In our last post we observed that the conjunction of bright Planet Venus with the crescent moon on July 18, 2015 looked like a “Semicolon in the Sky.” As our family members watched the crescent moon and Venus set, thanks to Earth’s rotation, the darkening skies revealed another assemblage of stars—The Big Dipper. This grouping of stars rates as an “asterism,” a conspicuous star pattern or grouping of stars which does not qualify as a constellation.
The Big Dipper is a favorite of young and old. To modern folks, this asterism looks like a container with a handle and a pan (the Dipper). To the ancients, The Big Dipper was only a part of a greater constellation known as Ursa Major—the Great Bear. When my grandson asked if the Big Dipper was visible in the darkening evening skies following our sighting of the Venus/moon conjunction, I first mistakenly replied, “No.” But a quick reappraisal revealed it actually was visible high in the northwest skies if we looked carefully. After a few hours, however, major changes were in store. The star group moved to a new position in the darkening skies.
The seven stars of the Big Dipper are never below the horizon, night or day. The entire Big Dipper asterism visually circles daily around a point on the dome of the sky called the north celestial pole (NCP). The famous star Polaris, the North Star, is fortuitously located almost exactly at that spot. If we journeyed to the Earth’s geographic north pole, the north celestial pole would be straight overhead continuously, night and day. “Why?” we may ask. Even though Earth’s revolution is a long journey around the sun once per year, the Earth’s axis of rotation always tilts to this single spot on the dome of the sky. Stars are incredibly distant so earth’s revolution seems minor in comparison. Earth’s axis is always directed at only one point on the dome of the sky—the NCP.
The apparent daily 360º journey of the Big Dipper around the NCP is caused by the real rotational motion of the Earth. Our real motion causes the stars such as The Big Dipper to appear to move. In reality, our Earth is moving—360º of rotation per day—but we do not sense any movement. Apparent movement of the Big Dipper and other stars signals we are moving. The confusion about what moves was settled in the days of Nicolas Copernicus (1473-1543). He established that the Sun is at the center of the Solar System while Earth and other planets revolve around it and rotate. These real motions provide us with the joy of witnessing sunrises, sunsets, seasonal changes and many other phenomena, and the interesting circular swings of the Big Dipper and all other visible stars in the sky.
The Creator has provided our system of revolving planets around a central Sun. He has enabled us to decipher the apparent motion of the sun, moon, planets, and stars as well as their real motions. The Big Dipper’s major movements tell us about the rotation of the Earth. Less obvious, more subtle movements of the stars have enabled astronomers to discover the facts about Earth’s revolution. We now know the Earth revolves around the sun, but how did the brilliant astronomers of several hundred years ago first figure it out? We leave this mystery for an upcoming post.