What Makes The Harvest Moon Special

“The Harvest Moon”, a circa 1833 oil painting by Samuel Palmer. Closely spaced moonrises meant extra light to bring in the crops in the days before electric lighting.

Today, September 8, the Harvest Moon will rise the color of a fall leaf and spill its light across deserts, forest, oceans and cities. The next night it rises only a half hour later. And the next, too. The short gap of time between successive moonrises gave farmers in the days before electricity extra light to harvest their crops, hence the name.

The Harvest Moon is the full moon that falls closest to the autumnal equinox, the beginning of northern autumn. As the moon orbits the Earth, it moves eastward about one fist held at arm’s length each night and rises about 50 minutes later. You can see its orbital travels for yourself by comparing the moon’s nightly position to a bright star or constellation.

Around the time of Harvest Moon, the full moon’s path is tilted at a shallow angle to the eastern horizon making with successive moonrises only about a half hour apart instead of the usual 50 minutes. Source: Stellarium

50 minutes is the usual gap between moonrises. But it can vary from 25 minutes to more than an hour depending upon the angle the moon’s path makes to the eastern horizon at rise time. In September that path runs above the horizon at a shallow angle. As the moon scoots eastward, it’s also moving northward this time of year.

This northward motion isn’t as obvious unless you watch the moon over the coming week. Then you’ll see it climb to the very top of its monthly path when it’s high overhead at dawn. The northward motion partly cancels out the eastward motion, keeping the September full moon’s path roughly parallel to the horizon with successive rise times only ~30 minutes apart.

The angle of the moon’s path to the horizon makes all the difference in moonrise times. At full phase in spring, the path tilts steeply southward, delaying successive moonrises by over an hour. In September, the moon’s path is nearly parallel to the horizon with successive moonrises just 20+ minutes apart. Times are shown for the Duluth, Minn. region. Illustration: Bob King

Exactly the opposite happened 6 months earlier this spring, when the moon’s path met the horizon at a steep angle. While it traveled the identical distance each night then as now, its tilted path dunked it much farther below the horizon night to night. The spring full moon moves east and south towards its lowest point in the sky. Seen from the northern hemisphere, that southward travel adds in extra time for the moon to reach the horizon and rise each successive night.

If all this is a bit mind-bending, don’t sweat it. Click HERE to find when the moon rises for your town and find a spot with a great view of the eastern horizon. You’ll notice the moon is orange or red at moonrise because the many miles of thicker atmosphere you look through when you gaze along the horizon scatters the shorter bluer colors from moonlight, tinting it red just as it does the sun.

A series of photos of the full moon setting over Earth’s limb taken by from space by NASA astronaut Don Pettit on April 16, 2003. Refraction causes a celestial object’s light to be bent upwards, so it appears higher than it actually is. The bottom half of the moon, closer to the horizon, is refracted strongest and “pushed” upward into the top half, making it look squished. Credit: NASA

The moon will also appear squished due to atmospheric refraction. Air is densest right at the horizon and refracts or bends light more strongly than the air immediately above it. Air “lifts” the bottom of the moon – which is closer to the horizon – more than the top, squishing the two halves together into an egg or oval shape.

How we perceive the moon’s size may have much to do with what’s around it. In this illustration, most of us seen the bottom moon as smaller, but they’re both exactly the same size. Crazy, isn’t it? Credit: NASA

You may also be entranced Monday night by the Moon Illusion, where the full moon appears unnaturally large when near the horizon compared to when viewed higher up. No one has come up with a complete explanation for this intriguing aspect of our perception, but the link above offers some interesting hypotheses.

Can you see craters with your naked eye? Yes! Try tonight through Wednesday night. Plato is the trickiest. Credit: Bob King

Finally, full moon is an ideal time to see several lunar craters with the naked eye. They’re not the biggest, but all, except Plato, are surrounded by bright rays of secondary impact craters that expand their size and provide good contrast against the darker lunar “seas”. Try with your eyes alone first, and if you have difficulty, use binoculars to get familiar with the landscape and then try again with your unaided eyes.

In contrast to the other craters, Plato is dark against a bright landscape. This one’s a real challenge – I’ve tried for years but still haven’t convinced myself of seeing it. The others are easier than you’d think. Good luck and clear skies!

4 Responses

  1. Edward M. Boll

    This seems like an early Harvest Moon. Next year’s Sept. 27, will be a total solar eclipse. Comets Jacques and Oukaimeden are winding down, with Jacques fading and my guess is only about 1 or 2 more mornings to see the other one till it drops into the pre sunrise glare.

    1. astrobob

      Edward,
      Indeed it is. I did catch Oukaimeden once last time over the weekend VERY low near Alpha Mon. Still easy to see with a short tail. It will be gone for us soon. Meanwhile, PANSTARRS K1 has returned. In two weeks it should be easy (though low) in twilight.

  2. Giorgio Rizzarelli

    Yesterday moonrise was excellent, but when Moon rose high a and I saw the exceptional moonlight I remembered this was the 3rd of this year’s 3 supermoons in a row. I could finally really enjoy a supermoon – after the two months (July and August) of unusual cloudy weather of this zone of Europe we finally had some Sun days. Yesterday there were just some scattered clouds, giving some iridescence as ice on the cake.

    Speaking about the Sun, it’s active. AR2158 released an M4 flare and, thanx also the more earth directed position, the central spot in the group has fattened to a diameter of 1.2% of the solar disc: it should now be definitely visible without optics using goggles for eclipse or other artificial filtering.

Comments are closed.