Principle behind Solar and Lunar Eclipses!

  

    Eclipses are one of the cosmological events that occur due to the celestial bodies. An eclipse is one, in which the light from one celestial body is blocked partially or fully by another celestial body, to the observer. The eclipses are mainly caused by the moons of the planets. A planet with more moons has more eclipses. On Earth, the eclipse occurs rarely because the earth has only one moon. So on earth, it is observed as a rare cosmological event. Now let’s look at the types of eclipses and how does it occur on earth.

Difference between a solar and lunar eclipse

    Generally, the moon, which is causing all the eclipses, can make it in two different forms, namely

• Solar Eclipse
• Lunar eclipse 

     Both types of eclipses are caused by the shadow effect. A Solar Eclipse occurs, when the moon passes between the sun and the earth. Here the moon’s shadow is made to fall on the earth thereby blocking the light from the sun. Whereas, a lunar eclipse occurs, when the earth passes between the sun and the moon. Now the earth’s shadow is made to fall on the moon, thereby the moon is blocked from receiving the sunlight. Now let’s look at the detailed process of the solar and lunar eclipse.

 

Solar Eclipse

Lunar Eclipse

Solar eclipse

   The sun is the primary source for us. It gives us all the energy we need, in the form of light and heat. The light emitted by the sun is continuously received by us. But sometimes on an average of 18 months, the light is obscured by the known celestial body called the moon. In those days, the moon passes between the earth and the sun, thereby casting its shadow over the earth causing a solar eclipse. This solar eclipse occurs very rarely over a period and can appear in different forms. These forms can be represented as,

• Total solar eclipse
• Annular solar eclipse
• Partial solar eclipse
• Hybrid solar eclipse (a combination of the annular and total solar eclipse)

  All these eclipses represent the solar eclipse. But, how the moon obscures the sunlight to the observer, matters. 

Total solar eclipse

    A total solar eclipse is one, in which the moon completely hides the sun for a particular period (varies from a few seconds to 7.5 minutes). In this period, no sunlight will be received by the observer on the specific part of the earth. So that part of the earth becomes completely dark pretending it as a night. Usually, a very small portion of the earth experiences this total solar eclipse. Many organisms think of it as an original night and start to sound and carry out different activities that they originally do at night.

Total Solar Eclipse

    As the total solar eclipse doesn’t really darken all the part of the earth facing the sun dramatically, only a small portion of the earth darkens as night, whereas the other part pretends as an evening time. This occurs due to the geometry of the shadow effect which is given as umbra, penumbra and antumbra regions. Generally, when an object cast a shadow by a light source three shadows are formed as named before. This shadow forming depends upon the direction of the light given out from the source. Let’s see how these shadows are formed by considering different instances. Each different instance causes different shadows and different eclipses.

Instance -1

   At the first instance let’s consider the fundamental three objects to form a shadow, namely a light source, an object to cast a shadow, and an observer to experience the shadow effect. Let’s arrange the objects as shown in the figure.

Light is fully blocked to the viewer

    The important point to be considered is, at this instance the light source is punctual and smaller than the object, casting the shadow. Punctual light means the light is made to fall over the object at a specific point of an area (i.e., light forms a certain degree and not for 360°). When the light falls on the object it casts a shadow in an outward “V” shape. The reason for this V shape shadow depends upon the amount of light obscured by the object. If the object was too far from the light source, the shadow becomes too narrow as it obscures only a minimum light. When the object is near to the light source, it obscures more amount of light from the source and the shadow becomes enlarged. When an observer is standing inside the shadow region he will not be able to see the light source. In this condition, the light is completely obscured from the observer. This shadowy part, where the observer is standing is called the umbra region, which means umbrella, which completely protects us from sunlight. When we are experiencing a total solar eclipse, we will be standing in the umbra region. The umbra region usually covers a small part during a total solar eclipse. But a total solar eclipse doesn’t have only the umbra region. It has penumbra and may have antumbra too, which will be seen in the below instances. 

Instance-2 (Total and Partial solar eclipse)

   In this instance let’s consider the same setup we have considered before, but with a different thing, that the punctual light is replaced by a non-punctual light. A non-punctual light is a type of light that does not destine its light to a specific degree. It covers the light over 360°, which means it has a larger surface area.

Light is partially blocked to the viewer

   When the light from the non-punctual source falls on the object, it casts a shadow as same as before instance. But as the light has a larger surface area, some of the light waves travel into some part of the shadow region projected by the object. So at this condition, some of the light from the source enters into the shadow and causes the shadow to get lighter causing the penumbra region. The nearer the object becomes to the light source, the larger the penumbral region becomes, and far away the object becomes, the smaller the penumbral region becomes. The penumbral region usually covers a large surface area during an eclipse time. The penumbra region starts at the verge of the umbra region and proceeds over a certain distance. The penumbral region nearer to the umbra region will be slightly darker which is less than the umbra, and goes on decreasing as it proceeds over the outer part. Now the person who is standing in the umbra region will experience a total eclipse, while the person standing in the penumbra region will experience a partial eclipse.

Instance-3 (Annular and Partial eclipse)

   This instance rarely occurs over the period. This instance produces a rare event of shadow called the antumbra. Consider the event below, where the light source is bigger in diameter than the object casting the shadow. In such an event, the light passes through the maximum area of the shadow casted by the object. So now the shadow becomes inward “V” shaped and causes the shadow to be pinpointed and increasing again as shown in the figure.

The viewer sees the ring of light effect

   So when standing at the antumbra region the observer will see the annular solar eclipse and when standing at the penumbral region the observer will see the partial eclipse.

Partial Solar Eclipse

     As seen above, a partial solar eclipse can be observed only when we are standing in the penumbra region. In this effect, the moon does not completely cover the sun, and some amount of light gets to the earth. So when a partial solar eclipse is occurring the observer will experience a light shadow effect on the penumbra region, he/ she are standing. The observer experience only a minimal amount of sunlight and the maximum amount of sun is hidden from the view. The principle of the partial solar eclipse can be observed from instance 2.

Partial Solar Eclipse

Annular Solar Eclipse

    An annular solar eclipse can be observed when the observer is standing at the antumbra region. This type of eclipse merely resembles the total eclipse but the size of the moon does not fit generally into the size of the sun. So in this situation, the moon will not be very big to cover the sun. The reason causing this type of eclipse is the distance between the earth and the moon, which you can observe in the figure. So the eclipse will look like the total eclipse with a light called “ring of fire” surrounding the moon. The principle of an annular solar eclipse can be observed from instance 3.

Credit (Annular Solar Eclipse): By Rehman Abubakr, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=85283761

Hybrid Solar eclipse

  A hybrid solar eclipse is a rare type of eclipse which is a combination of both annular and total eclipse. Some people in the eclipse path experience an annular eclipse while some other people in the eclipse path experience a total solar eclipse. This happens due to the curvature of the earth and the distance travelled by the shadow. As shown in the figure, when the moon’s shadow starts to fall on the earth in the first place, the shadow has to travel a long distance which makes it to appear as an annular solar eclipse to the observer. As the orbit of the moon continues the shadow of the moon takes lesser time to reach earth which makes it to appear as a total solar eclipse to the observer. This combination of both the annular and the total eclipse is called a hybrid eclipse. 

Hybrid Solar Eclipse

Lunar Eclipse

   A lunar eclipse is one, which happens when the moon passes into the earth shadow. As seen in the solar eclipse, the region of umbra and Penumbra is also present on the lunar eclipse. There is no antumbra region in the lunar eclipse due to the diameter of the earth. There is also various type of lunar eclipse depending upon how the moon passes into the earth’s shadow. They are named as,

• Total Lunar Eclipse
• Partial Lunar eclipse.
• Penumbral Lunar Eclipse

   An annular and hybrid lunar eclipse does not occur, as the diameter of the earth is bigger. Because the umbra region made by the earth is itself greater than the moon’s size. So a separate annular eclipse or the annular eclipse in the hybrid eclipse does not occur on the moon. But as an alternative, there will be a penumbral lunar eclipse.

 

Total Lunar Eclipse

    Total Lunar eclipse is a type of eclipse in which the moon gets completely engulfed into the earth’s shadow. When the moon is in its orbit around the earth and getting to be eclipsed, the moon first enters into the earth’s penumbra region as shown in the figure. On the way through the penumbral region, the moon’s becomes slightly dimmed due to the partial shadow of the earth. Then the moon passes into the earth’s umbra region where it is completely hidden from receiving the sunlight. So it darkens completely. But if we look at a total lunar eclipse we will be able to see the moon, even if it was in the earth’s umbra region; but with a different look. The different look makes the moon to appear in red in colour, due to the sunlight scattering in the earth’s atmosphere. As the moon starts to move into the earth’s umbra region, this red colour scattering takes place.

 

Side View

Front View

    But there is another riddle, in which the moon keeps on orbiting the earth once in every month. So it should be crossing the earth’s shadow every time. Then why the lunar eclipse is not happening in every month? This is due to the moon’s orbiting tilt. The moon orbit is tilted to 5° with respect to the earth’s equator and this orbit varies with respect to our sun known as precession. So that even though the moon crosses the earth’s shadow it rarely collides with the shadow. This orbital tilt is also responsible for causing other types of lunar eclipse like partial and penumbral lunar eclipses. Suppose if the moon orbit is kept still in straight to the earth’s equator, then only a total lunar eclipse will be occurring every month as the moon crosses the earth’s umbra region.

Moon's orbital tilt with respect to earth's equator

 

Partial Lunar Eclipse

Credit (Partial Lunar Eclipse): By Vxfour11 - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=92494012

   A Partial Lunar Eclipse occurs when the moon slightly passes into the earth’s umbra region. So, some part of the moon that is inside the earth’s umbra region becomes red in colour due to the atmospheric scattering while the remaining part will be slightly dimmed, as it is in the earth’s penumbral region. It is the same as the partial solar eclipse where the moon’s shadow obscures some of the light from the sun to the earth. But in the lunar eclipse, some amount of light (reflected sunlight) from the moon is obscured by the earth’s shadow. The partial Lunar eclipse is shown in the below figure.

Side View

Front View

Penumbral Lunar Eclipse

Credit (Penumbral Lunar Eclipse): By Giuseppe Donatiello from Oria (Brindisi), Italy - Penumbral Lunar Eclipse on January 10, 2020, CC0, https://commons.wikimedia.org/w/index.php?curid=85799543

    In this type of lunar eclipse, the moon passes only into the earth’s penumbral region and does not pass into the earth’s umbra region. So passing into the earth’s penumbral region makes it to darken slightly. In this type of eclipse, the change in the effect of the moon cannot be easily noticed by us, as the brightness is reduced a little bit. The process of this type of eclipse is shown below,

 

Side View

Front View