Orthographic

Like the Gnomonic projection, the Orthographic projection is a geometric projection onto a plane which is tangent to the celestial sphere. The difference between the two projections lies in the point from which the projection proceeds. The Gnomonic projection proceeds from the centre of the celestial sphere, whereas the Orthographic projection proceeds from a point at infinity, diametrically opposite the tangent point (i.e. the projection lines are all normal to the tangent plane). The Orthographic projection is used (for example) for the IRAS Pointed Observation images. The area of the sky covered by a pixel changes across the image, and so this projection is not a good one to use for mapping large areas.