mined, 579; polar, 329; how dis-
tances are measursd, 567 et seq.
Double Stars, see Stars.

at

Earth, the, is round, 150, 151; rota-
tion proved by Foucault, 154, 157;
poles, 153; equator, ib.; diameters,
ib.; dimensions, 163; how deter-
mined, 573 et seq.; latitude and longi-
tude, 190, 191, 328; parallels and
meridians, 162, 328; tropics, circles,
and zones, 162; shape, 163, 196;
shape of orbit, 167; changes, 445:
inclination of axis, 168; day and
night, 164; how caused, 171;
the poles, 172; length of day and
night, 173; how to determine, 369;
seasons, 175 et seq 447; structure and
past history of, 181 et seq.; interior
temperature of, 193; once a star,
190; why an oblate spheroid, 196;
atmosphere, 197 et seq. 208; belts
of calms and rains, and trade
winds, 201; cause of the winds, 202;
elements in the earth's crust, 207;
in the earth's atmosphere, 208;
Apparent movements. The earth is the
centre of the visible creation, 322;
apparent movements of the heavens
are due to the real movements of
the, 325; effects of rotation, 326, 343:
apparent movements of the stars as
seen from different points on the
surface, 331 et seq.; effects of the
carth's yearly motion, 344 et seq.;
effects of attraction of, 605; motions
of axis, 651 et seq.
Earth-shine, 217.
Eccentricity of an orbit (ex. from,
and centrum, a centre), the distance
of a focus from the centre of an
ellipse. It is expressed by the
ratio the distance bears to the sun's
axis major. An eccentricity of o'1,
e.g., means that the focus is one-
tenth of the sun's axis major from
the centre.
Eclipses (ekdeifis, a disappearance),
233 et seq.
Ecliptic (so called because when

either sun or moon is eclipsed it is
in this circle), the great circle of the
heavens, along which the sun per-
forms his annual path, 363; plane of
the, 105, 300. The plane of the sun's
apparent, and of the earth's real,
motion, 105, 136, 300; obliquity of,
the angle between the plane of the

ecliptic and of the celestial equator,

553.

Egress, the passing of one body off
the disc of another; e.g. one of the
satellites off Jupiter, or Venus or
Mercury off the sun.

Elements, chemical, present in the
sun, 123; fixed stars, 69; earth's
crust, 207; meteorites, 317.
Elements of an orbit are the quan-
tities the determination of which
enables us to know the form and
position of the orbit of a comet or
planet, and to predict the positions of
the body,see Appendix, Tables II.-V.
Ellipses, 165 et seq. 624.
Elongation, the angular distance
of a planet from the sun: of Mer-
cury and Venus, 380.

Emersion, the reappearance of a
body after it has been eclipsed or
occulted by another; e.g. the emer-
sion of Jupiter's satellites from be-
hind Jupiter, or the emersion of a
star from behind the moon.
Enceladus, one of the satellites of
Saturn.

Envelopes of comets, 294a.
Ephemeris (étí, fɔr, nμépa, a day),
a statement of the positions of the
heavenly bodies for every day or
hour prepared soine time before-
hand, 557-

Epoch, the time to which calcu.
lations or positions of the heavenly
bodies are referred, 551, 555.
Equation of the centre, the differ-
ence between the true and mean
anomalies of a planet or comet; of the
equinoxes, the difference between
the mean and apparent equinox; of
time, the difference between true
solar and mean solar time, 415.
Equator, terrestrial, 153; celestial,
328.

Equatorial telescope, 482 et seq;
method of using, 535; horizontal
parallax. see Parallax.

Equinoxes (æquus, equal, and nox,
night); vernal or equinoctial, the
points of intersection of the ecliptic
and equator. When the sun occu-
pies these positions in Spring and
Autumn of the northern hemisphere,
there is equal day and night all
over the world, a small circle near
each pole excepted, 171; precession
of the, see Precession.

Errors, instrumental and clock, 530,

555.
Evection (evehere, to carry away).

One of the lunar inequalities which
increases or diminishes her mean
longitude to the extent of 1° 20'.
Evening star, 380.

Eye-pieces of telescopes, 471; their
various forms, 472-73; transit eye-
piece, 531.

Faculæ (Lat. torches), the brightest
parts of the solar photosphere, 119,

119a.

Field of view, the portion of the
heavens visible in a telescope.
Figure of the earth, see Earth.
Fixed stars, see Stars.

Focus (Lat. hearth), the point at
which converging rays meet, 458.
Foci of an ellipse, 166.
Foucault proves the earth's rota-
tion, 154; determines the velocity of
light, 450.

Fraunhofer's lines, 490.

Galaxy (yáλaктos, of milk), the
Greek name for the Milky Way, or
Via Lactea.

Geocentric (y, the earth, and Kév-
Tрov, a centre), as viewed from the
centre of the earth; latitude and
longitude, 360.

Geography, physical, 182 et seq.
Geology, 182.

Gibbous (Lat. gibbus, bunched)

Harvest moon, 373.
Head of comets, 291.

Heavens, how to observe the,

342.
Heliacal rising or setting of a star
is when it just becomes visible in
morning or evening twilight.
Heliocentric (Acos, the sun, and
Kévтρоv, a centre), as seen from, or
referred to, the centre of the sun;
latitude and longitude, 360.
Heliometer (ἥλιος and μέτρον, ο
measure), a telescope with a divided
object-glass designed to measure
small angular distances with great
accuracy. It is so called because it
was first used to measure the sun.
Hemispheres (μ, half, and
opaipa, a sphere), half the surface
of the celestial sphere. The sphere
is divided into hemispheres by great
circles such as the equator and
ecliptic.
Herschel, Sir W., discovers the
inner satellites of Saturn, 271; dis-
covers Uranus, 277.

Horizon (opíšw, I bound), true or
rational, 329; sensible, 152.
Horizontal parallax, see Parallax.
Hour angle, the angular distance
of a heavenly body from the meri-
dian.

Hour circle, the circle attached to
the equatorial telescope, by which
right ascensions are indicated, 535..
Huggins, Mr., his spectroscopic
observations, 499.

Hyperbola, the, one of the conic
sections, 624.

Immersion (immergere, to plunge
into), the disappearance of one
heavenly body behind another, or
in the shadow of another.
Inclination of an orbit, the angle
between the plane of the orbit and
the plane of the ecliptic: of the
sun, 106; of the earth, 168; of the
axes of planets, 253, 254.
Inequalities, Secular; perturba.
tions of the celestial bodies so small
that they only become important in
a long period of time, 643.
Inferior conjunction, see Conjunc-
tion; planet, see Planet.
Instruments, astronomical, 518 et

seg.
Irradiation, 217.

Jets in comets, 294.
Jovicentric (Jovis, of Jupiter, and
KÉVтроV, a centre), as seen from, or
referred to, the centre of Jupiter.
Julian period, calendar and style,

443.

Jupiter, distance from the sun and
period of revolution, 134, 139;
diameter, 140; volume, mass, and
density, 147, polar compression,
255; description of, 263 et seq.;
satellites, 267.

Kepler's laws, 614; proofs of, 619
et seq.

Kirchhoff's investigations on spec-
tra, 492.

Latitude (latitudo, breadth), terres-
trial, 160; how obtained, 560; celes-
tial, 360; how obtained, 554; latitude
of a place is equal to the altitude of
the pole, 336; Geocen.ric, Helio-
centric, Jov.centric, Saturnicentric,
latitude as reckoned from the centres
of the planets named.

Lens, its action on a ray of light,
458; convex and concave 461, 462;
bi-convex and bi-concave, &c. 463;
axis of a, 458; achromatic lenses,
464; chromatic and spherical aberra-
tion of, 465.

Leverrier discovers Neptune, 277.
Libration of the moon, 214.
Light, what it is, 448; velocity of,
16, 449; aberration of, 449; refrac-
tion and reflection, 450 et seq.; dis-
persion, 465; light curves of variable
stars, 54, et seq.

Limb, the edge of the disc of the
moon, sun, or a planet.
Line, of collimation, 518; of nodes,

the imaginary line be.ween the as-
cending and descending node of an
orbit.

Longitude (longitudo, length), ter-
restrial, 161; how determined, 554;
celestial, 360; how determined, 563
et seq.; mean, the angular distance
from the first point of Aries of a
planet or comet, supposed to move
with a mean rate of motion; Geo-
centric, Heliocentric, Jovicentric, or
Saturnicentric, longitude as reckoned
from the centres of the planets
named.

Lumière cendrée, 217.
Lunar distances, used to deter-
mine terrestrial longitudes, 565.

Lunation (lunatio), the period of
the moon's journey round the earth,

434.
Luni-solar precession, see Preces-
sion,

Magellanic clouds, 33.
Magnitudes of s ars, 22, 23.
Major axis, see Axis.

Maps of countries, how constructed,
572

Mars, 134; distance from the sun
and period of revolution, 139;
diameter, 140; volume, mass, and
density, 147; polar compression.
255; description of, 256; moons cf.
259a; seasons, 262; how presented
to the earth in different parts of its
orbit, 393; how its distance from the
earth is determined. 583.

Mass. The mass of a heavenly body

is the quantity of matter it contains:
of sun, 103; of planets, 147; of
comets, 295.

Mean distance of a planet, &c. is
half the sum of the aphelion and peri-
helion distances. This is equal to
the semi-axis major cf an elliptic
orbit, 139; mean anomaly, see Ano-
maly; mean obliquity is the obliquity
unaffected by nutation; mean time,
see Time; mean sun, 405.
Medium, resisting, 295.
Mercury, 134; distance from sun
and period of revolution, 139; dia-
meter, 140; volume, mass, and den-
sity, 147; polar compression, 255;
elongation of, 380.

Meridian (meridies, midday), the
great circle of the heavens passing
through the zenith of any place and
the poles of the celestial sphere,
162.

Metals and metalloids, list of, 207.
Meteorites, aërolites, aëroside.
rites, and aërosiderolites, 314; spo-
radic meteors, 315; remarkable
meteoric falls, 316; chemical con-
sti ution, 317 et seq.; meteoric orig.n
of nebulæ, 95; of comets, 287; of
all celestial to lies, 65, 504%.
Meteors, lumin us, their position in
the system, 134, divisions of, 298;
numbers seen in a star-shower, ib.;
explanation of star-showers, 301 et
seq.; the November ring, 308;
radiant point, 305: cause of bril-
liancy, 310; shape of orbits, 308,

312; weight of, 311; velocity of
310 detonating meteors, 313.
Micrometer (ukpós. small, and
μéτроv, measure), an instrument with
fine moveable wires attached to eye-
pieces to measure small angular dis-
tances, 473, 519.
Microscopes, 518.
Midnight Sun, 171.
Milky Way, 28; stars increase in
number as they approach, 29; ne-
bulæ do not, 95.
Minor axis, see Axis.

Minor planets, how discovered, 280,
284; sizes, 281; crbits and distances
from the sun, 282; eccentric.ty of
orbits, 283 brilliancy, 284; atmo-
spheres, 286.
Month, the, 434.

Moon, why its shape changes, 12;
dimension and distance of, 211-12;
line of revolution, 213; libration, 214;
nodes, 215, 244; moon's path con-
cave with respect to the earth, 216;
earth-shine, 217; brightness of, 218;
description of surface, 221 et seq.;
rotation, 228; no atmosphere, 227;
phases, 229; eclipses, 233 et seq.;
nodial and synodical revolution of,
241; apparent motions, 370 et seq.;
harvest moon, 373; how the distance
of the moon is determined, 579; ele-
ments of the moon, see Appendix,
Table V.

Morning star, 380.
Motion, proper, of stars, 43; appa-
rent, of planets, 374 et seq.; direct,
381; retrograde, 381; laws of, 399
et seq.; circular, 622.
Mountains, lunar, heights of, 224.

Nadir (natura, to correspond), 328
Neap tides, 660.

Nebulæ, why so called, 6, 76; are
swarms of meteorites, 13, 96; classi-
fication of, 81; light of, 92; vari-
ability of, 94; spectrum analysis of
the, 498, 501 et seq.
Nebular hypothesis, 98, 210, 504 a.
Nebulous stars, see Stars.
Neptune, distance from the sun
and period of revolution, 134, 139;
diameter, 140; volume, mass, and
density, 147; discovery of, 277 et

Object-glass of telescopes, c^n-
struction of, 466; aperture and illu-
minating power of, 470; accuracy
required in constructing, 480; largest
object-glass, 48t.

Obliquity of the ecliptic, see Ecliptic.
Occultation (occultare, to hide),

the eclipsing of a star or planet by
the moon or another planet.
Opposition. A superior plant is
in opposition when the sun, earth,
and the planet are on the same
straight line and the earth in the
middle, 378.

Optical double stars, see Stars.
Orbit (orbis, a circle), the path of
a planet or comet round the sun, or
of a satellite round a primary, 282.
Ordnance Survey of England, 570.
Orion, 353.

cone

Parabola, a section of a
parallel to one of its sides, 624.
Parabolic orbits of comets, 288.
Parallactic inequality, an irregu-
larity in the moon's motion, arising
from the difference of the sun's
attraction at aphelion and peri-
helion.
Parallax (Tapáλλağıs, a change),
542; corrections for, 543, 544; equa-
torial horizontal, 542; of the moon,
580; of Mars, 583; of the sun, 585 et
seq.; old and new values of, 593; of
the stars, 594.

Parallels of latitude, 162; of de-
clination, 328.
Penumbra (pene, almost, and
umbra, a shadow), the half-shadow
which surrounds the deeper shadow
of the earth, 237; of sun-spots,

earth.) (1) The point in the moon's
orbit nearest the earth, 212; (2) the
position in which the sun or other
body is nearest the earth.
Perihelion (πepí, near, and nλios),
the point in an orbit nearest the sun,
167; distance, the distance of a
heavenly body from the sun at its
nearest approach: longitude of, one
of the elements of an orbit; it
is the angular distance of the peri-
helion point from the first point of
Aries: passage, the time at which
a heavenly body makes its nearest
approach to the sun, 3.
Peri-Jove, Saturnium, &c., the
nearest approach of a satellite to
the primary named, Jupiter, Saturn,
&c.

Period (Tepí, round, and ódós, a
path), or periodic time, the time of
a planet's, comet's, or satellite's
revolution; synodic, the time in
which a planet returns to the same
position with regard to the sun and
earth, 384.

Perturbations (perturbare, to in-
terfere with), the effects of the
attractions of the planets, comets,
and satellites upon each other, con-
sisting of variations in their motions
and orbits described round the sun,
633.

Phases (dáois, an apperance), the
various appearances presented by
the illuminated portions of the moon,
(229) and inferior planets (377) in
various parts of their orbit with regard
to the earth and sun.

Phobos, one of the satellites of Mars,

sun, IIC.

Physical constitution of the stars,
65, 69; of the sun. 119 et seq.
Plane of the ecliptic, 105, 136, 300.
Planet (λavýτns, a wanderer), a
cool body revolving round a central
incandescent one.

Planets change their positions with
regard to the stars, 4; what they
are, 11; names of, 134; travel round
the sun in elliptical orbits, 135, 377;
and in one direction, 138; distances
of, from the sun, 139; periods of
revolution, 139; real sizes of, 140;

comparative sizes of, 141; mass,
volume, and density, 144-47; com-
pared with the earth, 251 et seq.;
apparent movements of, 374 et seq.;
varying distances from the earth,
376; brilliancy and phases, 377!
inferior and superior, 378; conjunc-
tion and opposition, 378; elonga-
tions, 380; direct and retrograde
motion, 381; stationary points, 382;
synodic periods, 384; inclinations
and nodes of orbits, 388; apparent
paths among the stars, 391 et seq.;
elements of the, see Appendix,
Table II.

Planetary nebulæ, see Nebulæ.
Plateau's experiment, 197.
Pointers, the, 341.

Polar axis of the earth, 153, 163;
compression (see Compression), 255;
distance, 329.

Polaris (Lat.), the pole-star, 341; is
not always the same, 547:
Poles (Toλew, I turn), the extremi-
ties of the imaginary axis on which
the celestial bodies rotate, 153, 261;
the poles of the heavens, 328; are
the extremities of the axis of the
celestial sphere which is parallel to
the earth's axis; the poles of the
ecliptic are the extremities of the
axis at right angles to the plane of
the ecliptic, 360; cf the earth,

519.

Precession (præcedere, to precede)
of the equinoxes, or luni-s lar pre-
cession, a slow retrograde motion
of the equinoctial points upon the
ecliptic, 361, 548; cause of, explained,
644 et seq.

Prime, vertical, see Vertical.
Prisms refract light, 453..
Prominences, red, of the sun, 118,
248.

Proper motion, see Motion.

Quadrant (quadrans, a fourth part),
the fourth part of the circumference
of a circle or 90°; of altitude, a
flexible strip of brass graduated into
90°, attached to the celestial globe
for determining celestial latitudes,
declinations being determined by the
brass meridian.

Quadrature. Two heavenly bodies
are said to be in quadrature when