Earth's shining satellite tonight is called a blue moon, although when it rises just after sunset it could look more like a fat bright orange -- a jack- o'-lantern perhaps, turning white as it climbs high in the sky, and fading away as it sets toward sunrise.

According to Bing Quock, the Planetarium director at the California Academy of Sciences in Golden Gate Park, what's so unusual about a blue moon on Halloween is that it last shone over California in 1944. And there won't be another until the year 2020 -- time for a new generation of costumed celebrants.

The blue moon was originally defined by the Maine Farmer's Almanac in 1819 as an extra full moon within any season. Later, however, the respected publication Sky and Telescope altered the almanac's definition and since then it has come to mean the second full moon within a single month. The first full moon this month was Oct. 2.

No one knows for sure why this rare full moon is called blue, but in Shakespeare's time, Quock believes, people may have called it blue because they likened it to the blue-green mold that marks some ripe cheeses.

Perhaps the most famous mysterious words recited at Halloween parties are those from Shakespeare's three witches in "Macbeth":

"Round about the cauldron go;

In the poison'd entrails throw.

Toad, that under cold stone

Days and nights hast thirty-one

Swelter'd venom sleeping got,

Boil thou first i' the charmed pot.

Double, double, toil and trouble,

Fire burn and cauldron bubble."

But there are many mixtures of evil in pre-Shakespeare Halloween legends, Quock notes.

Tonight, for example, as the blue moon rises in the east, so will the Pleiades, the "Seven Sisters," a cluster of stars in the constellation Taurus the bull. (Actually, there are about 3,000 stars in the Pleiades, not seven). The cluster will rise about a half-hour after the moon, and should move up the sky a little below and about 25 degrees to the left of the moon, according to Quock.

The ancient Celts celebrated the rise of the Pleiades on the night between the autumnal equinox and the winter solstice -- in other words, on Halloween night. Their Druids lit candles and bonfires on hilltops beneath the dark oak forests, and sacrificed crops and animals to begin the "season of darkness and cold."

In the year 834, the Catholic Church declared All Saints' Day on what is now Nov. 1. The night before was the eve of all those who had been hallowed, or consecrated -- Allhallows Eve, or Halloween.

Even earlier, according to some anthropologists, Mayan groups believed that the Pleiades reigned supreme over the darkness. At midnight each Oct. 31 when the star cluster was overhead, Mayan priests atop their temple would cut open a young boy's chest to remove his beating heart.

Somehow, perhaps from that macabre Mayan tradition, plus the original Allhallows Eve of the Catholic Church and an ancient Aztec festival presided over by the goddess known as "Lady of the Dead," comes the Mexican "Dia de los Muertos," whose annual celebration also begins tonight.

The Day of the Dead, celebrated according to Ricardo J. Salvador of Iowa State University, marks a "traditional blend of ancient aboriginal and introduced Christian features." It is ''a way of recognizing the cycle of life and death that is human existence, and ''it is not a morbid occasion but rather a festive time.''

E-mail David Perlman at dperlman@sfchronicle.com.