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Equation of Time. As the time, as shown by a good sun-dial, is sometimes before, and at others after, that shown by a well regulated clock or time-piece, which goes at a uniform rate, when only one of these times is known, it requires a slight reduction to find the other; and the following table shows what is to be added to solar time, or that shown by the dial, to obtain mean time, as it ought to be indicated by the clock at the same moment, for certain days, during the present month: the corresponding correction for any other day must be found by proportion, as in the above instance for the Sun's rising and setting.
Phases of the Moon.
Last Quarter 8th day, at 92 m. after 4 afternoon.
Moon's Passage over the Meridian. The centre of the Moon will pass the meridian of the Royal Observatory at the following times during this month, and which will therefore be convenient opportunities for observing her in that situation, if the weather prove favourable. Her passage during the other parts of the month are not well adapted for observation, on account of the light with which they are accompanied.
8th, · 29 · · 5 · · · · ·
9th; - 11 . . 6 . .. . .
11th,. 45 .
27th, · 15 · • 10 • • • •
• Phases of Venus. Our astronomical readers will recollect, that, in the volume of Time's Telescope for last year, we gave a simple rule for finding the phase of this beautiful and interesting planet at any given period, as well as illustrated the problem relative to its greatest brilliancy. We must therefore refer to that volume for these subjects, and particularly as affording good exercises for our youthful students in the simpler species of astronomical calculations; but we shall insert the result for each month, in its proper place.
5 Enlightened part • 11.5806 January 1st Dark part ..... 0.4194
Eclipses of Jupiter's Satellites. The following are the only two of the eclipses of Jupiter's first and second satellites that will be visi. ble at the Royal Observatory, Greenwich, this month, viz.
.. Form of Saturn's Ring. For the variations which this ring experiences, the causes from which they arise, and the method of
calculating them, we must refer to our volume for 1819: as this variation is much slower than that which takes place with respect to the phase of Venus, we shall insert the result of the calculation only every third month.
Transverse axis = • 1.000
Other Phenomena. Mercury will obtain his greatest elongation on the 13th of this month. Márs will be in opposition at half past 10 in the evening of the 16th. Venus and Jupiter will be in conjunction at 27 m. after 11 in the evening of the 18th, at which time Venus will be 47" south of Jupiter. The Moon will be in conjunction with Mars at 20 m. after 7 in the morning of the 2d. With the star marked «, in Virgo, at 1 m. after midnight of the 8th. With Q, in Scorpio, at 23 m. after 11 in the morning of the 12th. With Saturn at 7 m. after 8 in the evening of the 19th. And with B, in Tarrus, at 2m, after 4 in the morning of the 25th. The Moon will also be in perigee on the 16th, and in apogee on the 30th.
EXPLANATIONs in PRACTICAL ASTRONOMY.
Having in the former volumes of Time's Telescope given a familiar explanation of many of the leading principles in the science of astronomy, we shall now avail ourselves of this renewed opportunity to add a few brief explanations, examples, and illustrations of the practical part of that science; and as White's Ephemeris is one of the most useful and practical compendiums, as well as one which is familiar to all who study this part of the subject, we shall principally confine our observations to the terms and tables inserted in that work; referring, however, to some of the other almanacs, whenever such a reference has a tendency to render our remarks more useful.
- The Sun's Declination - Is his distance either northward or southward from the equinoctial line, which is measured on a celestial meridian passing through his centre. Declination, therefore, corresponds with latitude, or distance from the equator, in geography. This declination is readily found for any given time by computation; for, knowing the Sun's place in the ecliptic, the general rule is,
To the declination required. Astronomers, therefore, compute this declination for every day at apparent noon, or the moment when the centre of the Sun passes the first meridian of the country for which the computation is made: in England, this is done for the Royal Observatory, at Greenwich. The declination thus found, is then formed into tables for practical use; that is, to be employed in such other astronomical calculations as require it as one of their elements. One of the first and most frequent uses which the young astronomer is required to make of this declination, is that of finding the altitude of the Sun from having the latitude of the place of observation given. This is at once so easy and obvious an operation, that a simple illustration will be sufficient; for since the height of the equator is always equal to the co-latitude of the place of observation, when the latitude is known, the height of the equator is also given; and consequently, if the declination of the Sun be added to this height, or subtracted from it, as circumstances may require, and the refraction also be taken into the account, the apparent meridian altitude of the Sun's centre will be obtained. This is the first thing to be done in making observations with a transit instrument; for then the instrument may be placed at its
proper elevation, so that the centre of the wires may
Height of the equator . . . . . . . . 38° 31' 20'
Altitude of the Sun - ..... • Sum 58 32 55 i Now, as the refraction always renders the apparent altitude of any of the heavenly bodies greater than the true altitude, this quantity, which for the above altitude is about 35", being added to the above sum, gives 58° 33' 30" for the apparent al. titude of the Sun's centre at the time required.
If the declination had been south, it must have been subtracted from the co-latitude, and the difference would have been 18° 2945"; to which the refraction being added, would give 19° 32' 37" for the apparent altitude of the Sun's centre, in that case.
If the place at which the observation is to be made be not upon the meridian for which declination is calculated, and it be thought necessary to correct it for this difference of longitude, this may be done by a simple proportion; for as 24 hours is to the time between the Sun's passing the meridians of the two places, so is the difference between the declinations answering to the two days to the correction required; which must be added to declination in the table, or subtracted from it, according as it is increase ing or decreasing..
The Sun's declination is also used in a variety of
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