A Harvest Moon Penumbral Eclipse

Skywatchers in the Eastern Hemisphere will see tonight's full moon skirt through Earth's dusky outer shadow.

The fourth and final eclipse of 2016 would be easy to overlook. It brings up the rear after a dramatic total solar eclipse last March 9th and an annular eclipse on September 1st seen across south-central Africa, Madagascar, and the Indian Ocean.

Sept 16th's penumbral lunar eclipse

On September 16, 2016, skywatchers in the Eastern Hemisphere can watch the full Harvest Moon skirt through Earth's penumbra.
Sky & Telescope diagram / source: eclipsewise.com

Tonight's full Harvest Moon features a "barely there" lunar eclipse. As with the solar eclipses, its viewing area is restricted to the Eastern Hemisphere. Mid-eclipse occurs on September 16th at 18:54 Universal Time. Skywatchers in Europe and Africa will see it this evening, while for those in eastern Asia and Australia it's a predawn event on September 17th.

The Moon will pass deeply into the dusky penumbra of Earth's shadow and barely misses clipping the much darker umbra. Although the entire eclipse lasts 4 hours, geometrically speaking, attentive skywatchers might pick up a subtle, dusky shading on the full Moon's northern half only 30 or 40 minutes on either side of mid-eclipse.

Ajay Talwar will webcast tonight's penumbral lunar eclipse from his home in Gurgaon, India, a program hosted by Slooh.

November 2012's penumbral lunar eclipse

The penumbral lunar eclipse of November 28, 2012. From Quezon City in the Philippines, Raven Yu used identical camera settings to shot the lunar disk at mid-eclipse and afterward the event ended. Note the changes in appearance of the lunar disk, which here is shown north up.

This year's other lunar eclipse, back on March 23rd, was also a penumbral affair. That will also be the case at the next lunar eclipse on February 11, 2017. In fact, there won't be another total lunar eclipse until January 31, 2018.

Meanwhile, tonight's the full Harvest Moon. Traditionally, this is the full Moon that falls closest to the autumnal equinox. The Harvest Moon gets this name from a geometric oddity. Because the lunar orbit makes a shallow angle with respect to the eastern evening horizon at this time of year, the Moon rises only about 30 minutes later on successive evenings, not the usual 50 minutes or so. So it never really gets dark between sunset and moonrise for several successive evenings — a boon to farmers working late to try to harvest their crops at day's end.

6 thoughts on “A Harvest Moon Penumbral Eclipse

  1. misha17

    For years I’ve had problems accepting the “shallow angle” explanation for the Harvest Moon’s small night-to-night change in moonrise. I think it’s more because the Harvest Moon is following the same path as the Springtime Sun. In the Spring, there is big change in the “earliness” of sunrise day-by-day as the Sun path moves it northward. Figure the moon moves about 13 degrees per day, about 13 days’ worth of solar motion. This year’s lunar daily motion would put about it where the sun moves between March 16th and March 29th. Here in Los Angeles the sun rises about 14 minutes earlier on March 29 than it did 13 days earlier on March 29th, These 14 minutes in September would reduce the average 50-minute night-to-night delay in moonrise to a little more than 30 minutes each night.

    By the same token, the sun sets about 14 minutes later on March 29 than it did on March 13 (for Los Angeles). You ADD these 14 minutes to the daily 50-minute delay in ~moonset~ and there is a large night-to-night ~increase~ in ~moonset~ around the time of the Harvest Moon, even though the moon’s path near moonset also makes a shallow path with the horizon.

    Go out near dawn the next few mornings, and again in a month around the time of the Hunter’s Full Moon and watch the Moon set; the moon will seem to “pop up” (or linger) in the western morning sky as it sets over an hour later each day.

    1. misha17

      Oops, that sentence near the end of the 1st paragraph should read, “Here in Los Angeles the sun rises about 14 minutes earlier on March 29 than it did 13 days earlier on March 16th, …”

    2. Kelly BeattyKelly Beatty Post author

      hi, Misha17… yes, you’re on the right track. the “shallow angle” refers to the angle of the ecliptic with respect to the horizon. another way of saying this is (as you suggest) that the Moon’s orbital motion in that part of the sky carries it rapidly northward in declination (about 4° per night, in fact) — it’s the same geometry, just stated in different ways. so, for us northerners, since the Moon is moving rapidly northward, then, at a given right ascension, it’ll come up sooner than it would if positioned farther south. here in Boston around the September equinox, the Moon’s rise times are about 37 minutes later each night — whereas they’re almost an hour later round the March equinox, when that favorable ecliptic geometry becomes unfavorable.

      1. misha17

        Oh, OK, I thought the “shallow angle” explanation for the Harvest Moon effect was that:

        Because of it (the shallow angle) the Moon’s motion did not have as much West-East (Right Ascension) daily motion as other parts of its orbit, so its RA “hourly” position did not change much night-to-night.

        But this would apply to its setting too; and this smaller daily RA hourly change by itself would not explain the large change in Harvest Moon’s setting times from one dawn tot the next.

        1. misha17

          … so it’s actually a combination of both the small daily/nightly change in R.A. (or is it hour angle?) ~and~ the large daily/night change in declination, but I usually don’t read about the factor that the change in declination plays in the Harvest Moon effect.

          1. Kelly BeattyKelly Beatty Post author

            the reduced daily shift in R.A. near the equinoxes is only about 1° (of the usual 13° ). it’s really all about the rapid change in declination, which has a highly leveraged affect on rise time. for comparison, around this equinox, the Moon’s declination changes as much in 1 day as the Sun’s does in ~1½ weeks.

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