LECTURES ON ASTRONOMY.
BY PROF. ORSON PRATT.
LECTURE EIGHTH.
MERCURY.
We shall next proceed to give a brief sketch of the primary planets of the solar system; the names of which we here give in the order of their distances from the sun, namely: Mercury, Venus, Earth, Mars, Flora, Vesta, Iris, Metis, Hebe, Astraea, Juno, Ceres, Pallas, Jupiter, Saturn, Uranus, and Neptune. Of these, the earth has already been under our investigation; and the principal phenomena exhibited in relation to its motions, the form of its orbit, the time of its revolution, &c., have been explained. And as there are many of these characteristics similarly manifested by all the rest of the planets, it will be unnecessary to repeat the illustrations in each individual case of these bodies.
The planet Mercury is the first in order. This planet performs a revolution around the sun at the mean distance of about 36,000,000 of miles; its periodic time is about 88 days; its diameter is about 3,200 miles; its bulk is about 16 times less than our globe. This planet is supposed to be placed the nearest to the sun, yet it is possible that there may be several planetary bodies still nearer, as it would be very difficult to observe such bodies, even though we were assured of their existence; the bright glare of the sun would probably shield them from our view.
It is very likely, that if such bodies existed, we should occasionally behold them crossing, like small black dots, the sun’s disc; but no such phenomena have been observed. It is also very probable that if such bodies existed, their perturbations upon the planet Mercury would be rendered visible. The gravitating force of each planet affects every other planet, more or less, according to the distance and quantity of matter in the disturbing planet. These mutual disturbances are such as to cause the planets to deviate from the true elliptic paths which they would pursue if no such disturbance existed. Now the planet Mercury does not appear to be disturbed by any planetary body within its orbit; and, therefore, from these negative evidences we have grounds to suppose Mercury to be the nearest planet to the sun.
The orbit of Mercury is enclosed within that of the earth’s, being about 59,000,000 of miles from the path pursued by our globe. It follows, therefore, that the illuminated hemisphere of Mercury will assume every variety of position in relation to the earth; for Mercury is an opaque body and only shines as it is shone upon. That hemisphere which is turned towards the sun will be highly illuminated, while the opposite hemisphere will be in the dark and consequently invisible to us. Now the illuminated hemisphere will be turned from us, when Mercury is in that portion of its orbit between the sun and earth, consequently it will be invisible. In this position the planet is said to be in its inferior conjunction. When the planet is in that portion of its orbit most distant from the earth, or when it is nearly on the opposite side of the sun from us, it is said to be in its superior conjunction; it is then that the sun illuminates the hemisphere of the planet which is turned towards the earth; and if the bright glare of the sun did not prevent us from seeing the planet, its disc would appear round and full. At all intermediate positions between the inferior and superior conjunctions, the planet will assume every phase exhibited by our moon between her conjunction and opposition; if it were possible to perceive through the telescope, the planet, when near its inferior conjunction, it would appear like a crescent, becoming more and more slender the nearer it approaches the line between the earth and sun; this crescent would have precisely the shape of the new moon. As the planet recedes from its inferior conjunction, so as to be in a line drawn from the sun at right angles to the line of vision by which we see the planet, it will then appear like a half moon, one-half of the enlightened hemisphere being visible, and the other half invisible because it is turned from us. From this point as it still recedes from us towards its superior conjunction, a greater portion of the enlightened hemisphere gradually heaves in view, exhibiting a gibbous phase precisely of the shape of the moon between the full and second and third quarters; but as it now approaches its superior conjunction, the great intensity of the sun’s rays will overpower the feeble rays of reflected light from the planet, and it will be hid in the glorious splendor of day. It is therefore, only for a few days, when near its greatest eastern and western elongation, that it can be easily seen with the naked eye.
What we mean by the planet’s greatest eastern and western elongation is that position in its orbit, when just one-half of the enlightened hemisphere is turned towards the earth; this happens, on an average, about six or seven times during the year, alternately on the east and then on the west of the sun. The greatest distances at which the planet is seen on the east and west of the sun, varies from 16 deg. 12 min. to 28 deg. 48 min. The principal cause of this variation is the great eccentricity of the elliptic orbit of Mercury, which is nearly one-fourth of the planet’s mean distance from the sun; and as the sun is situated in one focus of the ellipse, it is evident, that when the earth is in a line at right angles to the major axis of Mercury’s orbit, the elongations which happen at that time near the perihelion, will be only a little over one-half what they would be at or near the aphelion point of the orbit.
When the planet is at its greatest eastern elongation, it will be seen in the west just after sunset; and when it is at its greatest western elongation, it will be seen in the morning just before sunrise. – When the planet is seen with the naked eye, it exhibits a very brilliant white light, like Venus, only much smaller. The best or most favorable seasons of the year to view this planet is when its greatest elongations happen, in the months of March or April, or in August or September.
The further you recede from the orbit of Mercury the more difficult will it be to perceive this planet. It is extremely probable that the inhabitants on the most distant planets of our system have never been favored with a view of Mercury, and are, it is presumed, altogether ignorant of its existence, unless they have seen it apparently crossing the sun’s disc, like a small black point; for in all other positions it would be overwhelmed in the brightness of the solar rays.
We have already observed that Mercury revolves around the sun in about 88 days; this, therefore, is the length of one year to the inhabitants of that planet; each of the four seasons will alternately take place in the short period of 22 days. During the time that Mercury performs one complete revolution around the sun, the earth performs about one-quarter of its revolution; hence if they both set out together from the inferior conjunction, that planet has to describe one complete revolution and about one-third of another, in order to bring itself again into conjunction with the sun and earth; this requires a period of about 116 days, and is called the synodical period. In this period happens all the phases which have been described. The real time of one complete revolution is called the sidereal period, and is exactly equal to 87 days, 23 hours, 15 min., 43.9 sec.
If the orbit of Mercury were in the same plane as that of the earth, at each inferior conjunction, that planet would through the telescope be observed to enter the sun’s disc on the eastern limb, and pass across the same to the western limb being observed as a small dark spot, and requiring from five to seven hours to traverse the disc. But as the orbit of Mercury, instead of being coincident with the plane of the earth’s orbit, is considerably inclined to that plane, namely, 7 deg. 0m 9.1s, the transits do not happen only at intervals of several years. The transits of Mercury that will happen during the present century, will take place as follows:
HOURS. MINUTES.
Visible, 1861, Nov.12, 11 52 A.M.
Invisible, 1868, Nov 4, 11 16 P.M.
Visible, 1878, May 6, 11 10 A.M.
Visible for a
short time, 1881, Nov. 7, 5 12 P.M.
Visible for a
short time, 1891, May 9, 7 17 P.M.
Visible, 1894, Nov. 10, 10 49 A.M.
The time of the transits here expressed is the mean civil time at Great Salt Lake City, which place we have assumed to be 7 hours and 28 minutes west of Greenwich. We have only given the time in which they will be seen in the middle of their paths at Greenwich. They will be seen nearly in the same position on the sun’s disc from this city, as from Greenwich. The few seconds deviation will be principally owing to the parallax. – These transits will require the telescope to render them visible.
Owing to Mercury’s dazzling appearance, it is very difficult to discover any prominent marks upon its surface, yet Schroeter, a German astronomer, has been enabled, not only to discern spots, but mountains also upon its disc. The height of two of these mountains, he has calculated; one of which is about 1 1-4 miles high; the other about 10 3-4 miles, or about 8 times higher than the twin peaks on the east of this valley. The highest mountains on Mercury are said to be situated in the southern hemisphere of that planet.
When the horns or cusps of this planet are carefully observed at the time when it appears of a crescent form, it is ascertained that they vary from day to day. By these variations, the time of the rotation of the planet is determined to be 24 hours, 5 minutes, and 28 seconds. Hence, the length of days and nights upon that planet do not differ, only by a very small fraction from those enjoyed upon our globe.
The intensity of light which Mercury enjoys, is far greater than what is enjoyed on the surface of the earth. It can easily be demonstrated, that the intensity of light varies inversely as the square of the distance. Therefore, if we divide the square of the earth’s distance from the sun by the square of Mercury’s distance, the quotient will be about 6 2-3. Consequently the intensity of light upon that planet is about 6 2-3 times greater than upon the earth. As the apparent disc of the sun is also in proportion to the square of the distance, it follows that the resplendant orb of day will appear nearly 7 times greater to the inhabitants of Mercury than what he appears to us. Consider, for a moment, the mountains, and vallies, and all the objects with which we are surrounded, illuminated with a seven fold splendor. Such a brilliancy would be far too great for the present constitution of our eyes; if the pupils of our eyes were contracted to about one-seventh part of their present dimensions, we should still be able, under the influence of a seven fold intensity of light, to perceive every object with the same ease and distinctness that we do now.
The splendor of the scenery upon Mercury, must be magnificently grand; the vividness of colors, radiated from surrounding objects, must be exquisitely beautiful, and the whole landscape must be adorned, as if with a gorgeous robe of light.
While Mercury enjoys 6 2-3 times more light than the earth, the planet Neptune only enjoys the 1-900 part as much as the earth; for Neptune is about 30 times the distance of the earth from the sun. The square of 30 is 900; hence the intensity of the sun’s light on Neptune is 900 times less than what we receive; 900 multiplied by 6 2-3 is equal to 6000; therefore the light on Mercury is 6000 times greater than the light on Neptune. The sun at the distance of the outermost planet, discovered in the system, will subtend an angle of 1 deg. 4 sec.; and consequently will only appear about the size of the planet Venus when nearest to the earth; or, in other words, the sun’s apparent diameter will be only about one-half greater than the planet Jupiter’s when in opposition.
Many have supposed heat to follow the same law as light; if so, Neptune would have 900 times less heat from the sun than what we experience; and the proportion of the sun’s heating power at the extremities of our system, would be as 1 to 6000. – But independent of the heating power of the sun, the planetary spaces and worlds no doubt have a natural temperature of their own, modified more or less, by the heat of the sun, according to their proximity to that great luminary. One cause of the common temperature, which we have great reason to believe, exists in the celestial regions, is the combination or united heat, emanating from the fixed stars, which are known to be great suns similar to our own.
Heat is no doubt generated, or, rather, set free by the chemical action of the materials of which the planets consist. From these two sources, it is very probable that the planets are maintained at a temperature far greater than what they would enjoy if they were dependent upon the sun alone.
The amount of common temperature, existing in the celestial regions, is very likely in proportion to the amount of star light; if so, it is comparatively easy to calculate the amount or degree of this temperature; this has been calculated by a great number of different methods, and they all concur in showing that it does not differ much from 58 degrees below the zero of Fahrenheit’s scale, or about 90 degrees below the freezing point of water; now this is a degree of cold much less than what we are capable of producing artificially. Such a common temperature would operate to greatly diminish the cold that would otherwise exist in the more distant extremities of our solar system.
As Mercury is the nearest planet to the sun, its velocity, according to the law of mechanics governing centrifugal forces, is greater than that of any other planet. Its average velocity is equal to about 109,800 miles every hour; but as its orbit is much more eccentric than the earth’s, its velocity varies from the mean to a much greater extent than that of the earth’s. Its average velocity is 1830 miles every minute, and over 30 miles every second.
The density of Mercury is about 6 times greater than water, or about the density of lead ore, that is a globe of lead ore 3140 miles in diameter would just balance the planet Mercury; this is considerably greater than the density of the earth, and greater than the density of any other planet in the system.
The mass of this planet is 4,865,751 times-less than that of the sun’s. But as the materials of which Mercury consists is much heavier than the sun’s materials, its bulk is about 4 1-2 times less than it would be if composed of materials similar to the sun’s; consequently it would require about 22,000,000 of globes of the size of Mercury to compose one as large as the sun. The weight of bodies on the surface of Mercury is only about one half as much as they would weigh at the surface of the earth.
Venus.
The planet Venus is the brightest and most conspicuous luminary, the sun and moon excepted, that shines in the heavens. Her diameter is 7800 miles; she is, therefore, about the size of the earth, and revolves in an orbit from west to east, at the distance of 68,000,000 of miles from the sun; consequently, when situated in the nearest point of her orbit, she is only about 27,000,000 of miles from the earth; and is the nearest to us of all the primary planets.
Her orbit being enclosed within that of the earth’s, she never departs over about 48 deg. from the sun; hence she is never seen in the south during the night; and is never seen east of south for many hours after sunset; and never appears west of south for many hours before sunrise. When in that point of her orbit the most distant from us, she is in her superior conjunction, being 163,000,000 of miles from the earth; and when in that point of her orbit between the earth and sun, she is in her inferior conjunction, and is then over 6 times nearer to us than when in the other conjunction. If the whole of the hemisphere of Venus, turned towards us, when at the inferior conjunction, were enlightened, so as to be visible, she would present a disc over 36 times greater than she appears at her superior conjunction. But when at the inferior conjunction, her dark hemisphere is turned towards the earth, which renders her invisible.
The time occupied by Venus in passing from her inferior conjunction to the same conjunction again, is about 584 days. For about 35 hours previous to and after the inferior conjunction, Venus cannot by the telescope be easily seen; and for about 6 3-4 days preceding and following the superior conjunction, it is difficult, if not impossible, to see Venus in consequence of the splendor of the sun’s rays. Therefore, by the aid of the telescope, Venus may be rendered visible 567 1-2 days out of 584; while to the unassisted eye, she will be visible only about 3-4 of her synodical period, or 440 days. The real sidereal period of Venus is 224.7007869 mean solar days. One synodical revolution of Venus is more than 2 1-2 times longer than her real period.
Venus, like the planet Mercury, passes through every variety of phase, similar to the moon; these phases are easily perceived by the telescope; some times they appear of a crescent shape; sometimes half the disc is seen; at other times they are gibbous; and at other times they appear full.
Soon after this planet passes her inferior conjunction, she will appear a short distance west of the sun, and consequently will be seen just before sunrise in the east; each succeeding morning she will rise a little earlier, moving gradually to the westward of the sun, until it attains to its greatest western elongation, when, for a few days, it apparently remains nearly stationary, and then moves gradually to the east until it arrives at its superior conjunction.
From the inferior to the superior conjunction embracing a period of about nine months, Venus is called the morning star, though for several days when near the conjunction, she is not visible to the naked eye. After passing her superior conjunction and emerging from the sun’s rays, she will appear on the east of the sun, and will be seen soon after sunset. And each succeeding evening she will appear to have advanced to the eastward until she attains to her greatest eastern elongation, when she will, for a short period, appear to remain stationary and then to apparently retrograde towards the sun or to the west; when near her inferior conjunction she will again be lost in the glare of the sun’s light. At the greatest eastern and western elongations, Venus never rises above the eastern and western horizons over about 48 deg, or about one-half the distance from the horizons to the meridian.
By the motions of the telescopic spots observed on Venus, it has been ascertained that it has a rotation upon its axis in 23 hours and 21 minutes.
The position of the axis of a planet is determined by observing the direction of the spots across its disc; this has not been, as yet, very accurately observed in regard to Venus; but from the few imperfect observations which have been taken, it is believed, that the axis of Venus has a position in reference to its orbit very different from that of the earth; while the axis of the earth is inclined only 23 deg. 28 min. from the perpendicular, the axis of Venus appears to be inclined 75 deg. from the perpendicular to the plane of its orbit. This circumstance will render the seasons on Venus far more changeable than upon the earth. The inhabitants in the same latitude, will, in the period of 225 days experience every vicissitude of climate, manifested in the torrid, temperate, and frigid zones of the earth. The inclination of the axis of Mercury is also believed to be much greater than that of the earth; if so, the variety of changes in Mercury’s seasons will be much greater than what is experienced on our globe.
The degree of heat and light received from the sun on Venus will be nearly double the amount which we receive; and the sun’s disc will appear about as large again to the inhabitants of Venus as it does to us. But, the temperature will doubtless be greatly modified by surrounding circumstances such as the density of the atmosphere–the amount of clouds surrounding the body, and the nature of the materials composing its surface.
The telescope reveals several large mountains on the planet Venus, one of which is stated to be 22 miles in perpendicular height; another 19 miles, a third 11 1-2 miles, and a fourth 10 3-4 miles in elevation. These mountains are far higher than any upon our globe.
We can form some conception of the awful grandeur of these towering elevations, by imagining the chain of mountains bounding this valley on the east to be raised up 17 times higher than they are now. Such a mountain scenery would be worth visiting while the view from the top of such an elevation would scope in many hundreds of miles in all directions.
From careful observations of the twilight observed on Venus, it is believed that her atmosphere is very dense for some 3 or 4 miles above the surface and consequently that her atmosphere extends far above the highest of our mountains. If these observations and calculations can be depended upon, the surface of Venus will be in a measure protected from the intense glare of the sun’s rays, and consequently it can be inhabited by beings not differing materially in their constitution, from us.
Several observers have been of the opinion that Venus is accompanied by a satellite, although seldom seen. A luminous appearance has occasionally been seen a short distance from the disc of Venus, exhibiting the same kind of phase as Venus and about one-fourth the diameter of the planet; it has been observed to move, and its supposed period is about 11 days, 5 hours, and 13 minutes. Its distance from Venus is supposed to be about 259,000 miles. The inclination of its orbit to the ecliptic is very great, being about 63 3-4 degrees. The observations on which these calculations are founded, being very imperfect, cannot fully be relied upon, therefore astronomers are still doubtful whether such a satellite exist. It is very evident that if Venus have such a satellite, it could not very easily be discovered. The most favorable positions of Venus for discovering its satellite, if it have any, are when that planet is near its greatest elongations east and west, or within about 40 deg. of the sun; for then the amount of light reflected from the satellite and reaching the eye would be greater than in any other position.
The transit of Venus across the sun’s disc takes place about twice in one century. In 1874, December 8th, at 8 hours and 40 minutes in the evening mean time at Salt Lake, there will be a transit of Venus; but being after sunset it will not be visible in this city. There will be another transit which will be visible in Utah Territory—the middle of the transit will happen about 8 hours 48 minutes in the morning of December 6, 1882 mean time at Salt Lake city. We have already observed in some of our former lectures, that the transit of Venus affords the surest and best method of finding the true distance of the sun from the earth.
Venus revolves in an orbit 433,800,000 miles in circumference. Its average velocity per hour is about 80,000 miles; equal to about 1330 miles every minute, and above 22 miles every second. The density of Venus and the earth is about the same and bodies will weigh nearly the same at their respective surfaces. The orbit of Venus approaches nearer to a circle than any of the rest of the planets its eccentricity being only about 492,000 miles, or the 1-138 part of its mean distance from the sun. The inclination of the orbit of Venus to the ecliptic is only 3 deg. 23 min., 28.5 sec.; therefore its greatest deviation from the ecliptic either north or south, never exceeds seven apparent diameters of the sun. Its mean apparent diameter is 17 sec., and its greatest about 57 1-3s. Its mean arc of retrogradation from east to west, contrary to the order of the signs, is 16 deg., 12m., and its mean duration about 42 days.
We have thus given a very brief outline of the principal phenomena characterizing the two interior planets of the solar system. Much more might be said upon these interesting subjects; but we shall necessarily have to be very brief in order to include within the small compass of twelve lectures the most striking and interesting particulars, pertaining to this grand and sublime science of the heavens.
[Transcribed by Cheryl Brawn, Marlene Peine, and Mauri Pratt; June 2012]
Orson Pratt. “Lectures on Astronomy: Lecture Eighth.”, Unknown newspaper, May 29, 1852.