Crux Southern Cross Mike SalwayFEATURE

A Journey From Cassiopeia To The Southern Cross

The W of Cassiopeia the Queen is due north and low in the sky at mid-northern latitudes in May. Created with Stellarium

Walking with my older daughter the other night, we noticed how low Cassiopeia had dropped in the  northern sky. It’s also back to looking like a “W”  instead of a zigzag. The familiar constellation reaches its nadir or lowest point above the northern horizon around 11 p.m. in mid-May. For most sky watchers that means it’s lost in the trees or hidden by buildings.

Cassiopeia’s height also depends on one’s latitude. From Duluth, Minn. the W never sets, but from Phoenix, Arizona it nearly scrapes the horizon. Travel further south to Miami and you won’t see it at all on May and June evenings.

The North Star is as high above the horizon as your latitude. In Duluth, that’s 47 degrees or more than halfway between the horizon and the overhead point or zenith.  Since Cassiopeia is only 30 degrees from the pole star, it cycles around it as the Earth rotates and misses the horizon by 17 degrees, ie. never sets. Constellations that never set are called circumpolar. In addition to Cassiopeia, the Big Dipper, Little Dipper, Cepheus, Draco and parts of others are all circumpolar from where I call home.

Cassiopeia is below the horizon in May and June for Florida skywatchers. This map shows the view looking north from Miami around 11 o'clock. Credit: Chris Marriott's SkyMap software

Now let’s pretend we’re in Miami. Its latitude is 26 degrees (21 degrees south of Duluth, Minn.). That means the North Star is just 26 degrees above the horizon in the northern sky. Since the W is 30 degrees from the pole star, it’s below the northern horizon this time of year.

Before we pity the poor Miamians, let’s expand our view. While the northern sky is cut short by their southerly latitude, they’re well compensated by a deeper look into the southern sky.

In Duluth, our southern horizon cuts off constellations not far below the trapezoid-shaped Corvus the Crow. Floridians get to see another 20-plus degrees of sky beyond that.

From southern Florida and points south, the Southern Cross (Crux) and the brightest stars of Centaurus the Centaur are visible. Sky shown around 11 p.m. local time.

And guess what’s there? Only one of the starriest lots of real estate in the sky. From a boat on the Gulf, you’d be able to look due south and spy the Southern Cross just cresting above the horizon with bright Alpha and Beta Centauri in tow on May nights. You’ll recall that  the triple star Alpha is the closest star to Earth after the sun with a distance of 4.3 light years or 26 trillion miles.

In my opinion that more than makes up for the loss of the W. Besides, all Miamians need do to see Cassiopeia is go out at dawn, when Earth’s rotation brings the constellation back up in the northeastern sky.

The Southern Cross (center) and Alpha and Beta Centauri (left) are embedded in a rich section of the southern Milky Way. The dark patch to the left of the Cross is dark nebula called the Coalsack, composed of interstellar dust. Credit: Mike Salway

While clearly unfair to northerners, the Southern Cross and its cohorts are motivation for the astronomically-inclined to make a trip to the far south. After living most of one’s life where the North Star is always forever in one place in the sky, you’ll feel like a newborn babe under the southern stars.

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