Totally eclipsed moon (image by Doug Murray; courtesy Science@NASA and NASA's Goddard Space Flight Center)
According to American folklore, the full moon that appears closest to the autumnal equinox is called the Harvest Moon. This year, the autumnal equinox (also called the September equinox, recognizing that folks in the southern hemisphere will be marking the beginning of spring instead of autumn) will occur early in the morning on September 23. The full moon closest to that date will occur on September 28 at 02:50 UT (Universal Time). In time zones across the United States this moment will happen before midnight, so it will still be September 27.
The Harvest Moon this year will also be a perigee moon, a circumstance the media has dubbed "Super Moon." Although the moon orbits the earth at a distance of approximately 238,000 miles, its slightly elliptical orbit sometimes takes it out to a distance of more than 250,000 miles and also brings it as close as 225,000 miles. The most distant point is called the apogee, and the nearest point is called the perigee. On September 28 (or 27 depending on your time zone), the moon will reach perigee at 01:47 UT, just an hour before full. NASA has determined that a full moon at perigee appears fourteen percent bigger and thirty percent brighter than a full moon at apogee. Despite popular media excitement over super moons, however, the size difference isn't really enough for casual observers to note because there are no handy measuring tools in the sky to use for reference. Nevertheless, lunar observers can still enjoy knowing that this year's Harvest Moon is also a "super" moon.
On the other hand, this Harvest Moon promises to put on a display that many lunar observers will be able to notice with no trouble at all—a total lunar eclipse. A total lunar eclipse occurs when the sunlight that would normally reflect off the lunar surface is completely blocked by the darkest part of the earth's shadow (called the umbra). When this bright light is blocked, a coppery orange hue emerges. This coloration is created when sunlight passes through parts of the earth's atmosphere and is bent toward the moon. In essence, the color is a result of all the sunrises and sunsets occurring around the globe while the eclipse is in progress. Because of the color change, headlines often refer to an eclipsed moon as a "blood moon."
A total lunar eclipse occurs when the moon passes fully into the darkest part of the earth's shadow, the umbra. (Image courtesy NASA Lunar Eclipse Group)
Unlike solar eclipses, which require special equipment for safe viewing, lunar eclipses do not pose optical hazards. Just look up and enjoy! To help you plan your viewing, here are some times for eclipse events. These are based on Eastern Daylight Time, which is four hours behind UT. If you are in a different time zone, you'll need to adjust accordingly. If you're in the western half of the United States or Canada, the moon will already be in an eclipsed state when it rises:
Sunday night, September 27
9:07 p.m. EDT: The moon begins to enter the umbra
10:11 p.m. EDT: The moon is fully within the umbra
10:47 p.m. EDT: Time of greatest eclipse
11:23 p.m. EDT: The moon begins to emerge from the umbra
After midnight, during the first hour of Monday, September 28
12:27 a.m. EDT: The moon will have fully exited the umbra
For more information about the moon from NASA, visit these web pages:
- September 27, 2015 Total Lunar Eclipse: Shadow View provides additional details about the eclipse.
- For technical details, see Total Lunar Eclipse of 2015 Sep 28
- This article from NASA Science explains super moons, Super Full Moon
- To see the moon phase for any date and time this year, check out Moon Phase and Libration, 2015.
This article originally appeared in the September 2015 issue of Observations: A Pier Press® Newsletter. You can access the archived issue here, and if you're not already a subscriber, click here to sign up. It's free.