29 May 2010

High in the sky: The Big Dipper

The Big Dipper is one of the easiest groupings of stars in the sky to identify, and it serves as a guide to some of the more interesting stars in other parts of the sky. Late Spring evenings it is nearly overhead as seen from San Francisco, and its distinctive pattern provides an interesting exercise for understanding the motions of objects in the heavens.

The Big Dipper is not a constellation, by strict definition, because it is only the brightest 7 stars of the larger constellation Ursa Major. A named combination of stars within a constellation such as the Big Dipper is known as an "asterism." Because of its distinctive shape, the Big Dipper is a very well known asterism, one of several celestial groupings that lives up to its name (I put Leo, Scorpius, Cygnus and a few other constellations in this special class).

The Big Dipper points to the North Star (Polaris) if you follow the two stars at the side of the bowl of the dipper. This Wikipedia article illustrates this nicely. The line along the pointers from the Big Dipper to Polaris is helpful because this line is similar to an hour hand on a 24-hour clock. Every 24 hours the Big Dipper makes one counter-clockwise rotation around Polaris. From latitude 38 degrees north (approximately the latitude here in San Francisco) the Big Dipper is high in the sky when it is above Polaris (as it is now at sunset) and low in the sky when it is rotated half way around Polaris just above the horizon (as it will be in late Fall evenings).

The three stars in the handle of the Big Dipper form a curve, and if you think of this curve as an arc, you can follow it to a very bright star called Arcturus (in the constellation Bootes), and by continuing along this arc you end up at another bright star called Spica (in the constellation Virgo).

One more fun thing to find in the Big Dipper is the middle star of the handle, known as Mizar. This star has a very close companion, Alcor, next to it and if you want to test your eyesight, see if you can split the two without using binoculars or a telescope.

Enjoy learning about the Big Dipper in the pleasant weather of May and June. It's full of surprises and one of my favorite stops when sharing the sky with friends and guests at star parties.

08 May 2010

Planets and Bright Stars along the Ecliptic

This is a great time of year to enjoy the view of bright stars and planets demarcating the ecliptic. I am hosting a lot of star parties these days (last week in Tomales, this week in Fremont, next week in Healdsburg and the week after in San Mateo) and I always love to point out the ecliptic, the band across the sky where the planets and Moon are found in their wanderings across the heavens.

The ecliptic is the plane of the Solar System, the imaginary line across the sky that marks the orbits of the planets and the Moon. In a planetarium this can easily be shown, but under the heavens it is daunting to visualize this. I use a laser pointer to show the path across the sky, and that helps to visualize this, but right now the skies are cooperating to make this a bit easier for those of you without an amateur astronomer and a laser pointer :-)

Face South about 30-45 minutes after sunset and you will be looking toward the ecliptic. It stretches from the point of sunset to your right (West) where bright Venus gives you one reference point, then stretches up and toward the south to Castor and Pollux, the two twin stars of the zodiac constellation Gemini. Just to the upper left of the pair is bright orange Mars, and continuing left you encounter blue-white Regulus, the brightest star in the zodiac constellation Leo. Now the line of the ecliptic moves down toward the East, that is, down and to the left as you face South. Lower left of Leo is the planet Saturn, a bright, milky-white dot of light. And continuing to the lower left of Saturn is the bright star Spica, in the zodiac constellation Virgo.

Enjoy the tour, and if you have a star chart, put it to work so you can use these bright points of light to help you learn a few constellations. Even in the big city, all of these are visible.

The image on this page was copied from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version.