It’s that time of year again when the weather turns spooky. If you happen to be in DC, the chances are fairly good that you’ll be picking through the autumn foliage for jack-o-lanterns to hang in your home. But the fall foliage also gets a little ghoulish. On Oct. 10, the moon of the full moon will rise in the western sky during the morning twilight before illuminating the landscape with a mixture of light and dark.
One of the planets in the neighborhood will also align itself with the moon as the celestial transit of Mercury traverses the left side of its orbit between Oct. 10 and 13.
And now you know why we call that rare event a hunter’s moon. Here’s a little bit of information on it:
How and when it happens: Hunters’ moons are not uncommon: They occur when the moon’s proximity to the sun, or perigee, causes it to approach closest to Earth on its elliptical orbit around the Earth. For example, on Oct. 12, the full moon will rise at 5:36 a.m. in the east, so the observed moonset will take place at 5:38 a.m. (due to sunlight bending at certain angles due to atmospheric turbulence.)
Observing: By detecting changes in the tilt of the horizon, you can determine the exact moment the moon will rise and set. That’s important, as the proper time for ascension – the time at which the moon rises, so you can tell it to rise and set – will vary among varying degrees of latitude.
How often is it visible? Hunters’ moons can be more visible in the western sky because that’s where the transit of Mercury is taking place. On Oct. 10, at 9:34 a.m. local time, you’ll be able to see the solar system’s second-closest planet up close. The condition will also extend to the conclusion of the transit on Oct. 12, at 12:31 a.m.
When is it considered for viewing purposes? Hunters’ moons can be viewed during the moon phase between its apogee (northernmost point) and apogee-next (northernmost point of the lunar orbit). It should be noted that its late-night visibility comes during the time that the sunset of the full moon is already under way. The 15th, 18th, and 19th fall are its optimal viewing periods, during the waning crescent phase.
The Moon’s last hunting’s moon, happened exactly 36 years ago on Oct. 11, 1982.
Update: Sept. 4, 12:11 p.m. EST: Thanks to the bright sunlight in today’s air, the blood-red color of hunters’ moons was not as noticeable in early September as it was during the occasion in 1982.
For more information on the astronomers, check out the following discussion about Hunters’ moons from the Georgetown Astronomical Society.