Eclipses – an overview

By Parth Ghumare

What is it that not just astronomers, but all populace
looks forward to, happens twice a year, and is one of the most beautiful things
to experience? You guessed it right – An eclipse!

Solar Eclipse by Stargazing MumbaiWe have heard about the two types of eclipses – solar and lunar; today, lets take a deeper look at how things work and why we say that eclipses are not to be ‘seen’ but to be ‘experienced’! We are lucky that our sun and moon have highly similar apparent sizes, which is the sole reason we get such beautiful eclipses. A Solar Eclipse occurs when the moon comes between the Earth and the Sun, blocking it completely or partially. Lunar Eclipse occurs when Earth comes in between the Sun and the Moon; often making it appear red or sometimes, completely disappear. A major difference in the two eclipses is that whenever you see a solar eclipse from a given place, it does not necessarily mean it’ll be visible from all the other places the Sun is visible from. Whereas, a lunar eclipse is visible from all the places the moon is visible from, at a given instant. A lunar eclipse always occurs on a full moon – because the three bodies are aligned in a straight line. 

The three different terms that need to be explained while we speak on the topic are – Occultations, Transits an Eclipses. Occultation is the event when a bigger body completely covers a smaller body (thus making the smaller body disappear) while passing from in front of it. A transit is where a smaller body passes from in front of a larger body – thus both the bodies are visible in a transit. And finally, an eclipse is an event where a body casts its shadow onto another body – making is either
completely/partially invisible or appear out of normal. Thus, in case of an eclipse, the observer need not be a part of the system. For example, eclipses can also be seen of Jupiter’s moons (where the earth is not a part of the system). An interesting thing to note here is, when we say a total solar ‘eclipse’, its actually an occultation of the Sun by the Moon. And an annular solar ‘eclipse’, is the transit of the Moon on the Sun.

A person may wonder that even though we have new and full moon every month, we do not get to see eclipses that often. The reason being that during the new/full moons, the three bodies do not truly lie in a
straight line. This is because the plane in which Earth revolves around the Sun, and the plane in which the Moon revolves around the Earth – are not same. The two planes are inclined with each other at an angle of 5.145 deg. Every month, when the moon crosses the intersection of planes- going below from above (waning), it’s called as the descending node. And when the moon goes back up (waxing), crossing the intersection, it’s called as the ascending node. Rahu and Ketu (from Indian Mythology) are believed to have discovered the points where these nodes fall exactly on the line of Earth and the Sun and an
eclipse occurs (twice a year).

An ecliptic is an apparent path of the Sun on the celestial sphere. The name ‘ecliptic’ is derived from the word eclipse. This is because all the eclipses occur on the ecliptic. Umbra and Penumbra are the two categories we divide shadows into. Whenever an object is casting its shadow on a screen far away, its shadows’ edges become blurry. This creates some zones which are completely dark and some partially dark. When moon casts its shadow on Earth, the completely dark regions (or the inner shadow) is called as the Umbra and partially dark regions, or the outer shadows are called as Penumbra. So, during a solar eclipse, there always exists an umbral and a penumbral zone. But the eclipse is always referred to as a complete or partial solar eclipse, and never an umbral or penumbral eclipse. A complete solar eclipse is visible from umbra and this region is called as the totality belt. The thickness of this totality belt depends on the distance between the Earth and the moon during the eclipse. On the other hand, from the penumbra, the moon does not completely cover the Sun and just blocks it partially making it a partial solar eclipse.

 During a solar eclipse, we don’t notice any dimming until around 70% of blocked Sun (because our eyes compensate for that). But after that, we start noticing the light levels falling slowly. And after 80-90 % of covered Sun, when it is almost dark, the atmosphere becomes absolutely electrifying! If the eclipse is happening in mid-day, it’s even better. That’s because, a crescent Sun is present in the sky and the no. of photons is less but the brightness of the Sun is there in the sky. Which is not like the evening Sun, where you have hues of red and blue; instead the environment feels silverish! And if you’re lucky, you get to see the prominence (which are bright during the solar maxima). They’re mostly H-alpha, red in color. All of this is a very fast process. The Sun gets thinner and thinner, and finally when almost all it is covered and just a small thin line is left, what you see is the chromosphere- which is about 2000 kms above the photosphere. It has a red color, and is majorly consisting of Helium. The most beautiful moment, a diamond ring visible during a total Solar eclipse, is just after the moon starts showing the sun again. It lasts for less than a second!

  Prior to the diamond ring, one can see shimmering light patterns passing on the ground. These are called as shadow-bands. People in earlier times have often associated this phenomenon with snakes (unexplained things led to superstitions). The reason behind these wavy patterns is very basic. The light from the crescent Sun passes through the bubbling atmosphere, diffracting/bending it in different directions and making patterns on the ground. 

Another thing we get to see only during eclipses is the solar corona extending in different directions. A beautiful sight as the moon has covered the Sun completely. When there are large number of sun spots (which are magnetic), the corona (which too, is magnetic) is almost spherical. But during solar minima, when the sun spots are less, the corona looks stretched in opposite directions. The temperature of corona is about a million degrees which is much, much higher than the photosphere which is about 6000 degrees. The reason for this is yet unknown and the theories that have come up, are not that satisfactory. But why is it that even though it is so hot we do not see corona unless there is an eclipse? The reason is the low density (thus very few photons are present which are only visible when an eclipse blocks the photons from the Sun).

It is too long for the next solar eclipse to be visible from India, but lunar eclipses keep coming around! So, sit tight, and when the next eclipse is around, be prepared to ‘experience’ it!

 

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