PHY: Eclipses
Hello everyone, David here, and welcome back! I've been quite busy lately, but I finally got some time to sit down and talk about a very interesting topic that I would love to one day have the opportunity to experience: eclipses! Not a lot of people realize how awesome eclipses are and just how beautiful they can get, so let's get right into it!
What are Eclipses?
First and foremost, let's talk a little bit about the essentials here: what are these so-called eclipses. The word "eclipse" comes from the French word with the same spelling, and can mean a lot of things, however, for the sake of this astronomy-oriented post, let's look at two of its meanings: Obscure and/or Alignment. This makes total sense to us, as we will see that eclipses are nothing more than an alignment between two or more celestial bodies that can cause some interesting sights and events for us humans.
Generally speaking, whenever somebody says the word "eclipse", we usually come across the same image in our heads: the Sun being blocked, causing a huge shadow here on Earth, and that is indeed a type of eclipse, a Solar Eclipse, that we will talk more about later. What a lot of people don't know is that this kind of eclipse is not the only one, as there is one that is, in my opinion, equally as awesome, and that is the Lunar Eclipse. Let's investigate these phenomena more deeply, shall we?
Solar Eclipse
Let's start with the most popular of the two, the Solar Eclipse. As I said earlier, eclipses are alignments between two or more celestial bodies. As it turns out, in the case of a Solar Eclipse, the alignment occurs between the Sun and the Moon, therefore we can already conclude that the object that blocks the Sun is, you guessed it, the Moon!
There are a couple of things to talk about here, the first one being the reason for this phenomenon to be so rare. Let's start with the basics: the Moon, in a period of about 28 days, goes through different stages, called Lunar Phases, due to its relative position to the Sun observed here on Earth. Because of it, it's apparent shape changes and, most importantly, the time it rises and sets also changes (I will probably make a post focused solely on Lunar Phases, so stay tuned for that). As a matter of fact, the least angle between Sun and Moon is when the latter is going through the New Moon phase, in which it rises and sets with the Sun, that's why we can't see the Moon during the night on this phase.
With that said, a valid question can be asked: If the least angle between the Moon and the Sun occurs every 28 days or so, then why don't we have Solar Eclipses with the same frequency of the Lunar Phases? That would be the case if the Moon's orbital plane was aligned with the Earth's orbital plane (in this case, every New Moon would mean the giant rock would be blocking the Sun, which is clearly not the case), however, that is not true!
The picture above shows the 5° tilt between the orbital planes, and that is the answer to our question: because the planes are slightly tilted, a new condition for the Solar Eclipse arises, which is the fact that the Moon has to be passing through the intersection of the planes at the same time of its New Moon phase, lowering the probability of this happening. The same tilt is the reason for the next eclipse's rarity, so keep this in mind. We call the points of intersection between a reference orbital plane and another one the Orbital Node.
When all conditions are right, i.e, the Moon is on the orbital node and on its New Moon phase, it blocks the Sun, casting its shadow on a portion of the Earth's surface. If you happen to be inside the umbral part of the shadow, you will experience complete darkness, that's what we call a Total Solar Eclipse. On the other hand, if you are on the penumbral part of the shadow, the Sun will appear to be blocked partially by the Moon, and you will experience partial darkness, that's what we call a Partial Solar Eclipse.
There's also the Annular Solar Eclipse, which happens when the Moon is perfectly aligned with the Sun, on one of the orbital nodes AND in its apogee, which is the point in its longest distance from Earth. This long distance means it won't be able to cover the Sun entirely, creating a "ring" around the Moon, a beautiful sight to see.
Lunar Eclipse
Similarly to the Solar Eclipse, the Lunar Eclipse also involves alignment between celestial bodies, however, this time Earth and Moon need to be aligned, the latter being behind the Earth. This phenomenon is less popular then the first one we discussed, but it is equally beautiful in my opinion.
Because the Moon is behind our planet, this time it is not the Moon's shadow that is casted on Earth, it is actually the complete opposite: Earth's shadow is casted on the Moon! There is also something else to note here: because the Moon will be completely behind us, the angle between it and the Sun must be of 180°, which coresponds to its Full Moon phase! In other words, Lunar Eclipses can only happen during a Full Moon, just like Solar Eclipses can only happen during a New Moon.
With that being said, we could ask the same question as we did before, but we would get the same answer: Lunar Eclipses don't happen every Full Moon because of the disalignment between the orbital planes.
There are two types of Lunar Eclipses: partial and total. Partial Lunar eclipses occur when there is still a slight disalignment between the Moon's position on its plane and Earth's orbital plane. Because of this, the Moon doesn't completely enter our umbral shadow, only our penumbral shadow. When this happens, we see the Moon slightly darker in some areas, almost unnoticeable with the naked eye. A Penumbral Lunar Eclipse actually happened this Monday! It could be seen on most of South America and some of North America. I actually stayed awake and tried to experience the eclipse, with no success. It was a very faint and unnoticeable shadow, impossible to see with the naked eye, but hey, at least I get to say I witnessed an Eclipse!
The other type of Lunar Eclipse is the Total Lunar Eclipse, which happens when the Moon is completely envolted by our shadow. As the Moon enters our umbra, it starts getting red, until it is completely colored by this mysterious color out of nowhere! Well, turns out this happens because of our atmosphere!
Earth's atmosphere acts almost as a prism: it deflects the Sun's light into various wave lenght's, i.e, various colors. The color that is most deflacted is red, and that is no coincidence: red has the biggest wave lenght out of all the colors in the visible spectrum, that's why it's the most deflated! Because of this, a red shaded shadow is formed directly behind Earth, and the Moon is completely covered by it, making its surface appear to be red! That is also the reason this type of Lunar Eclipse is called the Blood Moon (due to its blood-like color it gets).
Well, that's it for today's post! I really recommend you guys check out the following links, they're simulations of the Lunar Phases and Eclipses, which make the whole topic way more visual and intuitive, and I must say it helped me a lot to understand this topic. Have a good day and stay safe everyone! Peace!
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