Part 13

Yes. If the sun were put back 150,000 times further than it is, it ought to look as brightly as that star. If it does not, it is because it is really smaller than the star.

What! 150,000 times 95 millions!

But that is nothing; for it is only to the first rank. What of the twentieth magnitude?

Yes. But you say the nebulæ are further off than that.

I may tell you that if the sun moved three times as fast as the world does, in its six hundred millions of miles a year, it would take two hundred and fifty millions of years to get to as far as Lord Rosse’s telescope could see.

That takes my breath away.

Hear a little more. Light comes from the sun to us in eight minutes. It will take sixty thousand years to come from one of those stars Lord de Rosse saw. In fact his telescope has enlarged our universe one hundred and twenty-five million times.

Then I think religious people ought to thank astronomers for showing them more of the greatness of God. Those who only thought of him as the Creator of the three thousand stars, to be seen by the naked eye, could not have such a notion of his vast power as those who know of millions upon millions of suns.”

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After this, James was left to digest such wonderful lessons. When his first astonishment had passed away, his curiosity was excited to know more about the distances of the stars, so that he might form a simpler idea of the thing. He took, therefore, another occasion of bringing up the subject in these words:--

“Father, do you really believe the stars are so far off?

I am obliged to believe many things I do not understand, upon the testimony of trustworthy witnesses; but in this case I can form a good guess of the truth. Do you remember what I once told you of the parallax, or angle of observation of the sun or moon?

Yes. That of the moon was 57 minutes, and the sun 8-3/4 seconds. A degree is 60 minutes, and a minute 60 seconds.

Very well. Then 57 minutes, or 3420 seconds, will be four hundred times as much as the other. If the moon be 240,000 miles off, the sun will be four hundred times further or 96,000,000.

But how do you get this parallax?

Distances are calculated by the angle made in looking at an object from two places. The two lines of sight cross one another. A great base is needed to view a distant object, or else no angle can be observed. Astronomers take the diameter of the earth’s orbit.

That is twice ninety-five millions of miles.

With that base--that is, looking at a star from both sides of our orbit, or at six months’ interval--we could get the two lines crossing one another, and so making an angle. The further the object, the more minute the angle. Only a few of the fixed stars could be observed in this way, as they generally are too far off to give an angle.

I know an equilateral triangle has its three angles equal to two right angles; and with ninety degrees for one right angle, each angle of the triangle will have sixty degrees. But I suppose no star parallax could be one degree.

No; nor a minute, the sixtieth part of one degree. When the object makes an angle of a second, or sixtieth of a minute, from a base line of one hundred and ninety millions of miles, the distance of the star will be about twenty millions of millions of miles.

Is there any star making the second angle?

The alpha of the Centaur is about that, and is one of the nearest of fixed stars.

That the nearest to us, and yet so far! Do tell me the distance of some others.

There is one, 61 Cygni, of the Swan, with one-third of a second; and, therefore, three times the distance of the alpha Centaur. There is a star in the Lyre which is one-fifth of a second. Grand Arcturus is one-eight. The North Polar Star is one-tenth. Pretty Capella, of the Kid, is one-twentieth; that is, twenty times farther back than a Centaur. As it looks one of the brightest stars, it must be very large.

What of old Sirius?

The angle he makes with our orbit diameter is one-fourth of a second; so that he is about 80,000,000,000,000 miles.

Thank you, dear father, for these terrible long figures.

Their great distance may give us a good guess of their great size.