DOI Page / Citation link:
https://doi.org/10.11588/diglit.68888#0353
If therefore we would come to any certainty in this point, we must go
other wayes to work 5 and as I have here examined the height and refra-
ctive property of the Air by other wayes then are usual, so must we
find the Parallax of the Planets by wayes not yet prastised 5 and to this
end, I cannot imagine any better way, then the Observations of them by
two persons at very far distant parts of the Earth, that lye as neer as may
be under the same Meridian, or Degree of longitude , but dissering as
much in latitude, as there can be places conveniently found : These two
persons, at certain appointed times, should ( as near as could be ) both
at the same time, observe the way of the Moon, Mars, Venus, Jupiter,
and Saturn, amongst the sixt Stars, with a good large Telefcope, and ma-
king little Iconismes, or pi&ures, of the small fixecl Stars, that appear to
each of them to lye in or near the way of the Center of the Planet, and
the exaCc measure of the apparent Diameter '5 from the comparing of
such Observations together, we might certainly know the true distance,
or Parallax, of the Planet. And having any one true Parallax of thefe
Planets, we might very easily have the other by their apparent Diame-
ters, which the Telefcope likewife affords us very accurately. And thence
their motions might be much better known, and their Theories more ex-
actly regulated. And for this purpose I know not any one place more
convenient for such an Observation to be made in, then in the Issandof
St. Helena, upon the Coast of Asrick^, which lyes about sixteen degrees
to the Southwards of the Line, and is very near, according to the latest
Geographical Maps, in the lame Meridian with London 5 sor though
they may not perhaps lye exactly in the same, yet their Obfervations,
being ordered according to what I shall anon (hew, it will not be diffi-
cult to find the true distance of the Planet. But were they both under
the same Meridian, it would be much better.
And because Obfervations may be much easier, and more accurately
made with good Telefopes, then with any other Instruftients, it will not,
Isuppose, feem impertinent to explain a little what wayes 1 judge most
fit and convenient for that particular. Such therefore as shall be the
Obsei waters for this purpose, ssiould be furnished with the best lelefcopes
that can be had, the longer the better and more exade will their Obser-
vations be, though they are somewhat the more difficultly manag’d.
These ssiould be fitted with a Rete, or divided Scale, plac’d at lucha di-
stance within the Eye-glals,that they may be distinssly seen,which ssiould
be the mealures of minutes and feconds 5 by this Instrument each Ob-
servator ffiould, at certain prefixt times, obferve the Moon, or other
Planet, in, or very near, the Meridian 5 and becaufe it may be very dissi-
cult to find two convenient Rations that will happen to be just under the
same Meridian, they shall, each of them, observe tire way of the Planet,
both for an hour before, and an hour after, it arrive at the Meridian 5
and by a line, or stroke, amongst the Imall fixed Stars, they shall denote
out the way that each of them observ’d the Center of the Planet to be
mov’d in for thofe two hours : These Obfervations each of them shall
repeat for many dayes together, that both it may happen, that both of
theisi
other wayes to work 5 and as I have here examined the height and refra-
ctive property of the Air by other wayes then are usual, so must we
find the Parallax of the Planets by wayes not yet prastised 5 and to this
end, I cannot imagine any better way, then the Observations of them by
two persons at very far distant parts of the Earth, that lye as neer as may
be under the same Meridian, or Degree of longitude , but dissering as
much in latitude, as there can be places conveniently found : These two
persons, at certain appointed times, should ( as near as could be ) both
at the same time, observe the way of the Moon, Mars, Venus, Jupiter,
and Saturn, amongst the sixt Stars, with a good large Telefcope, and ma-
king little Iconismes, or pi&ures, of the small fixecl Stars, that appear to
each of them to lye in or near the way of the Center of the Planet, and
the exaCc measure of the apparent Diameter '5 from the comparing of
such Observations together, we might certainly know the true distance,
or Parallax, of the Planet. And having any one true Parallax of thefe
Planets, we might very easily have the other by their apparent Diame-
ters, which the Telefcope likewife affords us very accurately. And thence
their motions might be much better known, and their Theories more ex-
actly regulated. And for this purpose I know not any one place more
convenient for such an Observation to be made in, then in the Issandof
St. Helena, upon the Coast of Asrick^, which lyes about sixteen degrees
to the Southwards of the Line, and is very near, according to the latest
Geographical Maps, in the lame Meridian with London 5 sor though
they may not perhaps lye exactly in the same, yet their Obfervations,
being ordered according to what I shall anon (hew, it will not be diffi-
cult to find the true distance of the Planet. But were they both under
the same Meridian, it would be much better.
And because Obfervations may be much easier, and more accurately
made with good Telefopes, then with any other Instruftients, it will not,
Isuppose, feem impertinent to explain a little what wayes 1 judge most
fit and convenient for that particular. Such therefore as shall be the
Obsei waters for this purpose, ssiould be furnished with the best lelefcopes
that can be had, the longer the better and more exade will their Obser-
vations be, though they are somewhat the more difficultly manag’d.
These ssiould be fitted with a Rete, or divided Scale, plac’d at lucha di-
stance within the Eye-glals,that they may be distinssly seen,which ssiould
be the mealures of minutes and feconds 5 by this Instrument each Ob-
servator ffiould, at certain prefixt times, obferve the Moon, or other
Planet, in, or very near, the Meridian 5 and becaufe it may be very dissi-
cult to find two convenient Rations that will happen to be just under the
same Meridian, they shall, each of them, observe tire way of the Planet,
both for an hour before, and an hour after, it arrive at the Meridian 5
and by a line, or stroke, amongst the Imall fixed Stars, they shall denote
out the way that each of them observ’d the Center of the Planet to be
mov’d in for thofe two hours : These Obfervations each of them shall
repeat for many dayes together, that both it may happen, that both of
theisi