« ZurückWeiter »
be 1' 50“, or, taking the nearest minute, 2 subtracted from the above declination to obtain that at 12 o'clock on the given meridian, which will therefore be 49' s. With this and the latitude we have the corresponding semidiurnal arc 5h, 57 m. Now as this is half the time of the planet's duration above the horizon, and Venus rises on the given day at 54m. past 5 in the morning at Greenwich, and consequently, as she varies only about a minute and a half a day, she will rise on the given meridian very nearly 1 h. 20 m. earlier, and therefore at 34 m. after 4 in the morning. Hence the semidiurnal arc being added to this time gives 31 m. after 10 in the morning for the times of the planet's southing at the given place.
It may be remarked here, that when the risings and settings of the planets, &e. are given, with their declinations and the latitude of the place, the time of their southing may be found from these by means of the semidiurnals arcs, as in the above instance, And, on the contrary, when the times of their southing are known, those of their rising and setting may be found by means of the same table, provided the planet's declination and the latitude of the place be also known. There are likewise other methods of finding the southing of the planets, which are explained in the following observations,
If it were therefore required to observe the transit of either of these planets over the meridian at the time above specified, the telescope must be elevated according to the result of the above computation, and the planet will be seen to pass through the field of view, with its centre nearly coinciding with the centre of the wires at the time specified. A similar calculation will also give the meridian altitude of any of the other planets, and also for any other meridian than that of the Royal Observatory, by only taking the difference of longitude into the account. This must first be turned into time at the rate of 15° to an hour, or 1° to 4 minutes of time, and then applied, by addition or subtraction, to the difference of time between the apparent noon at Greenwich and the time of the planet's passing the meridian of that place. The following illustration will, perhaps, make this clear.
Suppose it were required to find the meridian altitude of Mars on the 25th of July, 1820, in latitude 45° 26' N. and longitude 60° 45' W.
In this case the difference of longitude answers to 4h. 3 m., and the planet will therefore be later by this quantity on the meridian required than on that of the Royal Observatory; and hence, as the time at the latter place is 13m. past 3 in the afternoon, at the former it will be 16 m. after 7 in the evening. The declination of the planet at noon of the given day is 3° 49' N., and the change in 24 h. is 15'; hence the change answering to 7 h. 16 m. is 4' 31"; and as the declination is decreasing, and the time afternoon, this quantity must be subtracted, and therefore 3° 49 -4° 31'=3° 44' 29'N. for the declination for the given time for which the meridian altitude is required. Consequently, Colatitude
44° 34 Declination of the planet N. add 3 44 2941 Corresponding refraction add
51 The meridian altitude required
19 20 The transit instrument then being adjusted to this elevation, the planet will pass the wires.
The time that any of the planets come to the meridian may also be easily found, when the right ascensions of the Sun and the planets are given; and as these are both inserted in the Nautical Almanac for every day at noon, the 'method, when that work is at hand, is very convenient. For this purpose subtract the right ascension of the Sun, in time, from that of the planet, and the remainder will be the time when the planet will be upon the meridian of the
place for which the right ascensions were calculated, very nearly. If the result of this operation should not be thought sufficiently accurate, the right ascensions of the Sun and planet may be found again for the time of the day this obtained, and then subtracted as before, which will give the true time. It should be observed, however, that the first operation always giving the time within a few seconds, will generally be near enough for any practical purpose. When the right ascension of the planet is less than that of the Sun, add 24 hours to it, and then subtract the Sun's right ascension from the sum, and the remainder will be the time required.
At what time will Jupiter be on the meridian of Greenwich on the 25th of January, 1820 ? The right ascension of Jupiter for that day, as taken from the Nautical Almanac, is
21 45 0 That of the Sun for the same time, is
26 Time required, nearly
differ. 1 Again, let it be required to find what time Venus will be on the meridian on the 1st of July, 1820. Then
The right ascension of Venus for that day is 9 7
S. 0 12
2 25 48 Venus will therefore be on the meridian nearly at 45 m. 48 s, after 2 in the afternoon of the 25th. But if greater accuracy were required, the right ascension must be computed for this time instead of taking them for noon, as in this operation. Now, as the right ascension of the planet increases 2 m. in 6 days, it will increase nearly 2 s. in the above time. And as the increase of the Sun's right ascension in 24 hours is 4m. 8 s., its augmentation in 2 h. 25 m. 48 s. will be about 28 s.; and hence the corrected numbers will be,
The planet's right ascension
The time required
2 25 The corrections, therefore, obtained by the second operation, is only 26 s. But as the object of knowing when the planets come to the meridian is either for the sake of obtaining an observation of them in that position, or distinguishing them from the other heavenly bodies, this small difference of time cannot be of any importance; for even in the former case, all that is requisite is to be at the instrument half a minute, or a minute, before the time given by the first operation. The time of a planet's southing may also be rea
found, when the longitude of the Sun and the geocentric longitude of the planet are known; for by subtracting the former from the latter, and reducing the remainder into time, it will be the time at which the planet will come to the meridian. Taking the former of the above examples, we have
The planet's geocentric longitude = 10s. 230 504 Oui
Diff. 19° 19' 7" And which converted into time gives 1 h. 17 m. 16 s. afternoon for the time required. If the former method of working this example had been corrected by the second operation, it would not have differed more than 4 or 5 seconds from this result.
- = 10
The Naturalist's Diary
For JULY 1820.
To the Sun.
O'er the wide bosom of creative earth;
To thee the Persian offers up his vows,
Efficient means which make his bosom glow,
And makes the blossoms of his orchard grow.
His melons swell beneath thy vertie ray;
Oppose their blushes to the rip’ning day,
VALDARNO. Most persons, perhaps, receive a greater pleasure from fine weather than from any other sensual enjoyment of life. In spite of the auxiliary bottle, or any artificial heat, we are apt to droop under a gloomy sky, and taste no luxury like a blue firmament, and sunshine. I have often, in a splenetic fit,' observes an amiable writer, wished myself a dormouse during the winter; and I never see one of those snug animals wrapt up close in his fur, and compactly happy in himself, but I contemplate him with envy beneath the dignity of a philosopher. If the art of flying were brought to perfection, the use I should make of it would be to attend the sun round the world, and pursue the spring through every sign of the zodiac. This love of warmth makes my heart glad at the return of Summer. How delightful is the face of nature at this season, when the earth puts forth her plants and flowers, clothed with green, diversified with ten thousand various dies! how pleasant is it to exhale such fresh and charming odours, as fill every living creature with delight! At this season well may we exclaim with the poet,
Thrice happy he! who on the sunless side