From the Southworth Planetarium
“The psychic octopus said what?”
THE DAILY ASTRONOMER
July 8, 2010
Morning Sisters
On an ordinary day, we’d have only one problem to enjoy. Today, we’ve decided to be selfish and have two.
These problems pertain to the following sentence: The Pleaides is a “winter constellation.”
The first problem is that the Pleiades is not a constellation. Some call it an “asterism,” because it is a star pattern within a larger constellation (Taurus the Bull). Even “asterism” isn’t the best choice, however, since the Pleiades is a galactic star cluster: a relatively small, gravitationally bound system of a few hundred stars. Astronomers use the moniker “seven sisters” because most observers can resolve the cluster into seven distinct stars. (Many only see six.)
Mythologically associated with the seven daughters of Atlas, the Pleiades are among the night sky’s most famous stellar groupings.
The next problem concerns the word “winter,” which suggests that one can only observe these sisters during the winter months. In fact, they’re visible now: low in the eastern pre dawn sky. The Pleiades Cluster resembles a faint, nearly round smudge of faint white light. On July 8th, the thin sliver moon passes within less than a degree and a half of it. However, the brightening twilight might obscure the cluster, making it difficult to see. Unless they’re circumpolars, which are always visible, stars are observable for about ten months each year. They appear in the pre-dawn eastern sky and move gradually away from the Sun. Eventually, they’ll rise around midnight and then about two months later or so, rise around sunset. A few months later, they’re setting around midnight. Add a couple more months, they’re setting with the Sun in the west. Soon after, they’ll rise in the eastern pre-dawn sky to begin the cycle anew.
During the winter, the Pleiades is visible most of the night, hence the “winter constellation” malarky. The “winter” indicates that the star or constellation is best seen in the winter. It does not tell us that one should ONLY try to find the star or constellation in the winter.
This is why we astronomy types should avoid specifying seasons when we speak of stars and constellations. However, doing so has been a practice since Stonehenge was a shopping plaza, and we’re nothing if not shameless traditionalists.
Find the morning sisters this month, the time of year in which we can truly call them “morning” sisters.
