delta flight 900 from BOS descending to ATL. landing time 857am. estimated time of sighting 830-840 am. i (witness) was seated on left side of plane halfway between wing and rear of plane cabin. window spacing was aligned perfectly with my seat to accomodate an unobstructed view of the outside left of the plane. the bottom of the wing basically represented the top border of my open viewing field. i estimate the time of the sighting based on the time following it before which the plane landed (assuming an ontime landing). i had my iphone turned off for the flight (hence no time check/filming).i estimate we had descended from 32k ft to about 20k ft. the plane vibration had been continually decreasing in confluence with rpms from the engines as we slowed and the details of the ground came further into focus. we had decended from the cloud ceiling/layer, above and below whch i'd noticed other airtraffic. approx 10-12 other planes in 15 minute's time. these were obviously observed as airplanes (wings, fuselage and tail features clearly discernable)as we were approaching landing airport at a busy time of the day for flight takeoffs. direction of light was from behind the plane above my left shoulder, plane had been on more or less consistant course for duration of flight. sighting occurred close to airport and we "overshot" airport post-sighting and later changed course and further descended for landing approach.the sighting itself was brief. my best estimate for duration was between 1.5 and 2 seconds. it appeared from the "bottom of the wing", moving parralell to but opposite our direction of travel at a velocity way beyond the capacity of known aircraft. i would estimate the altitude of the object to be aout 5k feet below the plane (15k ft). if the relative speed of other sighted (identified) aircraft moving oposite the planes direction could be estimated at 500-600mph i would estimate the relative speed of this object at around 1500 mph (1200mph actual speed?).in appearance the object was highly reflective with either a white or metallic surface. the shading (sun was still not high angle in sky) was pronounced but diffcult to determine if object was purely spherical or posessed a saucer-esque dome. that it was round and certainly not an airplane and certainly travelling well beyond the capable speeds of an airplane were without question. the object was in view a short time and did not seem to deviate from a straight-line travel path. no sound could be heard from within the plane. occupancy of the fight was around 75% full capacity though i heard no other passengers comment at time of sighting; nor did i reach out to other passengers who were seated as well at windows on left side of plane to question whether they too had witnessed object. the sighting was purely visual and did not have immmediate or proceeding effects on psyche/emotions. the sighting was the only anomalous object ever witnessed in 34 years of life. i was not sleep deprived/under the influence of drugs or alcohol at time of sighting. the overwhelming sensation after the very unexpected though clear sighting was that i had seen a material object not made by (at least not conceived by) the minds of men.