The Big Dipper (Ursa Major) and the Little Dipper (Ursa Minor) are two of the best-known constellations for viewers in the Northern Hemisphere. They are easy to locate. And having found the Dippers, you can also track W-shaped Cassiopeia (KASS ee oh PEE yuh). All you need is a portable planetarium.
To make an Umbrella Planetarium (very portable), paint three constellations onto the underside of an umbrella, as shown. Point your umbrella at the North Star, and rotate the stick counterclockwise. Voila, the nightly and seasonal motion of the circumpolar stars.
Find the North Star
- If you are in Los Angeles (34 degrees latitude), Polaris is 34 degrees above the horizon.
- If you are in Seattle (47 degrees latitude), Polaris is 47 degrees above the horizon.
From anywhere in the Northern Hemisphere — any time of year, any time of night — Polaris is due north and x degrees above the horizon, where x = degrees north of the equator for your location. At the North Pole (90 degrees latitude), the North Star is at the top of the sky.
- Because of Earth’s rotation, the Big Dipper appears to circle Polaris every 24 hours.
- Because of Earth’s orbit around the Sun, the Big Dipper’s starting point each night appears to advance a little in the counterclockwise direction.
Knowing about the Big Dipper’s circle, you can predict where to find this constellation during each season of the year. If you are out for several hours, you can follow the Big Dipper’s slow progress around the circle and estimate the time.
With observations taken in mid-evening, Spring finds the Big Dipper high in the northern sky, well above the North Star (more than two hand widths, held at arm’s length). By Summer, the Big Dipper’s nightly starting point has inched one-quarter of the way around its circle. Notice how the two forward stars of the pot always point to Polaris. In the Fall, the Big Dipper approaches the horizon and may disappear for viewers in southern latitudes (in Florida, for example). Look for Cassiopeia, which travels around the same circle as the Big Dipper but on the opposite side. Near the middle bump of this W-shaped constellation, astronomer Tycho Brahe observed a supernova in 1572. In Winter, the Big Dipper climbs toward the top of the sky again, now three-quarters of the way around its circle. The Little Dipper, with Polaris at the tip of its tail, sweeps around the circle like the hand of a clock running backwards.
Disclaimers: The North Star is visible from the Northern Hemisphere. Sorry, Southern Hemisphereans!
The seasonal positions shown for the Big Dipper and Cassiopeia assume a viewing time in the mid-evening. When viewed at later times, these constellations will be seen to have moved along their circular paths in the counterclockwise direction.
A line drawn through the South and North Poles points directly to the North Star.
Coming soon . . .