Last night Mars blazed in the treetops. I’m still amazed at how bright the planet’s become in the past few weeks. Paired up with the bright, blue-white star Spica in Virgo, it’s unmistakable. The two clear the southeastern horizon together around 9:30 and become very obvious an hour later.
Reach your arm toward Mars and move “two fists” to its upper left and you’ll spot Arcturus, the brightest star in the constellation Bootes the Herdsman. Together the three form a striking “lightsaber” of luminaries.
Looking more closely, you’ll notice that each is a different brightness. Thanks to fortunate circumstance, they differ from each other by almost exactly one magnitude, the standard unit of measurement astronomers use to measure star brightness.
First magnitude stars are 2.5 times brighter than second magnitude stars, which are 2.5 times brighter than stars of third magnitude and so on.
Mars rules the roost at magnitude -1.2, Arcturus is next at 0 and Spica at 1.0. If you’ve ever wondered what a magnitude of difference between celestial objects looks like, check this convenient live demonstration the next clear evening.
Arcturus and Spica will remain fixed in their brightness – at least for thousands of years – but the light of Mars and the other planets vary depending on their distance from Earth. In astronomy, there’s a simple rule: when close, objects appear brighter than when farther away. Mars’ magnitude varies more than most of the planets with extremes of -3.0 when nearest to 1.6 when most distant. That’s a difference of nearly 100 times.
These large variations directly relate to Mars’ more elliptical (oval) orbit. When the Red Planet is on the far end of its elliptical orbit at the same time Earth’s on the opposite side of the sun, it’s farthest and faintest. When on the far end of its orbit on the same side of the sun as our planet, it handily outshines every star and planet in the sky except Venus.
Even if you don’t give two shakes about magnitudes, make sure you take in a view of all three of these night sky gems. If you have a telescope and are observing from the western hemisphere, this is the best week to see the bright false south polar cap.
The “cap” is really Hellas, the largest crater on Mars, covered in frost and bedecked with clouds during southern hemisphere winter.
Look for a bright lens-shaped spot on the planet’s south end around midnight early this week and closer to 1-2 a.m. next weekend. It’ quite obvious in 6-inch and larger telescopes in steady air.