Which star is used to determine latitude by measuring its altitude?

• A. Sirius
• B. Polaris
• C. Betelgeuse
• D. Vega

Answer: (B)

The key idea is that a star located very close to the north celestial pole serves as a stable reference for latitude. Polaris sits almost directly above Earth's axis, so the angle between Polaris and the horizon equals the observer’s latitude in degrees. Measuring Polaris’s altitude with a sextant or similar instrument gives you your latitude in the Northern Hemisphere: at the equator Polaris would be at the horizon, and at the North Pole it would be overhead. The small offset of about 0.7 degrees from the true pole means a tiny rounding error, but practically it’s a reliable latitude marker.

The other stars listed aren’t suitable for a direct latitude reading because their positions in the sky aren’t fixed relative to the pole. Sirius, Betelgeuse, and Vega chart paths across the sky that depend on time and date, so knowing just their altitude won’t give a straightforward reading of latitude without additional information about time, date, and the stars’ positions. Polaris, by contrast, provides a simple, nearly constant reference for latitude in the northern sky.