PROBLEM LXXII.

To find the Time when any Star will rife or fu acronycally, in a given Latitude,

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A ftar is faid to rife or fet acronycally when its rifing or fetting happens at fun-fet. Hence the day in the calendar agreeing with the point of the ecliptic which fets with the ftar, is that of the ftar's acronycal fetting: and the day which coincides with the point in the ecliptic oppofite to that which rifes with the ftar, is that on which the ftar rifes acronycally.

Rectify the globe for the given latitude, and bring the propofed ftar to the Eastern edge of the horizon, and then obferve what degree of the ecliptic is in the Western part of the Lorizon; the day in the calendar correfponding with this laft will be the time when the given ftar rifes acrony. cally, or at fun-fet, and confequently when it begins to be visible in the evening. For the acronycal fetting, turn the globe about till the ftar comes to the Weftern verge of the horizon, and remark what degree of the ecliptic is, at the fame time, fetting with it; the day in the calendar agreeing therewith is the period when the ftar fets acronycally, or with the fun, and of course is no longer confpicuous in the evening.

From thefe confiderations it is plain that those months of the year which intervene between the acronycal rifing and fetting of a ftar, are the space of time in which that ftar may be feen above the horizon, in the given latitude, in the evening.

EXAMPLES.

EXAMPLES.

In what days does the brighteft of the seven s rife and fet acronycally at London, lat. 514? Anf. Rifes acronycally, Nov. 6; fets acronyy, May 23; confequently the Pleiades are visible is fome part of every night during the interliate months.

Required when the following ftars rife and fet onycally at London.

The centre ftar in Orion's Girdle-y, in the exnity of the Wing of Pegafus-8, in the Beam Libra-a, in Scorpio-6, in Canis Major-a, Hydra, in Hydra-B, Deneb, in Leo-a, Spica, Virgo-a, Menkar, in the mouth of Cetusin Lepus-the North ftar in the Horn of Caprinus- in Corvus.

PROBLEM LXXIII.

find what Stars rife and fet acronycally on any particular Day in a given Latitude.

Adjust the globe to the propofed latitude, and ing the fun's place for the given day to the eftern fide of the horizon: all thofe ftars hich are then on or near the Eaftern fide of the rizon rife acronycally; and those which are at at time on the Western verge of the horizon Et acronycally.

EXAMPLES.

What stars rife acronycally Dec. 10, at London? -Anf. The Hyades.

What

What ftars fet acronycally Dec. 23, at London? -Anf. Arcturus in Boötes, and, in Serpens.

Required what ftars rife and fet acronycally London on the following days:

Jan. 30-Feb. 12-March 8-April 8-May June 8-July 6-August 8-Sept. 2-October 25-Nov. 17-Dec. 17.

PROBLEM LXXIV.

To determine the Time when a Star rifes and fe heliacally, at any given Place.

The heliacal rifing is when a ftar, once im merfed in the fun's beams, emerges out of them, and becomes vifible at the Eastern verge of the horizon, juft before fun-rifing; and it fets helia cally when it is at that period fo near the fun's rays as to appear, for the laft time, in the even ing before it is entirely hidden and become incon fpicuous.

The heliacal rifing and fetting of the ftars vary according to their different magnitudes; for it is evident, that the ftronger the brilliancy of a fur is, the greater degree of the fun's light is re quired to render it invifible whilft above the hori zon. Stars of the first magnitude are seen at rifing and fetting when the fun is about 12 degrees below the horizon; ftars of the fecond magnitude, when that luminary is 1g; thofe of the third, when he is 14; of the fourth, when he is 15; of the fifth, when he is 16; of the fixth, when he is 17; and the nebulous, or fmall ones, when he is 18 degrees below the horizon, namely, about the beginning and ending of twilight.

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To folve this Problem; rectify the globe for e latitude, bring the given ftar to the Eaftern de of the horizon, and, having fixed the quadrant altitude to the zenith, turn it to the Weftern de till it interfects the ecliptic in the degree to hich the ftar's light requires the fun to be dereffed obferve the point of the ecliptic which then cut by the quadrant; for the point of the liptic oppofite to it will fhew, by its correponding day in the calendar, when the ftar rifes eliacally.

Bring the ftar to the Western edge of the horizon, nd the quadrant to the Eaftern, and notice the egree of the ecliptic, as before; and the degree the ecliptic, which is oppofite to it, will fhew n the calendar the time of the ftar's fetting heacally.

EXAMPLES.

On what day does Sirius, or the Dog-Star, rife heliacally at London; and on what day does it fet heliacally at the fame place?

Anf. Rifes heliacally on Auguft 23; and fets heliacally on May 2.

Required the days on which the following stars rife and fet heliacally at London:

Rigel, in Orion's Heel-Procyon, in Canis Minor -Aldebaran, in Taurus-Cor Scorpionis, in Scorpio -Spica, in Virgo-Ar&urus, in Boötes-Cor Hydra, in Hydra-Alcair, in Aquila-Caftor, in Gemini-the Pleiades, in Taurus-Deneb, in Leo Markab, in Pegafus.

PROBLEM

PROBLEM LXXV.

To find when any Star culminates, or comes upon the Meridian with the Sun.

Bring the given far to the meridian, and ob ferve what degree of the ecliptic comes to the meridian with it; find the correfponding day of the month in the calendar, and the Problem folved.

EXAMPLES.

When does Cor Hydra come to the meridian with the Sun-Anf. August 9.

Required the days on which the following ftars come to the meridian with the fun.

ɛ, in the mouth of Pegafus-, in CaffiopeiaB, in Andromeda-y, in the Ear of Aries-, in Caput Medufa-, in Urfa Major, in the Skirt of Boötes-, in the North Wing of VirgoCorona Borealis-a, in Delphinus-6, in Canis Major-B, in Canis Minor.

PROBLEM LXXVI.

To find at what Time of the Year a Star is upon the Meridian at Midnight.

Bring the given ftar to coincide with the notturnal part of the meridian, and then obferve what point of the ecliptic is interfected by the diurnal part of the meridian; and the day when the fun poffeffes that part of the ecliptic, which may be feen in the calendar, is the time required. EXAMPLES.