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49 Equation of Time. The time shown by a good sun-dial, increased by the numbers in the following table, will give the time that should be shown by a well regulated clock at the same moment.
5 32 Friday, 11th,
4.53 Wednesday 16th,
4 0 Monday, 21st,
2 53 Saturday, 26th,
1 34 Thursday, 31st,
9 evening First Quarter 17th
1 morning Full Moon 23d
10 evening Last Quarter - 30th
2 afternoon. Moon's Passage over the Meridian. The Moon will pass the first meridian of this country at the following times during this month; which are selected for the reasons already assigned. August 19th, at 7 m. after 8 in the evening 20h, - 9
9 21st, 11
5 31st, - 18
August 1st Enlightened part
Phase of Venus.
= 0.0150 Dark part
11.9850 Eclipses of Jupiter's Satellites. The following eclipses of Jupiter's first and second satellites will be visible at the Royal Observatory this month : viz.
3 22d, 16.
10 evening 30th, 11
O inorning 2d Satellite 10th, 14
11 night 18th, 51
1 morning. Other Phenomena. Mercury will be stationary on the 5th, and in conjunction at 45 m. after 9 in the evening of the 19th of this month. He will also be stationary again on the 29th; and Venus will be stationary on the 21st. The Moon will be in conjunction with ß in Taurus at 45 m. after 8 in the morning of the 4th ; with ein Leo, at 9 m. after 8 in the evening of the 9th; with Mars, at 28 m. after 9 in the evening of the 12th ; with a in Virgo, at 52 m. after 9 in the morning of the 14th ; with & in Scorpio, at 38 m past 2 in the morning of the 18th; with Jupiter, at 54 m. after 5 in the morning of the 25th; and with 8 in Taurus again, at 23 m. after 2 in the afternoon of the 31st. The Moon will likewise be in apogee on the 11th, and in perigee on the 24th. DECLINATION and MERIDIAN ALTITUDES of the
PLANETS. The declination of a planet will be easily understood by considering that it is the same as that of the Sun, which we have already described; and that its use is also the same; but it is not so easy in its ap
plication, especially for the first meridian; for both being calculated for the instant of apparent noon, the Sun is on that meridian at that moment, but the planet may be any distance less than 180° from it; and consequently its declination requires a correction before it can be applied to the finding of the meridian altitude of the planet. If the declination of a planet was increasing, for instance, and it did not come to the meridian for some hours after the apparent noon, the time for which its declination inserted in the ephemeris is calculated, its declination at the moment of its passage over the meridian, would be greater than that in the table; but if it was decreasing, the contrary would take place, in consequence of the difference of time which elapsed between the two instants of computation and observation. On the other hand, if the declination of the planet were diminishing at that time, the opposite of the above statement would take place in both cases. To illustrate these remarks, let it be required to find the altitude of Venus at the Royal Observatory, when she is on the meridian on the 1st of March, 1820.
Now the declination of the planet at noon of that day is 5° 27' N., and the daily increase about 30' 30"; and her passage over the meridian also takes place at 5 m. past 2 in the afternoon; and hence, as 24h. : 2h, 5m.:: 30'} : 2' 39", which being added to the above declination, gives 5° 29 39" for the declination of the planet when on the meridian. Hence we haye Co-latitude of the place
38° 31° 20" Declination of the planet, N.
5 2939 Refraction answering to the sum of these two quantities o
0 59 Meridian altitude required
Sum 44° 1' 581" Again, if it were required to ascertain the meridian altitude of Jupiter in latitude 53° 30', when he passes the first meridian on the 7th of May, 1826.
The declination of the planet at noon of that day is
6° 8 S., and the decrease of this declination about 4' a day; and consequently, as he passes the meridian at 16 m. after 8 in the morning, we shall have 37" for the change of declination answering to the 3 h. 44 m. before noon; and as the declination was decreasing, and the time was earlier than that for which the declination in the table is calculated, this quantity must, of course, be added to the former, which then becomes 6° 8' 37" S. Now The co-latitude to 53° 30'
is 360 30' Jupiter's declination, S.
subtract 6 8 37"
difference 30 Corresponding refraction
- add 0 Correct altitude required
30° 23' 3" Eclipses of JUPITER'S SATELLITES. The first discoverer of these satellites early perceived that they exhibited phenomena that deserved to be ranked among the most interesting and useful movements of the heavenly bodies; and since telescopes have been so greatly improved, and accurate tables of the planetary motions constructed, the eclipses of these satellites have claimed great attention from the most eminent astronomers. Besides furnishing data for determining, with considerable accuracy, the distance of Jupiter from the Earth, and supplying materials for improving the theory of their motions, they are well known to afford the readiest and best method of finding the longitude of all places where steady and accurate observations can be made. In this way they are peculiarly calculated to give the longitude of all places on land with great ease and accuracy; but the motion of a ship precludes the observations from being made with sufficient correctness at sea, to render these eclipses available for finding the longitude on that unstable element. It is chiefly on this account, and in consequence of the great improvements that have been made and may
yet be made in geography by their means, that they are calculated and inserted on the 3d
page every month in the Nautical Almanac; they also very properly find a place in White's Ephemeris; and such of them also as are visible at the Royal Observatory, may also be found under the head of ASTRONOMICAL OCCURRENCES for every month in Time's TELESCOPE.
As these eclipses are seen at the same moment of absolute time by all the observers situated within the limits of the space where they are visible, it is evi. dent that the difference of time at which they occur, as estimated at the situations of any two observers, 'when converted into degrees, &c., will give the difference of longitude between them. Hence it is desirable that travellers should embrace every opportunity of making observations of this kind with all the care and accuracy that their circumstances will permit; for by this means, the positions of numerous places would soon be determined with great precision, which are at present very doubtful. In making observations of this kind, the following circumstances must be attended to.
If the person is situated under any other meridian than that of the Royal Observatory, for which the mean time of their happening set down in the tables is calculated, it is necessary to convert the difference of longitude between his situation and the observatory into time, and either add it to that in the tables, or subtract it from it, as the place is east or west of the first meridian, and the result will be the time, nearly, when he will see the eclipse. For this purpose, it is not necessary that he should know the longitude of the place where he is with accuracy; for as this is only to ascertain nearly at what time the eclipse will happen, that the observer may be in readiness to observe it at the required moment, a few minutes error in the estimate will not create any error in the result of the observation, provided the eclipse is not suffered