Mars is so tiny and so far away from Earth right now, few skywatchers would ordinarily pay it any attention. That might change the next two mornings, when the Red Planet slides right up to Regulus, the brightest star in the constellation Leo the Lion. I saw the two of them on Monday and thought they looked close together then. But that was only the prelude.
They’ll be much closer the next two days and make an eye-catching sight in the east as the night sky gives way to dawn. Add in an impossibly-bright Venus and the return of Jupiter and you’ve got a scene worth at least a cave painting, or in the current era, a digital photograph.
Close pairings like this one are called conjunctions. They occur when a planet lines up directly above or below a star (or another planet or the moon). Regulus sees more moon and planet conjunctions than most bright stars because it happens to lie almost directly on the ecliptic, the path taken by the moon, sun and planets. Like a good-natured Canadian border crossing guard, Regulus extends a friendly greeting to wandering planets before bidding adieu. Regulus will next meet up with Venus and the crescent moon on October 8.
Regulus is something of an exceptional star. Located 79 light years from Earth, it shines at magnitude +1.3, a bit brighter than the current Mars. The color difference between Regulus, a white star hotter than the sun, and Mars should be obvious if you plan to get up for the conjunction. Having them so close together makes their hues that much more striking.
Regulus shines 360 times brighter than the sun and spins so fast it’s stretched into an oblate spheroid 4.3 times the sun’s diameter.
Not every star has a spherical shape like the Sun. Because stars are made of gas and not rock, they’re “stretchable”. Fast-rotating stars get stretched into flattened spheres called oblate spheroids. Regulus is a classic example with wildly fast rotation rate of nearly 200 miles per second (317 km/sec). It makes one complete spin every 16 hours compared to the approximately 27 days for the sun to do the same. If we could zoom in for a look, we’d see that Regulus has an equatorial diameter (girth) of 4.3 suns, 32% larger than its polar diameter. Yes, it would look something like an egg.
Wishing you clear skies for the conjunction, which you’ll see best starting about an hour, hour-15 minutes before sunrise. But if it’s cloudy through Friday, Mars and Regulus will still be pretty close on Saturday morning, too.