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Conceptual Questions


Explain why diurnal motion is counterclockwise for a star we observe toward the north but clockwise for a star we observe toward the south.

Suppose an observer in the northern hemisphere determines that the diurnal motion of a star keeps it above the horizon for 16 hours. Is the star in the northern or southern hemisphere of the celestial sphere? Does the star rise in the northeast or the southeast? How much time elapses between the time the star rises and the time it crosses the meridian?

What is the local hour angle of a star at the moment it crosses the meridian?

The equatorial coordinate system is very similar to the terrestrial coordinate system. Which terrestrial coordinate is the counterpart of right ascension? Which terrestrial coordinate is the counterpart of declination?

An observer, working at the summer solstice, notes that the Sun circles about the sky at a constant altitude (23.5°). The observations are interrupted by a bear. What color is the bear?

What is the orientation of the celestial equator for observers at the Earth's equator?

At what latitude is the altitude of the south celestial pole greatest?

Suppose an observer finds that Aries is the constellation just above the horizon as the stars fade at sunrise. What constellation would be seen just above the horizon at sunrise 1 month later? How about 1 month later still?

Suppose the ecliptic weren't tilted with respect to the celestial equator. How would the azimuth of sunrise vary during a year? How would the length of day and night vary throughout a year?

Suppose the ecliptic were tilted by 40° rather than 23.5° with respect to the celestial equator. What effect would this have on the variation of the azimuth of sunrise during a year?

Suppose you and your roommate had built a monument with piles of rocks to mark the azimuths of sunrise at the solstices. How could you determine where to place a pile of rocks to mark the azimuth of sunrise at the equinoxes? (Note, there are several correct answers to this question.)

Describe why it would be difficult to use sidereal time for civil timekeeping.

Why would it be difficult to build a wristwatch that keeps apparent solar time?

The Julian calendar, instituted by Julius Caesar in 46 B.C. and replaced by the modern Gregorian calendar beginning in 1582, averaged 365.25 days in length. How did annual events, such as the vernal equinox, move through the calendar while the Julian system was in effect?

Suppose the Moon moved westward rather than eastward among the stars. Would the sidereal month be longer or shorter than the synodic month? Explain.

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