🔍 Key Concepts

• sidereal rotation vs synodic rotation • the reference point used when measuring a synodic period (e.g., phases as seen by an observer) • how the apparent position of a body changes due to the observer’s own orbital motion

💭 Think About

• Ask yourself: a synodic period is usually linked to what changing appearance as seen from Earth (for example, think of the Moon’s cycle of phases)? • If the observer is on Earth, which reference direction must the body realign with for us to say one synodic cycle is complete? • How does the Earth’s own motion around the Sun affect the choice of reference for a synodic period?

✅ Before You Answer

• Be clear on the difference: sidereal uses a fixed, distant background; synodic uses a changing alignment seen by an observer. • Think about which option is associated with observable cycles like phases or conjunctions, not fixed star positions. • Verify which reference gives the period that is slightly longer or shorter than the true rotation because of the observer’s orbital motion.