Mars Gets Tight With Neptune Tonight

Use this map to find Mars tonight and through next week. It’s set for the end of twilight (around 6-6:30 p.m.) today, Dec. 7. Notice that Markab and Scheat (in the Pegasus Square), the planets, and bright Fomalhaut lie along a nearly straight line 57° or nearly 6 fists long. Stellarium

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.

Mars may look a lot brighter, but it’s 7 times smaller than Neptune in real life — 30,000 miles across vs. 4,200 for Mars. Neptune only appears smaller because it’s so much further away. Anthony Wesley (left) and NASA

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.

Here’s the view in binoculars tonight with Mars, Neptune and several other stars in the field of view. Brighter stars are labelled, so 81 Aqr is the star 81 Aquarii in the constellation Aquarius. Stellarium

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.


And here’s the view tonight in a small telescope. A fainter magnitude 9 star lies to the upper right of brighter Neptune. North is up. Stellarium

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.

This map shows Mars’ eastward travels over the next few nights. SkyMap

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.

4 Responses

  1. Edward M Boll

    I wish that I had seen this post before the planets had set. I suppose using Mars as a guide tomorrow will be a little tougher to find Neptune since they will be over another half a degree apart.

    1. astrobob

      I’ve got a map at the end of the blog showing Mars and Neptune through the 10th, so you can use that if you like.

  2. Edward M. Boll

    I plan to. And it will be my first look at a dim planet since the time Uranus was close to a lunar eclipse. Thanks

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