Full Hunter’s Moon Horizon Hijinks

A previous October full moon rises orange and distorted over Lake Superior in Duluth, Minn. Bob King

With the approach of the firearms deer hunting season it won’t be long before blaze orange becomes the rage in my state. The full Hunter’s Moon will rise a less lurid hue tonight around sunset and stay up all night. The name comes from the time when hunters stalked their prey by moonlight. If your prey is the moon itself then check the moonrise and sunset times for your location and find a place with a great view to the east to watch tonight’s big, juicy moonrise.

There’s always so much to see at moonrise, including the distorted shape of the lunar disk caused by the bending of light by the denser, thicker air near the horizon and changes in color. Typically, the full moon rises a deep orange or pinkish-orange because the blues, purples and greens are scattered away by that thick air again.

The stepped appearance in the bottom quarter of the moon reveals horizontal layering in the lower atmosphere. These and other effects are best seen at and shortly after moonrise. Bob King

We don’t often think about it, but when we gaze toward the horizon we’re looking through many miles of air at the very bottom of the atmosphere, where the density of air is greatest.  Looking higher, our line of sight cuts through less air and overhead, the least. The amount of air directly overhead is called 1 air mass. For a star 30° or three fists above the horizon, you’re looking through 2 air masses. At 10°, 5.6 air masses, and for an object on the horizon, 40 air masses.

As the moon rises, its top shines through significantly less air than its bottom. Air can bend light much like a lens in a telescope. The thicker or denser the air, the more the bending. Air at the horizon bends the light of celestial objects upward to the tune of a half-degree. If you’ve ever seen the sun rise over a true, flat, cloudless horizon, you saw a mirage caused by dense air “lifting” the sun into view shortly before its true rising.

When light passes from one medium to another — water to air or air to water — or from denser air to thinner air, it gets bent or refracted. The “broken” pencil helps us picture why the moon appears distorted at moonrise and set. Imagine the water as the thickest layer of air right on the horizon. Bob King

Anyway, air at the horizon bends the light from the bottom portion of the moon more than air at its top, squishing the bottom into the top to create an egg-shaped moon. The effect doesn’t last. Within a half-hour, as the moon rises higher, the difference in air density between the lower and upper parts of the moon becomes insignificant. The moon’s proper round shape returns and its color pales as we view it through much thinner air. Thin air scatters less but it still scatters away some blue light, giving both the sun and moon ever-so-slight yellowish casts.

If possible bring binoculars to watch moonrises — you’ll see all these and other fascinating distortions as the ever fluid and changeable atmosphere washes this way and that over our hapless satellite.

I should mention that tonight is not officially full moon. That happens tomorrow at 11:45 a.m. (Central Time), but it will appear just shy of full this evening and just past full tomorrow night (Weds. Oct. 24). So, two fulls moons in a row … sort of!

Compare this moonrise to the first photo. This was taken closer to sunset in hazy air. The moon was faint! Bob King

The brightness of the moon around moonrise depends on where the sun is in the sky. If a full moon rises just before, at or slightly after sunset, it’s appears pale and won’t be easy to spot unless you know just where to look. That’s because the sky is still lit up and the moon’s light is dimmed as it traverses not only thick air but air laden with natural and human-made pollutants like dust, soot and salt. But if it rises a little later, about 15 minutes or more after sunset, it comes up bright. The sky is darker then, so the contrast between moon and sky is much greater, making the moon appear so much brighter in comparison.

What does mean for the Hunter’s Moon? Tonight’s moon will be weaker and little harder to see compared to tomorrow night’s which rises a half-hour later. Hope you have clear skies and good weather.

3 Responses

  1. Mat Drummen

    Dear Bob King,

    I live in the Netherlands. I’m a writer on amateur astronomy. I would like to review your new book: Wonders of the Night Sky. Could you send me a printed copy? The review will be published in the astronomical magazine Zenit (in Dutch). I’ll send you a pdf of this review.

    Mat

    1. astrobob

      Hi Mat,
      Thank you for offering to review my book. Very kind of you. I will check with my publisher to see if they can send you a copy. Do you have an address?

  2. Address is the 1st sentence, Bob. The Netherlands, it’s the alternate spelling of The Underworld, ruled by Pluto, not to be confused with Ploutos, the god of wealth.
    so just send the book, to Pluto, The Netherlands, C/O Met Drummen, you know it’ll get there ;p

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