[geuis]

Not obvious. Astronomers are actively looking for signatures of exomoons around exoplanets. So "sun plus moon" could mean that too.

[xattt]

The OP said moon + sun, rather than sun + moon. We have no idea yet if celestial math is non-communicative.

[BenjiWiebe]

*commutative

[godelski]

Well you find the signature by looking for a dip in but sun's luminosity. So minus might be the better relationship here