Hi,

When one object passes in front of another and appears smaller than the object in the background it is called a transit. One recent example is last year's transit of Venus across the Sun (see http://apod.nasa.gov/apod/ap120607.html). Venus transits of the Sun are quite rare, with less than two every century.

One unusual transit was a transit of the moon by the Sun seen from the point of veiw of the spacecraft STEREO-B in 2007. STEREO-B was leaving Earth orbit and further away from the moon than Earth is. As a result the moon appeared smaller than the Sun An image of this transit across the Sun seen in the ultraviolet can be seen here: http://apod.nasa.gov/apod/ap070303.html

From Earth the Sun's disk and Moon appear to be the same size resulting in our unusual and beautiful total eclipses.

Cheers,

Terry