Astronomy Picture of the Day

Explanation: An unusual triangle of light is visible this time of year just before dawn, in the northern hemisphere. Once considered a false dawn, this triangle of light is actually Zodiacal Light, light reflected from interplanetary dust particles. The bright reflecting triangle is clearly visible on the right of the above image taken from Laguna Verde near Valparaíso, Chile in late July. The band of our Milky Way Galaxy on the left mirrors the zodiacal band. Zodiacal dust orbits the Sun predominantly in the same plane as the planets: the ecliptic. Zodiacal light is so bright in the north this time of year because the dust band is oriented nearly vertical at sunrise, so that the thick air near the horizon does not block out relatively bright reflecting dust. Zodiacal light is also bright for people in Earth's northern hemisphere in March and April just after sunset. In the southern hemisphere, zodiacal light is most notable after sunset in late summer, and brightest before sunrise in late spring.

Here’s a further explanation from Space.com...

“What it is

It was once thought to be solely an atmospheric phenomenon: perhaps reflected sunlight shining on the very high atmosphere of Earth.

We now know, however, that while the phenomenon indeed involves reflected sunlight, it is being reflected not off our atmosphere, but rather off a nonuniform distribution of space debris left over from the formation of the planets some 4.5 billion years ago.

These countless millions of particles — ranging in size from meter-sized miniasteroids to micron-sized dust grains — seem densest around the immediate vicinity of the Sun, but extend outward, beyond the orbit of Mars and are spread out along the plane of the ecliptic (the path the Sun follows throughout the year).

Hence the reason for the name “zodiacal” light, is because it is usually seen projected against the zodiacal constellations, which were conjured by astronomers and poets to fit the ecliptic.

When to see it

The best time to see the Zodiacal Light is when the ecliptic appears most nearly vertical to your local horizon.

For those in the Northern Hemisphere, this occurs in the western evening sky after sunset from early February to late March. The best morning view in the eastern sky comes from late September into the early part of November.

Conversely, for those who live in the Southern Hemisphere, the best view in the western evening sky comes after sunset from early August to late September, while the best morning view in the eastern sky comes from late March into the early part of May.

Those who live in the tropics or at the equator are luckiest of all, since it has been said that the Zodiacal Light is bright and very conspicuous from these regions. This is probably because the ecliptic is always favorably oriented there, allowing views of the Zodiacal Light both in the western evening sky and eastern morning sky all year long.

What it looks like

To the discerning eye, its diffuse shape resembles almost a tilted cone, wedge or slanted pyramid. At the base of the cone, the light may extend some 20 to 30 degrees along the horizon (a fist on your outstretched arm covers about 10 degrees of sky).

At its best, the display can approach or even equal the Milky Way in brightness, yet it is typically so faint that even a small amount of atmospheric haze can obscure it. On exceptionally clear nights, the tapering cone might be seen to stretch more than halfway to the zenith, an imaginary point in the sky directly overhead from wherever you are.

Finer points to look for

Should you be blessed with optimum sky conditions — absolutely no artificial lighting, smoke or haze — you should also try to see the Zodiacal Band, which runs along the entire ecliptic and usually averages about 5 to 10 degrees in apparent width.

Also difficult to see, though actually perhaps a trifle brighter than the Zodiacal Band, is the “counterglow” or Gegenschein. This is a very faint oval patch of light about 10 to 20 degrees long and 6 to 8 degrees wide (overall, comparable in size to the Great Square of Pegasus) and situated exactly on the ecliptic at that point diametrically opposite to the Sun in the sky. If the Sun has just dipped below the western horizon, for example, the counterglow would be just above the eastern horizon.

The counterglow is also caused by material wafting through space, but this stuff is beyond the orbit of Earth. It may appear ever-so-slightly brighter than the Zodiacal Band because the miniasteroids and meteoroids that reflect the light are on the exact opposite side of the Sun, so individually theyre illuminated in much the same manner as the Moon at full phase. The maximum possible return of light to the Earth results, producing a concentrated glow at that particular portion of the band.

To see the Gegenschein with certainty is no small achievement. Not only does it require absolutely black skies, but unusual perception and visual acuity. Moreover, if it occurs anywhere in or near the Milky Way, it will be hopelessly lost in its light.

Fortunately this week it is located near the constellation of Aries, the Ram, which is well to the south of the Milky Way. The Gegenschein should be centered about 10 to 15 degrees below and to the left of the second-magnitude star, Hamal, in a rather star-poor region of the sky.

Because of its extreme faintness, your best chance of glimpsing it is to use averted vision. Try this: look directly toward that spot in the sky where the Gegenschein should be, then turn your eyes slowly to one side. Slowly returning your eyes to the spot, you just “might” be able to discern this large albeit exceedingly faint hazy patch.

Good Luck! (Youll need it.)”

http://www.space.com/spacewatch/zodiacal_light_021101.html

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