For observers on the equator, which statement is true?

• A. The length of day varies throughout the year.
• B. The azimuth of sunrise and sunset is constant.
• C. The sun never crosses the observer's meridian.
• D. None of the above.

Answer: (D)

At the equator, day length stays essentially 12 hours all year because the Sun’s path across the sky and the way the horizon cuts it lead to equal time above and below the horizon. In the simple model, the sunrise hour angle is 90 degrees regardless of the Sun’s declination, so the day length is about 12 hours (small differences arise only from atmospheric refraction and the Sun’s finite size). The sunrise and sunset azimuths aren’t constant either; only on the equinox are sunrise due east and sunset due west, while on other dates the Sun rises a bit northeast or southeast and sets correspondingly to the northwest or southwest. Also, the Sun does cross the local meridian each day at solar noon. Because each of the first three statements isn’t correct, the right choice is that none of the above describe the true behavior.