The earth and the moon are non-luminous objects, while the sun is a luminous object. When any opaque object blocks the rays of light, a shadow is formed. As the earth goes round the sun and the moon goes round the earth, at some moment the sun, the earth and the moon fall in one straight line. When they fall in one straight line, either the shadow of the moon falls on the earth or the shadow of the earth falls on the moon. That happening is called an eclipse occurring. An eclipse occurs when the sun, the moon and the earth fall in one straight line.
You were asked to hold on to this. At every full moon the earth is between the sun and the moon, and at every new moon the moon is between the sun and the earth, so the three bodies come into a line twice every single month. That is exactly the arrangement described above. So why is there not an eclipse twice a month? The answer is one detail that nothing in the book mentions. The path of the moon round the earth is tilted by about five degrees against the path of the earth round the sun, so the three bodies are usually not quite in line: at most new moons the moon passes a little above or a little below the sun as we see it, and at most full moons it passes above or below the shadow of the earth and misses it altogether. Only when the moon happens to be crossing that tilted plane at the same moment as a new moon or a full moon do the three line up properly, and that happens a few times a year rather than twenty four. So the rarity of eclipses is not caused by anything dramatic. It is caused by a slight tilt.
Shadow
When an opaque object blocks light, a shadow is formed. The dark, thick shadow that is formed where the object blocks the light completely is called the umbra. The faint shadow that is formed where the object blocks the light only partially is called the penumbra. The study of eclipses is done on this same basis.
Activity 11.6 asks whether the shadow of the ball on the wall is uniform, and the honest answer is no, there is a dark middle and a fuzzy grey edge around it. It is worth knowing why, because everything about eclipses follows from it. A candle flame is not a single point of light; it is a small object with a top, a bottom and two sides, and each part of it is throwing light past the ball from a slightly different direction. At the middle of the shadow, every part of the flame is blocked, so no light at all arrives and the shadow is completely dark: that is the umbra. Round the edge, some parts of the flame are blocked and others still get past, so a little light arrives and the shadow is only partly dark: that is the penumbra. If the light source really were a single point, there would be no penumbra and every shadow would have a razor sharp edge. This is why your own shadow on a sunny day has a soft edge, and why the sharpness changes with a cloudy sky, and it is exactly why an eclipse has a total region and a partial one.
The reasons an eclipse occurs
The moon orbits the earth and the earth orbits the sun without a break. In that sequence these three celestial bodies sometimes fall in a straight line. In such a state, when the moon falls between the sun and the earth, the shadow of the moon falls on the earth; and when the earth falls between the sun and the moon, the shadow of the earth falls on the moon.
| An eclipse needs all five of these |
|---|
| Light moving only in a straight line. |
| The moon orbiting the earth, and the earth orbiting the sun. |
| The sun, the earth and the moon falling in one straight line. |
| The earth and the moon being opaque. |
| Among the sun, the earth and the moon, only the sun being a luminous object. |
The process in which the shadow of one celestial body falls on another celestial body is called an eclipse. The eclipses that are seen from the earth are of two kinds.
Lunar eclipse
When the sun, the earth and the moon fall respectively in one straight line, the process in which the shadow of the earth falls on the moon is called a lunar eclipse. The conditions for a lunar eclipse to occur are these: the sun, the earth and the moon falling respectively in one straight line; the earth falling between the sun and the moon so that the shadow of the earth falls on the moon; and light always moving in a straight line.
When the umbra of the earth falls on the whole part of the moon and the whole moon looks dark, it is called a total lunar eclipse. If the umbra of the earth falls on only some part of the moon, only some part of the moon looks dark, and that is called a partial lunar eclipse. A lunar eclipse occurs only at the full moon.
The book states this and moves on, but it is something you can prove to yourself, and doing so is worth more than remembering it. A lunar eclipse needs the shadow of the earth to fall on the moon. The shadow of the earth always points directly away from the sun, because that is where a shadow goes. So the moon can only enter that shadow if it is on the far side of the earth from the sun. And what did the last topic call the arrangement where the earth sits between the sun and the moon? A full moon. So a lunar eclipse must happen at a full moon, because that is the only time the moon is anywhere near the shadow. The same argument run the other way explains why a solar eclipse can only happen at a new moon: for the moon to block the sun, the moon has to be between us and the sun, and that is the definition of a new moon. Two facts, one line of reasoning.
Solar eclipse
When the moon is between the earth and the sun and all three bodies fall in one straight line, the light going from the sun to the earth is blocked by the moon and the shadow of the moon falls on the earth. That state is called a solar eclipse, and a solar eclipse occurs at the new moon.
From the place where the umbra of the moon falls on the earth, the whole part of the sun is seen to be blocked, and this state is called a total solar eclipse. From the place where the penumbra falls on the earth, only some part of the sun is seen to be blocked when the sun is looked at, and this state is called a partial solar eclipse. A solar eclipse must not be looked at with the naked eye; when it is looked at in that way, the radiation coming out of the sun damages our eyes.
Where the book gives the Nepali term khagras suryagrahan, it prints the English in brackets as total lunar eclipse. That is a misprint; the correct English is total solar eclipse, and the paragraph is plainly describing the sun being blocked. Write total solar eclipse in an examination. It is worth noticing that the mistake is one of those slips that a reader who is following the sense, rather than copying words, will catch immediately, because a passage about the moon covering the sun cannot suddenly be about a lunar eclipse.
The two kinds of eclipse feel very different in practice, and the reason is a matter of which body is casting the shadow. In a lunar eclipse the earth throws its shadow on the moon. The earth is much bigger than the moon, so its shadow is far wider than the moon is, and the moon can sit entirely inside it for over an hour. And because it is the moon that goes dark, anybody on the whole night side of the earth can look up and see it at the same time; roughly half the world watches the same event. In a solar eclipse the moon throws its shadow on the earth, and the moon is much smaller than the earth, so its umbra reaches the ground as a dark patch only a couple of hundred kilometres wide. That patch races across the surface, and totality at any one place lasts a few minutes at most. So a total solar eclipse is not rare because it seldom happens; it is rare because you have to be standing in a narrow strip when it does. A great many people travel across the world to stand inside that strip, and most never see one from home in a lifetime.
Look at the two numbers you already have. The sun is about four hundred times wider than the moon. And the sun is also about four hundred times farther away from us than the moon is. Those two facts have nothing to do with one another, and yet they cancel out almost exactly, with the result that the sun and the moon appear almost the same size in our sky. That is why the moon can cover the sun so neatly, hiding the blinding disc while leaving the faint outer glow visible around the edge. If the moon appeared even slightly smaller, no total solar eclipse would ever be possible anywhere; if it appeared much larger, a total eclipse would be a common and less remarkable thing. As far as anybody knows, no other planet in the solar system has a moon that fits its sun so exactly. It is not a design and it is not a message. It is a coincidence, and it happens to be a spectacular one.
This is the last topic in the last unit, so it is worth looking back at what has been built. Eclipses were frightening for most of human history, because a sun that goes out in the middle of the day is a genuinely alarming thing when you have no idea why. They are not frightening now, and the reason is not that people became braver. It is that somebody worked out that light travels in straight lines, that the earth and the moon are opaque balls, and that all three bodies move on paths that can be measured. From those few unremarkable facts it follows that eclipses must happen, that they must happen only at full moon and new moon, and exactly when and where each one will occur. Eclipses are now predicted centuries in advance, to the minute, and nobody has ever been surprised by one in living memory. That is what this whole book has been teaching, one unit at a time: that the world is made of ordinary things behaving in regular ways, that careful observation finds the rule, and that once you have the rule you can say what will happen next. A stone, a lemon, a magnet, a bulb and a shadow on the moon are all the same subject.
The full lesson is waiting for you
Create a free account to unlock activities, practice tests, presentations and videos - and to track your progress and confidence score for every subject.
The full lesson is waiting for you
Create a free account to unlock activities, practice tests, presentations and videos - and to track your progress and confidence score for every subject.