fix divide by zero

src/orbits.cpp

@@ -104,9 +104,13 @@ orbitGetElementsFromStateVectors(glm::dvec3 r, glm::dvec3 v, double mu)
 		if (ecc_v.z < 0) el.omega = 2 * M_PI - el.omega; // quadrant check
 	}
 
-	// FIXME: el.e == 0 would be undefined here
+	if (el.e == 0) {
+		el.nu = 0;
+	} else {
 		double cos_theta = glm::dot(r, ecc_v) / (el.e * r_mag);
 		el.nu= acos(cos_theta);
+	}
+
 	// FIXME: breaks test case
 	//if (glm::dot(r, v) < 0) el.nu *= -1; //quadrant check
 	if (glm::dot(r, v) < 0) el.nu = 2 * M_PI - el.nu; //quadrant check