This Sunday, May 15, 2022, there will be a total lunar eclipse visible throughout the national territory. The phenomenon actually starts on Sunday and goes into the early hours of Monday, May 16th.
The phenomenon will last for more than five hours and runs from 10:32 pm on Sunday to 3:51 am on Monday (Brasília time). Totality, the phase in which the Full Moon turns red (see detailed explanations below), runs from 00:28 to 01:54.
Shall we observe? (Cheering now for clear skies!). Before, how about understanding in detail what a total lunar eclipse is? See the text below.
Understanding a total lunar eclipse
I) A geometric “whim”
Solar and lunar eclipses occur when there is alignment between the Sun, Earth and Moon, a true geometric quirk. And alignment of three bodies (in strict terms of their centers) can seem like something easy to happen. But in practice this is not so. Two points, you know, can always be joined by a single line. But a third point, to be on the same line that already contains the two previous points, has to be on the same plane that contains that line and, obviously, on the line. Can you visualize the idea?
The following illustration, which shows a box in the shape of a parallelepiped and containing three points (in fact, three spheres), one orange, another blue and a third violet, will help a lot in the reasoning.
Depending on the point of view, three points appear aligned. But they may not be.
Image: Dulcidio Braz Jr/Física na Veia
Looking at the picture above, would you say that the three points (orange, blue and violet) are aligned? Watch carefully. And think carefully before answering.
If you look over the top of the box, in a direction perpendicular to the top face, you will say yes, the points are aligned. But if you look at it from the side, it will say that the blue and violet dots are aligned and, strangely, the orange dot seems to be gone! And if you look straight into the box, you’ll find that the violet dot is in the same vertical plane as the other two dots, but it’s not in the same horizontal plane. Therefore, it is not aligned with the other two!
In eclipses, the geometric problem is more or less the same because the three stars (Sun, Earth and Moon) must be perfectly aligned, wherever you look. But, as the plane of the Earth’s orbit around the Sun does not coincide with the plane of the Moon’s orbit around the Earth, everything is more complicated because there will only be a real (or almost) alignment of the three stars when they are contained in a direction that corresponds to the intersection of the planes of the Earth’s orbits around the Sun and the Moon around the Earth. We may even find that Sun/Earth/Moon are aligned depending on the point of observation. But perfect (or approximate) alignment only occurs in this preferred direction known as the Line of Nodes.
Complicated, right? But once I appeal to the drawing! Check out the idea below. This illustration, like others in this text, is deliberately out of scale.
The Moon’s orbit around the Earth is in a plane inclined to the plane of the Earth’s orbit around the Sun. This makes the alignment of the three stars something much rarer.
Image: Dulcidio Braz Jr/Física na Veia
If the Earth’s orbits around the Sun and the Moon around the Earth were in the same plane, we would have solar (with the Moon obstructing the Sun) and lunar eclipses (with the Earth obstructing the light that illuminates the Moon) every 14 days. approximately and eclipses would be commonplace phenomena. But the subtle inclination of 5.2 degrees between the orbital planes mentioned and above represented in the illustration make eclipses phenomena much rarer and that can only occur in the Line of Nodes.
II) Play of light and shadow: an optical “whim”
The Sun, the great beacon of the Solar System, illuminates all the other stars in the system. And behind the stars illuminated by sunlight, a shadow cone also called umbra forms. In fact, being more detailed and scientifically correct, as the Sun is not a point but an extensive sphere, behind the illuminated opaque star a region of penumbra must also form beyond the shadow.
To better understand this idea, we appeal again to an illustration, purposely out of scale. Note that the star illuminated by the Sun is and behind which a shadow (or umbra) and penumbra will form is the Earth.
As the Sun is an extensive source of light, both the umbra and the penumbra form behind the Earth.
Image: Dulcidio Braz Jr/Física na Veia
Note that I have highlighted the rays of light that depart from opposite extreme points of the Sun and touch opposite extreme points of the Earth. The Earth was treated as an opaque sphere that light cannot pass through. In this way, two distinct regions are formed behind the Earth:
A cone of shadow, also known as an umbra, where there should be no light at all. A region of “half light”, partially illuminated by the Sun, called a penumbra.
Now think: the Full Moon, orbiting the Earth (dashed green line in the illustration below), will cross the penumbra and then enter the umbra within which it will stay for a while. Then it will gradually leave the umbra and go through the penumbra on the opposite side of the umbra. What can we expect in the observable aspect of the Full Moon as it passes through these three regions?
The Moon will make the path (dashed green) around the Earth, passing through the penumbra, through the umbra (reddish) and again through the penumbra.
Image: Dulcidio Braz Jr/Física na Veia
It is to be expected that, when entering the Earth’s penumbra, the Full Moon will be less bright because it will be receiving less light from the Sun. This is exactly what happens, although it is not so easy to observe without instruments capable of measuring the intensity of sunlight reflected by the Full Moon.
But, as the Full Moon penetrates the Earth’s umbra, it is expected that it will have a dark “bite” aspect that gradually increases until it disappears completely when completely immersed in the Earth’s umbra cone, the apex of the total eclipse.
But that’s not exactly what will happen because we have here another cosmic whim. The Moon is being “bitten” by the Earth’s umbra but does not disappear completely in the totality of the eclipse. Check it out below.
III) The Earth is not just an opaque sphere: another “whim” of nature
The Earth is a sphere with good approximation. And it’s opaque. But it’s not just that. It has a thin, semi-transparent gaseous layer that surrounds it, which we know as the atmosphere. The atmosphere refracts white sunlight made up of all visible colors, from red to violet. This refracted light undergoes a deflection and therefore a little light will penetrate the region that should be completely without light, that is, in the umbra. And, as the rays touch the atmosphere, they end up traveling a longer path than when they fall perpendicular to it. In this way, light is scattered and absorbed.
And because it is a longer path, only the reddish and orange tones of the solar spectrum are left. It is this red-orange light that will “dye” the umbra cone region in these warmer tones. The umbra, therefore, ceases to be, literally, a true shadow as it starts to contain a faint orange-red light. And that’s why the Full Moon doesn’t disappear while inside the Earth’s umbra cone.
Contrary to what we could imagine, in the totality of the eclipse The Full Moon should have a reddish color, a tone similar to wet brick and that depending on the conditions of local atmospheric pollution.
It is precisely this optical subtlety that will be responsible for what some like to call the “Blood Moon”! But there’s no blood. There is indeed selective lighting, in shades of red-orange, thanks to the refraction and scattering and absorption of white sunlight in the atmosphere. Another beautiful whim of nature that makes lunar eclipses even more exciting.
Check out below montage with images I captured of the lunar eclipse of December 21, 2010, from beginning to totality. The rest of the phenomenon, on this date, cannot be observed from here in Brazil because it was late in the morning and the sun was up. But this Sunday’s lunar eclipse can be seen in full from here in Brazil.
From the sequence of my images you can get an idea of the temporal evolution of the visual aspect of the Full Moon during the eclipse.
Time evolution of the total lunar eclipse of 12/21/2010, from beginning to totality
Image: Dulcidio Braz Jr/ Physics in the Vein
Total Lunar Eclipse Timeline for May 16, 2022
Temporal evolution of the eclipse, with the Full Moon crossing the Earth’s penumbra and umbra
Image: Dulcidio Braz Jr/ Physics in the Vein
The infographic above, which I made with data from astronomers Paulo G. Varella and Regina A. Atulim, shows the temporal evolution of this Sunday’s lunar eclipse and will help you with your observations. Check out the main moments:
22:32: beginning of the phenomenon, when the Full Moon begins to enter the Earth’s penumbra; 23:27: the Full Moon will be completely inside the Earth’s penumbra; 00:28: beginning of totality, when the Full Moon will be completely inside the Earth’s umbra cone and will be very reddish;01:11: middle of the eclipse, still in totality;01:54: the Full Moon begins to leave the Earth’s umbra;02:55: the Full Moon leaves the Earth’s umbra completely;03:51: end of the phenomenon, when the Moon Full comes out of the Earth’s penumbra.
I will try to observe and photograph the phenomenon. I post possible astrophotos in real time on my social networks (@dulcidio on Twitter, and @prof_dulcidio on Instagram) and, who knows, right here at Physics in the vein throughout Monday.
Good remarks. Hugs from Prof. Sweetie! And Physics (and Astronomy) in the veins!
