1. A system for spacecraft attitude determination, the system comprising: a plurality of horizon sensors, each of the plurality of horizon sensors having a respective field-of-view (FOV), each of the plurality of horizon sensors having a sensitivity to the portion of the respective FOV obscured by Earth; an attitude determination module coupled to the plurality of horizon sensors, the attitude determination module configured to: obtain horizon sensor readings from the horizon sensors; determine an Earth disk radius in a spacecraft's body frame; convert the horizon sensor readings to nadir angles; and estimate a nadir vector using the nadir angles and the Earth disk radius.