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Def. 1. The Equator is a great circle equi-distant from the poles.

It divides the earth into the Northern and Southern Hemispheres.
Def. 2. The Poles are the extremities of the earth's axis.

The one which lies upon the same side of the equator as the Dominion is called the North Pole, while the other is termed the South Pole.

Def. 3. A Meridian is a great circle which passes through the Poles.

It is one of the two lines which define the position of any spot upon the earth.

Def. 4. The Ecliptic is a great circle showing the apparent path of the sun in the heavens.

This apparent movement of the sun amongst the stars is caused by the motion of the earth in its orbit.

Def. 5. The Tropics are two small circles parallel to the equator, eech cutting a vertex of the ecliptic.

There is always a point between these two circles where the sun is vertical. That one which lies to the Northward of the equator is called the Tropic of Cancer, while the other is the Tropic of Capricorn.

Def. 6. Latitude is the arch of a meridian intercepted between any given place and the equator.

It is reckoned from the equator towards the poles, and is nau.ed North or South, the same as the pole towards which it is reckoned.

Def. 7. Parallels of Latitude are small circles parallel to the equator.

It is the intersection of a parallel of latitude with a meridian at any given point which defines the position of that point upon the face of the Globe.

Def. 8. Longitude is the arch of the equator intercepted between what is called the First Meridian and the meridian passing through any given place.

Any meridian may be made the First Meridian, many nations using that which passes through their respective Capitals, but must maritime nations have adopted the English First Meridian, which is the one which passes through the Observatory at Greenwich. Long is reckoned from 0 at the First Meridian to 180, where it meets the other (or inferior) part of the First Meridian and is named East, if, when looking towards the North, it is reckoned to the right of the First Meridian, but West if the contrary.

Def. 9. The Visible Horizon is the circle in the open sea, by the limit of vision.

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It will expand with an increased elevation of the eye above the level of the sea, and contract as the eye approaches the surface.

Def. 10. The Sensible Horizon is a plane which passing through the eye of the observer, is parallel with the visible hori

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Def. 11. The Rational Horizon is the plane which passes

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through the centre of the earth parallel to the visible horizon.

Def. 12. The Artificial Horizon and its uses. The artificial horizon is a small shallow trough containing quicksilver. It is used when there is no visible horizon, for the purpose of measuring the altitude of an object

As the angle taken is the angle of r flect on, it is double the actual altitude, and must therefore be halved after the error of the sextant has been applied to it. Def. 13. True Course of a Ship is the angle contained between the ship's head and the TRUE meridian.

It is therefore the compass course corrected for all its errors.

Def. 14. Magnetic Course is the angle contained between the ship's head and the Correct Magnetic meridian.

Is in error by the amount of the deviation.

Def. 15. Compass Course is the angle contained between the ship's head and the meridian, as shown by her compass.

Consequently it is in error by the amount of the variation, deviation and

leeway.

Def. 16. Variation of the Compass is the angle between the true and magnetic meridians.

Every place has its own variation, and this again is also subject to a slow change in its amount. The variation of the compass is caused by the true and magnetic poles not being located in the same place, and the change which is always taking place in its amount appears to arise from a slow movement of the magnetic round the true pole.

Def. 17. Deviation of the Compass is the angle between the correct magnetic meridian and that shown by the compass.

Def. 18. The Error of the Compass is the combined effects of both variation and deviation upon a ship's compass.

As the variation is affected by a change of place, and the deviation, in addition, by any alteration in the direction of the ship's head, it is evident that the error of the compass found in the amplitudes or azimuths by getting the angle between the true and compass bearings of the sun, can only be used when in about the same locality, and with the ship lying upon the same course as when the compass bearing was taken.

Def. 19. Leeway is the angle formed by the ship's keel with her actual course through the water.

It is caused by the action of the wind upon a ship's side, which gives her a lateral as well as a progressive motion.

Def. 20. The Meridian Altitude of a Celestial object is its altitude when upon the meridian of the place of observation.

Def. 21. Azimuth is the angle contained between the North or South Pole and the vertical circle passing through the object.

It is called the true or magnetic azimuth, according whether the angle is reckoned from the true or magnetic pole. The azimuth found by computation is true.

Def. 22. Amplitude is the angle contained between the East or

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West point of the horizon and an object in the act of rising or setting.

It is called the true or magnetic amplitude accord ng whether the angle is reckoned from the true or magnetic East or West point of the horizon; the amplitude found by calculation is true.

Def. 23. Declination is the angular distance of any heavenly body North or South of the celestial equator.

Def. 24. Polar Distance is the angular distance of any celestial object from the pole of the observer.

Def. 25. Right Ascension is the arc of the Celestial Equator contained between the First Point of Aries and the Circle of Declination passing through any given heavenly body.

It is expressed in time, and is reckoned Westward from the First Point of Aries in continuation right through the whole cire e, that is, from 0h. to 24h. The First Point of Aries is that point in the Celestial Equator which is crossed by the Sun in March.

Def. 26. Dip is the angle contained between the sensible horizon and a line drawn from the eye of the observer to any point in the visible horizon.

As the visible horizon expands or contracts with any increase or decrease of the height of the observ r's eye above the level of the sea, it follows that the dip of the horizon is regulated by the height of thye.

Def. 27. Refraction is the amount by which the altitude of a heavenly body is increased by the effect of the earth's atmosphere.

The rays of light which form the image of an object, while forcing their way through our atmosphere, get more or less bent downwards from their proper straight line, and as an object is seen in the same direction as that with which the ray enters the eye, its having been curved downward from its true direction will naturally give the object an apparent altitude greater than really belongs to it

Def. 28. Parallax is a correction additive to an altitude to make it equal to what it would have been, supposing it had been observed from the centre of the earth.

Def. 29. Semi-diameter is the half of the apparent diameter of a heavenly body.

Def. 30. Augmentation of the Moon's Semi-diameter is the increase of the apparent semi-diameter of the Moon, caused by the place of the olserver getting closer to that body while she is rising from the horizon, to the zenith.

When the Moon is in the zenith, the observer is immediately underneath, and consequently at his shortest distance from her; but as the Moon sets (a motion due to the rotation of the earth eastward), the place of the observer is gradnally removed from the moon, until when that body is in the horizon, it is plain that the observer has increased his distance from her by the length of the earth's radius; and the moon's distance from us is not so great but that this amount has an appreciable effect upon her apparent size.

Def. 31. Observed Altitude is the angular distance of any heavenly body from the horizon as shown by a sextant.

Def. 32. Apparent Altitude is the angular distance of an object from the sensible horzon.

It is found by applying the index error and dip to its observed altitude.

Def. 33. True Altitude is the apparent altitude of an object corrected for refraction and parallax.

Def. 34. Zenith Distance is the angular distance of an object from that point in the heavens immediately over the observer. It is found by subtracting the true altitude from 90o

Def. 35. Vertical Circles are great circles passing through the Zenith.

They are consequently perpendicular to the horizon.

Def. 36. Prime Vertical is that vertical circle which is at right angles to the Celestial Meridian.

Therefore any object upon the Prime Vertical must bear due East or West. Def. 37. Civil Time is the ordinary way of reckoning time on shore.

The civil day is contained between two following midnights; it is 24 h. long, and is divided into two parts of 12 h. each; the first is called ▲. м. or ante meridian, the second P. M., or post meridian.

Def. 38. Astronomical Time is the interval of time from the preceding noon.

The astronomical day commences at noon of the civil day of the same date, and closes at the noon following; astronomical time is reckoned consecutively up to 24h.

Def. 39. Sidereal Time is the time clapsed since the preceding transit of the first point of Aries.

Def. 40. Mean Time is time as shown by an ordinary clock.

The interval between following transits of the sun is not regular, a mean sun therefore has ben imagined, which moving at one uniform rate is supposed to make the cirenil of the Celestial Equator in the sa ne time, exactly, that the sun requires to pass through the Ecliptic.

Def. 41. Apparent Time is the time elapsed since the preceding transit of the true sun.

Dcf. 42. Equation of Time. The interval of time between Mean and Apparent time.

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Def. 43. Hour Angle of a Celestiai object. The angle contained between it and the Celestial Meridian.

Def. 44. Complement of an Arc or Angle.-The difference between it and 90o.

Def. 45. Supplement of an Arc or Angle.—The difference between it and 180o.

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Alderney Island
Cape de la Hague
Cape Barfleur
Cape Grisnez
Calais

Skelligs
Calf Rock
Fastnet
Kinsale

1 F.

1 R.

Light

Every minute, three flashes in quick succession.

Electric.

Every twenty seconds. Red.

2 F. Electric.

1 R. Every fifteen seconds. Green. 1 F.

R.

Every twenty seconds.

3 F. Triangular.

I F.

I F.

3 R. Every twenty seconds. Triangular.

2 F. Red.

I F.

1 R. Every thirty seconds.

1 R. Every thirty seconds.

1 F. Flash every four minutes.

ST. GEORGE'S CHANNEL.

Daunts Rock Lt. V.

F.

F. Flash every fifteen seconds. 1 R. Every minute.

I F.
1. F. Red.

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