Jan 31st 2018, we are going to witness a rare phenomenon, the total lunar eclipse. It has been called the “Blue moon, super moon, blood moon lunar eclipse”, a rare phenomenon which only occurs once in 150 years… Here is the explanation for some of the terms used in the eclipse.
Blue moon: You might have heard the phrase ” once in a blue moon” which refers to a rare event. The time between 2 full moons is approximately thirty days. So, in a month, we usually have only one full moon. If the full moon occurs on the first or second day of the month, the next full moon will be on the 30th or 31st day of the same month. This second full moon is called a blue moon. In Jan 2018, the first full moon was on 2nd and the next full moon will be on 31st. There is absolutely no astronomical significance for a blue moon. It is just due to the fact that a month can have 31 days.
Supermoon: Supermoon is a full moon which is slightly (10-14%) bigger than the normal full moon. Since the moon travels around the earth in a slightly elliptical orbit, it comes close to the earth on some days, and is slightly far away on other days compared to the average distance. The farthest point is called the apogee and the nearest point is called the perigee. When the moon is at perigee, it looks the biggest. When it is at apogee, it looks the smallest. The difference between these is just 10-14%, and this happens every month. A supermoon is the day when the moon is at perigee and the phase of the moon is full… Again, this has no astronomical significance. The change is tides is barely noticeable and does not cause any threat to him an or marine life. This term was made popular by some western astrologer. The correct name for supermoon is “perigee full moon”.
Bloodmoon: when the moon raises on Jan 31st in our part of the world, it is already eclipsed. So it looks dark red-brown color (typically – the color can vary based on atmospheric conditions). The Moon appears to be reddish because of Rayleigh scattering (the same effect that causes sunsets to appear reddish) and the refraction of that light by Earth’s atmosphere into its umbra. But contrary to the name, it is not blood red in color. It is more like copper color. So the correct name would be copper moon than blood moon.
Rahu and Ketu:
Short answer: Rahu and Ketu are ascending and descending nodes of the lunar orbit over the ecliptic.
Long answer: Rahu and Ketu are not demons. If you look at old astronomical texts, they are called chaya grahas, which means they are invisible. They are not Uranus or Neptune. That’s another misconception.
Also, when you look at the 12 houses in the jaathaka, the rahu and ketu are always in opposite houses. Ex: if rahu is in Pisces, ketu will be in Virgo.
Also, if we look closely, on the day of a lunar eclipse, sun and rahu (or ketu) will be in the same house. Moon and ketu (or rahu) will be in the same house.
Ex: on this particular eclipse, moon and rahu are in the same house. Sun and ketu are in the same house. That’s why it is called rahu-grastha Chandra grahana.
All these point to the fact that, rahu and ketu are the points in space where the path of the sun meets the path of the moon.
Rahu is the “ascending node” and ketu is the “descending Node”
Why did our ancestors track these points? Simple. It helped them to predict eclipses.
They also built stories around rahu and ketu. Like, rahu is the head of the demon and ketu is the body of the demon, which is to say that they are always at opposite sides to each other.
Also the stories say that rahu and ketu hate the sun and the moon. So they chase sun and moon and eat them, thus causing eclipses.
Obviously this is just a funny story signifying the scientific facts about rahu and ketu and eclipses.