The Big Dipper: finding Polaris and telling the seasons

The Big Dipper may be the best-known star pattern in the world: seven stars of magnitude 2 to 3 in the shape of a ladle, visible even from light-polluted cities. Chinese astronomers used it to find north, to tell the season and even to name the months by where its handle pointed, and that is why a BaZi chart's month pillar starts from the Yin month.

The seven stars

Right next to Mizar (Kaiyang) sits a 4th-magnitude companion, Alcor, called Fu ("the Assistant") in Chinese. Being able to see it was an old test of eyesight.

Using the Dipper to find Polaris

Join the two stars at the front of the bowl, Merak and Dubhe (Tianxuan and Tianshu), and extend the line out of the bowl's mouth about five times: you reach Polaris. Our figures: Merak to Dubhe is about 5.4°, Dubhe to Polaris about 28.7°, exactly 5.3 times as far. Polaris is the end of the Little Dipper's handle, about magnitude 2, due north at an altitude roughly equal to your latitude.

When the Dipper is too low to see, use Cassiopeia instead: five stars in a W on the other side of Polaris, high on autumn and winter evenings.

At low latitudes the Dipper sets

Far enough north the Big Dipper never sets: Dubhe stays up all night anywhere above about 28°N and Alkaid, at the end of the handle, above about 41°N, so from London or Toronto it circles the pole all year. At Taiwan's latitude, though, the whole Dipper spends part of every day below the horizon. Here is the altitude of Dubhe (first star of the bowl) and Alkaid (end of the handle) at 9 p.m. on the 15th of each month in Taipei:

Calculated by our sky engine, without refraction. In practice anything below about 10° is often hidden by buildings, hills or haze.

So in Taiwan at 9 p.m. in September, October, November and December, the Dipper is very low or already set, and Cassiopeia is the better way to Polaris. Spring evenings (March to May) put the Dipper highest and make it easiest to learn.

The handle tells the season

The ancient text Heguanzi says: "When the Dipper's handle points east, it is spring everywhere; when it points south, summer; when it points west, autumn; when it points north, winter." It means which side of the pole star the handle has swung to at dusk. Our figures for 8 p.m. in Taipei in 2027 put Alkaid, at the tip of the handle, here relative to Polaris:

When you face north, "up" leads toward the zenith and, past it, to the south, so a handle above the pole star is what the ancients called pointing south. The four seasons roughly match the old text; at the December solstice the handle swings below the pole star at dusk and, from Taiwan, below the horizon.

The Dipper and the calendar: why the month pillar starts at Yin

Chinese astronomers divided the horizon into the twelve Earthly Branches and named each month by the branch the handle pointed to at dusk, a practice called dou jian: the handle pointing to Yin marks the first month, to Mao the second, and so on. The Xia calendar made the Yin month the first of the year, and a BaZi chart's month pillar still follows that order: the year starts from the Yin month, and the months change at solar terms (Start of Spring, Awakening of Insects …). See Stems and Branches and solar-term boundaries.

Note that dou jian is how the ancients turned the sky into a calendar; a modern BaZi chart changes months by the Sun's position at each solar term, not by actually looking at the Dipper.

The Northern and Southern Dippers

The Southern Dipper is the six stars of the mansion Dou in Sagittarius, also ladle-shaped but smaller and fainter, low in the south on summer evenings. Chinese folklore says "the Southern Dipper records birth, the Northern Dipper records death", a saying from the Soushen Ji collection of myths and legends.

Taipei, April 15, 9 p.m.. The maps show the sky as you look up: north at the top, east on the left; the center is the zenith and the rim is the horizon. Planets and the Moon are left out because they move from year to year; use the tool on this page to see a given night.

Three common misconceptions