Through Magic Glasses and Other Lectures — An Hour with the Sun, Part 2

You will wonder no doubt how the spectroscope can show the height and the shape of the flames.

I will explain to you, and I hope to show them you one day.

You must remember that the telescope makes a small real image of the flame at its focus, just as in one of our earlier experiments you saw the exact image of the candle-flame upside down on the paper (see p. 33).

The reason why we only see a strip of the flame in the spectroscope is because the slit is so narrow.

But when once the sunlight was dispersed so as no longer to interfere, Dr. Huggins found that it is possible to open the slit wide enough to take in the image of the whole flame, and then, by turning the spectroscope so as to bring one of the bright hydrogen lines into view, the actual shape of the prominence is seen, only it will look a different colour, either red, greenish-blue, or indigo-blue, according to the line chosen.

As the image of the whole sun and its appendages in the telescope is so very small, you will understand that even a very narrow slit will really take in a very large prominence several thousand miles in length.

Fig 51 shows a drawing by Mr. Lockyer of a group of flames he observed very soon after Dr. Huggins suggested the open slit, and these shapes did not last long, for in another picture he drew ten minutes later their appearance had already changed.

[Illustration: Fig. 51. Red prominences, as drawn by Mr. Lockyer during the total eclipse of March 14, 1869. ] These then are some of the facts revealed to us by our magic glasses.

I

scarcely expect you to remember all the details I have given you, but you will at least understand now how astronomers actually penetrate into the secrets of the sun by bringing its image into their observatory, as we brought it to-day on the card-board, and then making it tell its own tale through the prisms of the spectroscope; and you will retain some idea of the central light of the sun with its surrounding atmosphere of cooler gases and its layer of luminous lambent gases playing round it beyond.

Of the corona I cannot tell you much, except that it is far more subtle than anything we have spoken of yet; that it is always strongest when the sun is most spotted; that it is partly made up of self-luminous gases whose bright lines we can see, especially an unknown green ray;

while it also shines partly by reflected light from the sun, for we can trace in it faint dark lines; lastly it fades away into the mysterious zodiacal light, and so the sun ends in mystery at its outer fringe as it began at its centre.

And now at last, having learnt something of the material of the sun, we can come back to the spots and ask what is known about them.

As I have said, they are not always the same on the sun's face.

On the contrary, they vary very much both in number and size.

In some years the sun's face is quite free from them, at others there are so many that they form two wide belts on each side of the sun's equator, with a clear space of about six degrees between.

No spots ever appear near the poles.

Herr Schwabe, who watched the sun's face patiently for more than thirty years, has shown that it is most spotted about every eleven years, then the spots disappear very quickly and reappear slowly till the full-spot time comes round again.

Some spots remain a very short time and then break up and disappear, but others last for days, weeks, and even months, and when we watch these, we find that a spot appears to travel slowly across the face of the sun from east to west and then round the western edge so that it disappears.

It is when it reaches the edge that we can convince ourselves that the spot is really part of the sun, for there is no space to be seen between them, the edge and the spot are one, as the last trace of the dark blotch passes out of sight.

In fact, it is not the spot which has crossed the sun's face, but the sun itself which has turned, like our earth, upon its axis, carrying the spot round with it.

As some spots remain long enough to reappear, after about twelve or thirteen days, on the opposite edge, and even pass round two or three times, astronomers can reckon that the sun takes about twenty-five days and five hours in performing one revolution.

You will wonder why I say only _about_ twenty-five, but I do so because all spots do not come round in exactly the same time, those farthest from the equator lag rather more than a day behind those nearer to it, and this is explained by the layer of gases in which they are formed, drifting back in higher latitudes as the sun turns.

It is by watching a spot as it travels across the sun, that we are able to observe that the centre partlies deeper in the sun's face than the outer rim.

There are many ways of testing this, and you can try one yourselves with a telescope if you watch day after day.

I will explain it by a simple experiment.

I have here a round lump of stiff dough, in which I have made a small hollow and blackened the bottom with a drop of ink.

As I turn this round, so that the hollow facing you moves from right to left, you will see that after it passes the middle of the face, the hole appears narrower and narrower till it disappears, and if you observe carefully you will note that the dark centre is the first thing you lose sight of, while the edges of the cup are still seen, till just before the spot disappears altogether.

But now I will stick a wafer on, and a pea half into, the dough, marking the centre of each with ink.

Then I turn the ball again.

This time you lose sight of the foremost edge first, and the dark centre is seen almost to the last moment.

This shows that if the spots were either flat marks, or hillocks, on the sun's face, the dark centre would remain to the last, but as a fact it disappears before the rim.

Father Secchi has tried to measure the depth of a spot-cavity, and thinks they vary from 1000 to 3000 miles deep.

But there are many difficulties in interpreting the effects of light and shadow at such an enormous distance, and some astronomers still doubt whether spots are really depressions.

For many centuries now the spots have been watched forming and dispersing, and this is roughly speaking what is seen to happen.

When the sun is fairly clear and there are few spots, these generally form quietly, several black dots appearing and disappearing with bright streaks or _faculae_ round their edge, till one grows bigger than the rest, and forms a large dark nucleus, round which, after a time, a half-shadow or _penumbra_ is seen and we have a sun-spot complete, with bright edges, dark shadow, and deep black centre (Fig. 52).

This lasts for a certain time and then it becomes bridged over with light streaks, the dark spot breaks up and disappears, and last of all the half-shadow dies away.

[Illustration: Fig. 52. A quiet sun-spot.

(Secchi.

)] But things do not always take place so quietly.

When the sun's face is very troubled and full of spots, the bright _faculae_, which appear with a spot, seem to heave and wave, and generally several dark centres form with whirling masses of light round them, while in some of them tongues of fire appear to leap up from below (Fig. 53).

Such spots change quickly from day to day, even if they remain for a long time, until at last by degrees the dark centres become less distinct, the half-shadows disappear, leaving only the bright streaks, which gradually settle down into luminous points or _light granules_.

These light granules are in fact supposed by astronomers to be the tips of glowing clouds heaving up everywhere, while the dark spaces between them are cooler currents passing downwards.

[Illustration: Fig. 53. A tumultuous sun-spot.

(Langley.

)] Below these clouds, no doubt, the great mass of the sun is in a violent state of heat and commotion, and when from time to time, whether suddenly or steadily, great upheavals and eruptions take place, bright flames dart up and luminous clouds gather and swell, so that long streaks or _faculae_ surge upon the face of the sun.

Now these hot gases rising up thus on all sides would leave room below for cooler gases to pour down from above, and these, as we know, would cut off, or absorb, much of the light coming from the body of the sun, so that the centre, where the down current was the strongest, would appear black even though some light would pass through.

This is the best explanation we have as yet of the formation of a sun-spot, and many facts shown in the spectroscope help to confirm it, as for example the thickening of the dark lines of the spectrum when the slit is placed over the centre of a spot, and the flashing out of bright lines when an uprush of streaks occurs either across the spots or round it.

And now, before you go, I must tell you of one of these wonderful uprushes, which sent such a thrill through our own atmosphere, as to tell us very plainly the power which the sun has over our globe.

The year 1859 was remarkable for sun-spots, and on September 1, when two astronomers many miles apart were examining them, they both saw, all at once, a sudden cloud of light far brighter than the general surface of the sun burst out in the midst of a group of spots.

The outburst began at eight minutes past eleven in the forenoon, and in five minutes it was gone again, but in that time it had swept across a space of 35,000 miles on the sun!

Now both before and after this violent outburst took place a magnetic storm raged all round the earth, brilliant auroras were seen in all parts of the world, sparks flashed from the telegraph wires, and the telegraphic signalmen at Washington and Philadelphia received severe electric shocks.

Messages were interrupted, for the storm took possession of the wires and sent messages of its own, the magnetic needles darting to and fro as though seized with madness.

At the very instant when the bright outburst was seen in the sun, the self-registering instruments at Kew marked how three needles jerked all at once wildly aside; and the following night the skies were lit up with wondrous lights as the storm of electric agitation played round the earth.

We are so accustomed to the steady glow of sunshine pouring down upon us that we pay very little heed to daylight, though I hope none of us are quite so ignorant as the man who praised the moon above the sun, because it shone in the dark night, whereas the sun came in the daytime when there was light enough already!

Yet probably many of us do not actually realise how close are the links which bind us to our brilliant star as he carries us along with him through space.

It is only when an unusual outburst occurs, such as I have just described, that we feel how every thrill which passes through our atmosphere, through the life-current of every plant, and through the fibre and nerve of every animal has some relation to the huge source of light, heat, electricity, and magnetism at which we are now gazing across a space of more than 93,000,000 miles.

Yet it is well to remember that the sudden storm and the violent eruption are the exceptional occurrences, and that their use to us as students is chiefly to lead us to understand the steady and constant thrill which, never ceasing, never faltering, fulfils the great purpose of the unseen Lawgiver in sustaining all movement and life in our little world.

CHAPTER VII AN EVENING AMONG THE STARS