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will offer opportunities of observing her in that situ-
7 in the evening
Phase of Venus.
= 4.6367 Eclipses of Jupiter's Satellites. The following visible eclipses of Jupiter's first and second satellites will take place this month at the Royal Observatory; and they will not differ much for other parts of the kingdom: viz.
Other Phenomena. Mercury will attain his greatest elongation on the 7th, and be stationary on the 26th of this month. Jupiter will also be stationary on the 6th, and Georgium Sidus on the 11th. The Moon will be in conjunction with æ in Virgo at 51 m. after 4 in the morning of the 4th; with a in Scorpio at 32 m. after 9 in the evening of the 7th ; with Jupiter at 14 m. after midnight of the 14th; with B in Taurus at a
6 7 10
quarter past 5 in the afternoon of the 21st; with Pollux at 19 m, after midnight of the 230; and with u in Leo at 15 m. after 11 at night on the 26th. The Moon will also be in apogee on the 1st; in perigee on the 17th; and again in apogee on the 29th. ECLIPSES of JUPITER'S SATELLITES.
(Conçląded from p. 202.] As it is evident, from the number of eclipses that happen almost every month, the times of which are inserted in the 3d page of the month in the Nautical Almanac, or on the 36th and 37th pages of White's Ephemeris, and the few that are marked with an asterisk, as being visible at the Royal Observatory, that all are far from being visible in the same place; hence it becomes requisite to ascer. tain what are the circumstances which distinguish those that are visible from those that are not. Accordingly, therefore, the following particulars should be attended to:-
An eclipse of either of Jupiter's first or second satellite will be visible if his altitude above the horizon exceed 8°, and the Sun at the same time is as much below it, with respect to the place where the observation is intended to be taken; and this may readily be ascertained with sufficient accuracy by means of a celestial globe. Or by using the terres. trial globe, in conjunction with the former, the place on the Earth's surface where an eclipse of either of these satellites will be visible may easily be found in the following manner :--The place of the Sun, with the latitude and longitude of Jupiter, being given for the required time in the Ephemeris, find their declinations and right ascensions by the globe; then con, vert the difference between the time at which the eclipse is to happen and noon into degrees and minutes, and they will show the longitude of that meridian on the surface of the Earth where it is noon at the time the satellite is eclipsed, which may be called the meridional longitude of the eclipse, and is either east or west, as the eclipse happens before or after noon at Greenwich. Next, bring this meridional longitude to the brass meridian of the terrestrial globe, and elevate the pole which is nearest the Sun equal to his declination, and fix the globe in this position; then, if Jupiter be eastward of the Sun, draw a line along that part of the globe which coincides with the eastern horizon, which will pass over all those places where the Sun is setting at that time; but if Jupiter be westward of the Sun, draw the line along the western horizon, and it will pass over all the places where the Sun is then rising. When Jupiter is eastward of the Sun, add the difference of his and the Sun's right ascensions to the meridional longitude; bring the degree answering to their sum to the meridian, elevate the pole nearest Jupiter equal to his declination, and fix the globe in that position; then, another line being described on the globe, along the eastern horizon, the space included between this and the line of the Sun's setting before drawn, will comprise all the places where the eclipse will be seen during the interval between the setting of the Sun and that of the planet. But if Jupiter be to the westward of the Sun, the difference of the right ascensions must be subtracted from the meridional longitude, instead of being added to it, and the degree answering to the remainder brought to the meridian, the pole elevated, and the globe fixed as before. Thus, if a line be drawn along the western edge of the horizon, the space included between this line and that of the Sun's rising before drawn, will comprise all the places where the eclipse will be seen between the rising of Jupiter and that of the Sun. The eclipse will evidently be seen the best at those places that are most distant from these boundary lines; all the other circumstances attending it being the same.
visible, by computation, and without the use of the globe; and for this purpose find the time of the Sun's rising and setting, by means of a table of semidiurnal arcs, as before described, for the required latitude. Find also the time of Jupiter's rising and setting, from the time of his passing the meridian and his declination, both of which are given in the Ephemeris, and the same table of semi-diurnal arcs. The manner of doing this has likewise been already pointed out..
When the immersion or emersion of either the first or second satellite has been accurately observed, according to mean time, at the place of observation, the longitude from Greenwich is immediately found by taking the difference between the time of observation and that stated in the Ephemeris, as the time of the same eclipse happening at the first meridian; and this difference, converted into degrees, minutes, &c., will give the difference of longitude between the two places, which will be east or west, as the observed time of the eclipse was greater or less than that given in the Ephemeris.
To illustrate this, let it be supposed that an emersion of the first satellite was observed, at the Cape of Good Hope, to take place at 9 h. 46 m, 38 s. of mean time, the time in the Ephemeris being stated to be 8h. 33m. 5 s. The difference of these times is 1 h. 13 m. 33 s., which, being converted into degrees, is equal to 18° 23' 15", which is the longitude of the Cape east of Greenwich; since the time at which the eclipse is supposed to have happened at the Cape is greater than that stated in the Ephemeris, as the time of its happening at the Royal Observatory.
The most certain way, however, of deducing the longitude from an observation of this kind, is not to compare the time of its happening with that stated in the Ephemeris, but with that of an actual observation of the same eclipse, made at some piace at which the longitude is well known when such an
observation has been made; for such a comparison avoids the errors to which the computations are liable. But if no corresponding observation of the kind can be obtained, it is desirable to find, by the nearest observations to the given time that have been made, what corrections the calculations of the Ephemeris require; and then the application of these corrections to the calculation of the given eclipse in the Ephemeris, renders it almost equivalent to an actual observation.
It has already been observed, that the immersions of Jupiter's first and second satellites alone are visible from his conjunction with the Sun to his opposition with that laminary; and from this time to his conjunction again the emersions only can be seen. But within about fifteen days, both before and after the conjunction, both the planet and his satellites are lost in the Sun's light; and consequently the eclipses are altogether invisible. This accounts for their generally being omitted in Time's Telescope for one month in the year. The reason why both the immersion and emersion of Jupiter's first and second satellites are not visible in the same eclipse, as well as the reason of the particular periods during which each is visible, will be clearly explained by the following simple diagram :
Let S represent the place of the Sun, EE the Earth's orbit, mPn the orbit of Jupiter, and P the