pm
■ differience'iujjfljei riling and fe ttif ig .q f tlie ftars ;in. different
Hates o f the atmofphere ; but he made, no alio nr—
ance for it in his commutations. A lha-zen, an Arabian
optician, in the n t h century-, obfeiVed |he effedt’ upon
the circumpolar ftara; but T ycho iwas tKtefiril perfopi
who conftrufted a table for fW'refraffions ’at different
altitudes, for th'e refraftion decreafes from the Jsori-
Zon tct the zenith, whdre i t ia. nothing. -In the mean
flate o f our. air, the refraftion in the horizon is 33'.
13- Another property of-thefrefraiflibn, o f the air is
this, that it caufes ail!the heavenly ‘.bodies to appearIn
the morning above the horizon, when they are acbojlly,
below'-it ; and in the -evening they appear above, a little
after they are a (filially f i t ; fgrJthe_diameter o f the fun
•being about 32', th e .refraction in the horizon aerating
it 33', will caufe it to appear above the horizon when
the whole body is below. In climates nearer the equa-
. fd t, the refra'Aion is lefs than it is here ; and in colder
-regions it is much greater, and thisfs aLappy.provifioni
for lengthening fhe appearance- o f ithel light at thofe
parts. G asshnd.us relates, that feme Hallanilirs who
■ wintered in Nova 'Zambia, in -latitude 7 7 ° , .were agreeably
furprifed with a fight o f the fun 17 days before they
eXpefted .him. T o the fame caufe‘ we muff attribute
another phenomenon, mentioned b y Pu n y ,. that the
-moon had been vifibly eclipfed when, Ihe was in-the weft,
at the'faipe time i^attjje fun appeared above(.the hpfi-
Zon- in' the eaft- M vestlinus, in KEP,Lgjt,3^tj6estan-
■ Qther inftance of the fame kind which fell Under his
own OhfervatiDn. the decreafe q£ refra&ipn as
the altitude above the-horizon increafes,.makes the fun
and moon appear o f an oval form, more .particularly in
the.horizon.- .For, figjpgfg i-jpp,- diraTuf. t>f ithe' fihr to •
be ga', and the- lowerdimb to touc^tthc; horizon, djeri
the mean; refraftion o f that limb >8. 33' ; but thevalti-
tudefhf the upper limb being then -32', its-jefra&iqn (is
only 28'- 6'', _ differing 4' 54." from the refra£tion o f the
■ lower limb ; by this quantity therefore the vertical dia-
meter, is fhorten^d_Vfjthe lower limb being fo much .more
elevated than the upper. , The like is true at-any other
altitude, only in a fmaller degree.
O n P a r a l l a x . |
14. When you refer an ;objeótmto fomething behind
i t , it will not appear in the fame fituation to; two fpec-'
tators fituated at different places, unlefs/the objeft be at
.an almoft indefinitely great difiance when compared with
the diftance o f the two fpe&ators; and the diftance o f
thefe apparent places is- called the parallax o f rhat-ob-
je ft. From the jmiiienfe diftance o f the fixed Aars
therefore in refpeA to the diampter o f the earth's orbit,
they never appear to change their relative fituatipns; on
which account we may- confider them as a back-ground
to which we may refer all the bodies in ourfyftem ; and
we may confider them as placed in the: concave fuiface
o f a fphere, o f which the earth is the center. ' If. therefore
a planet, when .it is in the fame ipar’t o f its orbit, \
be viewed fromitheitwo extremities o f i diamefêf bfifltfe
.earth’s orbit, it .will appear in' two different placés
arrongltythe fixed liars, and the diftajreebe tween 'thefe
two places' is called the annual parallax. In like mannier,
i f a- planet, or any o f the ; bodies in our fyftemj,
were pbferved fronf-f.be earth’ s center and Curface, they
wonldibe referred tcüjdifferent.places amongft the fixed
liars, and the diftance o f thole places is .called till diur-
■ nal parallaZ, ancl this is ivliatv we have, noiv soccalion to
t e t Û \ t fc| «center, oftthe. partk S V , S the place o f
a fpeâatqV, -Z his .zfpith,; and^qnceJfp the jbircle Z T
to reprefcnt A e ^ & p r e ^ l f j ie -fif.ed ftars, an d ie t ',ffSR
be a .plaqe-toqchtng the earth at d.jlihpn th jt plane a
CfHcd thq pyripon ; , î t priding the .vififijs part
E Z R o f tire -heaveds froffl the irryÿtbleqjart f fT R . I f
a plane P C J V b t d t p i ; through the center o f thè çarth.
parallel to $ Z R , l^ c a l le d the ratt^aJ bprfzpn'. . 1
th e a rp '^ ^ .q rg p ^ f t jb^idkjed .fi^s-
angle at the earth, yii«l thence .we may^ugipBfe.jtJie tvya
bt^izqnljtheVe lo^pqiqÿde.’ . .L et A-be-ï-Jilânet^jand
draw CPn, SPm ; then ithe pljiqet feen, fi-onj.1! appears
at m, and.Lon^L,it?Iriguld appear a t ^ aqd am i|.ejh§d
taèiïËumàl.parallax ; bec?ufe at- différent «parts o f the
day, when the planet is taf different altitudes., the arc
mn will be different. I f the planet be in the horizon
at p , and we draw, Cpa,. then Ra is the hof-iiontai, parallax,
which is the greateft'of.ali ; and fi-qWv.thé horizon
to the zenith,it gradu.aflyAdtereafes, jaild is nothing in
the zenith. A Ho, the nearer a .planet, is.ito the earth, .
the greater is its parallax ; for- the nearer 'P is 'to C,
the.grea'ter is the àngje C l’S, or «ƒ>,«, which is the pa-'
rallax, as -that angle is meafured.';by,'the;atop»n. . Now
aftronomers refer âiLtheir obfervàtiqns .'tp thccènter..pf .
.tlie earth,..ànd confider the place as feen from thence,to.
be the true place ; therefore the! apparent placé »1. feen
I I !SpuH
■ frdtn- the ffirfâce is below tlie trite place a Hence,-
■ after! ahJaltitude is taken lip.qn! the furface of'the earth,
we mult:a‘M thç parallax coi 1 efpondmg to that altftpde,
fif agifude feen.
ÇÇïftL: q? .e.av.thl nbo^çi the ' rqtipduï hqrizo.n.'
o f a ils dutaiiqe
i swç kiJOlf Âbg
SpC, then by plane trigonometi j , fin. lypÇ ,-j radius r:
^ us Ç?1- A 8. il1 |*^fW3Bi Ja !
st.fhiaetej ïf®ni WS haye ôhfefEed
Xd,rt.i toe. the Afijtodèrnf w - h tw n j p
rign^,^oi^|or'>#ra^(qi}r tód tîve”*^®er'har pardlax ;
the fo l f j& tó fife fpbtW^pik and ^rç ls ftAj^fe-ad d éfl.
c-objeryeJ.to the I w i l l S b 'A S
the dxsd ftars baye no p^Hax^ the qnly coi réélyjn there,
uepefibiy* â f refra^îôq.
, a s the, ^Ititjjdpa o f thn ,‘hepiyiniyj bpd'W,
termined by an inftrument called a quadrant, it may.be
hers .proper Ip give a -general dçfcrjfetipnof it. -
' , 17. Let- C.'repreferit the center o f the quadrant, C A ,
GR two radii perpendicular to-.èaeh other, thereby including
an arc A B ó f 90. degrees ; T L is a Telefcope
moveable abouti the -center C ; in the. principal, focus
ƒ o f the objeft.-glafs-, there are fixed two fine wires at'
’ right, angles to each other, interfering each other in the
■ center of the telefcope ; one o f thefe wires is adjufted
parallel to the horizon, and confeqiiently the other w ill.
be ppipèndiculai'; t o lt ; the line joining the interfeftion■
o f thefe wires, and the cencer of the objeft glafs, is called
the axis o f the telefcopiè, and fometimes the line o f
collimahon. The telefcópé' moves againft .the limb o f the
quadrant, and carries with it a fmall graduated piece o f
brafs rv, called a vernier, having a mark at 0 pointing to
. ItkAb.' T h u (point ö .is fo adjufted,,
that when it is fet to point to o o,n -the limb, the axis o f>
t.he telefcope is horizontal, and therefore an object in
the hprizon will appear up'on the horizontal wire. .When .
therefore -the telefcope- is put into ’any other litigation,,
'^ n # a i ^j|^v^o 1a,ght upon the horizontal wire, thé •
point 0 o f the vernier wilï;he direéted to a point ó f the
limb .which fhows how many degrees high the objeét is *
above the horizon. The limb is generally divided into >
degrees,. and each degree into three equal parts, by .
which the whole limb is divided into .every ip minutes.
T he vernier £as alfo a .certain number of divifions upon .
itj-fo that b y ^obferving .which • two divifions ó f the *
vernier ap'd limb coincide, ypu .cah tell' to what minute-
\ o f -the -limb the mark o o £ the vernier is direéled, and
.therefof^ know- the altitude o£‘ th£ object abovje the--
llónzqnj'. ip. degrees and minlifes1.'1; I f ^ho two divifions
mould ; coincide* there-', i s anothër apparatus prefixed.
tO-the telefcoj^ ahthe lfmb o f th? quadrari-t, by which-
y^u1 ^an ftéll to a.fecoqd, the point o f the limb againft.
which ,^on the vernier;ftands ; and . thus you:can afeer- -
anfóbféêpif^ahïecönd.'. P o r a full
^xplaha|ipn ^óf tfiefe .matters, we referthe reader to M r, -
Y/j n-c e’ si on PraSical jijlronomy\ This inftrument
is fometimes fixed to a perpendicular axis, and 'can-
be placed in any fituation, fo that the altitudes o f any,
o f the' heavenly vbö^eg can-’.be determined^by it. Some-' •
times/it is fixed agaïnfb a‘ very him ftone wall, having ,
its’ plate exaflly. in the mefidiap,-Td that only meri- •
dian altitudes can be taken; by it. This is called , a «
muralquadrant; and a ll. very large, quadrants are thus'
fixed up>'^|qff ‘the-, r a o f t accu rate obferv-alions which -
a'ftróhiÓmers want,' are thofe up'diTthe meridian, 'b y
w f i T t h e ^declintitioks &£ ■
th.e h!eayenlyvbp'dies may,^be' found..' A fter ' an altitude
is taken, it muft (art. 15) ^e'.’horreiSted by fubtraéling [
‘the refi;a4Ü9n ' and adding the' parallax, by which we
-get tbs true altitude "of the' 'óbjèét above the rationale
horizon! corlrefppndipg,;to. the,'place.ofithe -obferyef»-.
Oh‘.