P^lar Star ? ax j g causes the whole sphere of the fixed
stars, &c., to appear to move round the earth every twenty-
four hours from east to west. To the inhabitants of the
northern hemisphere, the immovable point on which the
whole seems to turn is the Pole Star. To the inhabitants
r/f the southern hemisphere there is another and a cor-
responding point in the heavens.
What is the 1852. Certain of the stars surrounding the
dtde- of per- UOT t\ l po le never set to us. These are in-
petual appar- , 1,7.11
ition and of eluded hi a circle parallel with t!;o equator,
^nd in every part equally distant from tiro
Citation? north pole star. This circle is called the
circle of perpetual apparition. Others never rise to us.
These are included in a circle equally distant from the
south pole ; and this is called the circle of perpetual oo-
cullation. Some of the constellations of the southern
hemisphere are represented as inimitably beautiful, par-
ticularly the cross.
What is par- 1353. The parallax of a heavenly body
allax* ig the angular distance between the true
and the apparent situation of the body.
Describe 1354. la Fig. 196, A G B represents the earth
f \g. 196. am i (j the moon. To a spectator at A, the moon
would appear at F; while to another, at B, the moon woul;!
appear at D ; but, to a third spectator, at G, the centre of the
earth, the moon would appear at E, which is the true situation,
The distance from F to E is the parallax of the moon when
vicvTed from A, and the distance from E to D is the parallax
when viewed from B.
1355. From this it appears that the situation of thf heavenly
bodies must always be calculated from the centre of the earth ;
and the observer must always know the distance between the
place of his observation and the centre of the earth, in order tc
make the necessary calculations to determine the true situation
of the body. Allowance, also, must be made for the refraction
jl the atmosphere.
<*6fc NATURAL PHILOSOPHY.
lions of miles every year. This will give him, at the same time,
*: motion of more than 68 000 miles in an hour in a different
direction. If the question be asked, why each individual is not
sensible of these tremendously rapid motions, the answer is>
that no one ever knew what it is to be without them. We can-
not be sensible that we have moved without feeling our motion,
as when in a boat a current takes us in one direction, while a
gentle wind carries us, at the same time, in another direction.
It is only when our progress is arrested by obstacles of some
kind that we can perceive the difference between a state oi
motion and a state of rest.
What would 1297. The rapid motion of a thousand miles in
le the conse- an hour is not sufficient to overcome the centri-
yutnce if the peta i f orce cause( i by gravity ; but, if the earth
sarth should r J \ ?
revolve on its should revolve around its axis seventeen times in
axis once in a day, instead of once, all bodies at the equator
would be lifted up, and the attraction of gravita-
tion would be counterbalanced, if not wholly overcome.
1298. Certain irregularities in the orbit of the earth have
been noticed by astronomers, which show that it is deviating
from its elliptical form, and approaching that of a circle. In
this fact, it has been thought, might be seen the seeds of decay.
But Laplace has demonstrated that these irregularities proceed
from causes which, in the lapse of immensely long periods,
counterbalance each other, and give the assurance that there is
no other limit to the present order of the universe, than the will
of its great Creator.
Describe the 1299. OF MARS. Next to the earth is
flatiet Mars, the planet Mars. It is conspicuous for its
fiery-red appearance, which is supposed by Sir Johi)
llersclier* to be caused by the color of its soil.
* Sir John Herschel is the son of Sir "William Herschel, the discov^rr
oi' *he piauet Uranu.
The degree of heat and light at Mars is less than half of
that received by the earth.
1300. OP THE MINOR PLANETS. It. has already been mention
ed that between the orbits of Mars and Jupiter one hundred and
thirteen small bodies have been discovered, which are called the
minor planets. It is a remarkable fact, that before the discovery
of Bode's law (see No. 1232) certain irregularities observed in the
motions of the old planets induced some astronomers to sup-
pose that a planet existed between the orbits of Mars aLd Jupi-
tei. The opinion has been advanced that these small bodies
originally composed one larger one, which, by some unknown
force or convulsion, burst asunder. This opinion is maintained
with much ingenuity and plausibility by Sir David Brewster.
(See Ed'm. Encyc., art. ASTRONOMY.) Dr. Brewster further
supposes that the bursting of this planet may have occasioned
"the phenomena of meteoric atones ; that is, stones which have
fallen on the earth from the atmosphere.
Describe the 1301. OF JUPITER. Jupiter is the largest
planet Jupiter. p i anet O f tm3 so i ar gj ste m, and the most bril-
liant, except Venus. The heat and light at Jupiter art
about twenty-five times less than that at the earth. This
planet is attended by four moons, or satellites, the shadows
of some of which are occasionally visible upon his surface.
1302. The distance of -those satellites from the planet are
two, four, six and twelve hundred thousand miles, nearly.
The nearest revolves around the planet in less than two days ;
the next, in less than four days ; the third, in less than eight
days ; and the fourth, in about sixteen days.
These four moons must aiford considerable light to the inhab-
itants of the planet ; for the nearest appears to them four times
the size of our moon, the second about the same size, the third
somewhat less, and the fourth about one-third the diameter of
at another time, the whole surface appearing resplendent,
This is caused by the relative position of the moon with
regard to the sun and the earth. The moon is an opaque
body, and shines only by the light of the sun. When,
therefore, the moon is between the earth and the sun, it
presents its dark side to the earth ; while the side presented
to the sun, and on which the sun shines, is invisible to the
earth. But when the earth is between the sun and the
moon, the illuminated side of the moon is visible at the
'Describe 1364. In Fig. 197, let S be the sun, E the earth,
Fig, 197. an d A B C D the moon in different parts of hei
orbit When the moon is at A, its dark side will be towards
the earth, its illuwvtated part being always towards the sun.
Hence the moon will appear to us as represented at a. But
when it has advanced in its orbit to B, a small part of its
illuminated fide coming in sight, it appears as represented at b.
and is said to be horned. When it arrives at C, one-half its
illuminated side is visible, and it appears as at c. At C, and
in the opposite point of its orbit, the moon is said to be in quad'
At D its appearance is as represented at d, and it is
to be gibboits. At E all the illuminated side is towards
.is, aud wo have a full moon. During the other half of it*
revolution, less and less of its illuminated side is seen, till it
again becomes invisible at A.
Wfiat is (he 1365. The mean difference in the rising of the
mean differ- moon, caused by its daily motion, is a little lusr
ruing of the ^ an an ^ our - But, on account of the different
moon from day angles formed with the horizon by different parts
to day . Q f tne ec iipti C) it happens that for six or eight
nights near the full moons of September and October the moon
rises nearly as soon as the sun is set. As this is a great con-
venience to the husbandman and the hunter, in-
^the l Harvest asmuca as li affords tn ein light to continue their
and the Hunt- occupation, and, as it were, lengthens out thei/
er'sMoon, and day, the first is called the harvest moon, and the
occur t second the hunter's moon. These moons are
always most beneficial when the moon's ascending
node is in or near Aries.
1366. The following signs are used in our common almanacs
to denote tle different positions and phases of the moon. } or
}) denote the moon in the first quadrature, that is, the quad-
rature between change and full ; C or < denotes the moon in
the last quadrature, that is, the quadrature between full and
cnange. $ denotes new moon ; Q denotes full moon.
1367. When viewed through a telescope, the surface of the
moon appears wonderfully diversified. Large dark spots, sup-
posed to be excavations, or valleys, are visible to the eye;
some parts also appear more lucid than the general surface
These are ascertained to be mountains, by the shadows whicV-
they cast. Maps of the moon's surface have been drawn, on
which most of these valleys and mountains are delineated, and
names are given to them. Some of these excavations are
thought to be four miles deep, and forty wide. A high ridge
generally surrounds them, and often a mountain rises in the
centre, These immense depressions probably very much re-
semble what would be the appearance of the earth at the m >OP
were all the seas and lakes dried up. Some of the mountain*
are supposed to be volcanic.
What are the 1368. OF THE TIDES. The tides are the
Tides f regular rising and falling of the water of
the ocean twice in about twenty-five hours. They are
occasioned by the attraction of the moon upon the
matter of the earth ; and they are also affected by that
of the sun.
Explain 1369. Let M, Fig. 198, be the moon revolving in
Fig. 198. her orbit ; E, the earth covered with water ; and S,
the sun. Now, the point of the earth's surface, which is nearest
to the moon, will gravitate towards it more, and the remoter
point less, than the centre, inversely as the squares of their re-
spective distances. The point A, therefore, tends away from
the centre, and the centre tends away from the point B ; and in
each case the fluid surface must rise, and in nearly the same
degree in both cases. The effect must be diminished in propoi-
tion to the distance from these points in any direction ; and at
the points C and D, ninety degrees distant, it ceases. But
there the level of the waters must be lowered, because of the
exhaustion at those places, caused by the overflow elsewhere.
Thus the action of the moon causes the ocean to assume
the form of a spheroid elongating it in the direction of the
Thus any particular place, as A, while passing from under the
moon till it comes under the moon again, has two tides. But
the moon is constantly advancing in its orbit, so that the earth
must a little more than complete its rotation before the place A
comes under the moon. This causes high water at any place
about fifty minutes later each successive day.
As the moon's orbit varies but little from the ecliptic,
the moon is never more than 29 from the equator, and is
generally much less. Hence the waters about the equator,
being nearer the moon, are more strongly attracted, and
fche tides are higher than towards the poles.
1370. The sun attracts the waters as well as the moon. When
the moon is at full or change, being in the same line of direction
(see Fig. 198), the sun acts with it ; that is, the sun and moon
tend to raise the tides at the same place, as seen in the figure.
The tides are then very high, and are called spring tides.
Explain But when the moon is in its quarters, as in Fig. 199,
Fig. 199. the sun and moon being in lines at right angles, tend
to raise tides at different places namely, the moon at C and D,
and the sun at A and B. Tides that are produced when the
moon is in its quarters are low, and are called neap tides.
1371. There are so many natural difficulties to the free prog-
ress of the tides, that the theory by which they are , accounted
for is, in fact, and necessarily, the most imperfect of all the
theories connected with astronomy. It is, however, indisputable
that the moon has an effect upon the tides, although it be not
392 NATURAL PHILOSOPHY.
equally felt in all places, owing to the indentations of the coast,
the obtructions of islands, continents, &c., which prevent the
free motion of the waters. In narrow rivers the tides are fre-
quently very high and sudden, from the resistance afforded by
their banks to the free ingress of the water, whence what would
otherwise be a tide, becomes an accumulation. It has been con-
stantly observed, that the spring tides happen at the new and
full moon, and the neap tides at the quarters. This circum
stance is sufficient in itself to prove the connexion between the
influence of the moon and the tides.
1372. An Eclipse is a total or partial ob-
What is an ,. ,, , 1111,1-
Eclipse ? scuration of one heavenly body by the interven-
tion of another.
The situation of the earth with regard to the
When does an ,1^.1 . , -,
eclipse of the moon or rather of the moon with regard to the
sun or of the earth, occasions eclipses both of the sun and
moon take place? moon< Thoge of fhe gun take p]ace wheQ ^
moon, passing between the sun and earth, intercepts his rays.
Those of the moon take place when the earth, coming between
the sun and moon, deprives the moon of his light. Hence, an
eclipse of the sun can take place only when the moon changes,
and an eclipse of the moon only when the moon fulls ; for, at
the, time of an eclipse, either of the sun or the moon, the sun
earth, and moon, must be in the same straight line.
If the moon revolved around the earth in the
Why is there . . , . , , ,
not an eclipse at samc plane m which the earth revolves around
every new and the sun, that is, in the ecliptic, it is plain that
full moon? ^ gun wou i ( j b e eo iip sec i a t every new moon,
anl the moon would be eclipsed at every full. For, at each of
these times, these three bodies would be in the same stiaight
line. But the moon's orbit does not coincide with the ecliptic,
but is inclined to it at an angle of about 5" 20'. Hence, since
the apparent diameter of the sun is but about a degree, and
that of the moon about the same, n } eclipse will take place at
new or full moon, unless the moon be within ^ a deg/ee of the
ecliptic, that is, in or near one of its nodes. It is found tbit
if the moon be within 16 of a node at time of change, it will
be so near the ecliptic, that the sun will be more or le&a
eclipsed ; if within 12 at time of full, the moon will be more
or less eclipsed.
Why are there 1373. It is obvious that the moon will be
more eclipses of oftener within 16^ at the time of new moon,
^moo^ini than withiu 12 at the time of ful1 ; cons
given course of quently, there will be more eclipses of the sun
years? than of the moon in a course of years. As the
nodes commonly come between the sun and earth but twice in
a year, and the moon's orbit contains 360, of which 16^-, the
limit of solar eclipses, and 12, the limit of lunar eclipses, are
but small portions, it is plain there must -be many new and full
moons without any eclipses.
Although there are more eclipses of the sun
E*P lain Ft 8- than of the moon, yet more eclipses of the
inoon will be visible at a particular place, as
Boston, in a course of years, than of the sun. Since the sun is
very much larger than either the earth or moon, the shadow of
these bodies must always terminate in a point ; that is, it must
always be a cone. In Fig. 200, let S be the sun, m the moon,
and E the earth. The sun constantly illuminates half the earth's
surface, that is, a hemisphere ; and consequently it is visible to
a') in this hemisphere. But the moon's shadow falls upon a
part only of this hemisphere ; and hence the sun appears
eclipsed to a part only of those to whoa it is visible, Some-
times, when the moon is at its greatest distance, its shadow,
394 NATURAL PHILOSOPHY.
m., terminates before it reaches the earth. In eclipses of this
kind, to an inhabitant directly under the point 0, the outermo*'.
edge of the sun's disc is seen, forming a bright ring around the
moon ; from which circumstance these eclipses are called annu-
lar, from anrndus, a Latin word for ring.
Besides the dark shadow of the moon, m O, in which all the
light of the sun is intercepted (in which case the eclipse is
called total], there is another shadow, r C D S, distinct from
the former, which is called the penumbra. Within this, only a
part of the sun's rays are intercepted, and the eclipse is called
partial. If a person could pass, during an eclipse of the sun,
from to D, immediately on emerging from the dark shadow,
m, he would see a small part of the sun ; and would con-
tinually see more and more till he arrived at D, where all
shadow would cease, and the whole sun's disc be visible. Ap-
pearances would be similar if he went from to C. Hence
the penumbra is less and less dark (because a less portion of
the sun is eclipsed), in proportion as the spectator is more re
mote from 0, and nearer G or D. Though the penumbra be
continually increasing in diameter, according to its length, 01
the distance of the moon from the earth, still, under the most
favorable circumstances, it falls on but about half of the illu
minated hemisphere of the earth. Hence, by half the inhab
tants on this hemisphere, no eclipse will be seen.
1374. Fig. 201 represents an eclipse of th
Explain Fig. mooilt The j nstant tlie moon enters the earth's
shadow at x, it is deprived of the sun's light
and is eclipsed to all in the un illuminated hemisphere of the
earth. Hence, eclipses of the moon are visible to at least twice
as many inhabitants as those of the sun can be ; generally the
proportion is much greater. Thus, the inhabitants at a par-
ticular placo, as Boston, see more eclipses of the moon than of
The reason why a lunar eclipse is visible to all to whom
the moon at the time is visible, and a solar one is not so to all to
whom the sun at the time is visible, may be seen from tho
nature of these eclipses. We speak of the sun's being eclipsed ;
but, properly, it is the earth which is eclipsed. No change
takes place in the sun ; if there were, it would be seen by all
to whom the sun is visible. The sun continues to diffuse its
beams as freely and uniformly at such times as at others. .But
these beams are intercepted, and the earth is eclipsed only
where the moon's shadow falls, that is, on only a part of a
hemisphere. In eclipses of the moon, that body ceases to
receive light from the sun, and, consequently, ceases to reflect
it to the earth. The moon undergoes a change in its appear-
ance ; and, consequently, this change is visible at the same time
to all to whom the moon is visible ; that is, to a whole hemis-
phere of the earth.
1375. The earth's shadow (like that of the moon) is encom-
passed by a per^jaDra, C R S D, which is faint at the edges
towards R and S, but becomes darker towards F and Gr. The
shadow of the earth is but little darker than the region of the
penumbra next to it. Hence it is very difficult to determine
the exact time when the moon passes from the penumbra into
the shadow, and from the shadow into the penumbra ; that is,
when the eclipse begins and ends. But the beginning and end-
ing of a solar eclipse may be determined almost instantaneously.
1376. The diameters of the sun and moon
KgJ* ^ap- are su PP sed to be divided into twelve P* ]
plied to 'eclipses parts, called digits. These bodies are said tc
of the sun and have as many digits eclipsed as there are of
those parts involved m darkness
1377. There must be an eclipse of the sun fk sften, al least,
as the moon, being near one of its nodes, comes between the
aun and the earth.
The greatest number of both solar and lunar eclipses that can
take place during the year is seven. The usual number is four
two solar and two lunar.
1378. A total eclipse of the sun is a very remarkable phe
June 16, 1806, a very remarkable total eclipse took place at
Boston. The day was clear, and nothing occurred to prevent accu-
rate observation of this interesting phenomenon. Several stars were
visible ; the birds were greatly agitated ; a gloom spread over the
landscape, and an indescribable sensation of fear or dread pervaded
the breasts of those who gave themselves up to the simple effects of the
phenomenon, without having their attention diverted by efforts of
observation. The tiret gleam of light, contrasted with the previous
darkness, seemed like the usual meridian day, and gave indescribable
life and joy to the whole creation. A total eclipse of the sun can
last but little more than three minutes. An annular eclipse of the
sun is still more rare than a total one.
1379. OF TIME. When time is calcu-
ference 'between ^ ate( ^ *>y tne 8un ' ' lt is calle( l solar time ? an( ^
the solar and the the year a solar year ; but when it is calcu-
ndereal ear ? gidereal
and the year a sidereal year. The sidereal year is 20 min-
utes and 24 seconds longer than the solar ^ :ar.
1380. The solar year consists of 365
days, 5 hours, 48 minutes, and 48 seconds;
sidereal but our common reckoning gives 365 days
by only to the year. As the difference amounts
to nearly a quarter of a day every year, it
is usual every fourth year to add a day. Every fourth
year the Romans reckoned the 6th of the calends of
March, and the following day as one day ; which, on
that account, they called bissextile, or twice the 6th day ;
whence ^e derive the name of bissextile for the leap v^>.
in which we give to February, for the same reason, 29
days every fourth year.
1381. A solar year is measured from the time the earth
sets out from a particular point in the ecliptic, as an equi-
nox, or solstice, until it returns to the same point again.
A sidereal year is measured by the time that the earth
takes in making an entire revolution in its orbit; or, in
other words, from the time that the sun takes to return into
conjuction with any fixed star.
What is the pre- 1382 - Ever J equinox occurs at a point,
cession cj the 50" of a deg. of the great circle, preceding
the place of the equinox, 12 months before j
and this is called the precession of the equinoxes. It is
this circumstance which has caused the change in the situ-
ation of the signs of the zodiac, of which mention has
already been made.
1383. The earth's diurnal motion on an inclined axis,
together with its annual revolution in an elliptic orbit,
occasions so much complication in its motion as to pro-
duce many irregularities; therefore, true equal time
cannot be measured by the sun. A clock which is
always perfectly correct will, in some parts of the year,
be before the sun, and in other parts after it. There are
When do the ^ ut * ur P eri ds m which the sun and a
sun and dock perfect clock will agree. These are the
agree? 15th of ^prf], the 15th of June, the 1st oi
September, and the 24th of December.
What is the 1384. The greatest difference between
greatest dif- true and apparent time amounts to between
s i xteen an d seventeen minutes. Tables of
apparent equation are constructed for the purpose of
* f pointing oat and correcting these differences
398 NATURAL PHILOSOPHY.
between solar time and equal or mean time, tne denomina-
tion given by astronomers to true time.
1385. As it may be interesting to those who have access to a
celestial globe to know how to find any particular star or con-
stellation, the following directions are subjoined.
There is always to be seen, on a clear night, a beautiful clus-
ter of seven brilliant stars, which belong to the constellation
" Ursa Major," or the Great Bear. Some have supposed that
they will aptly represent a plough ; others say that they are
more like a wagon and horses, the four stars representing the
body of the wagon, and the other three the horses. Hence
they are called by some the plough, and by others they are
called Charles 1 wain, or wagon.
Fig. 202 represents these seven stars ; K B- aoa -
b a g represent the four, and e z B \
the other three stars. Perhaps they
may more properly be called a large F \\
dippei of which e z B represent the j \
handle. If a line be drawn through the I '* . a
stars I *nd a, and carried upwards, it ( j
will pass a little to the left, and nearly +/z e i +