The Story of the Solar System

by George F. Chambers

Refraction is what it professes to be, a bending, and what is bent is the ray of light coming from a celestial object to a terrestrial station. Olmsted has put the matter in this way:—“We must consider that any such object always appears in the direction in which the last ray of light comes to the eye. If the light which comes from a star were bent into fifty directions before it reached the eye, the star would nevertheless appear in a line described by the ray nearest the eye. The operation of this principle is seen when an oar, or any stick, is thrust into the water. As the rays of light by which the oar is seen have their direction changed as they pass out of water into air, the apparent direction in which the body is seen is changed in the same degree, giving it a bent appearance—the part below the water having apparently a different direction from the part above.” The direction of this refraction is determined by the general law of optics that when a ray of light passes out of a rarer into a denser medium (for instance out of air into water, or out of space into the Earth’s atmosphere) it is bent towards a perpendicular to the surface of the medium; but when it passes out of a denser into a rarer medium it is bent from the perpendicular. The effect of refraction is to make a heavenly body appear to have an apparent altitude greater than its true altitude, so that, for example, an object situated actually in the horizon will appear above it. Indeed it sometimes happens that objects which are actually below the horizon and which otherwise would be invisible were it not for refraction are thus brought into sight. It was in consequence of this that on April 20, 1837, the Moon rose eclipsed before the Sun had set.

Sir Henry Holland thus alludes to the phenomenon:—“I am tempted to notice a spectacle, having a certain association with this science, which I do not remember to have seen recorded either in prose or poetry, though well meriting description in either way. This spectacle requires, however, a combination of circumstances rarely occurring—a perfectly clear Eastern and Western horizon, and an entirely level intervening surface, such as that of the sea or the African desert—the former rendering the illusion, if such it may be called, most complete to the eye. The view I seek to describe embraces the orb of the setting Sun, and that of the full Moon rising in the East—both above the horizon at the same time. The spectator on the sea between, if he can discard from mental vision the vessel on which he stands, and regard only these two great globes of Heaven and the sea-horizon circling unbroken around him, gains a conception through this spectacle clearer than any other conjunction can give, of those wonderful relations which it is the triumph of astronomy to disclose. All objects are excluded save the Sun, the Moon, and our own Globe between, but these objects are such in themselves that their very simplicity and paucity of number enhances the sense of the sublime. Only twice or thrice, however, have I witnessed the sight in its completeness—once on a Mediterranean voyage between Minorca and Sardinia—once in crossing the desert from Suez to Cairo, when the same full Moon showed me, a few hours later, the very different but picturesque sight of one of the annual caravans of Mecca pilgrims, with a long train of camels making their night march towards the Red Sea.”[3]

It is due to the same cause that the Sun and the Moon when very near the horizon may often be noticed to exhibit a distorted oval outline. The fact simply is, that the upper and the lower limbs undergo a different degree of refraction. The lower limb being nearer the horizon is more affected and is consequently raised to a greater extent than the upper limb, the resulting effect being that the two limbs are seemingly squeezed closer together by the difference of the two refractions. The vertical diameter is compressed and the circular outline becomes thereby an oval outline with the lesser axis vertical and the greater axis horizontal.

Though the foregoing information merely embraces a few general principles and facts, the reader will have no difficulty in understanding that refraction exercises a very inconvenient disturbing influence on observations which relate to the exact places of celestial objects. No such observations are available for mutual comparison, however great the skill of the observer, or the perfection of his instrument, unless, and until certain corrections are applied to the observed positions in order to neutralise the disturbing effects of refraction. In practice this is usually done by means of tables of corrections, those in most general use being Bessel’s. Inasmuch as refraction depends upon the aqueous vapour in the atmosphere, its amount at any given moment is affected by the height of the barometer and the temperature of the air. Accordingly when, for any purpose, the utmost precision is required, it is necessary to take into account the height of the barometer and the position of the mercury in the thermometer at the moment in question. At the zenith there is no refraction whatever, objects appearing projected on the background of the sky exactly in the position they would occupy were the earth altogether destitute of an atmosphere at all. The amount of the refraction increases gradually, but in accordance with a very complex law, from the zenith to the horizon. Thus the displacement due to refraction which at the zenith is nothing and at an altitude of 45° is only 57″ becomes at the horizon more than ½°. One very curious consequence is involved in the fact that the displacement due to refraction is at the horizon what it is; the diameter both of the Sun and Moon may be said to be ½°, more or less, so that when we see the lower edge of either of these luminaries just touching the horizon in reality the whole disc is completely below it, and would be altogether hidden by the convexity of the earth were it not for the existence of the earth’s atmosphere and the consequent refraction of the rays of light passing through it from the Sun (or Moon) to the observer.

Twilight is another phenomenon associated with astronomical principles and effects which depends in some degree on the Earth’s atmosphere and on the laws which regulate the reflection and refraction of light. After the Sun has set it continues to illuminate the clouds and upper strata of the air just as it may often be seen shining on the tops of hills long after it has disappeared from the view of the inhabitants of the plains below, and indeed may illuminate the chimneys of a house when it is no longer visible to a person standing in the garden below. The air and clouds thus illuminated reflect some of the Sun’s light to the surface of the earth lying immediately underneath, and thus produce after sun-set and before sun-rise, in a degree more or less considerable according as the Sun is only a little or is much depressed below the horizon, that luminous glow which we call “twilight.” This word is of Saxon origin and implies the presence of a twin, or double, light. As soon as the Sun has disappeared below the horizon all the clouds overhead continue for a few minutes so highly illuminated as to reflect scarcely less light than the direct light of the Sun. As, however, the Sun gradually sinks lower and lower, less and less of the visible atmosphere receives any portion of its light, and consequently less and less is reflected minute by minute to the Earth at the observer’s station until at length the time comes when there is no sunlight to be reflected—and it is night. The converse of all this happens before and up to sun-rise; night ceases, twilight ensues, gradually becoming more definite; the dawn appears, and finally the full Sun bursts forth. It may here be stated as a note by the way that the circumstances under which the Sun first shows itself after it has risen above the horizon has some bearing on the probable character of the weather which is at hand. When the first indications of day-light are seen above a bank of clouds it is thought to be a sign of wind; but if the first streaks of light are discovered low down, that is in, or very near the horizon, fair weather may be expected.

Twilight is usually reckoned to last until the Sun has sunk 18° below the horizon, but the question of its duration depends on where the observer is stationed, on the season of the year, and (in a slight degree) on the condition of the atmosphere. The general rule is that the twilight is least in the tropics and increases as the observer moves away from the equator towards either pole. Whilst in the tropics a depression of 16° or 17° is sufficient to put an end to the phenomenon, in the latitude of England a depression of from 17° to 20° is required. As implied above, it varies with the latitude; and as regards the different seasons of the year, it is least on March 1 and October 12, being three weeks before the vernal equinox and three weeks after the autumnal equinox. The duration at the equator may be about 1 hour 12 minutes; it amounts to nearly 2 hours at the latitude of Greenwich, and so on towards the pole. At each pole in turn the Sun is below the horizon for 6 months, but as it is less than 18° below the horizon for about 3½ of those 6 months it may be said that there is a continual twilight for those 3½ months. Something of the same sort of thing as this occurs in the latitude of Greenwich, for there is no true night at Greenwich from May 22 to July 21, but constant twilight from sunset to sunrise, or 2 months of twilight in all. Though twilight at the equator is commonly set down as lasting about an hour, this period is there, as elsewhere, affected by the elevation of the observer above the sea-level. Where the air is very rarified, as at places situated as Quito and Lima are, the twilight is said to last no more than 20 minutes, and this would accord with the theory that where there is no air at all (e.g., on the Moon) there is no twilight at all. The greater purity and clearness of mountain air, rarified as it is, is another cause which contributes to vary by reducing the duration of twilight.

It is sometimes stated that a secondary twilight may be noticed, and Sir John Herschel has spoken of it as “consequent on a re-reflection of the rays dispersed through the atmosphere in the primary one. The phenomenon seen in the clear atmosphere of the Nubian Desert, described by travellers under the name of the ‘afterglow,’ would seem to arise from this cause.” I am not acquainted with any records which throw light on these remarks of Sir John Herschel.

The phenomenon of twinkling is a subject which has been much neglected, possibly on account of its apparent, but only apparent, simplicity. The familiar verse of our days of childhood—

“Twinkle, twinkle little star,

How I wonder what you are,