(_For the Mirror._)
It has been computed, that all the celestial orbs perceived by the
unassisted eye (which on a clear night never exceed 1,000,) do not form
the 80,000 part of those which may be descried by the help of a
telescope, through which they appear prodigiously increased in number;
seventy stars have been counted in the constellation of the _pleiades_,
and no fewer than 2,000 in that of _Orion_.
The _galaxy_, or _via lactea_, (milky way,) is a remarkable appearance
in the heavens, being a broad ray of whitish colour surrounding the
whole celestial concave, whose light proceeds from vast clusters of
stars, discoverable only by the telescope. Mr. Brydone, in his journey
to the top of Mount Etna, found the phenomenon make a most glorious
appearance, "like a pure flame that shot across the heavens."
Dr. Herschel made many observations on this portion of the heavens,
using a Newtonian reflector of twenty feet focal length, and an aperture
of eighteen inches. With this powerful telescope he completely resolved
the whitish appearance into stars, which the telescopes he had formerly
used had not light enough to do. In the most vacant place to be met with
in that neighbourhood, he found sixty-three stars; other six fields, or
apparent spaces in the heavens, which he could see at once through his
telescope, averaged seventy-nine stars in each field: thus he found that
by allowing 15 min. of a deg. for the diameter of his field of view, a
belt of 15 deg. long, and 2 deg. broad, which he had often seen pass
before his telescope in an hour's time could not contain less than
50,000 stars, large enough to be distinctly numbered, besides which he
suspected twice as many more, which could be seen only now and then by
faint glimpses, for want of sufficient light. In the most crowded part
of that region he informs us, he has had fields of view which contained
no less than 588 stars, and these were continued for many minutes, so
that in one quarter of an hour's time there passed no less than 116,000
stars. He also intimates the probability of the sun being placed in this
great stratum, though perhaps not in the very centre of its thickness.
From the appearance of the galaxy it seems to encompass the whole
heavens, as it certainly must if the sun be within the same. From
succeeding observations made by Dr. Herschel, he gathers that the milky
way is a most extensive stratum of stars of various sizes, and our sun
evidently one of the heavenly bodies belonging to it. In viewing and
gauging this shining zone in almost every direction, he found the number
of stars composing it, by the account of those gauges constantly
increase and decrease in proportion to its apparent brightness to the
naked eye.
The _nebulæ_, or small whitish specks, discoverable by telescopes in
various parts of the heavens are owing to the same cause. Former
astronomers could only reckon 103, but Herschel counts upwards of 1,250.
He has also discovered a species of them, which he calls planetary
nebulæ, on account of their brightness, and shining with a well
defined disk.
The sun enters _Virgo_ on the 23rd at 11h. 42m. evening.
Mercury comes to his inferior conjunction on the 13th at 1-1/4h.
morning, becomes stationary on the 22nd, and is at his greatest
elongation on the 31st, when he passes his ascending node; he may be
seen early on that morning rising at 3-1/2h.
Venus is in conjunction with Mars on the 21st at 3h. afternoon; she
rises on the 1st at 2h. 38m., and on the 31st at 4h. 10m. morning.
Jupiter still continues a conspicuous object in the western part of the
heavens, setting on the 1st at 9h. 43m., and on the 31st at 8h. None of
the eclipses of his satellites are visible during the month in
consequence of his being so near the sun.
Herschel comes to the south on the 1st at 11h. 6m., and on the 31st at
9h. 43m. evening.
_Spica virginis_ (the virgin's spike), in the constellation Virgo
culminates on the 1st at 4h. 32m. afternoon, being situated 10 deg. 13m.
south of the equator, at a meridional elevation of 28 deg. 26m.
_Arcturus_ in Bootes south at 5h. 23m. with 20 deg. north delineation,
and at an altitude of 58 deg. 46m. _Antares_ in the heart of Scorpio at
7h. 34m., declination 26 deg. south, elevation 12 deg. 38m. _Altair_ in
the Eagle at 10h. 57m., declination 8 deg. 24m. north, altitude 47 deg.
3m. _Fomalhaut_ in the most southern fish of the constellation Pisces at
2h. 6m. morning, having a southern declination of 30 deg. 34m., being
elevated only 8 deg. 5m. above the horizon. The above stars come to the
meridian 4 min. earlier every evening; they are all of the first
magnitude (with the exception of _Altair_, which is of the second,) and
may be easily distinguished any hour of the day with a magnifying power
of thirty times; stars of the second magnitude require a power of 100,
but when the sun is not more than two hours above the horizon, they may
be seen with a power of sixty.