Eclipse – Concept, how it occurs and types of eclipse

We explain what an eclipse is and how the phenomenon occurs. We also talk about the differences between a solar eclipse and a lunar eclipse.

What is an eclipse?

An eclipse is an astronomical phenomenon in which the light of an incandescent star, such as the Sun, is totally or partially covered by another opaque star that interposes (known as eclipsing body) and whose shadow is projected on planet Earth. The name comes from the Greek ekleipsis, or “disappearance”.

In principle, eclipses can occur between any set of stars, as long as the dynamics of light and interposition described above take place. However, as there are no off-planet observers, we generally speak of two types of eclipse: lunar, and eclipses, depending on which celestial body is obscured.

Since ancient times, eclipses both fascinated and disturbed human beings. Ancient civilizations saw in them a sign of change, catastrophe or rebirth, if not a bad omen. Most religions worshiped the Sun in one way or another and were bound to observing the sky, looking for signs.

These phenomena were understood and predicted by ancient civilizations with some astronomical knowledge. They have studied the recurrence of astral movements, and noted their cycles in various calendars. Some of them even used them to separate political, religious or social periods.

Why do eclipses occur?

In the lunar eclipse, the Earth casts a shadow that obscures the Moon.

The logic of eclipses is simple: a celestial body comes between us and some source of light, producing a shadow that sometimes blocks much of the glare. It is something similar to what happens when we pass an object in front of the light of an overhead projector: the shadow is also projected against the backdrop.

For eclipses to occur, however, we need a confluence of spatial factors between the Moon, the Earth and the Sun, which happens once every certain number of orbital repetitions. That is why they occur in a specific frequency.

Today they can be predicted with the help of computers: since we know the time it takes for the Earth to rotate around the Sun and around its axis, as well as the time it takes for the Moon to orbit our planet, we can build a computer model to calculate the cycles and find out the next occurences.

Solar eclipse

In solar eclipses, the Moon comes between the Sun and the Earth.

The solar eclipse occurs when the Moon stands between the Earth and the Sun, casting its shadow over a portion of the Earth’s surface. The phenomena gradually covers, then uncovers the planet with a shadow, and takes several minutes to complete. Such events can only take place during the new moon, and it can happen in three different ways:

• Partial solar eclipse: the Moon partially obscures light from the Sun or a visible segment of its circumference, leaving the rest visible.
• Total solar eclipse: in a certain place on Earth, the Moon positions in a way that covers the whole surface of the solar disk. The Sun is totally obscured, creating a sort of an artificial night for a few minutes.
• Annular solar eclipse: although the Moon positions itself on the right trajectory from the Earth to the Sun, it is too far from Earth to cover all the surface of the Sun, and allows to view some part of the solar corona (Sun’s external edges).

Solar eclipses are pretty frequent but, since the Moon is much smaller than the Earth, they can only be seen from a specific point on our planet. This means that a solar eclipse of same kind may be seen every 360 years to 410 years on average.

Lunar eclipse

During lunar eclipses, the Earth comes between the Sun and the Moon.

Unlike the solar one, the lunar eclipse occurs when the Earth stands between the Moon and the Sun, casting its shadow on the Moon and obscuring it to some extent. As we have seen in the case of solar eclipses, the phenomenon is seen from a certain terrestrial point.

The durations of the eclipses are variable and depend on the position of the Moon within the cone of shadow cast by the Earth, which is divided into Umbra (the darkest section) and Penumbra (the least dark section).

We have between two and five lunar eclipses every year. There are three possible types of lunar eclipses:

• Partial lunar eclipse: when only a small part of the Moon gets submerged in the cone of shadow cast by the Earth. The Moon is showing slightly obscured or only obscured in some segment of its circumference.
• Penumbral eclipse of the Moon: occurs when the Moon passes through the Earth’s shadow cone, but only through the Penumbra region or the least dark section. The shadow can slightly obscure the image of the Moon on the sky, or it can alter its color, going from white to red or orange. Sometimes, when the Moon enters only partially in the penumbra, we can have a “partial eclipse of the Moon”, a combination between the partial eclipse and the penumbral eclipse.
• Total lunar eclipse. It happens when the Earth’s shadow completely obscures the Moon, a gradual process, passing first from a penumbral eclipse to a partial one, then the total one, and again the partial one, the penumbral and the end of the eclipse, in a full cycle.