While you’re waiting for your shot at seeing asteroid 2005 YU55 , let’s talk about something most all of us will see with only a little effort. Tomorrow morning before dawn and for the next few days the planet Mars will shine within a degree or two of the bright star Regulus in the constellation Leo the Lion. The two will be closest on the mornings of the 10th and 11th, when they’re separated by only about 1.5 degrees.
This is a fine opportunity for sky watchers to compare the two orbs’ colors and brightnesses. Mars is pinkish-orange and shines at magnitude 1.0, a tad brighter than white Regulus at magnitude 1.3. Regulus marks the end of the asterism called the Sickle of Leo which happens to look like a backwards question mark. The reason Mars gets so close to the star is because it lies almost exactly on the same “highway” in the sky called the ecliptic. This is the path the planets, moon and sun take through the 12 zodiac constellations.
Although they appear similar except for color, the two couldn’t be more different. Mars is a small planet shining by reflected sunlight, while Regulus is 4.3 times the size of the sun and spins so fast – 200 miles per second – that its shape resembles that of an ostrich egg. Through a telescope, Regulus, like nearly every other star, is too far away to show its actual shape. Except for color and brightness, all stars look the same through most telescopes. When observed through a circular aperature, stars show only an Airy disk, a tiny bright spot surrounded by several delicate rings of light. The disk is not the star itself but rather the smallest, sharpest view of an object so distant its shape can’t be resolved.
Mars on the other hand will gladly oblige if you pump up your magnification to 150x and higher. Provided the air turbulence is low, you’ll spy a tiny ochre disk with a white dab at one end – that’s the north polar ice cap. It’s currently winter in Mars’s northern hemisphere and the ice cap is very prominent. Additional dark markings similar to the ones in the photo at right will be in view during the hours of 4-6 a.m. in the coming week.
Sharp-eyed observers will also notice that the planet’s disk is not a perfect circle but more like a gibbous moon. Except around the time of opposition, when it’s closest to Earth, Mars shows a slight phase effect, because we see the planet well off to one side of the sun-Earth-Mars line. From that angle, a bit of shadow or shading darkens an edge of the planet.
Mars is a breeze to see and should make a pretty sight so close to a similarly bright star. I hope you’ll have clear skies this week to enjoy the view.