The Little Dipper
The Little Dipper is visible at any hour on any night of the year from most locations
in the Northern Hemisphere. Seven faint stars make up the Little Dipper. The four
faintest stars can be blotted out with very little light pollution, making it an
excellent indicator for rating the darkness of a night sky. If you can see all four
stars in the bowl, it's an excellent night sky.
Polaris, a.k.a. the North Star
Polaris, the brightest star in the Little Dipper, sits almost directly over the
North Pole, the Earth's axis of rotation. In fact, if you stood at the North Pole,
Polaris would lie directly overhead. Every twenty-four hours, the Earth completes
a full rotation on its axis. Polaris remains fixed in the North, while the other
stars appear to rotate around it. This special position has earned Polaris the designation
of North Star - serving as nature's navigational aid for explorers and migrating
birds.
There is popular misconception that the North Star is the brightest star in the
night sky—it actually ranks 49th in brightness.
The Big Dipper
The Big Dipper is a group of stars that resemble a soup ladle—three stars
form the curved handle of the Dipper and the other four form the bowl. Merak and
Dubhe are known as the Pointer Stars because they so useful for finding the all-important
North Star.
Limiting Magnitude
The magnitude of a star is a measure of its brightness. The smaller the magnitude,
the brighter the star. For example, at magnitude 1.81, Dubhe appears brighter to
the observer than Polaris, which has a magnitude of 1.97.
The limiting magnitude is the magnitude of the faintest star a person can see from
a given place at a given time. In good viewing areas, typical values range from
+6.0 to +7.0. In cities, the values are often much lower—from +4.0 to +2.0
in severe cases (e.g., downtown region of a city with more than 1 million residents).
This animation of the night sky (see right) compares visibility of stars in the
Little Dipper under varying degrees of light pollution. The number in the top right
shows limiting magnitude.
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