In practice, since MISR is an optical sensor that measures the reflectance of the Earth in the solar spectral range, it is acquiring useful data only while the Terra platform is over the illuminated (day) side of the planet, i.e., during one half of the complete orbit or a bit less. Of course, the Earth itself keeps turning around its own axis while Terra proceeds on its orbit. As a result, when Terra completes an orbit and initiates the next one, it actually flies over quite different regions. The orbit number thus also indicates the areas of the planet observed. For these reasons, the orbit number is explicitly included in the name of many MISR data and product files.
The sun-synchronous orbit of Terra was selected in such a way that after 233 revolutions around the planet, or some 16 days, the platform returns to exactly the same locations and observes them under nominally identical angular conditions. Because of the 360 km instantaneous swath width of the MISR instrument, it is possible to gather multiple sets of observations (each with 9 cameras and 4 spectral bands) of a particular site in 2 days (Poles) to 9 days (Equator), depending on its latitude, but of course under a variety of angular conditions.
An Orbit/Date Conversion tool is available from the ASDC web site