🔍 Key Concepts

• GHA (Greenwich Hour Angle) is measured westward from the Greenwich meridian from 0° to 360° • Relationship between Earth’s rotation (15° per hour) and the apparent motion of celestial bodies • Difference between the Sun’s hourly GHA change and the stars’ hourly GHA change

💭 Think About

• Think about how fast the Earth rotates in degrees per hour, and how that relates to hour angle changes on the celestial sphere • Recall whether stars appear to move slightly faster or slightly slower in hour angle than the Sun, and why sidereal time matters • Compare the given rates (4° vs 15°) and ask which one matches the Earth’s rotation, and which one is more like a small correction

✅ Before You Answer

• Be sure you know that Earth rotates 360° in about 24 hours and convert that to degrees per hour • Recall that GHA of a star is tied to sidereal time, which is slightly different from solar time by about 4 minutes per day • Check whether GHA for stars mainly changes at about the Earth’s rotation rate, with only a small daily difference from the Sun’s motion