The History of the Royal Society of London/Chapter 9

by Lawrence Rooke

DIRECTIONS

FOR THE

Observations of the Eclipses of the Moon.

By Mr. ROOKE.

EClipses of the Moon are observ'd for two principal Ends; one Astronomical, that by comparing Observations with Calculations, the Theory of the Moon's motion may be perfected, and the Tables thereof reform'd: The other Geographical, that by comparing among themselves Observations of the same Ecliptical Phases, made in diverse Places, the difference of Meridians, or Longitudes of those places may be discover'd.

The Knowledge of the Eclipse's Quantity and Duration, the Shadows, Curvity and Inclination, &c. conduce only to the former of these Ends: The exact time of the Beginning, Middle, and End of the Eclipses, as also in total ones, the Beginning and End of total Darkness, is useful for both of them.

But because these Times considerably differ in Observations made by the bare Eye, from those with a Telescope, and because the Beginning of Eclipses and the End of total Darkness are scarce to be observed exactly, even with Glasses; (one not being able clearly to distinguish between the true shadow and Penumbra, unless one have seen, for some time before, the Line, separating them, pass along upon surface of the Moon: And lastly, because in small partial Eclipses, the Beginning and End (and in total ones of short Continuance in the Shadow, the Beginning and End of total Darkness) are unfit for nice Observations, by Reason of the slow change of Appearances, which the oblique Motion of the Shadow then causeth: For these Reasons I shall propound a Method particularly designed for the Accomplishment of the geographical End in observing Lunar Eclipses free (as far as is possible) from all the mentioned inconveniences: For,

First, It shall not be practicable without a Telescope.

Secondly, The Observer shall always have Opportunity, before his principal Observation, to note the Distinction between the true Shadow and Penumbra.

Thirdly, It shall be applicable to those Seasons of the Eclipse, when there is the suddenest Alteration in the Appearances. To satisfy all which Intents,

Let there be of the eminentest Spots, dispersed over all Quarters of the Moon's surface, a select Number generally agreed on, to be constantly made use of to this Purpose, in all Parts of the World: As for Example, those which Hevelius calleth

Mons	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	Sinai
		Ætna	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$	Besbicus	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$	Mæotis
		Porphyrites Insula		Creta Palus		Maræotis
		Serrorum
Lacus Niger Major.

"Let in each Eclipse (not all, but for instance) three of these Spots, which then lye nearest to the Ecliptic, be exactly observ'd, when they are first touched by the true Shadow, and again when they are just completely entred into it; and (if you please) also in the Decrease of the Eclipse, when they are first fully clear from the true Shadow: For the accurate Determination of which Moments of Time (that being in this Business of main Importance) let there be taken Altitudes of remarkable fixed Stars, on this side the Line, of such as lye between the Æquator and Tropic of Cancer; but beyond the Line, of such as are situated towards the other Tropic; and in all Places, of such, as at the time of Observation, arc about four Hours distant from the Meridian.

 

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