The Evolution of Worlds/Notes 3

by Percival Lowell

3

Variation in Spectroscopic Shift

In the case of a body reflecting light, the shift differs from that for a body emitting it. If the planet be on the further side of the Sun, the approaching rim advances both toward the Sun and toward the Earth, thus doubling the shift. The receding rim recedes in like manner. At elongation the rims approach or recede with regard to the Earth, but not the Sun, and the shift is single as for emission. At inferior conjunction rotational approach to the Earth implies rotational recession from the Sun, and the two effects cancel.