Penumbral Lunar Eclipse Prajval Shastri

The Penumbral Lunar Eclipse 10 January 2020

A Factsheet for Everyoneby

Prajval Shastri

Penumbral Lunar Eclipse of 10 January 2020 21. When and why? 22. Did you know? 23. Additional FAQs 2

a. Why do lunar eclipses occur? 2

b. Why does a lunar eclipse not occur during every full moon? 3

c. Are Lunar Eclipses Safe to View? 3

d. How is a Lunar Eclipse different from a Solar Eclipse? 3

e. What are Total, Partial and Penumbral Lunar Eclipses? 4

f. Why does the penumbral shadow occur and how can this be demonstrated? 4

g. Why does the moon appear copper-coloured during a total eclipse? 5

h. Are Rahu and Ketu scientific terms? 6

i. What is meant by the term ‘Wolf moon’? 6

4. More detailed authentic information on the eclipse of 10 Jan 2020: 6

Page� of �1 6

Except where images are sourced from elsewhere as specified or otherwise noted the content is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License�

Penumbral Lunar Eclipse Prajval Shastri

Penumbral Lunar Eclipse of 10 January 2020

1. When and why?

A penumbral lunar eclipse will occur on the 10 January 2020, lasting about four hours, which will be visible throughout South Asia and other parts of the world. Lunar eclipses occur on some (but not all) full-moon nights when the Sun, earth and moon exactly line up. Since the Sun is an extended source of light rather than a point source, the shadow of the earth that it casts has both a dark central part (the umbra) and a lighter surrounding shadow (the penumbra). During a penumbral eclipse, the moon only passes through the penumbral part of the shadow. Therefore the visible effect will be a very slight dimming of the full moon, not easily discernible by naked eye. This is different from a partial or total lunar eclipse: in the latter cases the moon passes through the darker umbral part of earth’s shadow, and therefore the eclipsing by the earth’s shadow is very clearly visible. If the whole of the moon passes through the umbral shadow, a total lunar eclipse results, and the moon acquires a copper hue because it is only lit by the predominantly red light from the sun that is refracted onto it by the earth’s atmosphere.Quick Facts:

•Beginning of the penumbral lunar eclipse:  Fri, 10 Jan 2020, 22:37 IST•Maximum of the eclipse: Sat, 11 Jan 2020, 00:40. IST•End of the of the penumbral lunar eclipse: Sat, 11 Jan 2020, 02:42 IST

•23:25 IST on 10Jan & 01:55 IST on 11Jan are instances when about 50% of the lunar disc will be within the penumbral shadow  •It is COMPLETELY SAFE for ANYONE to view the moon during the eclipse.

•The moon will appear like any full moon, only very slightly dimmer during the duration of the eclipse. •The dimming is not sufficient to be easily discernible by the naked eye.

2. Did you know?

• The moon is at an average distance of about 384,400 km from the earth. • Therefore light takes about 1.3seconds to travel between the moon and the earth.• The difference between the closest and farthest moon’s distance from earth is about 50,000km.• The moon revolves around the earth at a speed of about 1km/second.• The earth’s diameter is about four times larger than the diameter of the moon• The Suns diameter is about 109 times that of the earth and about 400 times that of the moon

3. Additional FAQs

a. Why do lunar eclipses occur? A lunar eclipse occurs when the bright moon visible on a full moon night is obscured temporarily because the Sun casts a shadow of the earth on the moon. A lunar eclipse can only occur during full moon, because that is when the side of the moon that is lit up by the Sun is facing us on earth, and therefore the Sun is “behind” the earth and the earth can cast its shadow on the moon.

Page� of �2 6

Penumbral Lunar Eclipse Prajval Shastri

b. Why does a lunar eclipse not occur during every full moon? We do not see a lunar eclipse at every full moon. This is because the orbit of the moon around the earth is slightly tilted (by 5º with respect to the orbit of the earth around the sun, and therefore during most full moons, the moon’s position is just outside the earth’s shadow. It is only during some full moons, when the Sun, earth and Moon are perfectly aligned, that the moon will pass through the earth’s shadow and a total eclipse of the moon can occur. In a calendar year, i.e., during one orbit of the earth around the Sun, typically two lunar eclipses will occur.

c. Are Lunar Eclipses Safe to View? Absolutely! During a lunar eclipse, since the earth’s shadow falls on the moon, the moon reflects less light than a normal full moon. Therefore, lunar eclipses, whether total, partial or penumbral, are completely safe to view, both directly with the unaided eye as well as through binoculars and telescopes.

d. How is a Lunar Eclipse different from a Solar Eclipse? A solar eclipse occurs when the Sun casts a shadow of the moon on earth, so that as seen from the earth, the Sun is eclipsed or hidden by the moon. On the other hand, a lunar eclipse occurs when the Sun casts the earth’s shadow on the moon. This is why solar eclipses only occur during New Moon (when the moon is between the Sun and earth), whereas lunar eclipses only occur during full moon (when the earth is between the Sun and the Moon). However, we do not see a solar eclipse at every New Moon for the same reason that we do not see a total lunar eclipse at every Full Moon: the orbit of the moon around the earth is tilted slightly (about 5º) to the orbit of the earth around the sun, and so the Sun, Moon and earth align in exactly a straight line only during some New Moons. Furthermore, some of these exact alignments may result in an annular solar eclipse and not a total solar eclipse. The reason for this is the same as the reason for why we sometimes see a ‘super moon’, namely, that the moon’s orbit around the earth is elliptical and not circular. If the alignment of the Sun, moon and earth occur close to moon’s apogee (position where it is farthest from the earth), then the apparent size of the moon in the sky will be smaller, and will not obscure the full disc of the sun but only its central part. The outer ring-shaped edge of the Sun will still be visible - this is called an annular solar eclipse. This is often referred to as being ‘in the antumbra of the moon’s shadow.

Page� of �3 6

A diagram (not to scale) showing the moon’s orbit around the earth at various positions of the earth during its orbit around the sun. All the four configurations shown correspond to full moon, since the face of the moon that is facing and lit up by the sun is also facing the earth. However, in cases A and C, the moon, earth and Sun are not in a straight line, and so on these full moons, the earth’s shadow will not fall on the moon and a lunar eclipse will not occur. On the other hand in cases B and D, all three bodies are in a straight line and therefore a lunar

Penumbral Lunar Eclipse Prajval Shastri

When a solar eclipse occurs, those on earth who are located in the umbral region of the shadow see a total eclipse of the Sun, and those in the penumbral region see a partial eclipse (i.e., the Sun with a piece “bitten off”.) A unique feature of a total lunar eclipse is that, the moon does not disappear from sight, but instead appears to acquire a faint copper colour. While some care needs to be taken while viewing solar eclipses, lunar eclipses are completely safe to view. Care needs to be taken during solar eclipses because, just as it is not safe for the eyes to stare at the Sun on a regular day, it is also not safe to stare at the Sun during an eclipse. Even though the Sun is eclipsed by the moon, which reduces the light we see from the Sun, the amount of light is still a lot during the partial phase of the eclipse, and could be injurious to the eyes. Therefore solar eclipses should be viewed through safety glasses that are certified as safe for viewing the Sun. Solar eclipses are not rarer than lunar eclipses. However, because the shadow cast by the moon on earth is relatively small, a solar eclipse is visible only from a small fraction of the earth. The size of the umbra on earth could be at most about a 1000 km in size. If we stand on the moon during a solar eclipse we would see the shadow of the moon move across the earth, and the penumbral part of the shadow would be lighter and surround the umbral part, but would be much smaller than the surface of the earth. On the other hand, during a lunar eclipse, the shadow of the earth cast on the moon is relatively large, and the whole moon is bathed in shadow during both the penumbral phase and the phase of totality. Therefore the lunar eclipse will be visible from all locations that are on the night side of the earth during the period of the eclipse.

e. What are Total, Partial and Penumbral Lunar Eclipses? The phase of totality is when the whole of the moon is within the earth’s shadow. Before and after the phase of totality, a partial eclipse occurs. This is when the Sun, earth and Moon are not quite exactly aligned, and therefore the shadow of the earth on the moon appears to cut a piece out of the moon. Before and after the phases of the partial eclipse are the phases of the penumbral eclipses. The penumbra is a ‘lighter’ shadow of the earth that surrounds the main shadow, which is also called umbra. The phase of totality is therefore also referred to as the umbral phase. A penumbral shadow is formed whenever the source of light casting the shadow (in this case the Sun) is not a point source but an extended source, and it surrounds the main shadow or umbra. The penumbral phase of a lunar eclipse occurs when the moon passes through the penumbral shadow of the earth. The effect of the penumbral shadow is that the brightness of the full moon is very slightly dimmed overall, an effect that is not easily discernible by mere naked-eye observations. For a given lunar eclipse, some regions on earth may only see the partial and penumbral phases of the eclipse, as happened in India during the lunar eclipse of August 2017, and other regions may only see the penumbral phase as will happen on the night of the 10th January 2020. The lunar eclipse of January 2018 was a total one when all of South Asia saw a “Copper” coloured moon.

f. Why does the penumbral shadow occur and how can this be demonstrated? Penumbral shadows occur if the source of light is extended. This phenomenon can be easily demonstrated with a torch light from a cell phone. Use the cell phone to cast a shadow of a small dense object (such as a pencil) on a plane light-coloured surface, e.g., a wall or table top. Step1: Hold the light (cell phone) about 20cm or more from the surface, and the pencil very close (a few cm) to the surface so that a shadow of the pencil is cast on the surface. Notice that the shadow of the pencil has a sharp outline. Step 2: Now keep the source of light in the same position as before and move the pencil away from the surface and much closer to the light while keeping the shadow of the pencil at roughly the same position. Notice that the shadow of the pencil becomes more blurred. A careful look will reveal a dark shadow surrounded by a much lighter “narrower” shadow. The latter is the penumbra. If the light were a point source, such a penumbra will not occur. The reason the

Page� of �4 6

Penumbral Lunar Eclipse Prajval Shastri

penumbra is not visible in Step 1 above is that, even though the source of light is extended, the pencil is sufficiently far away from the source that the source functions as if it is almost a point source (see diagram).

g. Why does the moon appear copper-coloured during a total eclipse? During a total lunar eclipse, the Sun’s direct light is blocked by the earth and does not reach the moon. A small amount of the sunlight that passes through earth’s atmosphere, however, is

Page� of �5 6

The sketches show how the phenomenon of penumbral shadow can be demonstrated using an extended light source such as the ‘torch’ light from a cell phone, and an object with a sharp outline such as a pencil, casting a shadow on a light-coloured surface. When the object whose shadow is cast is far away from the light source the shadow is seen as sharp because the light source behaves almost point-like with respect to the object and so the penumbral shadow is negligible. When the light source is brought relatively close to the object, the shadow begins to blur at the edges and lighter shadow, viz., the penumbral shadow can be seen around the main shadow. (Figure by Juny Wilfred)

The diagram above shows the umbra and penumbra of the earth’s shadow cast by the Sun. The motion of the moon shown by the green curve. When the moon crosses the boundary between the umbral and penumbral regions we see a partial eclipse. When the moon is fully within the umbral region we see the total eclipse. In this phase the Earth blocks most of the sunlight and if the earth had no atmosphere, the Moon would not be visible. Because the earth’s atmosphere refracts and reddens the sunlight passing through it, some of this light enters the umbral region, and is reflected by the moon, making it appear copper-coloured.

Penumbral Lunar Eclipse Prajval Shastri

refracted or “bent” towards the moon by the atmosphere and is reflected towards us on earth by the moon. Particles in the atmosphere also preferentially scatter away the bluer, light of shorter wavelengths and only the redder light that have relatively longer wavelengths gets through, similar to the sunlight we see at sunset and sunrise, and this redder light reaches the moon. Therefore the moon appears copper-coloured during a total lunar eclipse. The exact colour will depend on the nature and quantity of the dust particles, water droplets and aerosols present in the earth’s atmosphere at that time. During a normal full moon we do not see this copper tinge because the direct sunlight reflected by the moon is far more than the sunlight refracted and reddened by the earth’s atmosphere and then reflected towards the earth by the moon.

h. Are Rahu and Ketu scientific terms? ‘Rahu’ and ‘Ketu’ are the technical terms used in ancient Indian astronomy for the imaginary geometrical points known as ‘ascending node’ and ‘descending node’ in modern astronomy, which are explained below.The earth’s rotation axis, which determines the North and South directions for us, is tilted at about 23º to the plane of the earth’s orbit around the Sun (known as the ecliptic). The orbit of the moon around the earth is tilted with respect to the orbit of the earth around the sun by about 5º. So, if the plane of the earth’s orbit around the Sun is imagined to be ‘horizontal’, then, the moon will sometimes be ‘below' (or more precisely, south of ) and sometimes be ‘above’ (or more precisely, north of) this plane. There are of course only two points where the moon’s orbit around the earth intersects the earth’s orbit around the Sun. While the moon orbits around the earth, at one of these intersecting points the moon goes from being ‘below’ (or south of) the ecliptic plane to above (or north of) it, and therefore this intersection point is known as the ‘ascending node’. Similarly the intersecting point where the moon goes from being ‘above’ the ecliptic to below it is known as the ‘descending node’. Thus the moon passes through these two nodes once during every one of its orbits around the earth. On the rare occasions when the passage of the moon through one of these points coincides with new moon or full moon, is when the moon becomes aligned with the Sun-earth line, and we get eclipses.

i. What is meant by the term ‘Wolf moon’? The full moon in the month of January is referred to as “Wolf moon” in some folklore. Some of the folk names for full moons such as Harvest moon or Snow moon may refer to the seasons when they occur.

4. More detailed authentic information on the eclipse of 10 Jan 2020:

Authentic detailed information about the eclipse may be obtained from the following websites:

https://www.timeanddate.com/eclipse/lunar/2020-january-10 https://earthsky.us1.list-manage.com/track/click?u=e56e7a92b1c5790f7343ef95a&id=2d759af308&e=d26e727471 https://en.wikipedia.org/wiki/Lunar_eclipse

https://www.eclipsewise.com/lunar/LEhelp/LEglossary.html

Page� of �6 6