As twilight gives way to darkness tonight, look up at the waxing moon in the south. Just above it you’ll see the planet Mars. If you’re game, whip out a pair of binoculars and see if you can spot Mars before sunset using the moon as guide.
It’s been two months now since Mars made its most recent closest approach to Earth. While the planet has faded a full magnitude and shrunk in size since opposition, it will remain the brightest ‘star’ in the evening sky until June 27, when Arcturus will outshine it by a hair.
Mars has resumed its normal eastward motion across the sky and is now on the move across Virgo. Watch for it to glide above bright Spica in mid-July and below Saturn in late August.
Through a telescope it’s easy to see that its phase has changed from full to gibbous.
The inner planets Venus and Mercury show phases from crescent to half to full as they alternatively pass between Earth and sun, but the outer planets are limited to full and gibbous phases because they’re forever outside the orbit of our own planet. No passing between the sun and Earth for them.
Full phases happens around the time of opposition when Earth and an outer planet like Mars are lined up on the same side of the sun and nearest each other. We face the planet square-on and it appears fully illuminated. Several months past opposition, sunlight strikes Mars at a very different angle than what we see on Earth. We look ‘off to one side’ instead of directly at the planet; from our perspective a portion of its globe is hidden in shadow and we see it as little gibbous ‘egg’.
The shadowing effect is most extreme at ‘quadrature’ when an outer planet lies 90 degrees from the sun, ie. it’s due south at sunrise or sunset. Mars reaches eastern quadrature on July 19.
Jupiter and Saturn also show a phase effect but it’s very, very slight because they’re so far away that Earth and sun appear in nearly the same direction from their perspective. There’s very little ‘looking off to one side’ perspective compared to much closer Mars.
Although the moon is getting brighter by the night as it approaches full phase on June 13, I see it’s time for a new map showing the ramblings of comet C/2012 K1 PANSTARRS. This reliable comet has been slowly getting brighter all spring and now has a nice 1/2-degree tail visible in 6-inch and larger telescopes. At magnitude +8, I’ve seen it plainly with 40mm binoculars from a dark sky.
This month it moves from the obscure constellation Leo Minor into Leo the Lion and will continue to slowly brighten. The best time to view K1 PANSTARRS is at nightfall when it’s highest in the southwestern sky.
Moonlight won’t interfere too much with viewing tonight but will be an issue in the coming nights. Dark skies return around June 15.
Give it a try – we’ve got until mid-July. After that northern hemisphere observers won’t see the comet again until morning twilight in early September.