DEPARTURE
, in Navigation, is the easting or westing of a ship, with regard to the meridian she departed or sailed from. Or, it is the difference in longitude, either east or west, between the present meridian the ship is under, and that where the last reckoning or observation was made. This departure, any where but under the equator, must be accounted according to the number of miles in a degree proper to the parallel the ship is in.
The Departure, in Plane and Mercator's Sailing, is always represented by the base of a right-angled plane triangle, where the course is the angle opposite to it, and the distance sailed is the hypothenuse, the perpendicular or other leg being the difference of latitude. And then the theorem for finding it, is always this: As radius is to the sine of the course, so is the distance sailed, to the departure sought.
DEPRESSION of the Pole. So many degrees &c as you sail or travel from the pole towards the equator, so many it is said you depress the pole, because it becomes so much lower, or nearer the horizon.
Depression of a Star, or of the Sun, is its distance below the horizon; and is measured by an arc of a vertical circle, intercepted between the horizon and the place of the star.
Depression of the Visible Horizon, or Dip of the Horizon, denotes its sinking or dipping below the true horizontal plane, by the observer's eye being raised above the surface of the sea; in consequence of which, the observed altitude of an object is by so much too great.
Thus, the eye being at E, the height AE above the surface of the earth, whose centre is C; then EH is the real horizon, and Eh the visible one, below the former by the angle HEh, by reason of the elevation AE of the eye.
In the right-angled triangle CEh, are given Ch the earth's radius = 21000000 feet, and the hypothenuse CE = the radius increased by the height AE of the eye; to find the angle C which is = the angle HEh, or depression sought; viz, as Ch : CE :: radius : sec. < C, or as CE : Ch :: radius : cosin. < C.
By either of these theorems are computed the numbers in the following table, which shews the depression or dip of the horizon of the sea for different heights of the eye, from 1 foot to 100 feet.
| Height of the eye | Dip of the horizon | Height of the eye | Dip of the horizon | Height of the eye | Dip of the horizon | |||
| feet | ′ | ″ | feet | ′ | ″ | feet | ′ | ″ |
| 1 | 0 | 57 | 13 | 3 | 26 | 26 | 4 | 52 |
| 2 | 1 | 21 | 14 | 3 | 34 | 28 | 5 | 3 |
| 3 | 1 | 39 | 15 | 3 | 42 | 30 | 5 | 14 |
| 4 | 1 | 55 | 16 | 3 | 49 | 35 | 5 | 39 |
| 5 | 2 | 8 | 17 | 3 | 56 | 40 | 6 | 2 |
| 6 | 2 | 20 | 18 | 4 | 3 | 45 | 6 | 24 |
| 7 | 2 | 31 | 19 | 4 | 10 | 50 | 6 | 44 |
| 8 | 2 | 42 | 20 | 4 | 16 | 60 | 7 | 23 |
| 9 | 2 | 52 | 21 | 4 | 22 | 70 | 7 | 59 |
| 10 | 3 | 1 | 22 | 4 | 28 | 80 | 8 | 32 |
| 11 | 3 | 10 | 23 | 4 | 34 | 90 | 9 | 3 |
| 12 | 3 | 18 | 24 | 4 | 40 | 100 | 9 | 33 |
See Robertson's Navigation, book 9 appendix; and Tables requisite to be used with the Nautical Ephemeris, pa. 1. See also Levelling.
