Time for the Queen to hit the hay. I’m referring to Cassiopeia, the constellation best known for its resemblance to the letter ‘W’. Mid-May evenings Cassiopeia the Queen reaches its lowest point in the sky due north below the polestar. This is referred to as its ‘nadir’ as opposed to the ‘zenith’ or point directly overhead. Pleasant dreams in the netherworld, queenie.
From the northern U.S., southern Canada and much of Europe, Cassiopeia swoops low but never sets. That’s because it’s not far from Polaris the North Star, which stays buttoned to one place in the sky.
How high Polaris is above the horizon depends on your latitude. Here in Duluth, Minn., we’re at 47 degrees north latitude, so Polaris is always 47 degrees above the northern horizon. At the north pole, it’s 90 degrees high or at the zenith. At the equator, it’s 0 degrees and sits directly on the northern horizon.
Cassiopeia is fairly close to Polaris, so at least from the northern U.S., it goes ’round the polestar during the year without ever setting. Draco, the Little Dipper and several others behave the same way and are called circumpolar constellations. What’s circumpolar for you might be different for someone else.
If you live in the southern U.S., Polaris is much lower in the northern sky, meaning the ‘W’ gets chopped off by the northern horizon when it’s at nadir below the North Star. If your hometown is San Diego, Cassiopeia sets in May and June – definitely not one of your circumpolars.
The Big Dipper is on the opposite side of the North Star from Cassiopeia, so at nightfall in May, when the Queen is low, the Dipper reaches its highest elevation of the year. In Duluth, its striking 7-star outline brushes the zenith.
Stay up late and Cassiopeia swings to the east like the hour hand of a clock. By dawn, it begins to climb the northeastern sky once again.
Now that Mars is slowly pulling away from the sun in the morning sky and visible once again in binoculars, I thought it would be a good time to check out some recent photos of the fascinating Martian slope streaks. Early on, it was thought they might be dark staining from water flows on or just below the surface, but the consensus is now leaning toward avalanches of dry dust. They form on the down sides of steep slopes like crater rims. Cascading dust and debris tumbling down slope under the influence of gravity remove the older layers of bright surface dust, revealing darker material beneath.
The streaks are around 1000 feet wide and some 1.5-2 miles long and typically start at a point and widen down slope. The dark interiors are lower than their surroundings indicating that material has been removed. Streaks become lighter with age as they’re slowly covered by surface dust blown across the landscape by Martian winds.
Since change is one of the defining characteristics of our own planet, it always gives me a thrill to see changes on another planet. It makes Mars less an imagined place and more a real world.