31st of January 2018 has given, the Earthlings on certain parts of the planet, a rare combination of four celestial events all happening simultaneously, viz, a total lunar eclipse, a super moon, a blood moon and a blue moon. Except for the fact that it is not a super huge moon partially blue and partially red in colour. And nope, it is not bad at all to witness this even with naked eyes. This even does not indicate anything inauspicious. All it gives you is a dose of goose bumps for good.
So, let’s understand in simple terms without complicated mathematics, why it happens and what really happens beyond what meets our eyes. To make it easy to understand, let’s break the title of this post into a few simple terms and understand them individually.
Basics of celestial motion:
A massive celestial body has strong gravitational field. That makes other bodies sufficiently within its gravitational field get attracted towards its centre. However, the bodies that have high enough speed directed off centre, don’t really come down crashing to the centre. Rather they get accelerated in a way that the direction of motion gets continually bent towards the centre, like a stone tied to a string revolving around a pivot (or your finger, if you are the one swirling it). That path of periodic revolution is called an orbit. This is how the members of our solar system should be moving around each other. But the real motions happen with a slight difference. Instead of moving in a sweet simple circle, they move in individual elliptical orbits. That’s because every individual body is under the influence of multiple gravitational fields of the bodies around them. Thus they eventually find a balance and move accordingly.
Take two fixed points on a plane sheet of paper. Fix the two ends of a long string at those points, one end at each. Now, hold the string tight using a pencil and draw a curve so that the string remains taut throughout the curve. This curve is an ellipse. Every point on the curve has two distances, one from each fixed point. The total of these two distances for any point on the curve is the same as the total length of the string. This means that an ellipse is the tracing of all points on a two dimensional plane which are at a constant sum of distances from two fixed points respectively. There will be two points on this ellipse that are the farthest from each other, called the vertices. Similarly there will be two points that are the closest, called the co-vertices. The vertices form a line called the major axis and the covertices form minor axis. These axes are perpendicular to each other.
We all understand a circle fairly well. In a circle all points on the circle are at equal distance from the centre. That means whatever force moves a particle in a circle has a constant value along all the directions. When this force becomes directional as in space the huge gravitating bodies are not uniform, neither are they uniformly spaced around a centre, the revolving periodic motion (if any) of almost every celestial object follows an elliptical curve, called an orbit. And this object revolves directly around the object which has the strongest of the gravitational fields experienced by the revolving object. Instead of being at the centre of revolution, the driving object remains at one of the focal points or foci (fixed points of an ellipse as discussed above). Thus the driven (revolving) object periodically comes to a closest point (one vertex) as well as to a farthest point (the other vertex).
The Earth’s orbit around the Sun is elliptical with the Sun being at one of the fixed points called the focal points. The point on the major axis where Earth comes to the closest to the Sun (one of the fixed point) is called Perigee. The other vertex is then called Apogee, where Earth is at the farthest from the Sun. Earth crosses the apogee once every solar year (365 days on normal and 366 days for leap years) or sidereal year (365 days, 5 hours, 48 minutes, 45 seconds).
Similarly, for the moon, at its Perigee of its elliptical orbit around Earth, it comes the closest. Thus it also appears almost 14% bigger than its average sighting. This is called a Super Moon.
The super moon of March 19, 2011 (right), compared to a more average moon of December 20, 2010 (left), as viewed from Earth
As we have been common sensed it, a Full Moon comes once every month, i.e., 30 days consisting a lunar month. However, the concept of the lunar month is not purely applicable to the celestial lunar events. Rather, it is the synodic month (29.5 solar days). A synodic month is the period for the moon to come back to the same point along its orbit around Earth while all measurements are done with respect to the Sun. A lunar month is just an easy approximation to see things purely as they seem from Earth. In a lunar month the Moon has two phases as per its visibility to Earth, viz, waxing phase and waning phase. What really happens during these phases has nothing to do with Earth’s shadow casted due to the Sun. Rather, it is the amount of area of the moon that appears illuminated as seen from the earth.
Other than being in Earth’s shadow a few times (lunar eclipses), the Moon is always half exposed to direct solar radiation (sunlight). However as the Moon, during its revolution around Earth gradually comes between the Sun and the Earth, the part of its bright hemisphere visible from Earth gradually decreases. That’s the waning phase. It reaches its peak when the moon is totally in between the Sun and the Earth as seen from above the Sun, Earth and Moon projected on a two dimensional plane as they move celestially in space. As the only part of the moon facing the Earth is totally unlit by the Sun, the Moon having no light of its own, appears the darkest. Some call it a No Moon. But usually it is called a New Moon. Then comes the crescent moon and so on. When the moon begins to move out to a side (Earth’s side of course), it enters a waxing phase, means its bright hemisphere gets increasingly visible to us. This phase reaches its peak with a Full Moon. This means that considering Earth’s plane of revolution around the sun, as seen from above the plane, the moon is behind Earth. Thus its fully lit hemisphere is totally visible to the Earth.
Then why does it not fall in Earth’s shadow? Why not every full moon is a lunar eclipse?
Note carefully here, there are two simultaneous revolutions happening. One is that of the Earth around the Sun in a plane (say A). Another is the revolution of the Moon around the Earth in another plane (say B). Actually these planes A and B are inclined to each other by approximately 5 degrees. 5 degrees may seem very small but when projected over to the astronomical distances between the Earth and the Moon, it enables the moon to gain some height above the plane A, thus escaping Earth’s shadow by a consistently good margin to appear fully lit by the Sun (except for the lunar eclipses).
A Lunar eclipse happens when the Moon is captured by Earth’s shadow by the Sun during its revolution around Earth. This means, that The Sun (S), The Earth (E) and The Moon (M) are almost aligned in a straight line in the order S-E-M. This also means a full moon. Lunar eclipse is actually caused by Earth’s apparent motion when we treat the moon and the Sun as two fixed points. This apparent motion is simple. Apart from Earth revolving around the Sun, it also moves up and down crossing the line joining the Sun and the Moon. Lunar Eclipse happens during these crossings only if there is a Full Moon, i.e., the Earth is directly aligned almost exactly in between the Sun and the Earth. When not just almost but exact alignment happens we call it a perfect or Total Lunar Eclipse.
The phrase, “Blue Moon” might have come from a few sightings of the Moon where an otherwise Full Bright White Moon appeared with a bluish tinge. But that tinge is because of certain local atmospheric conditions where the components of white light must have been absorbed by the particles of the local atmosphere, leaving mostly the blue end of the spectrum (out of the VIBGYOR of white) free. Alternatively, it may have happened that the blue part of the spectrum of white light from the moon got scattered more than the rest of the spectrum, like the scattering of Sun light that makes the Sky sometimes appear blue. That’s really rare though for the moon to literally appear a bit blue. That’s why a rare event gets complimented by the phrase, “once in a Blue Moon”.
There is a less rare event when there are two Full moons in a lunar month. Well, it is rare because of us and not due to any celestial occasion. Remember that there is a difference between a lunar month and a synodic month? A lunar month consists of 30 solar days. A synodic month consists of 29.53 solar days. There actually is a Full Moon strictly once a synodic month. But as we take a lunar calendar month to be 30 solar days, there is an error we always ignore. As per our lunar calendar (even Gregorian), we have 12 months in a year. Every month has one lunar cycle. Thus in a year we expect 12 lunar cycles. The actual number of solar days for 12 lunar cycles is 29.53 X 12 = 354.36. Yet in our solar year we have 365.24 days. Thus every year we accumulate a lunar error of 365.24 – 354.36 = 10.88 solar days. Thus an extra full moon pops out of our approximated lunar calendar every 29.53/10.88 = 2.7 years (approx). Thus as we see, we can predict when the next blue moon will occur.
During a total lunar eclipse, especially during a Super Moon, the moon is totally shadowed by Earth. This means the Moon gets deprived of direct sunlight. Yet, due to Earth’s atmosphere, a part of the solar radiation near earth’s surface edge, gets slightly bent towards the Moon due to refraction. As we know from the dispersion of white light by a prism, the red end of the spectrum gets least deviated than the rest. This reddish orange part of the white sun light manages to reach the moon while the rest of the spectrum happens to miss it. This gives the moon a reddish or orange tinge. The actual tinge in any case depends upon the moon’s distance from Earth. As every colour from ROYGBIV takes increasingly narrower path enveloped by the red, between Moon’s perigee and apogee, its position during an eclipse determines which tinge of colour it takes and when.