Gaze at the harvest moon on September 16, and you may notice the normally bright lunar orb start to darken. Don’t panic—the ominous sight is a perfectly normal penumbral eclipse.
This month’s full moon has been dubbed the harvest moon because it is the closest one to the autumn equinox in the Northern Hemisphere. Rising about half an hour later each night, the added light from the full moon’s shine is said to have given farmers more time to harvest their crops.
Last year, the harvest moon was also a supermoon—when our natural satellite made its closest approach to Earth—and it was turned a spectacular ruddy hue by a total lunar eclipse. This year, Earth’s shadow will again darken the moon but in a more ethereal event known as a penumbral eclipse.
A total eclipse of the moon is highly dramatic affair, since the moon turns dark red as it glides through the deep inner shadow cone, or umbra, of Earth.
Because the sun is a large disk rather than a single point of light, our planet’s shadow also has a lighter outer cone, or prenumbra, that can also envelop the moon. When this penumbral eclipse happens, it creates a subtle shading of the lunar disk.
The best views of this week’s harvest eclipse should be across Europe, Africa, Asia, Australia, and the western Pacific basin. The deepest and darkest phase of the eclipse will come at 2:54 p.m. ET (18:54 GMT). For detailed charts and times in your location, visit EclipseWise.com.
Because the darkening will be so slight, the best bet for viewers will be to use binoculars or telescopes to catch the creep of Earth’s shadow as it blankets the moon’s usual glare. Expect to see the darkening effect start over the northern portion of the moon’s limb and envelope about 91 percent of its disk during its maximum phase.
While not as flashy as a super blood moon eclipse, you should be sure to enjoy this week’s lunar show—it’s the last harvest moon eclipse of any kind that we’ll see until 2024.
The next big lunar event for the Eastern Hemisphere will be January 31, 2018, when there will be a total eclipse of the moon.
Also This Week
Neptune and Moon. After darkness falls on September 15, look for the moon to guide binocular viewers to the nearby planet Neptune.
The two worlds will appear to be only three degrees apart, a separation that’s equal to six full moon disks side by side. But don’t be fooled by their celestial coziness. While the moon is about 240,000 miles from us, on average, Neptune is the most distant planet from the sun at about 2.7 billion miles away.
Shining at a feeble magnitude 7.8, the ice giant is invisible to the naked eye, but it can be glimpsed with binoculars as a very faint bluish disk among a backdrop of white pinpoint stars.
Venus Meets Spica. Here’s an observing challenge for skywatchers in tropical latitudes: After sunset on September 17 and 18, look for the bright planet Venus to pair up with Spica, the lead star of the constellation Virgo. The celestial pair will only be 2.5 degrees apart, or equal to the width of five full moon disks.
What will make this a bit tricky for viewers is that this conjunction will happen less than 10 degrees above the southwestern horizon. This will make the viewing time critical, as the duo will follow the sun quickly and sink below the horizon within 45 minutes of local sunset. Binoculars will also help keep the pair in easy view.
Tune in to National Geographic Facebook LIVE each Monday afternoon Eastern Time to see Andrew share the week’s best sky events.