What is "depression of the visible horizon"

Depression \De*pres"sion\, n. [L. depressio: cf. F. d['e]pression.]

1. The act of depressing.

2. The state of being depressed; a sinking.

3. A falling in of the surface; a sinking below its true place; a cavity or hollow; as, roughness consists in little protuberances and depressions.

4. Humiliation; abasement, as of pride.

5. Dejection; despondency; lowness.

   In a great depression of spirit.
   --Baker.

6. Diminution, as of trade, etc.; inactivity; dullness.

7. (Astron.) The angular distance of a celestial object below the horizon.

8. (Math.) The operation of reducing to a lower degree; -- said of equations.

9. (Surg.) A method of operating for cataract; couching. See Couch, v. t., 8.

   Angle of depression (Geod.), one which a descending line makes with a horizontal plane.

   Depression of the dewpoint (Meteor.), the number of degrees that the dew-point is lower than the actual temperature of the atmosphere.

   Depression of the pole, its apparent sinking, as the spectator goes toward the equator.

   Depression of the visible horizon. (Astron.) Same as Dip of the horizon, under Dip.

   Syn: Abasement; reduction; sinking; fall; humiliation; dejection; melancholy.