Category: AstroPhotography

What is a Lunar Eclipse?

What is a Lunar Eclipse?

There’s flood of images of blood moon on social media since 31st Jan. The popular term being used for the phenomenon is “Super Blue Blood Moon”.  That phenomenon was nothing but a Total Lunar Eclipse. Now we will understand what happens and how it happens. First of all lets understand what is a lunar eclipse and then we will discuss its types.

What is a Lunar Eclipse?

The most common definition we hear is, when the sun earth and moon are aligned in the same line/ when the moon enters the Earth’s shadow. That all is fine, but the most natural question that pops up in our mind is; then why doesn’t eclipse happen every month? As it takes about 27.3 days for one complete revolution around Earth shouldn’t the sun, moon and earth line up twice in a month which would lead to one total solar eclipse and one total lunar eclipse every month? But that doesn’t happen right? Eclipses doesn’t happen that often. Well that’s because the Moon’s orbital plane is inclined by about 5.1° to the ecliptic plane (the plane in which Earth orbits around sun). So Eclipses occur only when the moon passes through that exact point where these two planes coincide. And this doesn’t happen that often.

By Peter Sobchak - Own work, CC BY-SA 4.0,
The Moon’s orbital plane differs a little than the Earth’s orbital plane.

Types of Lunar Eclipses

Umbra – The region where no sunlight reaches.
Penumbra – Region where sunlight reaches partially.

Penumbral Lunar Eclipse

Penumbral Lunar Eclipse happens when the moon passes through the penumbra region of the Earth’s shadow. This type of eclipse is usually very hard to notice as there is only slight change in the brightness of moon.

Partial Lunar Eclipse

When part of the moon passes through Umbra we get a partial Lunar eclipse. It is easy to notice as there is significant drop in Moon’s brightness. It might happen once or twice in a year.

Total Lunar Eclipse

Now this is something which doesn’t happen quite often. Total Lunar Eclipse happens when the moon passes through the umbra i.e all the sunlight is blocked by Earth. This event happened recently on 31st January 2018.

The types of Eclipses. Image credits: Addison Wesley.

So what was so special about the Lunar eclipse of 31st January? And why was it called Super Blue Blood Moon? Lets understand it word by word.

Super moon

The moon’s orbit around earth is not exact circle, its an ellipse. So there must be two points; when the moon is farthest from Earth and when it is closest to Earth. The farthest point is called apogee (406,300 km) and closest point is called perigee (356,700 km). Super moon is when its a full moon at perigee and its called super moon because it looks a little bit bigger than usual.

Comparison of Moon at Apogee and Perigee.

Blue Moon

This has nothing to do with the colour blue. When two full moons appear in one month then the second full moon of the month is called a blue moon! There is a phrase “once in a blue moon” so its evident that this is something rare. Blue moon happens once in 2 to 3 years. And whats more rare is a double blue moon which is two months in a year which has two full moons each! And this happens 3-5 times in a century!! The last double blue moon happened in 1999 and next will happen in 2037!

Blood Moon

Blood moon sounds so surreal! When a total lunar eclipse occurs all the sunlight is blocked by earth and moon is in the umbra region. Now Earth has a thick layer of atmosphere. Rayleigh’s scattering of sunlight in atmosphere is what gives sky its blue colour. The atmosphere scatters blue light more efficiently than red light (In better terms, the scattering is inversely proportional to the fourth power of wavelength). So the Red light passes through the atmosphere without getting scattered and enters the umbra region onto the moon! That’s what gives the moon a reddish hue! Hence the name blood moon.

The atmosphere bends red light towards the umbra region.
Image credits:

Super Blue Blood Moon

Now what are the odds of super moon, blue moon and blood moon happening at the same time !! That’s why people were hyped for the Super Blue Blood Moon!!! I got to watch the eclipse and also clicked images at different stages of eclipse. The first image is when the totality occurred, the moon was visibly red even with naked eyes. And then gradually it turned white into a full moon! Isn’t it beautiful!?

Composite image of different stages of Total Lunar Eclipse.

The next Blue Blood Moon will happen on 31st December 2028 and the next Super Blue Blood Moon will happen on 31st January 2037! So if you missed this event you need to have a lot of patience.