Accessible and Inaccessible. See Geometry.

Altitude of the Eye, in Perspective, is a right line let fall from the eye, perpendicular to the geometrical plane.

Astronomy, is the distance of a star or other point in the mundane sphere from the horizon. This altitude may be either true or apparent. If it be taken from the rational or real horizon, the altitude is said to be true or real; if from the apparent or sensible horizon, the altitude is apparent, or rather, the apparent altitude is such as it appears to our observation, and the true is that from which the refraction has been subtracted.

The true altitudes of the sun, fixed stars, and planets, differ but very little from their apparent altitudes, because of their great distance from the centre of the earth, and the smallness of the earth's semidiameter when compared therewith. But the difference between the true and apparent altitude of the moon is about 5°.

Altitude Instrument, or Equal Altitude Instrument, is that used to observe a celestial object when it has the same altitude on the east and west sides of the meridian.