HALF-HOURS WITH THE SUN AND MOON.
The moon perhaps is the easiest of all objects of telescopic
observation. A very moderate telescope will show her most striking
features, while each increase of power is repaid by a view of new
details. Yet in one sense the moon is a disappointing object even to the
possessor of a first-class instrument. For the most careful and
persistent scrutiny, carried on for a long series of years, too often
fails to reward the observer by any new discoveries of interest. Our
observer must therefore rather be prepared to enjoy the observation of
recognised features than expect to add by his labours to our knowledge
of the earth's nearest neighbour.
Although the moon is a pleasing and surprising telescopic object when
full, the most interesting views of her features are obtained at other
seasons. If we follow the moon as she waxes or wanes, we see the true
nature of that rough and bleak mountain scenery, which when the moon is
full is partially softened through the want of sharp contrasts of light
and shadow. If we watch, even for half an hour only, the changing form
of the ragged line separating light from darkness on the moon's disc, we
cannot fail to be interested. "The outlying and isolated peak of some
great mountain-chain becomes gradually larger, and is finally merged in
the general luminous surface; great circular spaces, enclosed with rough
and rocky walls many miles in diameter, become apparent; some with flat
and perfectly smooth floors, variegated with streaks; others in which
the flat floor is dotted with numerous pits or covered with broken
fragments of rock. Occasionally a regularly-formed and unusually
symmetrical circular formation makes its appearance; the exterior
surface of the wall bristling with terraces rising gradually from the
plain, the interior one much more steep, and instead of a flat floor,
the inner space is concave or cup-shaped, with a solitary peak rising in
the centre. Solitary peaks rise from the level plains and cast their
long narrow shadows athwart the smooth surface. Vast plains of a dusky
tint become visible, not perfectly level, but covered with ripples,
pits, and projections. Circular wells, which have no surrounding wall
dip below the plain, and are met with even in the interior of the
circular mountains and on the tops of their walls. From some of the
mountains great streams of a brilliant white radiate in all directions
and can be traced for hundreds of miles. We see, again, great fissures,
almost perfectly straight and of great length, although very narrow,
which appear like the cracks in moist clayey soil when dried by the
sun."[14]
But interesting as these views may be, it was not for such discoveries
as these that astronomers examined the surface of the moon. The
examination of mere peculiarities of physical condition is, after all,
but barren labour, if it lead to no discovery of physical variation. The
principal charm of astronomy, as indeed of all observational science,
lies in the study of change--of progress, development, and decay, and
specially of systematic variations taking place in regularly-recurring
cycles. And it is in this relation that the moon has been so
disappointing an object of astronomical observation. For two centuries
and a half her face has been scanned with the closest possible scrutiny;
her features have been portrayed in elaborate maps; many an astronomer
has given a large portion of his life to the work of examining craters,
plains, mountains, and valleys, for the signs of change; but until
lately no certain evidence--or rather, no evidence save of the most
doubtful character--has been afforded that the moon is other than "a
dead and useless waste of extinct volcanoes." Whether the examination of
the remarkable spot called Linné--where lately signs were supposed to
have been seen of a process of volcanic eruption--will prove an
exception to this rule, remains to be seen. The evidence seems to me
strongly to favour the supposition of a change of some sort having taken
place in this neighbourhood.
The sort of scrutiny required for the discovery of changes, or for the
determination of their extent, is far too close and laborious to be
attractive to the general observer. Yet the kind of observation which
avails best for the purpose is perhaps also the most interesting which he
can apply to the lunar details. The peculiarities presented by a spot upon
the moon are to be observed from hour to hour (or from day to day,
according to the size of the spot) as the sun's light gradually sweeps
across it, until the spot is fully lighted; then as the moon wanes and the
sun's light gradually passes from the spot, the series of observations is
to be renewed. A comparison of them is likely--especially if the observer
is a good artist and has executed several faithful delineations of the
region under observation, to throw much light upon the real contour of the
moon's surface at this point.
In the two lunar views in Plate 7 some of the peculiarities I have
described are illustrated. But the patient observer will easily be able
to construct for himself a set of interesting views of different
regions.
It may be noticed that for observation of the waning moon there is no
occasion to wait for those hours in which only the waning moon is
visible _during the night_. Of course for the observation of a
particular region under a particular illumination, the observer has no
choice as to hour. But for generally interesting observations of the
waning moon he can wait till morning and observe by daylight. The moon
is, of course, very easily found by the unaided eye (in the day time)
when not very near to the sun; and the methods described in Chapter V.
will enable the observer to find the moon when she is so near to the sun
as to present the narrowest possible sickle of light.
One of the most interesting features of the moon, when she is observed
with a good telescope, is the variety of colour presented by different
parts of her surface. We see regions of the purest white--regions which
one would be apt to speak of as _snow-covered_, if one could conceive
the possibility that snow should have fallen where (now, at least) there
is neither air nor water. Then there are the so-called seas, large grey
or neutral-tinted regions, differing from the former not merely in
colour and in tone, but in the photographic quality of the light they
reflect towards the earth. Some of the seas exhibit a greenish tint, as
the Sea of Serenity and the Sea of Humours. Where there is a central
mountain within a circular depression, the surrounding plain is
generally of a bluish steel-grey colour. There is a region called the
Marsh of Sleep, which exhibits a pale red tint, a colour seen also near
the Hyrcinian mountains, within a circumvallation called Lichtenburg.
The brightest portion of the whole lunar disc is Aristarchus, the peaks
of which shine often like stars, when the mountain is within the
unillumined portion of the moon. The darkest regions are Grimaldi and
Endymion and the great plain called Plato by modern astronomers--but, by
Hevelius, the Greater Black Lake.
The Sun.--Observation of the sun is perhaps on the whole the most
interesting work to which the possessor of a moderately good telescope
can apply his instrument. Those wonderful varieties in the appearance of
the solar surface which have so long perplexed astronomers, not only
supply in themselves interesting subjects of observation and
examination, but gain an enhanced meaning from the consideration that
they speak meaningly to us of the structure of an orb which is the
source of light and heat enjoyed by a series of dependent worlds whereof
our earth is--in size at least--a comparatively insignificant member.
Swayed by the attraction of this giant globe, Jupiter and Saturn, Uranus
and Neptune, as well as the four minor planets, and the host of
asteroids, sweep continuously in their appointed orbits, in ever new but
ever safe and orderly relations amongst each other. If the sun's light
and heat were lost, all life and work among the denizens of these orbs
would at once cease; if his attractive energy were destroyed, these orbs
would cease to form a _system_.
The sun may be observed conveniently in many ways, some more suited to
the general observer who has not time or opportunity for systematic
observation; others more instructive, though involving more of
preparation and arrangement.
The simplest method of observing the sun is to use the telescope in the
ordinary manner, protecting the eye by means of dark-green or
neutral-tinted glasses. Some of the most interesting views I have ever
obtained of the sun, have resulted from the use of the ordinary
terrestrial or erecting eye-piece, capped with a dark glass. The
magnifying power of such an eye-piece is, in general, much lower than
that available with astronomical eye-pieces. But vision is very pleasant
and distinct when the sun is thus observed, and a patient scrutiny
reveals almost every feature which the highest astronomical power
applicable could exhibit. Then, owing to the greater number of
intervening lenses, there is not the same necessity for great darkness
or thickness in the coloured glass, so that the colours of the solar
features are seen much more satisfactorily than when astronomical
eye-pieces are employed.
In using astronomical eye-pieces it is convenient to have a rotating
wheel attached, by which darkening glasses of different power may be
brought into use as the varying illumination may require.
Those who wish to observe carefully and closely a minute portion of the
solar disc, should employ Dawes' eye-piece: in this a metallic screen
placed in the focus keeps away all light but such as passes through a
minute hole in the diaphragm.
Another convenient method of diminishing the light is to use a glass
prism, light being partially reflected from one of the exterior
surfaces, while the refracted portion is thrown out at another.
Very beautiful and interesting views may be obtained by using such a
pyramidal box as is depicted in fig. 11.
[Illustration: _Fig. 11._]
This box should be made of black cloth or calico fastened over a light
framework of wire or cane. The base of the pyramid should be covered on
the inside with a sheet of white glazed paper, or with some other
uniform white surface. Captain Noble, I believe, makes use of a surface
of plaster of Paris, smoothed while wet with plate-glass. The door _b
c_ enables the observer to "change power" without removing the box,
while larger doors, _d e_ and _g f_, enable him to examine the image; a
dark cloth, such as photographers use, being employed, if necessary, to
keep out extraneous light. The image may also be examined from without,
if the bottom of the pyramid be formed of a sheet of cut-glass or oiled
tissue-paper.
When making use of the method just described, it is very necessary that
the telescope-tube should be well balanced. A method by which this may
be conveniently accomplished has been already described in Chapter I.
But, undoubtedly, for the possessor of a moderately good telescope there
is no way of viewing the sun's features comparable to that now to be
described, which has been systematically and successfully applied for a
long series of years by the Rev. F. Howlett. To use his own words: "Any
one possessing a good achromatic of not more than three inches'
aperture, who has a little dexterity with his pencil, and a little time
at his disposal (all the better if it be at a somewhat early hour of the
morning)" may by this method "deliberately and satisfactorily view,
measure, and (if skill suffice) delineate most of those interesting and
grand solar phenomena of which he may have read, or which he may have
seen depicted, in various works on physical astronomy."[15]
The method in question depends on the same property which is involved in
the use of the pyramidal box just described, supplemented (where exact
and systematic observation is required) by the fact that objects lying
on or between the lenses of the eye-piece are to be seen faithfully
projected on the white surface on which the sun's image is received. In
place, however, of a box carried upon the telescope-tube, a darkened
room (or true _camera obscura_) contains the receiving sheet.
A chamber is to be selected, having a window looking towards the
south--a little easterly, if possible, so as to admit of morning
observation. All windows are to be completely darkened save one, through
which the telescope is directed towards the sun. An arrangement is to be
adopted for preventing all light from entering by this window except
such light as passes down the tube of the telescope. This can readily be
managed with a little ingenuity. Mr. Howlett describes an excellent
method. The following, perhaps, will sufficiently serve the purposes of
the general observer:--A plain frame (portable) is to be constructed to
fit into the window: to the four sides of this frame triangular pieces
of cloth (impervious to light) are to be attached, their shape being
such that when their adjacent edges are sewn together and the flaps
stretched out, they form a rectangular pyramid of which the frame is the
base. Through the vertex of this pyramid (near which, of course, the
cloth flaps are not sewn together) the telescope tube is to be passed,
and an elastic cord so placed round the ends of the flaps as to prevent
any light from penetrating between them and the telescope. It will now
be possible, without disturbing the screen (fixed in the window), to
move the telescope so as to follow the sun during the time of
observation. And the same arrangement will serve for all seasons, if so
managed that the elastic cord is not far from the middle of the
telescope-tube; for in this case the range of motion is small compared
to the range of the tube's extremity.
A large screen of good drawing-paper should next be prepared. This
should be stretched on a light frame of wood, and placed on an easel,
the legs of which should be furnished with holes and pegs that the
screen may be set at any required height, and be brought square to the
tube's axis. A large T-square of light wood will be useful to enable the
observer to judge whether the screen is properly situated in the last
respect.
We wish now to direct the tube towards the sun, and this "without
dazzling the eyes as by the ordinary method." This may be done in two
ways. We may either, before commencing work--that is, before fastening
our elastic cord so as to exclude all light--direct the tube so that its
shadow shall be a perfect circle (when of course it is truly directed),
then fasten the cord and afterwards we can easily keep the sun in the
field by slightly shifting the tube as occasion requires. Or (if the
elastic cord has already been fastened) we may remove the eye-tube and
shift the telescope-tube about--the direction in which the sun lies
being roughly known--until we see the spot of light received down the
telescope's axis grow brighter and brighter and finally become a _spot
of sun-light_. If a card be held near the focus of the telescope there
will be seen in fact an image of the sun. The telescope being now
properly directed, the eye-tube may be slipped in again, and the sun may
be kept in the field as before.
There will now be seen upon the screen a picture of the sun very
brilliant and pleasing, but perhaps a little out of focus. The focusing
should therefore next be attended to, the increase of clearness in the
image being the test of approach to the true focus. And again, it will
be well to try the effect of slight changes of distance between the
screen and the telescope's eye-piece. Mr. Howlett considers one yard as
a convenient distance for producing an excellent effect with almost any
eye-piece that the state of the atmosphere will admit of. Of course, the
image becomes more sharply defined if we bring the screen nearer to the
telescope, while all the details are enlarged when we move the screen
away. The enlargement has no limits save those depending on the amount
of light in the image. But, of course, the observer must not expect
enlargement to bring with it a view of new details, after a certain
magnitude of image has been attained. Still there is something
instructive, I think, in occasionally getting a very magnified view of
some remarkable spot. I have often looked with enhanced feelings of awe
and wonder on the gigantic image of a solar spot thrown by means of the
diagonal eye-piece upon the ceiling of the observing-room. Blurred and
indistinct through over-magnifying, yet with a new meaning to me,
_there_ the vast abysm lies pictured; vague imaginings of the vast and
incomprehensible agencies at work in the great centre of our system
crowd unbidden into my mind; and I seem to _feel_--not merely think
about--the stupendous grandeur of that life-emitting orb.
To return, however, to observation:--By slightly shifting the tube,
different parts of the solar disc can be brought successively upon the
screen and scrutinized as readily as if they were drawn upon a chart.
"With a power of--say about 60 or 80 linear--the most minute solar spot,
properly so called, that is capable of formation" (Mr. Howlett believes
"they are never less than three seconds in length or breadth) will be
more readily detected than by any other method," see Plate 7; "as also
will any faculæ, mottling, or in short, any other phenomena that may
then be existing on the disc." "Drifting clouds frequently sweep by, to
vary the scene, and occasionally an aërial hail- or snow-storm." Mr.
Howlett has more than once seen a distant flight of rooks pass slowly
across the disc with wonderful distinctness, when the sun has been at a
low altitude, and likewise, much more frequently, the rapid dash of
starlings, which, very much closer at hand, frequent his church-tower."
An eclipse of the sun, or a transit of an inferior planet, is also much
better seen in this way than by any other method of observing the solar
disc. In Plate 7 are presented several solar spots as they have appeared
to Mr. Howlett, with an instrument of moderate power. The grotesque
forms of some of these are remarkable; and the variations the spots
undergo from day to day are particularly interesting to the thoughtful
observer.
A method of measuring the spots may now be described. It is not likely
indeed that the ordinary observer will care to enter upon any systematic
series of measurements. But even in his case, the means of forming a
general comparison between the spots he sees at different times cannot
fail to be valuable. Also the knowledge--which a simple method of
measurement supplies--of the actual dimensions of a spot in miles
(roughly) is calculated to enhance our estimate of the importance of
these features of the solar disc. I give Mr. Howlett's method in his own
words:--
"Cause your optician to rule for you on a circular piece of glass a
number of fine graduations, the 200th part of an inch apart, each fifth
and tenth line being of a different length in order to assist the eye in
their enumeration. Insert this between the anterior and posterior lenses
of a Huygenian eye-piece of moderate power, say 80 linear. Direct your
telescope upon the sun, and having so arranged it that the whole disc of
the sun may be projected on the screen, count carefully the number of
graduations that are seen to exactly occupy the solar diameter.... It
matters not in which direction you measure your diameter, provided only
the sun has risen some 18° or 20° above the horizon, and so escaped the
distortion occasioned by refraction.[16]
"Next let us suppose that our observer has been observing the sun on any
day of the year, say, if you choose, at the time of its mean apparent
diameter, namely about the first of April or first of October, and has
ascertained that" (as is the case with Mr. Howlett's instrument)
"sixty-four graduations occupy the diameter of the projected image. Now
the semi-diameter of the sun, at the epochs above mentioned, according
to the tables given for every day of the year in the 'Nautical Almanac'
(the same as in Dietrichsen and Hannay's very useful compilation) is
16' 2", and consequently his mean total diameter is 32' 4" or 1924". If
now we divide 1924" by 64" this will, of course, award as nearly as
possible 30" as the value in celestial arc of each graduation, either as
seen on the screen, or as applied directly to the sun or any heavenly
body large enough to be measured by it."
Since the sun's diameter is about 850,000 miles, each graduation (in the
case above specified) corresponds to one-64th part of 850,000
miles--that is, to a length of 13,256 miles on the sun's surface. Any
other case can be treated in precisely the same manner.
It will be found easy so to place the screen that the distance between
successive graduations (as seen projected upon the screen) may
correspond to any desired unit of linear measurement--say an inch. Then
if the observer use transparent tracing-paper ruled with faint lines
forming squares half-an-inch in size, he can comfortably copy directly
from the screen any solar phenomena he may be struck with. A variety of
methods of drawing will suggest themselves. Mr. Howlett, in the paper I
have quoted from above, describes a very satisfactory method, which
those who are anxious to devote themselves seriously to solar
observation will do well to study.
It is necessary that the observer should be able to determine
approximately where the sun's equator is situated at the time of any
observation, in order that he may assign to any spot or set of spots its
true position in relation to solar longitude and latitude. Mr. Howlett
shows how this may be done by three observations of the sun made at any
fixed hour on successive days. Perhaps the following method will serve
the purpose of the general observer sufficiently well:--
The hour at which the sun crosses the meridian must be taken for the
special observation now to be described. This hour can always be learnt
from 'Dietrichsen's Almanac'; but noon, civil time, is near enough for
practical purposes. Now it is necessary first to know the position of
the ecliptic with reference to the celestial equator. Of course, at noon
a horizontal line across the sun's disc is parallel to the equator, but
the position of that diameter of the sun which coincides with the
ecliptic is not constant: at the summer and winter solstices this
diameter coincides with the other, or is horizontal at noon; at the
spring equinox the sun (which travels on the ecliptic) is passing
towards the north of the equator, crossing that curve at an angle of
23-1/2°, so that the ecliptic coincides with that diameter of the sun
which cuts the horizontal one at an angle of 23-1/2° and has its _left_
end above the horizontal diameter; and at the autumn equinox the sun is
descending and the same description applies, only that the diameter
(inclined 23-1/2° to the horizon) which has its _right_ end uppermost,
now represents the ecliptic. For intermediate dates, use the following
little table:--
--------------------------------------------------------------------------
Date. |Dec. 22|Jan. 5|Jan. 20|Feb. 4|Feb. 19|Mar. 5 |Mar. 21
(Circiter.) | |June 6|May 21 |May 5 |Apr. 20|Apr. 5 |
-------------------+-------+------+-------+------+-------+-------+--------
Inclination of |Left |Left |Left |Left |Left |Left |Left
Ecliptical Diameter| | | | | | |
of Sun to the |0° 0' |6°24' |12°14' |17°3' |20°36' |22°44' |23°27'
Horizon.[17] |Right |Right |Right |Right |Right |Right |Right
-------------------+-------+------+-------+------+-------+-------+--------
Date. | |Dec. 7|Nov. 22|Nov. 7|Oct. 23|Oct. 8 |
(Circiter.) |Jan. 21|July 7|July 23|Aug. 6|Aug. 23|Sept. 7|Sept. 23
--------------------------------------------------------------------------
Now if our observer describe a circle, and draw a diameter inclined
according to above table, this diameter would represent the sun's
equator if the axis of the sun were square to the ecliptic-plane. But
this axis is slightly inclined, the effect of which is, that on or about
June 10 the sun is situated as shown in fig. 14 with respect to the
ecliptic _ab_; on or about September 11 he is situated as shown in fig.
13; on or about December 11 as shown in fig. 12; and on or about March
10 as shown in fig. 15. The inclination of his equator to the ecliptic
being so small, the student can find little difficulty in determining
with sufficient approximation the relation of the sun's polar axis to
the ecliptic on intermediate days, since the equator is never more
_inclined_ than in figs. 12 and 14, never more _opened out_ than in
figs. 13 and 15. Having then drawn a line to represent the sun's
ecliptical diameter inclined to the horizontal diameter as above
described, and having (with this line to correspond to _ab_ in figs.
12-15) drawn in the sun's equator suitably inclined and opened out, he
has the sun's actual presentation (at noon) as seen with an erecting
eye-piece. Holding his picture upside down, he has the sun's
presentation as seen with an astronomical eye-piece--and, finally,
looking at his picture from behind (without inverting it), he has the
presentation seen when the sun is projected on the screen. Hence, if he
make a copy of this last view of his diagram upon the centre of his
screen, and using a low power, bring the whole of the sun's image to
coincide with the circle thus drawn (to a suitable scale) on the screen,
he will at once see what is the true position of the different
sun-spots. After a little practice the construction of a suitably sized
and marked circle on the screen will not occupy more than a minute or
two.
[Illustration: _Fig. 12._]
[Illustration: _Fig. 13._]
[Illustration: _Fig. 14._]
[Illustration: _Fig. 15._]
It must be noticed that the sun's apparent diameter is not always the
same. He is nearer to us in winter than in summer, and, of course, his
apparent diameter is greater at the former than at the latter season.
The variation of the apparent diameter corresponds (inversely) to the
variation of distance. As the sun's greatest distance from the earth is
93,000,000 miles (pretty nearly) and his least 90,000,000, his greatest,
mean, and least apparent diameters are as 93, 91-1/2, and 90
respectively; that is, as 62, 61, and 60 respectively.
Mr. Howlett considers that with a good 3-inch telescope, applied in the
manner we have described, all the solar features may be seen, except the
separate granules disclosed by first-class instruments in the hands of
such observers as Dawes, Huggins, or Secchi. Faculæ may, of course, be
well seen. They are to be looked for near spots which lie close to the
sun's limb.
When the sun's general surface is carefully scrutinised, it is found to
present a mottled appearance. This is a somewhat delicate feature. It
results, undoubtedly, from the combined effect of the granules
separately seen in powerful instruments. Sir John Herschel has stated
that he cannot recognise the marbled appearance of the sun with an
achromatic. Mr. Webb, however, has seen this appearance with such a
telescope, of moderate power, used with direct vision; and certainly I
can corroborate Mr. Howlett in the statement that this appearance may be
most distinctly seen when the image of the sun is received within a
well-darkened room.
My space will not permit me to enter here upon the discussion of any of
those interesting speculations which have been broached concerning solar
phenomena. We may hope that the great eclipse of August, 1868, which
promises to be the most favourable (for effective observation) that has
ever taken place, will afford astronomers the opportunity of resolving
some important questions. It seems as if we were on the verge of great
discoveries,--and certainly, if persevering and well-directed labour
would seem in any case to render such discoveries due as man's just
reward, we may well say that he deserves shortly to reap a harvest of
exact knowledge respecting solar phenomena.
THE END.
FOOTNOTES:
[Footnote 1: Such a telescope is most powerful with the shortest sight.
It may be remarked that the use of a telescope often reveals a
difference in the sight of the two eyes. In my own case, for instance, I
have found that the left eye is very short-sighted, the sight of the
right eye being of about the average range. Accordingly with my left eye
a 5-1/2-foot object-glass, alone, forms an effective telescope, with
which I can see Jupiter's moons quite distinctly, and under favourable
circumstances even Saturn's rings. I find that the full moon is too
bright to be observed in this way without pain, except at low
altitudes.]
[Footnote 2: Betelgeuse--commonly interpreted the Giant's
Shoulder--_ibt-al-jauza_. The words, however, really signify, "the
armpit of the central one," Orion being so named because he is divided
centrally by the equator.]
[Footnote 3: I have never been able to see more than four with a
3-3/4-inch aperture. I give a view of the trapezium as seen with an
8-inch equatorial.]
[Footnote 4: Sir W. Herschel several times saw [epsilon] Lyræ as a
double. Bessel also relates that when he was a lad of thirteen he could
see this star double. I think persons having average eye-sight could see
it double if they selected a suitable hour for observation. My own
eye-sight is not good enough for this, but I can distinctly see this
star wedged whenever the line joining the components is inclined about
45° to the horizon, and also when Lyra is near the zenith.]
[Footnote 5: They were so described by Admiral Smyth in 1839. Mr. Main,
in 1862, describes them as straw-coloured and reddish, while Mr. Webb,
in 1865, saw them pale-yellow and _lilac_!]
[Footnote 6: Or the observer may sweep from [omicron] towards [nu],
looking for R about two-fifths of the way from [omicron] to [nu].]
[Footnote 7: Here a single period only is taken, to get back to a
convenient hour of the evening.]
[Footnote 8: Here a single period only is taken, to get back to a
convenient hour of the evening.]
[Footnote 9: I have constructed a zodiac-chart, which will enable the
student to mark in the path of a planet, at any season of the year, from
the recorded places in the almanacs.]
[Footnote 10: It is convenient to remember that through precession a
star near the ecliptic shifts as respects the R.A. and Dec. lines,
through an arc of one degree--or nearly twice the moon's diameter--in
about 72 years, all other stars through a less arc.]
[Footnote 11: Mercury is best seen when in quadrature to the sun, but
_not_ (as I have seen stated) at those quadratures in which he attains
his maximum elongation from the sun. This will appear singular, because
the maximum elongation is about 27°, the minimum only about 18°. But it
happens that in our northern latitudes Mercury is always _south_ of the
sun when he attains his maximum elongation, and this fact exercises a
more important effect than the mere amount of elongation.]
[Footnote 12: It does not seem to me that the difficulty of detecting
Mercury is due to the difficulty "of identifying it amongst the
surrounding stars, during the short time that it can be seen" (Hind's
'Introduction to Astronomy'). There are few stars which are comparable
with Mercury in brilliancy, when seen under the same light.]
[Footnote 13: I may notice another error sometimes made. It is said that
the shadow of a satellite _appears_ elliptical when near the edge of the
disc. The shadow is _in reality_ elliptical when thus situated, but
_appears_ circular. A moment's consideration will show that this should
be so. The part of the disc concealed by a _satellite_ near the limb is
also elliptical, but of course appears round.]
[Footnote 14: From a paper by Mr. Breen, in the 'Popular Science
Review,' October, 1864.]
[Footnote 15: 'Intellectual Observer' for July, 1867, to which magazine
the reader is referred for full details of Mr. Howlett's method of
observation, and for illustrations of the appliances he made use of, and
of some of his results.]
[Footnote 16: As the sun does not attain such an altitude as 18° during
two months in the year, it is well to notice that the true length of the
sun's apparent solar diameter is determinable even immediately after
sun-rise, if the line of graduation is made to coincide with the
_horizontal_ diameter of the picture on the screen--for refraction does
not affect the length of this diameter.]
[Footnote 17: The words "Left" and "Right" indicate which end of the
sun's ecliptical diameter is uppermost at the dates in upper or lower
row respectively.]
LONDON:
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CROSS.