Satellite Parallax from a Solar-System Baseline: Error Propagation for an Observer Displaced by One Astronomical Unit

This paper computes how a second observer displaced by roughly an astronomical unit, as in the Spitzer and Kepler microlensing campaigns, breaks the degeneracies that a single ground-based observer cannot. For a representative lens of 0.5 solar masses at 4.0 kiloparsecs, in front of a source at 8.0 kiloparsecs, the angular Einstein radius is 0.713 milliarcseconds, the event timescale is 52.1 days at a relative proper motion of 5.0 milliarcseconds per year, the microlensing parallax is 0.175, and the physical Einstein radius projected at the lens is 2.85 astronomical units. The central finding is that a one-astronomical-unit baseline resolves the parallax for typical bulge events because the projected Einstein radius is of the same order as the baseline. All quantities are computed from stated assumptions and published survey values used as comparison points; the study is simulated and computed, not measured. Where the computation shows that an inference commonly drawn from such events is not supported at the stated precision, that negative result is reported in full.