Mars will do us a favor tonight. If you point your binoculars at the planet this evening, snugged next to it will be another planet — Neptune. Neptune, the most distant planet in the solar system, will appear just a quarter of a degree (half a full moon) to the lower right of Mars. Find Mars and you’re guaranteed a view of Neptune. The two are closest tonight and slowly separate in the coming nights.
You can see Mars (magnitude 0.1) from practically anywhere, even within a large city. Neptune is considerably fainter at magnitude 7.9, so you’ll need something like an 8×40 or 10×50 pair of binoculars and reasonably dark skies to spot it. But if you have a telescope, even a small 3-inch refractor, the remote planet will be easy to see even from light-polluted locations.
First, find Mars. It’s a snap — face south and look halfway up the sky for a bright, reddish “star.” Granted, the planet’s lost its summertime luster, but it’s still bright and obvious. Next, use the maps to pinpoint Neptune. If you’re using binoculars, Neptune will look like a faint star very close to the lower right of Mars. Through a telescope at low power, the planet also appears stellar but has a distinctive pale blue color due to the presence of methane in its atmosphere. If you increase the magnification to 150x or higher, Neptune will swell into a tiny blue disk and make a wonderful contrast with rusty, red Mars. Enjoy this rare opportunity to see the two planets at high power in the same field of view.
Mars has shrunk a lot since summer because of its increasing distance from Earth but still appears 4 times larger than Neptune, a planet so remote that it takes light more than 4 hours to slog its way from there to our eyes.
Both Neptune and Mars are moving east (to the left as viewed from the northern hemisphere) across the sky as they orbit the sun. Neptune crawls along at the speed of a sloth. Mars, being much closer, moves rapidly in comparison. Cruising along at 38 arc minutes a day — a little more than the apparent diameter of the full moon — it zips right by Neptune.
So here’s the fun part. If the sky’s clear tonight, you can find Neptune with a little pluck and luck and then watch Mars step further and further away from the planet in the next few nights. That way you’ll get a feel for their vastly different distances: 98.3 million miles for the red one and 2.8 billion miles for the blue one.
Have fun and wishing you clear skies to see it all unfold.