Harvest Moon Meets Mars in Stunning Oct. 2 Conjunction

The waning gibbous moon will pass under the planet Mars on the night of Oct. 2-3. Observers in parts of the southern hemisphere will see the moon occult the Red Planet. Stellarium

We're really in for a treat. While auroras can be iffy, conjunctions are a sure thing. Conjunctions occur when two celestial objects line up directly on top of each other along the same line of sight. Bright, close conjunctions are the most exciting to watch.

On Friday, Oct. 2, the moon, just one day past full, will shine beneath the evening sky's brightest planet, Mars. What a magnetic pair they'll make! No matter where you live these two beacons will bust through the light pollution and capture your attention. 

The pair should be high enough in the eastern sky to see with ease around 9 p.m. local time. They'll be closest around 11 p.m. CDT (midnight Eastern, 10 p.m. Mountain, 9 p.m. Pacific) at just 1° apart, equal to two moon diameters. Since the moon and Mars are currently the two brightest objects in the evening sky, we're in for an eyeful. 

The conjunction from Buenos Aires around midnight Oct. 2-3. Stellarium

The moon's apparent motion in the sky as it orbits the Earth is much greater than that of Mars because it's so much closer to us. If you're up the following dawn, their separation will have increased to nearly 3°. Skywatchers in the southern hemisphere will witness an even more amazing sight — Mars pinned to the edge of the moon like a ruby earring. From southern South America and southwestern Africa the moon will briefly cover the planet in an occultation. For a map and list of cities and times of its disappearance, click here.

This full moon is special for another reason — it's the Harvest Moon, the full moon closest to the autumnal equinox. It's also the first of two full moons in October. The second occurs on Halloween. That makes it a Blue Moon, defined as the second full moon in a calendar month.

The Harvest Moon, painted by George Mason, was first exhibited in 1872. CC-BY-SA 3.0 

The Harvest Moon harks back to our pre-electric, agrarian past when outdoor lighting at night meant a fire, the moon and the stars. During September and October the full moon's path is nearly parallel to the eastern horizon, so it rises at nearly the same time on successive nights instead of the usual 50 minutes to an hour. That meant extra light for gathering in the crops during the critical time of harvest.

Planets, moon and sun all travel on or near the ecliptic, an imaginary circle in the sky that defines the plane of the solar system. During the fall the ecliptic runs nearly parallel to the eastern horizon at the point where the full moon rises. While the moon continues to move eastward about one outstretched fist each night, its path doesn't take it very far below the horizon on successive nights, trimming the time from one moonrise to the next to 15-30 minutes.

The moon's path, called the ecliptic, intersects the horizon at a shallow angle around the time of full moon in fall (left), which causes the moon to rise only about 20 minutes later each night. Six months later, the path intersects the horizon at a steep angle, delaying rising times by more than an hour. Times shown are for Duluth, Minn. Stellarium with additions by the author

Exactly the opposite happens six months later in spring when the moon's path is tipped at a steep angle to the eastern horizon. While it moves the same amount eastward each night as it did in fall — one outstretched fist — the angle forces it to dip much farther below the horizon, delaying moonrise by more than an hour.

To find the time the moon rises for your location, click here. All this talk about harvesting reminds me I need to get the last potatoes out of my garden. Colder weather's coming!