Oct. 13 Dawn Celestial Trio / Why Do Some Stars Never Set?

Venus, the lunar crescent and Regulus join forces at dawn tomorrow morning (Oct. 13) in the eastern sky. Stellarium

I love October. The sun rises late which means you can get up early. For many of us sunup occurs between 7 and 7:30, making it easier to see pretty happenings in the dawn sky. For instance, tomorrow morning (Oct. 13) the waning crescent moon will join 1st magnitude Regulus, Leo's brightest star, and brilliant Venus in the east to make an attractive sight. 

 If you're up around 6 a.m. you'll see the trio in the growing light and may find it's just what you need to start the day. The moon is three days from new and will look like a bright rind edging the bottom of the darker, earth-lit disk. Direct sunlight illuminates the crescent; light reflected from the Earth dimly brightens the remainder. 

 

Venus is a tiny gibbous "moon" in a telescope this month. Stellarium   

As ever, Venus has a magnetic draw on the eyes, while dimmer Regulus completes the scene. All of this takes place in Leo the lion, one of the brightest constellations of the zodiac.

In spring, Leo stands high in the south during evening hours. In autumn, it's just recently returning to view at dawn after having spent the past couple months in the daytime sky hidden from view by the sun's glare. 

Most constellations, including Leo, rise and set like you'd expect, but a special group called circumpolar constellations do not. These groups lie relatively near the North Star (Polaris) and share one of its most cherished qualities — they never set. From where I live I can see the Dippers, Cassiopeia, Cepheus, Draco and others all year long. The altitude of Polaris — how high it stands above the northern horizon — is equal to your latitude. If you live at 40° north latitude, the star will be 40° or four fists high. 

Stars in the northern sky describe circular arcs around the North Star in this 45-minute time exposure. Stars closer to Polaris than your latitude circle it without setting and are called circumpolar. Bob King

Any star or constellation closer to Polaris than 40° will appear to circle above it and then drop toward to west like all stars do. But instead of setting it "misses" the horizon and climbs back up again in the eastern sky.

With a latitude of 32°, Tucson's circumpolar stars and constellations are restricted to a circle with a radius of 32° centered on the North Star. Stellarium with additions by the author

There are more circumpolar constellation visible from Anchorage, Alaska at latitude 62° compared to Tucson at latitude 32°. Also, if a star lies farther from the North Star than your latitude it will dip below the horizon for a time. In our 40° latitude example, a star 45° from Polaris will dip 5° below the horizon and then climb back into view. Stars farther from Polaris stay below deck longer. Good examples include the familiar constellations Orion, Scorpius and of course Leo, the site of tomorrow's celestial conclave.

Minneapolis (latitude 45°) has a much wider circle of circumpolar stars that includes all of the Big Dipper and Cassiopeia. If you were to travel to the North Pole (latitude 90°), all stars within 90° of Polaris would be circumpolar — every single one of them! Stellarium with additions by the author

To find out what your circumpolar cutoff is, click here to find your latitude. With that in hand, download the free planetarium-style program Stellarium, set your location and then use the measuring tool at the bottom of the screen (angle-shape) to measure off your latitude from Polaris. You'll see at a glance exactly what's circumpolar and what's not. 

I like the comfort of circumpolar stars. They're like friends you can always count on.