time there was a full moon, that is to say every month.
But, as the lunar orbit is inclined by 5º 9 ‘with respect to the plane of the ecliptic, in most of the times when there is a full moon, our satellite is a little above or a little below the plane of the Earth’s orbit, and the Moon does not enter the cone of shadow.
Eclipses only occur when the Moon is at one of the junctions (nodes) of both orbits.
That is, once or twice a year. Lunar eclipses can only occur in the full moon phase.
A lunar eclipse can last several hours. The Moon takes about an hour to cross the gloom.
Then it spends another hour crossing the shadow, in which time it observes the eclipse in its entirety.
In the final phase of the eclipse, the Moon will enter the other twilight zone.
Atmosphere influences what a lunar eclipse looks like
The Earth’s atmosphere has a great vital influence on the perception of eclipses.
If there were no atmosphere on Earth, the Moon would completely disappear during the eclipse.
During the eclipse, the Moon does not disappear completely, because some solar rays bend when passing through the Earth’s atmosphere, and strike the lunar surface, illuminating it weakly.
A total lunar eclipse takes place when the Earth comes between the Sun and the Moon, and covers the Moon with its shadow.
For this reason, the totally eclipsed Moon, acquires a characteristic reddish color.
When this happens, the Moon can turn red, earning it the nickname of Blood Moon.
Observing the curved shape of the Earth’s shadow cast on the Moon during eclipses, Aristotle, in the 4th century BC, came to the conclusion that the Earth is spherical, since only a sphere can always cast a circular shadow, when it is illuminated from different angles.
There can be several lunar eclipses in a year
There can be no more than seven lunar or solar eclipses in an Earth year, and at least two eclipses will occur.
Eclipses occur two by two, because lunar eclipses are preceded by a solar eclipse.
As a curious fact, it is interesting to know that Christopher Columbus, on his second trip to Hispaniola, observed the lunar eclipse from September 14 to 15, 1494.
Later, using the lunar ephemeris, he predicted the lunar eclipse of February 29 of 1504 and took advantage of this knowledge to impress the natives of Jamaica, and thus obtain from them the food that until that moment, they refused to provide him.