For the purpose of illustrating the descriptions now given, and for affording some information respecting celestial day observations, I shall select a few of the observations above alluded to, which I formerly published in Nicholson’s Journal, along with a few others which have been since made. These observations were made with a view to determine the following particulars:—1. What stars and planets may be conveniently seen in the day-time, when the sun is above the horizon? 2. What degrees of magnifying power are requisite for distinguishing them? 3. How near their conjunction with the sun they may be seen? and 4. Whether the diminution of the aperture of the object-glass of the telescope, or the increase of magnifying power, conduces most to render a star or a planet visible in day-light. Having never seen such observations recorded in books of astronomy or in scientific journals, I was induced to continue them, almost every clear day for nearly a year, in order to determine the points now specified. Some of the results are stated in the following pages.
Observations on fixed stars of the first magnitude. April 23, 1813, at 10h 15m, A.M., the sun being 5½ hours above the horizon. Saw the star Vega, or α Lyræ, very distinctly with a power of 30 times. Having contracted the aperture of the object-glass to 9/10 of an inch, saw it on a darker ground, but not more plainly than before. Having contracted the aperture still farther, to half an inch, I perceived the star, but not so distinctly as before. The sky being very clear, and the star in a quarter of the heavens nearly opposite to the sun, I diminished the magnifying power to 15, and could still perceive the star, but indistinctly; it was just perceptible. August 23, at 0h 12m, P.M., saw the star Capella, or α Aurigæ, with a power of 60, and immediately afterwards with a power of 30; the aperture undiminished. With this last power it appeared extremely distinct, but not so brilliant and splendid as with the former power. Having diminished the aperture to 9/10 of an inch, it appeared on a darker ground, though in the former case, it was equally perceptible. A few minutes afterwards, could distinguish it with a power of 15, the aperture being contracted to half an inch. It appeared very small; it was with difficulty the eye could fix upon it in the field of the telescope; but when it was once perceived, its motion across the field of view could be readily followed. It could not be perceived, when the diminished aperture was removed. The sun was then shining in meridian splendour.
August 10th, 9h 30m, A.M. Saw the star Sirius with a power of 60, the aperture contracted to 9/10 inch. Saw it likewise when the aperture was diminished to half an inch, but not so distinctly as through the aperture of inch. Having put on a power of 30, could distinguish it distinctly enough through each of the former apertures, and likewise when they were removed; but somewhat more distinctly with the apertures of nine-tenths and half an inch than without them. At this time the star was 2h 42m in time of Right Ascension west of the sun, having an elevation above the horizon of about 17° 10’; the sun shining bright, and the sky very much enlightened in that quarter of the heavens where the star appeared. There was also a considerable undulation of the air, which is generally the case in the hot mornings of summer—which renders a star more difficult to be perceived than in the afternoon, especially when it is viewed at a low altitude. June 4th, 1h 30m, P.M., saw Sirius with a power of 30 with great distinctness, the aperture not contracted. The star was then within 1h 50m, in time of Right Ascension east from the sun. August 24th, 9h 5m, A.M., saw the star Procyon, or α Canis-Minoris distinctly with a power of 60, the aperture not contracted. When diminished to 9/10 inch, it appeared rather more distinct, as the ground on which it was seen was darker. With a power of 30, and the aperture contracted to 9/10 inch, could perceive it, but somewhat indistinctly. When the equatorial motion was performed, in order to keep it in the field of view, it was sometime before the eye could again fix upon it. When the aperture was diminished to half an inch, it could not be perceived. Saw it when both the apertures were removed, but rather more distinctly with the aperture of 9/10 inch. The difference in the result of this observation, from that of Capella, above stated, was owing to the star’s proximity to the sun, and the consequent illumination of the sky in that quarter where it appeared. Its difference in Right Ascension from that of the sun was then about 2h 5m of time, and its difference of declination about 4° 50´.[40] This star may be considered as one of those which rank between the first and second magnitudes.
Similar observations to the above were made and frequently repeated on the stars Rigel, Aldebaran, Betelguese Cor-Leonis and other stars of the first magnitude, which gave nearly the same results. The stars Altares and Fomalhaut are not so easily distinguished, on account of their great southern declination, and consequent low elevation above the horizon. The following observation on Arcturus may be added. June 3rd, observed Arcturus very distinctly, a little before 7 in the evening, the sun being about 1h 40m above the horizon, and shining bright—with a power of 15; the aperture not contracted. It appeared very small but distinct. This star is easily distinguishable at any time of the day with a power of 30.
Observations on stars of the second magnitude. May 5, 1813, at 6h, P.M.; the sun being an hour and three quarters above the horizon. Saw Alphard, or α Hydræ, a star of the second magnitude, with a power of 60; the aperture diminished to 9/10 inch. A few minutes afterwards could perceive it, but indistinctly, with a power of 30, the aperture contracted as above. It could not be seen very distinctly with this power, till about half an hour before sun-set. It was then seen rather more distinctly when the aperture was contracted than without the contraction. May 7th. Saw the star Deneb, or β Leonis, distinctly with a power of 60, about an hour and a half before sun-set. August 20th. Saw Ras Alkague, or α Ophiuchi, at 4h 40m, P.M., with a power of 100, the sun being nearly 3 hours above the horizon, and shining bright. Perceived it about an hour afterwards, with a power of 60—with the aperture contracted to 9/10 inch, and also when this contraction was removed. The star was seen nearly as distinctly in the last case as in the first. August 27, 5h, P.M., the same star appeared quite distinct with a power of 60, the aperture not contracted. It did not appear more distinct when the aperture was contracted to 9/10 inch. The sun was then more than 2 hours above the horizon. August 28th. Saw the star Pollux, or β Gemini, 2 hours after sun-rise with a power of 60, aperture undiminished. November 12th, 1h 30´, P.M. Saw the star Altair, or α Aquilæ, with an 8½ inch telescope, 1 inch aperture, carrying a power of 45, the aperture not contracted. Having contracted the aperture a little, it appeared somewhat less distinct. This star is reckoned by some to belong to the class of stars of the first magnitude; but in White’s ‘Ephemeris’ and other Almanacks, it is generally marked as being of the second magnitude. It forms a kind of medium between stars of the 1st and of the 2nd magnitude.
Similar observations, giving the same results, were made on the stars Bellatrix, Orion’s Girdle, α Andromedæ, α Pegasi, Alioth, Benetnasch, North Crown, or α Coronæ Borealis, and various other stars of the same magnitude.
From the above and several hundreds of similar observations, the following conclusions are deduced.
1. That a magnifying power of 30 times is sufficient for distinguishing a fixed star of the first magnitude, even at noon-day, at any season of the year; provided it have a moderate degree of elevation above the horizon, and be not within 30° or 40° of the sun’s body. Also, that, by a magnifying power of 15, a star of this class may be distinguished, when the sun is not above an hour and a half above the horizon. But, in every case, higher powers are to be preferred. Powers of 45 or 60, particularly the last, were found to answer best in most cases, as with such powers the eye could fix on the star with ease, as soon as it entered the field of the telescope.
2. That most of the stars of the 2nd magnitude may be seen with a power of 60, when the sun is not much more than 2 hours above the horizon; and, at any time of the day, the brightest stars of this class may be seen with a power of 100, when the sky is serene, and the star not too near the quarter in which the sun appears.
3. That, in every instance, an increase of magnifying power has the principal effect in rendering a star easily perceptible. That diminution of aperture, in most cases, produces a very slight effect; in some cases, none at all; and, when the aperture is contracted beyond a certain limit, it produces a hurtful effect. The cases in which a moderate contraction is useful, are the two following:—1. When the star appears in a bright part of the sky, not far from that quarter in which the sun appears. 2. When an object-glass of a large aperture, and a small degree of magnifying power, is used. In almost every instance the contraction of the object-glass of the 8½-inch telescope with a power of 45, had a hurtful effect. But when the 20-inch telescope carried a power of only 15, the contraction served to render the object more perceptible.