Edmond Halley

those that desire to know how to construct Geometrically the Orb of a Comet, by Three accurate Observations given, may find it at the End of the Third Book of Sir Isaac Newton

I design to treat of all these Things in a larger Volume, and contribute my utmost for the Promotion of this Part of Astronomy, if it shall please God to continue my Life and Health.

and reduce their Motions to certain Rules.

from its Period, and the Manner of its Transit, I cannot think different from those I have just now mention'd. Hence I dare venture to foretell, That it will return again in the Year 1758. And, if it should then return, we shall have no …

the Space between the Sun and the fix'd Stars is so immense, that there is Room enough for a Comet to revolve, tho' the Period of its Revolution be vastly long.

become very obscure; and wanting Tails, pass by us unseen

there may be a far greater Number of them

it is clear, they are not carry'd about or mov'd in

By comparing together the Accounts of the Motions of these Comets

I have publish'd this Specimen of Cometical Astronomy, as a Prodromus of a designed future Work, left, happening to die, these Papers might be lost, which every Man is not capable to retrieve, by reason of the great Difficulty of the …

5. From these Things given (by the very same Rules that we find the Planets Places, from the Suns Place and Distance given) we may obtain the Apparent or Geocentrick Place of the Comet, together with the Apparent Latitude. And this it may …

These necessary Things premis'd, let it be propos'd to compute the apparent Place of any one of the mention'd Comets, for any Given Time.

this one Table of ours will serve for all Comets.

A General Table for Calculating the Motions of Comets in a Parabolical Orbit.

The Astreonomical Elements of the Motions in a Parabolick Orb of all the Comets that have been hitherto duly obferv'd. ...This Table needs little Explication, since 'tis plain enough from the Titles, what the Numbers mean. Only it maybe …

Next, Hevelius (a Noble Emulator of Ticho Brahe) following in Keplers Steps, embraced the same Hypothesis of the Rectilinear Motion of Comets, himself accurately observing many of them. Yet, he complain'd, that his Calculations did not …

But all those that consider'd Comets, until the Time of Ticho Brahe (that great Restorer of Astronomy) believ'd them to be below the Moon, and so took but little Notice of them, reckoning them no other than Vapours.

[I]n the Year 1472, which being the swiftest of all, and nearest to the Earth, was observ'd by Regiomantanus. This Comet (fo frightful upon the Account both of the Magnitude of its Body,and the Tail) mov'd Forty Degrees of a great Circle …

Wherefore, if, according to what we have already said, it should return again about the year 1758, candid posterity will not refuse to acknowledge that this was first discovered by an Englishman.