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.. Monthly Correspondent,
FEBRUARY 1, 1814.
ASTRONOMICAL OBSERVATIONS FOR FEBRUARY.
DIVISIONS OF TIME,
As the Sun is the most conspicuous body in our system, and as he appears to move regularly through the heavens, his motion has been fixed on as one of the best measures of time that is afforded to us by nature. By means of his apparent diurnal and annual revolutions, we obtain the two grand and iinportant divisions of time, into days and years. By combining the revolution of the Sun and Moon, we embrace the larger intervals; and thence form an idea of those grand periods of time, which will be noticed hereafter. .
In civil computations, a day is usually divided into twenty-four, or twice twelve hours ; reckoning from midnight to noon, and from noon to midnight: but an astronomical day is the interval between the noon on one day and that on the next, and on this principle the Nautical Almanack is constructed; but the Connoisance des Tems, a French work intended for the same purposes, viz. for the special use of practical astronomers and navigators,-follows the usual course of reckoning by twice twelve hours, beginning their day at 12 at midnight. In England then the astronomical day is behind the common day 12 hours: thus by the Nautical Almanack, the New Moon for February takes place on the 19th day, at 11 minutes past 10; but in common reca
koning we say it occurs on the 20th day, at 11 minutes past 7 in the morning.
An astronomical day is the interval between two successive transits of the Sun's centre over the same meridian, and is divided into 24 hours, reckoned from 1 to 24, without any interruption ; so that what is called 9 o'clock in the morning of the 10th of February, by the civil or usual mode of reckoning, is denominated by astronomers, February the 9th, at 21 hours. In some parts of Italy the clocks and watches are made to go from 1 to 24.
The Sun appears to go round the Earth in 24 hours, and the fixed stars in nearly four minutes less, or in 23h. 56m, 4s.; so that any given fixed star is found to gain 3m. 56s. upon the Sun every day, which amounts to one diurnal revolution in a year; and therefore in 365 days, as measured by the returns of the Sun to the meridian, there are 366 days as measured by the return of any given fixed star to the meridian ; the former of which are called solar days, and the latter sidereal. The difference between the solar and sidereal days is occasioned by the immense distance of the fixed stars; for the Earth's orbit, though 190 millions of miles in diameter, is but as a point compared with that distance; and, therefore, any meridian of the Earth will revolve from a fixed star to that star again, in exactly the same time as if the Earth had only a diurnal motion, and was to remain for ever in the same part of its orbit. This, however, is not the case with respect to the Sun; for as the Earth advances almost a degree eastward in its orbit in the same time that it turns eastward round its axis, or completes its diurnal revolution, whatever star passes over the meridian any day with the Sun, will pass over the same meridian the next day, when the Sun is 3m. 56s, short of it. If the year contained exactly 360 days, as the ecliptic does 360 degrees, the Sun's apparent place would change a degree every day; and in this case, the sidereal days would be just 4 minutes shorter than the solar ones,
The daily revolution of the Earth, which is known to be uniform, is always completed, when any particular meridian is exactly parallel to the situation which it had at a certain time of the preceding day. For the same meridian can never be brought round from the Sun to the Sun again, by one entire revolution of the Earth upon its axis, but it will require as much more of another revolution, as is equivalent to the space that the Earth has advanced in its orbit during that time; which is, at a medium, the 365th part of a circle. So that in 365 days, the Earth will have turned 366 times round its axis; and therefore as one complete rotation makes a Sidereal day, in a year there will be one more sidereal day than there are solar days, be that number what it may.
It may be observed, that the regular return of the fixed stars to the meridian affords an easy method of determining whether our chronometers keep true time. For if through a small hole in a window-shutter, or other fixed object, it be observed at what time a given star disappears behind any building at a small distance; then if the same star disappears the next night, 3m. 56s. sooner by the clock or watch, than it did the night before, and on the second night 7m. 52s. sooner, and so on, it-is a proof that the instrument goes right; but if it does not observe this rule, it is evidently not accurate, and requires to be regulated. As the disappearing of a star is instantaneous, this rule may be depended on to half a second.
The following are the times of Sun-rising and setting, at London, for this month:
1st. Sun rises 27m. past 7. Sun sets 33m. past 4.
9th. – 13m. 7. - - 47m. - 4. . 18th. - - 56m. - 6. + - 4m. - 5.
Equation of Time.—[See the month of January.]
The following table will show what is to be added to the time pointed out on the dial, to obtain true or equal time for every 5th day of February:Tuesday, Feb. 1, to the time on the dial add 13m. 565.? Sunday, - 6, -
14 26 Friday, — 11,
14 35 Wednesday, 16,
- - 14 26 Monday, - 21,
– 13 59 Saturday, — 26,
- 13 16 j
To obtain true time by the clock.
The Sun enters Pisces on the 19th, at 20m. past 5 o'clock in the morning. Venus appears stationary on the 18th.
The Moon is full on the 4th, at 46m. past 6 in the afternoon : it enters its last quarter on the 12th, at 45m. past 4 in the morning ; the preceding new moon is at 1lm. past 7 in the morning of the 20th, and it enters its first quarter at 26m. past 10 in the morning of the 27th. The time of the Moon's rising, for the first five days after she is full, will be as follows, viz. on the
5th of February, 34m. past 5 in the afternoon.
On the first day of this month the Moon will eclipse the star inarked v II. in astronomical catalogues. The immersion will take place, at 13m. past 11 in the evening, when the star will be 7m. south of the Moon's center; and the emersion will be at 13m. past 12, the star being 8m. south of the Moon's centre.
On the 12th day, the Moon will eclipse the star un. The immersion will happen at 8m. past 12 in the morning, when the star is 7m. north of the Moon's centre; and the emersion will take place at 16m. past 3 in the morning, the star being 10m. north of the Moon's centre.
At 20m. past 5 in the morning of the 18th, Mercury