2. Solar Eclipse

Solar eclipses are natural phenomena that occur when the Moon passes between the Earth and the Sun, obscuring the latter.
This event is so unnatural that it astonishes and amazes not just humans, but many others living creatures. Studies have shown that animals react strangely to solar eclipses. Their behaviour is driven by the absence of sunlight where there should be and, in fact, depending on the animal, they usually prepare to sleep.

 [This is a post which is part of the series: 5 unmissable celestial events]



Physical background:

The word Eclipse refers to an event in which a body is occulted by another body. Eclipses can happen with any astronomical object, but the most relevant ones, visible to naked eyes, are the lunar and solar eclipses. In lunar eclipses, the Moon is eclipsed by Earth's shadow and in solar eclipses the Sun is eclipsed by the Moon's shadow.
An eclipse happens when 3 celestial bodies are aligned, and perfectly aligned (also their centres of mass must be aligned) for total eclipses.

A total solar eclipse. The light
passing throught is the corona
of the Sun

Solar Eclipses are divided in different types; the main ones are:

- Partial Solar Eclipse: the most frequent (yearly or more) and the most common, the Sun is not totally obscured by the Moon, because not in perfect alignment with it.
- Annular Solar Eclipse: similar to the partial one, but the Sun is in line with Moon and Earth. It isn't totally obscured because the shadow created by the moon is smaller, then a "ring of sun" around the moon can be still seen.
- Total Solar Eclipse: a very rare event in witch the Sun is totally obscured by the Moon.
So called Hybrid solar eclipses can also happen, where both an annular and a total solar eclipse occur on the same alignment but on different timings.

Thinking about the dynamics of the Moon's motion around the Earth, one could assume that solar eclipses must happen every new moon and must be located somewhere on the equator.
We see different kinds of eclipses all around the world, anyway, and not that frequently! What's missing in this model, then?

Sketch showing Earth, Moon and Sun in conjunction.
Umbra and Penumbra regions are annotated.

The tilted rotational axis of the Earth and revolution axis of the Moon. As the first is inclined and it revolves around the sun with it axis (almost) unchanged, if not for some millennial motions, eclipses do not fall on the equator. Furthermore, the plane of revolution of the Moon around the Earth is not the same as the plane of revolution of the Earth around the Sun, but it is inclined by ~5°. The combination of these factors lead to the formation of different kinds of eclipses in very different places of the Earth, but they can be predicted very accurately, since we know this model.
Partial solar eclipses are seen in the penumbra region and total solar eclipses or annular eclipses in the umbra region. The first (total) occurs when the apparent sizes of the Moon and the Sun are about the same, while the second (annular) when the apparent sizes of the Moon is slightly smaller than the one of the Sun.

Shadow of the Moon on Earth

The apparent size of a body is the size of astronomical objects as seen from Earth. A very curious and mysterious fact is that Moon's apparent size as seen from Earth is almost equal to that of the Sun (apart from the fact that it slightly changes, because of our satellite's elliptical motion, that constantly changes its distance from Earth) even if their distance is significantly different and also their volume. They pretty much match in our skies though, and I find that seriously amazing. Because of this, during total solar eclipses, we can see our Sun's corona, its outer region formed by very hot gas constantly expelled to form solar winds (important in next blog).


Result:

Eclipses at sunsets are my favourite and the most beautiful. The sky darkens, some stars appears (especially Mercury, during total eclipses) and an unreal atmosphere falls on the Earth for some minutes, with the sun still in the sky. A very mystic atmosphere.


Locations:

Everywhere on Earth, you just need some luck (a lot, actually since they occur in the same place once every 370 years, on average! 18 months, everywhere).
The second link below list the upcoming solar and lunar eclipses, with their location and length. Often trips are organized to go and see them. Don't be patient and think of waiting for one in your country, if you have the occasion to catch a trip. For your information, there is only one total eclipse in central Europe for the present century, and it's in 2081!


Interesting Links:


All the types of solar eclipses explained using animations
Past and Future eclipses dates including locations
Ancient Greeks' ingenious machine to predict eclipses
Abstract about the effect of solar eclipses on animals