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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.

TABLE.

m. s. Saturday 1st, to the time by the dial add 3 33 Thursday 6th,

5 58 Tuesday - 11th,

7 59 Sunday 16th,

9 53 Friday 21st,

11 29 Wednesday 26th,

• 12 46 Monday 31st,

13 42

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Phases of the Moon.

Last Quarter 8th day, at 22 m. after 4 afternoon.
New Moon 15th

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First Quarter 22d

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8 morning. Full Moon 30th

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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.

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TABLE
Of the Moon's Passage over the Meridian.
January 7th, at 50 m. past 4 in the morning.

8th, 29
9th, 11
10th,
11th,
12th, · 40
21st, 10

5 in the evening.
22d, 57

5 23d,

6
2.4th, 36

7
25th, 29
26th, 22

9
27th, 15

10 28th, 6

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. January 1st { Balightened part • 11.5806

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.

Emersions.
1st Satellite, 7th day, at 12 m. after 5 afternoon.
2d Satellite, 3d

53 Form of Saturn's Ring. For the variations which this ring experiences, the causes from which they arise, and the method of

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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. January 1st { Conjugate axis = Transverse axis =

0.095 Other Phenomena. Mercury will obtain his greatest elongation on the 13th of this month. Mars 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 a, in Virgo, at I'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.

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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. De-
clination, therefore, corresponds with latitude, or
distance from the equator, in geography. This de-
clination is readily found for any given time by com-
putation; for, knowing the Sun's place in the ecliptic,
the general rule is,

As radius
Is to the sine of the Sun's longitude,
So is the side of the obliquity of the ecliptic

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 em-
ployed in such other astronomical calculations as re-
quire 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 lati-
tude of the place of observation given. This is at
once so easy and obvious an operation, that a sim-
ple 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 conse-
quently, 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 instru-
ment; for then the instrument may be placed at its

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proper elevation, so that the centre of the wires may correspond with the apparent centre of the Sun.

As an example of this computation, let the apparent meridian altitude of the Sun's centre be required, at the Royal Observatory, Greenwich, on the 30th of May, 1820. The latitude of the observatory being 51° 28' 40", the complement of this is 38° 31' 20", which is the height of the equator; and the declination on the 20th of May being north, the Sun will necessarily be above that circle, and consequently the declination must be added to its height; hence Height of the equator

38° 31' 201 Declination, 20th May, N.

1 35 Altitude of the Sun

Sum 58 32 55 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 altitude 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° 29'45"; to which the refraction being added, would give 18° 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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