will probably occupy a very prominent place in the annals of
astronomy. For this planet goes well within the orbit of Mars,
and this will bring it at times very close to the earth. In fact,
when the motions of the new planet and the earth combine to bring
them to their positions of greatest proximity, the new planet will
approach us closer than any other celestial body except our own
moon. Witt named his new planet Eros. Its size, though small, may
prove to be sufficient to bring it within the possibilities of
naked-eye observation at the time of closest approach to the earth.
To astronomers the great importance of this new planet is due to
the following circumstance: For certain reasons too technical to be
stated here in detail, the distance from the earth to any planet
can be determined with a degree of precision which is greatest for
planets that are near us. Thus in time we shall learn the distance
of Eros more accurately than we know any other celestial distance.
From this, by a process of calculation, the solar distance from the
earth is determinable. But the distance from earth to sun is the
fundamental astronomical unit of measure; so that Witt's discovery,
through its effect on the unit of measure, will doubtless
influence every part of the science of astronomy. Here we have
once more a striking instance of the reward sure to overtake
the diligent worker in science--a whole generation of men will
doubtless pass away before we shall have exhausted the scientific
advantages to be drawn from Witt's remarkable observation of 1898.
HOW TO MAKE A SUN-DIAL[A]
Long before clocks and watches had been invented, people began to
measure time with sun-dials. Nowadays, when almost everyone has a
watch in his pocket, and can have a clock, too, on the mantel-piece
of every room in the house, the sun-dial has ceased to be needed
in ordinary life. But it is still just as interesting as ever to
anyone who would like to have the means of getting time direct
from the sun, the great hour-hand or timekeeper of the sky. Any
person who is handy with tools can make a sun-dial quite easily, by
following the directions given below.
In the first place, you must know that the sun-dial gives the time
by means of the sun's shadow. If you stick a walking-cane up in the
sand on a bright, sunshiny day, the cane has a long shadow that
looks like a dark line on the ground. Now if you watch this shadow
carefully, you will see that it does not stay in the same place all
day. Slowly but surely, as the sun climbs up in the sky, the shadow
creeps around the cane. You can see quite easily that if the cane
were fastened in a board floor, and if we could mark on the floor
the places where the shadow was at different hours of the day, we
could make the shadow tell us the time just like the hour-hand of a
clock. A sun-dial is just such an arrangement as this, and I will
show you how to mark the shadow places exactly, so as to tell the
right time without any trouble whenever the sun shines.
If you were to watch very carefully such an arrangement as a cane
standing in a board floor, you would not find the creeping shadow
in just the same place at the same time every day. If you marked
the place of the shadow at exactly ten o'clock by your watch some
morning, and then went back another day at ten, you would not find
the shadow on the old mark. It would not get very far from it in
a day or two, but in a month or so it would be quite a distance
away. Now, of course, a sun-dial would be of no use if it did not
tell the time correctly every day; and in fact, it is not easy to
make a dial when the shadow is cast by a stick standing straight
up. But we can get over this difficulty very well by letting the
shadow be cast by a stick that leans over toward the floor just the
right amount, as I will explain in a moment. Of course, we should
not really use the floor for our sun-dial. It is much better to
mark out the hour-lines, as they are called, on a smooth piece of
ordinary white board, and then, after the dial is finished, it can
be screwed down to a piazza floor or railing, or it can be fastened
on a window-sill. It ought to be put in a place where the sun can
get at it most of the time, because, of course, you cannot use the
sun-dial when the sun is not shining on it. If the dial is set on a
window-sill (of a city house, for instance) you must choose a south
window if you can, so as to get the sun nearly all day. If you have
to take an east window, you can use the dial in the morning only,
and in a west window only in the afternoon. Sometimes it is best
not to try to fasten the dial to its support with screws, but just
to mark its place, and then set it out whenever you want to use
it. For if the dial is made of wood, and not painted, it might be
injured by rain or snow in bad weather if left out on a window-sill
or piazza.
[Illustration: Fig. 1.]
It is not quite easy to fasten a little stick to a board so that it
will lean over just right. So it is better not to use a stick or
a cane in the way I have described, but instead to use a piece of
board cut to just the right shape.
Fig. 1 shows what a sun-dial should look like. The lines to show
the shadow's place at the different hours of the day will be marked
on the board ABCD, and this will be put flat on the window-sill
or piazza floor. The three-cornered piece of board _abc_ is
fastened to the bottom-board ABCD by screws going through ABCD
from underneath. The edge _ab_ of the three-cornered board _abc_
then takes the place of the leaning stick or cane, and the time
is marked by the shadow cast by the edge _ab_. Of course, it is
important that this edge should be straight and perfectly flat and
even. If you are handy with tools, you can make it quite easily,
but if not, you can mark the right shape on a piece of paper very
carefully, and take it to a carpenter, who can cut the board
according to the pattern you have marked on the paper.
[Illustration: Fig. 2.]
Now I must tell you how to draw the shape of the three-cornered
board _abc_. Fig. 2 shows how it is done. The side _ac_ should
always be just five inches long. The side _bc_ is drawn at right
angles to _ac_, which you can do with an ordinary carpenter's
square. The length of _bc_ depends on the place for which the dial
is made. The following table gives the length of _bc_ for various
places in the United States, and, after you have marked out the
length of _bc_, it is only necessary to complete the three-cornered
piece by drawing the side _ab_ from _a_ to _b_.
TABLE SHOWING THE LENGTH OF THE SIDE _bc_.
--------------------------
Place. _b c_
Inches.
--------------------------
Albany 4-11/16
Baltimore 4-1/16
Boston 4-1/2
Buffalo 4-11/16
Charleston 3-1/4
Chicago 4-1/2
Cincinnati 4-1/16
Cleveland 4-1/2
Denver 4-3/16
Detroit 4-1/2
Indianapolis 4-1/16
Kansas City 3-15/16
Louisville 3-15/16
Milwaukee 3-11/16
New Orleans 2-7/8
New York 4-3/8
Omaha 4-3/8
Philadelphia 4-3/16
Pittsburg 4-3/8
Portland, Me 4-13/16
Richmond 3-15/16
Rochester 4-11/16
San Diego 3-1/4
San Francisco 3-15/16
Savannah 3-1/8
St. Louis 3-15/16
St. Paul 5
Seattle 5-9/16
Washington, D. C. 4-1/16
--------------------------
If you wish to make a dial for a place not given in the table,
it will be near enough to use the distance _bc_ as given for the
place nearest to you. But in selecting the nearest place from the
table, please remember to take that one of the cities mentioned
which is nearest to you in a north-and-south direction. It does
not matter how far away the place is in an east-and-west direction.
So, instead of taking the place that is nearest to you on the map
in a straight line, take the place to which you could travel by
going principally east or west, and very little north or south. The
figure drawn is about the right shape for New York. The board used
for the three-cornered piece should be about one-half inch thick.
But if you are making a window-sill dial, you may prefer to have it
smaller than I have described. You can easily have it half as big
by making all the sizes and lines in half-inches where the table
calls for inches.
After you have marked out the dimensions for the three-cornered
piece that is to throw the shadow, you can prepare the dial itself,
with the lines that mark the place of the shadow for every hour
of the day. This you can do in the manner shown in Fig. 3. Just
as in the case of the three-cornered piece, you can draw the dial
with a pencil directly on a smooth piece of white board, about
three-quarters of an inch thick, or you can mark it out on a paper
pattern and transfer it afterward to the board. Perhaps it will be
as well to begin by drawing on paper, as any mistakes can then be
corrected before you commence to mark your wood.
[Illustration: Fig. 3.]
In the first place you must draw a couple of lines MN and M′N′,
eight inches long, and just far enough apart to fit the edge of
your three-cornered shadow-piece. You will remember I told you
to make that one-half inch thick, so your two lines will also be
one-half inch apart. Now draw the two lines NO and N′O′ square
with MN and M′N′, and make the distances NO and N′O′ just five
inches each. The lines OK, O′K′, and the other lines forming the
outer border of the dial, are then drawn just as shown, OK and O′K′
being just eight inches long, the same as MN and M′N′. The lower
lines in the figure, which are not very important, are to complete
the squares. You must mark the lines NO and N′O′ with the figures
VI, these being the lines reached by the shadow at six o'clock in
the morning and evening. The points where the VII, VIII, and other
hour-lines cut the lines OK, O′K′, MK, and M′K′ can be found from
the table on page 78.
In using the table you will notice that the line IX falls sometimes
on one side of the corner K, and sometimes on the other. Thus for
Albany the line passes seven and seven-sixteenth inches from O,
while for Charleston it passes four and three-eighth inches from M.
For Baltimore it passes exactly through the corner K.
TABLE SHOWING HOW TO MARK THE HOUR-LINES.
-----------------+-----------------------------+------------------------
| Distance from O to the line | Distance from M to the
| marked | line marked
PLACE. +---------+---------+---------+-------+--------+-------
| VII. | VIII. | IX. | IX. | X. | XI.
-----------------+---------+---------+---------+-------+--------+-------
| Inches. | Inches. | Inches. |Inches.|Inches. |Inches.
Albany | 1-15/16 | 4-3/16 | 7-7/16 | | 3-1/16 | 1-7/16
Baltimore | 2-1/8 | 4-11/16 | 8 | | 2-7/8 | 1-7/16
Boston | 2 | 4-5/16 | 7-7/16 | | 3-1/16 | 1-7/16
Buffalo | 1-15/16 | 4-3/16 | 7-7/16 | | 3-1/16 | 1-7/16
Charleston | 2-7/16 | 5-3/8 | | 4-3/8 | 2-1/2 | 1-1/8
Chicago | 2 | 4-5/16 | 7-7/16 | | 3-1/16 | 1-7/16
Cincinnati | 2-1/8 | 4-11/16 | 8 | | 2-7/8 | 1-7/16
Cleveland | 2 | 4-5/16 | 7-7/16 | -- | 3-1/16 | 1-7/16
Denver | 2-1/8 | 4-1/2 | 7-11/16 | | 2-7/8 | 1-7/16
Detroit | 2 | 4-5/16 | 7-7/16 | | 3-1/16 | 1-7/16
Indianapolis | 2-1/8 | 4-11/16 | 8 | | 2-7/8 | 1-7/16
Kansas City | 2-1/4 | 4-11/16 | 8 | | 2-7/8 | 1-5/16
Louisville | 2-1/4 | 4-11/16 | 8 | | 2-7/8 | 1-5/16
Milwaukee | 1-15/16 | 4-3/16 | 7-7/16 | | 3-1/16 | 1-7/16
New Orleans | 2-11/16 | 5-3/4 | | 4-1/16| 2-5/16 | 1-1/8
New York | 2 | 4-5/16 | 7-11/16 | | 3-1/16 | 1-7/16
Omaha | 2 | 4-5/16 | 7-11/16 | | 3-1/16 | 1-7/16
Philadelphia | 2-1/8 | 4-1/2 | 7-11/16 | | 2-7/8 | 1-7/16
Pittsburg | 2 | 4-5/16 | 7-11/16 | | 3-1/16 | 1-7/16
Portland, Me | 1-15/16 | 4-3/16 | 7-1/8 | | 3-3/16 | 1-1/2
Richmond | 2-1/4 | 4-11/16 | 8 | | 2-7/8 | 1-5/16
Rochester | 1-15/16 | 4-3/16 | 7-7/16 | | 3-1/16 | 1-7/16
San Diego | 2-7/16 | 5-3/8 | | 4-3/8 | 2-1/2 | 1-1/8
San Francisco | 2-1/4 | 4-11/16 | 8 | | 2-7/8 | 1-5/16
Savannah | 2-9/16 | 5-9/16 | | 4-1/4 | 2-1/2 | 1-1/8
St. Louis | 2-1/4 | 4-11/16 | 8 | | 2-7/8 | 1-5/16
St. Paul | 1-15/16 | 4-1/16 | 7-1/8 | | 3-3/16 | 1-1/2
Seattle | 1-13/16 | 3-15/16 | 6-5/8 | | 3-3/8 | 1-1/2
Washington, D. C.| 2-1/8 | 4-11/16 | 8 | | 2-7/8 | 1-7/16
-----------------+---------+---------+---------+-------+--------+-------
The distance for the line marked V from O′ is just the same as the
distance from O to VII. Similarly, IV corresponds to VIII, III to
IX, II to X, and I to XI. The number XII is marked at MM′ as shown.
If you desire to add lines (not shown in Fig. 3 to avoid confusion)
for hours earlier than six in the morning, it is merely necessary
to mark off a distance on the line KO, below the point O, and equal
to the distance from O to VII. This will give the point where the
5 A.M. shadow line drawn from N cuts the line KO. A corresponding
line for 7 P.M. can be drawn from N′ on the other side of the
figure.
After you have marked out the dial very carefully, you must fasten
the three-cornered shadow-piece to it in such a way that the whole
instrument will look like Fig. 1. The edge _ac_ (Fig. 2) goes on NM
(Fig. 3). The point _a_ (Fig. 2) must come exactly on N (Fig. 3);
and as the lines NM (Fig. 3) and N′M′ (Fig. 3) have been made just
the right distance apart to fit the thickness of the three-cornered
piece _abc_ (Fig. 2), everything will go together just right. The
point _c_ (Fig. 2) will not quite reach to M (Fig. 3), but will
be on the line NM (Fig. 3) at a distance of three inches from
M. The two pieces of wood will be fastened together with three
screws going through the bottom-board ABCD (Figs. 1 and 3) and
into the edge _ac_ (Fig. 2) of the three-cornered piece. The whole
instrument will then look something like Fig. 1.
After you have got your sun-dial put together, you need only set
it in the sun in a level place, on a piazza or window-sill, and
turn it round until it tells the right time by the shadow. You can
get your local time from a watch near enough for setting up the
dial. Once the dial is set right you can screw it down or mark its
position, and it will continue to give correct solar time every day
in the year.
If you wish to adjust the dial very closely, you must go out
some fine day and note the error of the dial by a watch at about
ten in the morning, and at noon, and again at about two in the
afternoon. If the error is the same each time, the dial is rightly
set. If not, you must try, by turning the dial slightly, to get
it so placed that your three errors will be nearly the same. When
you have got them as nearly alike as you can, the dial will be
sufficiently near right. The solar or dial time may, however,
differ somewhat from ordinary watch time, but the difference will
never be great enough to matter, when we remember that sun-dials
are only rough timekeepers after all, and useful principally for
amusement.
FOOTNOTE:
[A] This chapter is especially intended for boys and girls and
others who like to make things with carpenters' tools.
PHOTOGRAPHY IN ASTRONOMY
New highways of science have been monumented now and again by the
masterful efforts of genius, working single-handed; but more often
it is slow-moving time that ripens discovery, and, at the proper
moment, opens some new path to men whose intellectual power is but
willingness to learn. So the annals of astronomical photography do
not recount the achievements of extraordinary genius. It would have
been strange, indeed, if the discovery of photography had not been
followed by its application to astronomy.
The whole range of chemical science contains no experiment of
greater inherent interest than the development of a photographic
plate. Let but the smallest ray of light fall upon its strangely
sensitive surface, and some subtle invisible change takes place.
It is then merely necessary to plunge the plate into a properly
prepared chemical bath, and the gradual process of developing
the picture begins. Slowly, very slowly, the colorless surface
darkens wherever light has touched it. Let us imagine that the
exposure has been made with an ordinary lens and camera, and that
it is a landscape seeming to grow beneath the experimenter's
eyes. At first only the most conspicuous objects make their
appearance. But gradually the process extends, until finally
every tiny detail is reproduced with marvellous fidelity to the
original. The photographic plate, when developed in this way, is
called a "negative." For in Nature luminous points, or sources
of light, are bright, while the developing negative turns dark
wherever light has acted. Thus the negative, while true to Nature,
reproduces everything in a reversed way; bright things are dark,
and shadows appear light. For ordinary purposes, therefore, the
negative has to be replaced by a new photograph made by copying it
again photographically. In this way it is again reversed, giving
us a picture corresponding correctly to the facts as seen. Such a
copy from a negative is what is ordinarily called a photograph;
technically, it is known as a "positive."
One of the remarkable things about the sensitive plate is its
complete indifference to the distance from which the light comes.
It is ready to yield obediently to the ray of some distant star
that may have journeyed, as it were, from the very vanishing
point of space, or to the bright glow of an electric light upon
the photographer's table. This quality makes its use especially
advantageous in astronomy, since we can gain knowledge of remote
stars only by a study of the light they send us. In such study the
photographic plate possesses a supreme advantage over the human
eye. If the conditions of weather and atmosphere are favorable,
an observer looking through an ordinary telescope will see nearly
as much at the first glance as he will ever see. Attentive and
continued study will enable him to fix details upon his memory,
and to record them by means of drawings and diagrams. Occasional
moments of especially undisturbed atmospheric conditions will allow
him to glimpse faint objects seldom visible. But on the whole,
telescopic astronomers add little to their harvest by continued
husbandry in the same field of stars. Photography is different.
The effect of light upon the sensitive surface of the plate is
strictly cumulative. If a given star can bring about a certain
result when it has been allowed to act upon the plate for one
minute, then in two or three minutes it will accomplish much more.
Perhaps a single minute's exposure would have produced a mark
scarcely perceptible upon the developed negative. In that case,
three or four minutes would give us a perfectly well defined black
image of the star.
[Illustration: Star-Field in Constellation Monoceros.
Photographed by Barnard, February 1, 1894.
Exposure, three hours.]
Thus, by lengthening the exposure we can make the fainter stars
impress themselves upon the plate. If their light is not able
to produce the desired effect in minutes, we can let its action
accumulate for hours. In this manner it becomes possible and
easy to photograph objects so faint that they have never been
seen, even with our most powerful telescopes. This achievement
ranks high among those which make astronomy appeal so strongly
to the imagination. Scientific men are not given to fancies; nor
should they be. But the first long-exposure photograph must have
been an exciting thing. After coming from the observatory,
the chemical development was, of course, made in a dark room, so
that no additional light might harm the plate until the process
was complete. Carrying it out then into the light, that early
experimenter cannot but have felt a thrill of triumph; for his hand
held a true picture of dim stars to the eye unlighted, lifted into
view as if by magic.
Plates have been thus exposed as long as twenty-five hours, and the
manner of doing it is very interesting. Of course, it is impossible
to carry on the work continuously for so long a period, since the
beginning of daylight would surely ruin the photograph. In fact,
the astronomer must stop before even the faintest streak of dawn
begins to redden the eastern sky. Moreover, making astronomical
negatives requires excessively close attention, and this it is
impossible to give continuously during more than a few hours.
But the exposure of a single plate can be extended over several
nights without difficulty. It is merely necessary to close the
plate-holder with a "light-tight" cover when the first night's
work is finished. To begin further exposure of the same plate
on another night, we simply aim the photographic telescope at
precisely the same point of the sky as before. The light-tight
plate-holder being again opened, the exposure can go on as if there
had been no interruption.
Astronomers have invented a most ingenious device for making sure
that the telescope's aim can be brought back again to the same
point with great exactness. This is a very important matter;
for the slightest disturbance of the plate before the second or
subsequent portions of the exposure would ruin everything. Instead
of a very complete single picture, we should have two partial ones
mixed up together in inextricable confusion.
To prevent this, photographic telescopes are made double, not
altogether unlike an opera-glass. One of the tubes is arranged for
photography proper, while the other is fitted with lenses suitable
for an ordinary visual telescope. The two tubes are made parallel.
Thus the astronomer, by looking through the visual glass, can watch
objects in the heavens even while they are being photographed. The
visual half of the instrument is provided with a pair of very fine
cross-wires movable at will in the field of view. These can be
made to bisect some little star exactly, before beginning the first
night's work. Afterward, everything about the instrument having
been left unchanged, the astronomer can always assure himself of
coming back to precisely the same point of the sky, by so adjusting
the instrument that the same little star is again bisected.
It must not be supposed, however, that the entire instrument
remains unmoved, even during the whole of a single night's
exposure. For in that case, the apparent motion of the stars as
they rise or set in the sky would speedily carry them out of the
telescope's field of view. Consequently, this motion has to be
counteracted by shifting the telescope so as to follow the stars.
This can be accomplished accurately and automatically by means
of clock-work mechanism. Such contrivances have already been
applied in the past to visual telescopes, because even then they
facilitated the observer's work. They save him the trouble of
turning his instrument every few minutes, and allow him to give his
undivided attention to the actual business of observation.
For photographic purposes the telescope needs to "follow" the
stars far more accurately than in the older kind of observing with
the eye. Nor is it possible to make a clock that will drive the
instrument satisfactorily and quite automatically. But by means of
the second or visual telescope, astronomers can always ascertain
whether the clock is working correctly at any given moment.
It requires only a glance at the little star bisected by the
cross-wires, and, if there has been the slightest imperfection in
the following by clock-work, the star will no longer be cut exactly
by the wires.
The astronomer can at once correct any error by putting in
operation a very ingenious mechanical device sometimes called
a "mouse-control." He need only touch an electric button, and
a signal is sent into the clock-work. Instantly there is a
shifting of the mechanism. For one of the regular driving wheels
is substituted, temporarily, another having an _extra tooth_.
This makes the clock run a little faster so long as the electric
current passes. In a similar way, by means of another button, the
clock can be made to run slower temporarily. Thus by watching
the cross-wires continuously, and manipulating his two electric
buttons, the photographic astronomer can compel his telescope
to follow exactly the object under observation, and he can make
certain of obtaining a perfect negative.