Thursday, March 3, 2011

Zodiacal Light

Seven years ago I observed the zodiacal light, which appears as a faint triangular glow from the horizon up along the ecliptic. This phenomenon is visible only at dark locations; I observed it from eastern Arizona where I could look out over New Mexico -- best dark sky I've ever seen. Here is a photograph of my color slide I took at the time (a digital photo of a film original):
Zodaical light; the bright star in the middle is Jupiter

The graininess in the photo is from the fast speed film I used (200 ISO), not knowing at the time that faint phenomena requiring minute or longer exposures appear best on slow-speed, low-ISO film. Though graininess shows as noise in digital photos at high ISO settings, a good digital camera doesn't have the film equivalent of reciprocity failure -- the tendancy for high ISO film to quickly lose light-sensitivity during a long exposure.

With the approach of March, northern hemisphere observers are reminded to get their best views of the zodiacal light by watching for it in the west 90 minutes after sunset. As my only observation was near the autumnal equinox while looking east, I wanted to determine if both morning and evening offer similar views at this time of year. (Short answer: they don't.)

Below I sketched the viewpoint from behind Earth, looking toward the sun at this time of year:

I drew the Earth large enough to show the latitudes but too small to appreciate that any observers horizon is perceieved as flat, so I added the dotted lines. And the viewpoint is slightly above the Earth-Sun plane to provide better context of the locations of the sun, earth and brightest zone of zodiacal light. Notice Earth's tilt and how the same observer at evening will have a different orientation to the ecliptic (zodiacal light) in the morning.

In the next illustration, I centered Earth so that it covers the sun. You can see how angle of the observer's horizon in respect to the ecliptic changes from evening to morning:

Though you can see how the angle to the ecliptic changes as we rotate on our tilted world, the scale may give a false impression that we see less of the zodiacal light's brighter center  in the morning. More likely, we see the phenomena lower, along the horizon, where it's harder to distinguish from haze and skyglow.

The following diagrams strike a compromise between Earth perceived as a  flat horizon and the circle that shows our latitude.
Imagine this view at sunset:

And the 45  minutes later:

As the Earth turns, the horizon blocks ever greater amounts of the skyglow, till about 90 minutes after sunset, the skyglow is effectively blocked while enough of the bright center of the zodiacal light is still visible:

My guess at what the same bleary-eyed observer (in the northern hemisphere) would see in the morning sky is this:

Such a faint effect is easily lost that close the horizon.

So, for northern hemisphere observers, the best views are after sunset looking west near the vernal equinox, or before sunrise looking east near the autumnal equinox.
For southern hemisphere observers it's reversed: best in the morning looking east near the vernal equinox and in the evening looking west near the autumnal eqninox.

Which gives me my next problem. since Los Angeles obscures the night sky to the west of the most likely darksky observing locations I have, I'll wait for Autumn to get some eastward, morning views.

And last, by accident I filled my photo with black, but before undoing, I saved a copy, as I like how the accident accentuated the zodiacal light:


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