Supposing sound to travel at the same rate as on the surface of the air, and at a uniform rate, it would take 15 years to accomplish this journey. If a railway were laid through space between the earth and the sun, a train travelling at express speed, 12½ leagues an hour, would not arrive at its destination until the end of 338 years. This imaginary train, if dispatched from the earth in January, 1872, would arrive at the sun in the year 2210. The light from the sun, which travels 77,000 leagues in a second, takes 7 minutes 13 seconds to reach the earth.
Theophilus. The distance between the earth and the sun is, then, 38,000,000 miles—that is our unit of measurement for the distances of the stars. Now let us hear about these distances.
The Author. I will deal first with those stars which are nearest to us. One of these is a star in the constellation of the Swan. This star is distant from the earth 551,000 times our unit of measurement, that is to say, that we must multiply 551,000 times the distance of the earth from the sun to represent the distance of the star which we are considering, and yet it is one of the nearest to the earth. If we wish to represent this distance by the time occupied in the transit of light, supposing this light to travel, like that of our sun, 77,000 leagues a second, it would take 9½ years to travel from the star to us.
Now, if you wish to know the distance of other stars, and remember that I only speak of the nearest, look at this table, which I found in an astronomical treatise:
DISTANCE OF CERTAIN STARS FROM THE EARTH.
| Names of the Stars. | Distances from the Earth. | Time of transit of light. |
| α Of the Swan | 551,000 times | 9 years and a half. |
| α Of the Lyre | 1,330,700 | 21 years. |
| α Of the Great Dog | 1,375,000 | 22 years. |
| α Of the Great Bear | 1,550,800 | 25 years. |
| Polar Star | 3,678,000 | 50 years. |
Thus, the star α of the Lyre is distant from us more than 1,330,000 times as far as the earth is from the sun, and its light takes 21 years to reach us. If, by any celestial catastrophe, star α of the Lyre were to disappear, to be annihilated, we should still see it for 21 years, as its light takes that time to reach us.
Theophilus. It is then possible that our astronomers are now observing stars which no longer exist, and are only visible to us because the light which they omitted is still travelling towards the earth.
The Author. Just so. But to continue. I have begun with the stars which are nearest to the earth. There are the stars of first and second magnitude. You know, I suppose, the signification of those terms first, second, and third magnitude in astronomy?
Theophilus. Yes, I know that the word magnitude is only applied to the luminous appearance of the star, and not to its real bulk. A star of the first magnitude is one which forms part of the group of the most luminous stars; a star of the second magnitude is one which comes next in point of brilliancy.