|
|
SKY MAP: Draco the dragon as seen at 9 p.m. local time this time of year from mid-northern latitudes. Credit: |
The celestial dragon is in its ascendancy this week for early-evening observers.
Have you ever wondered why a particular group of stars was made into a certain constellation? Sometimes a star pattern suggests an object, creature or person. Other constellations portray mythological creatures such as unreal monsters.
Draco, the Dragon, is one of these.
Draco is almost entirely circumpolar – that is, it always remains above the horizon, never rising or setting for skywatchers at most mid-northern latitudes. But right now is the best evening season for tracing out the windings of this unusual beast's snakelike body. This week, between 8:30 and 9:00 p.m. local daylight time, he appears to pass between both the Little and Big Dippers, with his head raised high above Polaris, almost to the overhead point (called the zenith).
The Dragon's head is the most conspicuous part of Draco: an irregular, albeit conspicuous quadrangle, not quite half the size of the Big Dipper's bowl. You can find it situated about a dozen degrees to the north and west of the brilliant blue-white star, Vega, the brightest of the three stars that make up the Summer Triangle (ten degrees is roughly equal to your clenched fist held at arm's length).
Draco is a very ancient grouping. The earliest Sumerians considered these stars to represent the dragon Tiamat. Later it became one of the creatures that Hercules killed. One of Draco's tasks was to guard the garden of Hesperides and its golden apples that Hercules was supposed to retrieve. In the stars, as Draco coils around Polaris we now see Hercules standing (albeit upside down) on Draco's head.
The brightest star is Eltanin, a second magnitude star, shining with an orange tinge. This star is famous for being the one with which the English astronomer, James Bradley, discovered the aberration of starlight – an astronomical phenomenon which produces an apparent motion of celestial objects ? in the year 1728. Interestingly, a number of temples in Ancient Egypt were apparently oriented toward this star.
The faintest of the four stars in the quadrangle is Nu Draconis, a wonderful double star for very small telescopes. The two stars are practically the same brightness, both appearing just a trifle brighter than fifth magnitude and separated by just over one arc minute (or about 1/30 the apparent diameter of a full Moon). I first stumbled across Nu as a teenager in the Bronx, using low power on a four-and-a-quarter-inch Newtonian reflecting telescope. I likened it to a pair of tiny headlights. Check it out for yourself.
The pole of the heavens is moving slowly among the constellations of the northern sky, once around a large circle. It is owing to a movement of the Earth for which the pull of both the Sun and Moon on our bulging equator is chiefly responsible, a movement known as "precession." This double attraction causes the Earth to wobble slightly like a slowing-down top does.
While the tilt of the axis to the Earth's orbit remains the same (tilted 23.5 degrees from the equator), the axis itself describes a funnel-shaped motion, completing one rotation in about 25,800 years. This time span – one complete wobble – is called a "Great" or "Platonic" Year.
Located in Draco's tail is the faint star Thuban. During the third millennium, BC, the Earth's axis was pointed almost directly at this star. As such, Thuban was the North Star when the Pyramids were being built, some 5,000 years ago. Thuban was nearest to the North Pole of the sky about 2830 B.C. It then shone in the sky almost motionless in the north near to where the current North Star, Polaris, now appears. Look roughly midway between the bowl of the Little Dipper and the star Mizar (where the Big Dipper's handle bends) and there you will find the former North Star.
And thanks to the oscillating motion of precession, Thuban will again be the North Star some 20,000 years from now.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.
