Observations on the Planets made in the day-time.
Some of the planets are not so easily distinguished in the day-time as the fixed stars of the first magnitude. The one which is most easily distinguished at all times, is the planet Venus.
1. Observations on Venus. My observations on this planet commenced about the end of August, 1812, about three or four weeks after its inferior conjunction. About that period, between ten and eleven in the forenoon, with a power of 45, it appeared as a beautiful crescent, quite distinct and well-defined, with a lustre similar to that of the moon about sun-set, but of a whiter colour. The view of its surface and phase was fully more distinct and satisfactory than what is obtained in the evening after sun-set; for, being at a high elevation, the undulation near the horizon did not affect the distinctness of vision. The planet was then very distinctly seen with a power of 7 times, when it appeared like a star of the first or second magnitude. I traced the variation of its phases, almost every clear day, till the month of May, 1813. As at that time, it was not far from its superior conjunction with the sun, I wished to ascertain how near its conjunction with that luminary it might be seen; and particularly whether it might not be possible, in certain cases, to see it at the moment of its conjunction.
The expressions of all astronomical writers previous to this period, when describing the phases of Venus, either directly assert, or, at least imply, that it is impossible to see that planet, in any instance, at the time of its superior conjunction. This is the language of Dr. Long, Dr. Gregory, Dr. Brewster, Ferguson, Adams, B. Martin, and most other writers on the science of astronomy. How far such language is correct will appear from the following observations and remarks.
April 24, 1813, 10h 50´ A.M. Observed Venus with a power of 30, the aperture not contracted. She was then about 31 minutes, in time, of right ascension, distant from the sun. Their difference of declination 3° 59´. She appeared distinct and well-defined. With a power of 100, could distinguish her gibbous phase. May 1st, 10h 20m, A.M. Viewed this planet with a power of 60; the aperture not contracted. It appeared distinct. Saw it about the same time with a power of 15, the aperture being contracted to 9/10 inch. Having contracted the aperture to ½ inch, saw it more distinctly. When the contracted apertures were removed, the planet could with difficulty be distinguished, on account of the direct rays of the sun striking on the inside of the tube of the telescope. The sun was shining bright, and the planet about 25´ of time in R.A. west of his centre, their difference of declination being 3° 7´. May 7th, 10h, A.M. Saw Venus distinctly with a power of 60, the sun shining bright. It was then about 19´ in time of R.A. and 4° 27´ in longitude west of the sun; their difference of declination being 2° 18´. I found a diminution of aperture particularly useful when viewing the planet at this time, even when the higher powers were applied. This was the last observation I had an opportunity of making prior to the conjunction of Venus with the sun, which happened on May 25th, at 9h 30m, A.M. Its geocentric latitude at that time being about 16´ south, the planet must have passed almost close by the sun’s southern limb. Cloudy weather for nearly a month after the last observation, prevented any further views of the planet, when it was in that part of the heavens which was within the range of the instrument. The first day that proved favourable after it had passed the superior conjunction, was June 5th. The following is the memorandum of the observation then taken.
June 5th, 9h, A.M. Adjusted the Equatorial Telescope for viewing the planet Venus, but it could not be perceived, on account of the direct rays of the sun entering the tube of the telescope. I contrived an apparatus for screening his rays, but could not get it conveniently to move along with the telescope; and therefore determined to wait till past eleven, when the top of the window of the place of observation would intercept the solar rays. At 11h 20m, A.M., just as the sun had passed the line of sight from the eye to the top of the window, and his body was eclipsed by it, I was gratified with a tolerably distinct view of the planet, with a power of 60. The aperture being contracted to 9/10 inch. The distinctness increased as the sun retired, till, in two or three minutes, the planet appeared perfectly well-defined. Saw it immediately afterwards, with a power of 30, the aperture contracted as before. Saw it also quite distinctly with a power of 15; but it could not be distinguished with this power, when the contracted aperture was removed. At this time Venus was just 3° in longitude, or about 13´ in time of R.A. east of the sun’s centre, and of course only about 2¾ degrees from his eastern limb; the difference of their declination being 27´, and the planet’s latitude 11´ north.
Several years afterwards, I obtained views of this planet, when considerably nearer the sun’s margin than as stated in the above observation, particularly on the 16th October, 1819, when Venus was seen when only 6 days and 19 hours past the time of the superior conjunction. At that time its distance from the sun’s eastern limb was only 1° 28´ 42´´. A subsequent observation proved that Venus can be seen when only 1° 27´, from the sun’s margin—which I consider as approximating to the nearest distance from the sun at which this planet is distinctly visible.—I shall only state farther the two or three following observations.
June 7th, 1813, 10h, A.M. Saw Venus with a power of 60, the aperture being contracted to 9/10 inch—the direct rays of the sun not being intercepted by the top of the window. The aperture having being further contracted to ½ inch, could perceive her, but not quite so distinctly. When the contractions were removed, she could scarcely be seen. She was then 3° 33´ in longitude, and nearly 15 minutes in time of R.A. distant from the sun’s centre. Some fleeces of clouds having moved across the field of view, she was seen remarkably distinct in the interstices—the sun at the same time, being partly obscured by them.—August 19th, 1h 10´, P.M. Viewed Venus with a magnifying power of 100. Could perceive her surface and gibbous phase almost as distinctly as when the sun is below the horizon. She appeared bright, steady in her light, and well defined, without that glare and tremulous appearance she exhibits in the evening when near the horizon. She was then nearly on the meridian. On the whole, such a view of this planet is as satisfactory, if not preferable, to those views we obtain with an ordinary telescope in the evening, when it is visible to the naked eye.
All the particulars above stated have been confirmed by many subsequent observations continued throughout a series of years. I shall state only two recent observations which show that Venus may be seen somewhat nearer the sun than what is deduced from the preceding observations, and at the point of its superior conjunction. March 10th, 1842, observed the planet Venus, then very near the sun, at 19 minutes past 11, A.M. It had passed the point of its superior conjunction with the sun, on the 5th March, at 1h 19m, P.M. The difference of right ascension between the sun and the planet was then about 6½ minutes of time, or about 1° 37½´, and it was only about 1° 21´ distant from the sun’s eastern limb. It appeared quite distinct and well-defined, and might perhaps have been seen on the preceding day, had the observation been then made.—The following observation shows that Venus may be seen still nearer the sun than in the preceding observations, and even at the moment of its superior conjunction. On the 2nd of October, 1843, this planet passed the point of its superior conjunction with the sun, at 4h 15m, P.M. At two o’clock, P.M.—only two hours before the conjunction, I perceived the planet distinctly, and kept it in view for nearly ten minutes, till some dense clouds intercepted the view. It appeared tolerably distinct and well-defined, though not brilliant, and with a round full face, and its apparent path was distinctly traced several times across the field of view of the telescope. I perceived it afterwards, about half past four, P.M., only a few minutes after it had passed the point of conjunction, on which occasion it appeared less distinct than in the preceding observation, owing to the low altitude of the planet, being then only a few degrees above the horizon. The observations, in this instance, were made not with an equatorial instrument, which I generally use in such observations, but with a good achromatic telescope 44½ inches focal distance, mounted on a common tripod, with a terrestrial power of 95 times. A conical tube about ten inches long was fixed on the object-end of the telescope, at the extremity of which an aperture, 1½ inch diameter was placed, so as to intercept, as much as possible, the direct ingress of the solar rays. The top of the upper sash of the window of the place of observation was likewise so adjusted as to intercept the greater part of the sun’s rays from entering the tube of the telescope. The sun’s declination at that time was 3° 26´ south, and that of Venus 2° 12´ south; consequently, the difference of declination was 1° 14´ = the distance of Venus from the sun’s centre; and as the sun’s diameter was about 16´, Venus was then only 58´ from the sun’s northern limb, or 6´ less than two diameters of the sun.
This is the nearest approximation to the sun at which I have ever beheld this planet, and it demonstrates that Venus may be seen even when within a degree of the sun’s margin; and it is perhaps the nearest position to that luminary in which this planet can be distinctly perceived. It shows that the light reflected from the surface of Venus is far more brilliant than that reflected from the surface of our moon; for no trace of this nocturnal luminary can be perceived, even when at a much greater distance from the sun, nor is there any other celestial body that can be seen within the limit now stated. This is the first observation, so far as my information extends, of Venus having been seen at the time of her superior conjunction.[41]