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Continuing work initiated in an earlier publication (Abe, ApJ, 725 (2010)787), we study the gravitational microlensing effects of the Ellis wormhole inthe weak-field limit. First, we find a suitable coordinate transformation, suchthat the lens equation and analytic expressions of the lensed image positionscan become much simpler than the previous ones. Second, we prove that twoimages always appear for the weak-field lens by the Ellis wormhole. By usingthese analytic results, we discuss astrometric image centroid displacements dueto gravitational microlensing by the Ellis wormhole. The astrometric imagecentroid trajectory by the Ellis wormhole is different from the standard one bya spherical lensing object that is expressed by the Schwarzschild metric. Theanomalous shift of the image centroid by the Ellis wormhole lens is smallerthan that by the Schwarzschild lens, provided that the impact parameter and theEinstein ring radius are the same. Therefore, the lensed image centroid by theEllis wormhole moves slower. Such a difference, though it is very small, willbe in principle applicable for detecting or constraining the Ellis wormhole byusing future high-precision astrometry observations. In particular, the imagecentroid position gives us an additional information, so that the parameterdegeneracy existing in photometric microlensing can be partially broken. Theanomalous shift reaches the order of a few micro arcsec. if our galaxy hosts awormhole with throat radius larger than $10^5$ km. When the source movestangentially to the Einstein ring for instance, the maximum position shift ofthe image centroid by the Ellis wormhole is 0.18 normalized by the Einsteinring radius. For the same source trajectory, the maximum difference between thecentroid displacement by the Ellis wormhole lens and that by the Schwarzschildone is -0.16 in the units of the Einstein radius.

Astrometric Image Centroid Displacements due to Gravitational Microlensing by the Ellis Wormhole