Astronomical distance

“ To form a conception of the stellar system, we must have a unit of measure not only exceeding any terrestrial standard, but even any distance in the solar system. For purely astronomical purposes the most convenient unit is the distance corresponding to a parallax of 1", which is a little more than 200,000 times the sun's distance. But for the purposes of all but the professional astronomer the most convenient unit will be the light-year—that is, the distance through which light would travel in one year. ”

“ That the determination of the sun’s distance is important, in an astronomical sense, will be seen at once when it is remembered that the ideas we form of the dimensions of the solar system are wholly dependent on our estimate of the sun’s distance. ”

“ The Sun is an enormous sphere whose diameter is over 108 times the diameter of our globe, or very nearly 860,000 miles. Its radius is nearly double the distance from the Earth to the Moon. If we suppose, for a moment, the Sun to be hollow, and our globe to be placed at the centre of this immense spherical shell, not only could our satellite revolve around us at its mean distance of 238,800 miles, as now, but another satellite, placed 190,000 miles farther than the Moon, could freely revolve likewise, without ever coming in contact with the solar envelope. ”

“ At certain times some of them approach near enough to the earth to merit the attention of those astronomers who are specially interested in determining the dimensions of the solar system. The observations are of such a nature that they can be made with considerable precision; they can also be multiplied to any extent that may be desired. Some of these little bodies have consequently a great astronomical future, inasmuch as they seem destined to indicate the true distance from the earth to the sun more accurately than Venus or than Mars. ”

“ The measure of the Moon's distance involves no principle more abstruse than the measure of the distance of a tree on the opposite bank of a river. The principles of construction of the best Astronomical instruments are as simple and as closely referred to matters of common school-education and familiar experience, as are those of the common globes, the steam-engine, or the turning-lathe ”

“ The case of the stars must next be considered, and now we find that the distances involved are so enormous that the diameter, or semi-diameter, of the earth is altogether too insignificant a quantity to afford an available base-line for the measurement. We should have remained forever ignorant of star distances but for the effects produced by the earth's change of place due to its annual revolution round the sun. The mean diameter of the earth's orbit is about 186,000,000 miles, and we are able to make use of this immense distance as a base-line for ascertaining the parallax of a star. ”

“ Yet these stars are separated by inconceivable distances from each other, and it is one of the greatest triumphs of modern astronomy to have mastered, so far, the scale of the universe. For several centuries astronomers have known the relative distances from each other of the sun and the planets. If they could discover the actual distance of any one planet from any other, they could at once tell all the distances within the Solar System. ”

“ The mean distance of the Earth from the Sun may be taken to be about 93 millions of miles, and this distance is employed by Astronomers as the unit by which most other long celestial distances are reckoned. ”

“ There is no problem more important to the astronomer than that of the Sun’s distance from the Earth. Every change in the estimate changes everything. Now, since modern astronomers, in their estimates of this distance, have gone all the way along the line of figures from three millions of miles to a hundred and four millions—to-day, the distance being something over 91,000,000 ”

“ The sun's distance being ascertained, its absolute diameter is determined from the apparent by the process applied to the moon, A near approximation to the sun's radius is 430,680 miles. Imagine the earth at the sun's centre: its surface would appear as far distant as does now the celestial vault, and the moon's orbit would fall nearly 200,000 miles within the surface, or little more than half-way from the centre out, A locomotive at thirty miles an hour would run from centre to surface in 1.63 years. ”

“ It is important to note that this apparent distance of two stars, i.e. their distance from one another on the celestial sphere, is an entirely different thing from their actual distance from one another in space. In the figure, for example, Q is actually much nearer to S than it is to P, but the apparent distance measured by the arc q s is several times greater than q p. ”

“ If both Sun and Moon’s sizes are known, can we know at which distance is each from the Earth?
2 · At what distance must be the Moon from the Earth for a total and an annular eclipse to occur? Does it depend on Earth-Sun distance?
Answers
The Earth is an elliptical (almost circular) orbit around the Sun, just like the Moon around the Earth. The maximum and minimum distances from the Earth to the Sun are 147 and 152 million kilometers, respectively, and the Sun has a diameter of 1,391,400 km. ”

“ A kilometre is, however, an inappropriate unit for celestial distances. When dealing with distances in our planetary system, the astronomers, since the time of Newton, have always used the mean distance of the earth from the sun as universal unit of distance. Regarding the distances in the stellar system the astronomers have had a varying practice. ”

“ It is wonderful to contemplate the probability that of some of the more distant stars discovered, the rays of light which have found rest in the eye of the Astronomer, through the aid of the telescope, may have left their native sun thousands of years ago, and travelled at the rate of 192,000 miles a second ever since. "A certain cluster of stars was estimated by Sir William Herschel to be 700 times the distance of a star of the first magnitude—therefore at least 700 times nineteen billions of miles!" ”

“ Who cares whether the moon is 250,000 or 2,500,000 miles distant? Mathematicians try to impress upon us that the distance of the fixed stars is only expressible by a row of figures which stretches across a page; suppose it stretched across two or across a dozen pages, should we be any the wiser, or have, in the least degree, a clearer notion of the superlative distances? We civilly say, "Dear me!" when the astronomer looks to us for the appropriate stare, but we only say it with the mouth ”

“ If the stars were comparatively near us, the process would be a very simple one; but, unfortunately, the stars are so extremely far off that this triangle, even with a base of only one foot, must have its sides many miles long. Indeed, astronomers will tell you that there is no more delicate or troublesome work in the whole of their science than that of discovering the distance of a star. ”