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Sunday ...... 1st, from the time by the dial subtract 2 42
0 56 Monday..... 16th, to the time by the dial add 0 6 Saturday.... 21st,
1 10 Thursday... 26th,
Phases of the Moon.
11 at vight
3 in the afternoon Full Moon.... 23d
Moon's Passage over the Meridian. The Moon will pass the meridian of the Royal Observatory at the following times during this month, which may afford opportunities for observing her in that position: the times refer to the passage of her centre over that circle.
Phases of Venus.
part = 9.00453 digits Eclipses of Jupiter's Satellites. The eclipses of these small bodies are not visible this month, Jupiter being too near the Sun.
7th 13th 19th 25th
h. m. Mercury 1 39 aft. 1 29 1 6 0 33 11 56 mor. Venus 2 44 aft.
2 50 2 54 2 57 3 0 Mars 10 56 mor.
10 49 10 42 10 35 10 29 Jupiter 0 29 aft.
0 10 11 52 mor. 11 32 11 13 Saturn 10 27 mor.
10 5 943 9 21 8 59 G. Sidus 2 15 mor. 1 50 1 24 0 58 032
ALTITUDES. Mercury 63016
61°57' 60021' 58047' 57037" Venus 62 50
61 46 60 21 58 36 56 34 Mars 57 24
58 27 59 24 60 12 60 54 Jupiter 60 54
61 3 61 10 61 17 61 22 Saturn 53 22
53 34 53 45 53 56 54 5 G. Sidus 15 6
15 4 15 3 15 2 15 1
Other Phenomena. Jupiter will be in conjunction at a quarter past 3 in the afternoon of the 10th of this month. Mercury will be stationary on the 11th. The Moon will be in conjunction with e, in Scorpio, at 1 m. after 12 on the 21st, and with Georgium Sidus at 50 m. past 5 in the morning of the 24th. Mercury will also be in his inferior conjunction at half past 2 on the same day.
DESCRIPTION AND USE OF ASTRONOMICAL
(Continued from p. 141] We shall now show the method of applying the observations made with this instrument to the correction of a clock or watch, which is one of the most useful as well as frequent of its applications. If the clock or watch to be regulated be designed to keep mean solar time, like those employed for the common purposes of life, when the instrument has been properly adjusted, observe the transit of the Sun, in the manner which has already been described, by taking the mean of all the times in which his eastern and western limbs pass the different wires. This
will be the moment of apparent noon, and which must be corrected, for the equation of time, as already explained in several parts of our work, by which it will be reduced to mean solar time, and give the moment which should be indicated by the clock at the same instant: it will, thus be ascertained how much the clock is too fast or too slow. By repeating this observation the next day, and for several days in succession, and comparing the results with the moments indicated by the clock at the same time, the comparison will give the quantity which it gains or loses per day. If this be considerable, the pendulum must be lengthened or shortened by turning the proper screw, according as it gains or loses; but if the gain or loss in 24 hours be not more than one or two seconds, and either uniform, or very nearly so, for equal intervals of time, the pendulum may be allowed to remain, and the gain or loss in any given interval applied as a correction to the result obtained at a future time. As this observation requires a little dexterity in the observer to make it correctly, especially when he is unassisted by a second person, we shall explain the process more fully. The best method of observing the transit of any of the heavenly bodies, is first to watch it into the telescope, and then to note the hour, minute, and second by the clock, which is supposed to be placed near the instrument for the purpose of being readily observed; then, with your eye applied to the telescope, count the beats of the clock till the body passes the first wire, and note down the exact time of its transit. When this is done, observe the time by the clock again, and note the moment of its passage over the second wire, in the same manner as before; and so on, in succession, till it has passed all the wires; then, adding all these results together, and dividing by the number of them, will give the mean, which will be the instant of passing the central wire, and consequently that with which the time by the clock or watch is to be compared.
If the clock be adjusted to sidereal time (which is the most convenient for a regular observatory), that is, so as to register exactly 24 hours from one transit to another of any fixed star, observations on the stars are preferable to those of the Sun, as being attended with less reduction. In this case, the same star ought to pass the meridian at the same instant on each successive day; and the deviations from this, if any, will consequently be the gain or loss of the clock in a given time, and which must be corrected or allowed for as above described. Observations on the fixed stars may also be made in conjunction with a clock that is set to mean solar time; but then a slight computation is necessary for arriving at the true result: for, as the length of the sidereal day, when expressed in mean solar time, is only 23h.56m. 41.1s., the clock ought only to show this period between the consecutive transits. It is consequently with this, and not with 24 hours, that the time of the transit is to be compared, and the gain or loss deduced accordingly.
This instrument may likewise be conveniently used for ascertaining the Right Ascension of the heavenly bodies, which is an arc of the equator comprised between the first point of Aries, or that where it is intersected by the ecliptic at the vernal equinox, and the point where a secondary to the equator, passing through the body, meets the latter circle. Now, as the motion of the earth on its axis, or the apparent motion of the heavens, is uniform, right ascension may be denoted by the interval of time that elapses between the moment that the first point of Aries passes the meridian, and that of the transit of the body. This astronomical quantity, therefore, admits of a double measure, that is, either in degrees or time, but the latter is the most usual, and this is generally registered in sidereal hours, minutes, and seconds: it is given in these for every day at noon in the Nautical Almanac. To ascertain the right ascension of any body, set the
clock to 0h. at the moment the first point of Aries passes the meridian of the place of observation; then the clock being adjusted to correct sidereal time by the preceding observations, the hour indicated by the clock at the moment the body passes the meridian will be the right ascension required. If the clock does not show exact sidereal time, but has a regular daily deviation, this is called its rate, and must be added to the indicated hour, or subtracted from it, as the clock was too slow or too fast; and then the result will give the right ascension sought. Should the clock, however, not be so adjusted as to show 0h. Om. Os. at the moment the first point of Aries passes the meridian, it is evident that the difference of the times will be the right ascension required. When the measure is required in degrees instead of time, it is easily obtained by saying, as 23h. 56 m. 41 ls. is to the time given by the clock, so is 15° to the measure required.
It should be remarked, however, for the information of our youthful readers, that what is called the first point of Aries is not a fixed point in the heavens, but changes its place by a slow retrograde motion, which is called the Precession of the Equinoxes : the right ascension of any given star is therefore not a constant quantity, but requires frequent corrections. Many of the fixed stars have had their right ascensions determined with great precision by astronomers, particularly 36 of them, which, with their annual variations, were ascertained by the late Astronomer Royal, Dr. Maskelyne, and are now chiefly used by astronomers for their observations of this kind. The apparent places of 24 principal stars, corrected for precession, aberration, and nutation, are also given for every 10th day of 1823, in the Nautical Almanac. Those places are determined by their right ascensions in time, and the north polar distances in degrees, miputes, and seconds.