🔍 Key Concepts

• Relationship between observed altitude (Ho) and the angle from the zenith to the celestial body • Difference between altitude (measured up from the horizon) and an angle measured down from the zenith • How a 90° angle relates to the horizon–zenith geometry on the celestial sphere

💭 Think About

• When you measure Ho with a sextant, from where to where is that angle measured? Up from the horizon or down from the zenith? • If an angle is 30° above the horizon, what is the complementary angle from the zenith to that same body? How does that compare to 90° - 30°? • Which of the choices names the angle that is the complement of altitude (i.e., 90° minus altitude)?

✅ Before You Answer

• Identify which term is defined as the angle between the observer’s zenith and the celestial body • Confirm that Ho is an altitude measured from the visible horizon, not from the zenith • Eliminate any choice that represents a longitude/latitude concept or a time/angle from a reference meridian rather than the complement of altitude