What do you call the vast dark plains on the moon?

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Factual information for the teacher

1. Hints for creating the sharpest possible shadows

If a light bulb is used as a light source, the filament should be as small as possible so that the light source is as point-like as possible. Then sharp shadows are created. Halogen table lamps, for example, meet this condition. However, the halogen bulbs get very hot, so if possible only use lamps with protective glass or protective rod.
Ordinary frosted 230-volt incandescent lamps provide a relatively extended source of light that provides soft, blurred shadows when the distance between the object and the wall increases.
Far better results are achieved with a 40-watt incandescent lamp with a clear glass bulb, if you orient the filament perpendicular to the wall. The projection of the filament on the wall is then very small. (See picture.)

Overhead projectors also provide sharp shadows. But they have the disadvantage that the light source is not as clearly visible to the students as it is with an incandescent lamp.

2. Shadow creation

In order to be able to sufficiently explain the formation of shadows, it is first necessary to talk about the straight-line propagation of light, in order to then be able to understand the process of the formation of shadows.
Even small children draw the sun as a round disk that emits straight rays of light. Exactly this childish idea already corresponds to a large extent to the physical truth. If you hold an opaque object between your eye and a light source for illustration, you can no longer see the light source. This means the light does not flow in an arc around the object, but is stopped by it. From this it follows that light must propagate in a straight line. This also becomes clear in experiment V1 in the teaching unit "How is shadow created??"Here a dusty blackboard cloth is placed over a ray of light or. tightly bundled light channel shaken out. The straightness of light propagation is clearly visible through the tiny particles that enter the light beam.
Accordingly, shadows are caused by the fact that light does not pass through an object unless it is translucent. The space behind the object (seen from the light source) can therefore not be illuminated. This dark area is clearly separated from the brightly illuminated surroundings and we see a shadow. Simplified, the shadow is always in the extension of the connecting lines between light source and object.

Drawing after Wiesner and Claus, 1985

Only the rays of light that are not blocked by the object continue to reach the projection surface, z.B. a white wall. You can determine the shadow boundary by drawing a line from the light source to the edge of the object and continuing it to the projection surface. On this straight line there is the shadow border.
Even if we always perceive the shadow only as a surface, the logical conclusion is that it must be a whole space behind the object, which remains unlit. What we see is only the section through this space, which a projection surface forms by virtually pushing itself into this space. In this way we see a two-dimensional image of the outline of the shadow-casting object, which resembles it in terms of shape.

Drawing according to Wiesner and Claus , 1985

3. Size of shadows

The size of a shadow changes when one or more of the three parameters: Light source position, object position, projection surface position. In the following, for reasons of simplification, only the position shifts will be discussed, in which the planes of the object and the projection surface always remain perpendicular to the central connecting line to the light source. Basically, if the distance between the light source and the plane of projection is constant, the closer the object is moved to the light source, the larger its shadow will be. Conversely, the further away the object is moved from the light source, the smaller the shadow becomes. This physical effect can be well and simply explained with the rectilinear propagation of light. So it quickly becomes clear why the size of a shadow varies under certain circumstances.

Drawings according to Wiesner and Claus , 1985

If you observe what happens when you walk under a streetlight at night, the above theorem "The closer an object gets to a light source, the larger its shadow becomes" can no longer be applied. Because the closer you get to the streetlight, the smaller, or rather shorter, your own shadow becomes. If we move away from it again, the shadow becomes longer again, or rather, it becomes longer. greater. D.h., In the example of the street lamp, it must also be taken into account that the projection surface (the street) is now perpendicular to the figure and no longer parallel. However, this will be left out of the series of lessons. However, if you draw pictures analogous to the above sketches for this case, they are also easy to follow with the help of the straight-line light propagation and provide information about the type of size change of a shadow.

4. Colored shadows

First of all one thinks that shadow is darkness resp. the color black means. This is only correct as long as the shadow area is not further illuminated either. But as soon as you bring two lamps into play, which have different colored bulbs, things look a little different. If an object that is illuminated by a red lamp casts a shadow, it is initially dark (black). However, if this shadow area is then illuminated by a blue lamp, it turns blue. Conversely, of course, on the extension line blue lamp – object a shadow is created, which is illuminated by the red lamp and is therefore red. Outside of the two colored shadows, the colors of the lamps, being almost complementary, mix additively and complement each other to a bright purple (shown in white in the figure).

5. Origin of day and night

In everyday language we use two terms for day, which must first be distinguished: On the one hand we speak of a day as the unit, which comprises 24 hours, in which the earth rotates once around its own axis (exactly it is something over 23 h 56 min, which the earth needs for this rotation). On the other hand, we designate by a day only the bright ones of these 24 hours, in order to distinguish them from the dark ones of the night. If in the following of a day one speaks, then the counterpart is meant to the night.

The origin of day and night is to be explained quite simply:
Since the daylight comes from the sun, it is daytime on that part of the earth which is illuminated by the sun at the moment. The other part is in the so-called own shadow of the earth, there no sunlight reaches it, it is dark and thus night (see illustration below). Since the earth rotates around its own axis (in the direction of the arrow in the figure), the change from day to night always takes place within this period of time, i.e. within 24 hours. The fact that the days are not always of the same length, but shorter in winter and longer in summer, is due to the different angle of incidence of the sunlight, which is caused by the oblique position of the Earth’s axis. (This is treated in detail with the origin of the seasons).

At this point, the phenomena of twilight, blue sky color and sky redness at sunrise and sunset should be briefly discussed:

The twilight usually lasts for one or two hours and does not happen relatively fast, as it might be expected due to the rather sharp border of the shadow of the sunlight on the earth. This is due to the scattering of sunlight by small gas particles in the earth’s atmosphere. This is because the scattering of light causes the incident sunlight to be scattered in all directions. Thus, a part of the sunlight is scattered to the earth when the sun has already disappeared behind the horizon and it still remains bright for a certain time.

The red color of the sky at sunrise and sunset and the blue color of the sky also have to do with the scattering of light in the atmosphere. Sunlight contains light of different colors due to different wavelengths. When entering the atmosphere, the blue parts of the light are scattered more, because they are short wavelengths, the scattering is most intensive with light of small wavelengths. That is why the day sky appears blue to us.
The lower the sun is in the sky, the longer is the path of the sunlight through the atmosphere. When the sun rises or sets, much of the blue component of sunlight is lost before it reaches us because the short wavelength blue light has already been scattered out. Then the long-wave red light reaches us and the sky appears in red tones.

6. The seasons

The origin of the seasons is not – as is often wrongly assumed – due to the different distance of the earth from the sun. The earth moves on an ellipse around the sun, but this ellipse is nearly circular and the difference between sun proximity (perihelion) and sun remoteness (aphelion) is comparatively so small that this has no influence on the temperature on earth. We notice that the sun is closest to us on the second of January, i.e. in the middle of winter, with a distance of 147.1 million km and on the sixth of July with a distance of 152.1 million km it is the farthest away from the earth. That the emergence of the seasons has nothing to do with the distance of the earth to the sun is also plausible, since on the two hemispheres always the straight opposite seasons prevail. The change of the seasons results from the oblique position of the earth axis in relation to the orbit around the sun:

Picture after Wiesner and Claus , 1985

The earth turns in a year – exactly it is 365 days and 6 hours – once around the sun.
The orientation of the Earth’s axis remains unchanged, i.e. it always points in the same direction; however, it is not perpendicular to the orbit, but inclined to it (the angle between the Earth’s equator and the orbital plane is 23.5°). Therefore the northern and the southern hemisphere are inclined to the sun. During the European summer months the northern hemisphere is inclined to the sun (see drawing: 21. June), the southern hemisphere is then tilted away from the sun and receives less sunlight, it is winter. If it is winter, the northern hemisphere is turned away from the sun (see picture) and the southern hemisphere is tilted towards it, gets more light and the days are longer.

However, the difference in temperature between summer and winter is not determined by the duration of the sun’s rays, but mainly by the angle of the incoming sunlight. The area illuminated by a beam of light coming from the sun is much smaller in summer than in winter. the light hits the ground much more concentrated in summer than in winter and therefore warms it up more.

7. The phases of the moon

Image after Wiesner and Claus , 1985

This experimental sand arrangement illustrates in a vivid way the occurrence of the light shapes of the moon.

Since the moon is not a star, it does not shine itself. It appears bright to us, because it is illuminated by the sun. On its orbit around the earth one can see from the earth depending on the position of the moon a certain part of this illuminated side, the different moon phases:

Picture after Wiesner and Claus , 1985

If the moon is between the sun and the earth (position 1), the illuminated side of the moon is turned away from the earth, we see the unlit side: it is new moon. On positions 2 and 4 half of the bright side is visible, to us it appears as a half moon. If the moon is behind the earth seen from the sun (position 3), we see it as full moon in the sky, because the whole illuminated side is turned to us.

Since the self-rotation of the moon and an orbit of the earth take place in approximately the same time, i.e. approx. 27 days (an orbit of the earth takes exactly 27 days and 8 hours), one always sees the same side of the moon from the earth.

Sometimes shortly after or before new moon not only a narrow crescent can be seen in the night sky, but also in dim light the whole moon can be seen. One speaks then of ‘ashen moonlight’. The reason for this lies in the fact that around the time of the new moon the illuminated side of the earth is turned completely to the moon – one says the earth appears to the moon as full earth – and the sunlight reflects. The moon is illuminated by the sunlight reflected from the earth and so we can see the whole moon sphere, although we see only a small part of the illuminated half.

Here now the phenomenon of the apparently larger moon at the horizon is to be explained briefly:

You can often observe the rising moon close to the horizon and notice that it appears to be larger then than when it is high in the sky. This is a pure optical illusion, it is also known as ‘moon illusion’. This is explained by the fact that the size of the moon is perceived differently when objects of known size, such as houses and trees, etc., are in view., on the horizon are to be seen, which do not exist in the sky. The fact that the size of the moon does not change, however, can be easily checked by comparing the moon standing on the horizon and high in the sky, for example, with a coin on the outstretched arm.

8. Solar and lunar eclipse

When working on this topic, it should be pointed out that the illustrated models for the solar and lunar eclipse do not reflect the real size and distance relationships. To the clarification of the actual conditions the following diagram can serve:

Image after Wiesner and Claus , 1995

This situation can be represented with the pupils possibly in the break yard.

First, the events of both a solar and lunar eclipse can be explained quite simply: A solar eclipse occurs when the moon moves between the sun and the earth, obscuring the sun and casting a shadow on the earth.

Picture after Wiesner and Claus , 1995

In the umbra area (see picture above) a total solar eclipse can be observed, where in the so called totality phase nothing of the sun can be seen at all. Thereby the corona, the very thin solar atmosphere, becomes visible. (As visible on the picture at the beginning of this hour. See copy templates M 14) This is only possible because from the earth the sun and the moon appear to be the same size, which is due to the fact that the sun is 400 times larger than the moon, but also 400 times farther away from the earth. In the penumbral regions (see picture above) always a part of the sun remains visible, the moon does not push itself here completely in front of the sun disk. One speaks of a partial solar eclipse.

The formation of a lunar eclipse can be explained similarly:
A lunar eclipse can be observed when the moon passes through the earth’s shadow on its orbit and the sunlight can not reach it. A lunar eclipse can be observed from the whole night side of the earth.

Picture after Wiesner and Claus , 1995

So a solar eclipse can only occur at new moon, when the moon is between the sun and the earth, and a lunar eclipse only at full moon, when the earth is between the sun and the moon. (See also phases of the moon)

That it does not come however in each new moon phase to a solar eclipse and in each full moon phase to a lunar eclipse, is because the course, on which the earth turns around the sun and the course, on which the moon turns around the earth in different planes lie. The orbital plane of the moon is inclined by 5° to the orbital plane of the earth, where also the sun is located. The two planes have an intersection line in common, they intersect in the so-called nodal line. The moon is on this nodal line twice during each orbit around the earth.

Picture after Wiesner and Claus , 1995

But for an eclipse to occur, all three celestial bodies must lie on one line. This is the case when the moon lies on the nodal line and at the same time the line points to the sun. However, since the nodal line is not fixed in space, but also rotates (in a year about 20° opposite to the rotation of the earth around the sun), this does not happen on a regular basis.

In summary this means: Eclipses can occur when the earth and the moon are on the nodal line, this line points to the sun and at the same time it is new moon (for a solar eclipse) or full moon (for a lunar eclipse). On this background it is understandable why solar as well as lunar eclipses are to be observed so rarely.

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