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Zodiacal Light

by Clark Muir

Have you seen the Zodiacal Light?

Many observers have never seen it. This is unfortunate since it does not require any instruments or imaging equipment. In fact it is easiest seen using the naked eye. Although darker sites are always preferable the zodiacal light can be seen from observing locations a short drive from the KW and Guelph areas.

What is Zodiacal Light?

A very brief explanation of the phenomena is in order. It is simply caused by dust suspended in the inner solar system. Since this dust is concentrated along the ecliptic with the planets then we can expect to see it near this line and within the zodiac constellations. We see it when sunlight reflects off the particles and from an observer's perspective its collective light gives it the appearance of a large white haze or a brightening in the sky.

This seems pretty straight forward but there is an important caveat. The conditions and orientation must be favorable for its detection. Your location on the planet, time of year and time of night are all factors. Other considerations are well known to amateur astronomers. They include things such as the Moon phase and light pollution etc.

Observing Zodiacal Light

Perhaps the first consideration should be the angle of the ecliptic. Since the dust is seen near the Sun it is best observed immediately after evening twilight ends in the western sky or before the start of twilight in the morning eastern sky. We prefer the ecliptic be at a steep angle relative to the horizon. If you happen to live in the tropics this is never a problem. In the tropics the above requirements are always met for the duration of the entire year. At our latitude (40-45 degrees north) the optimum angle is reached in February/March for the evening sky and in September/October for the morning.

There are however, a few reasons that I believe that the evening delivers a better opportunity to see the zodiacal light than the morning. For starters, most of us will find it more convenient to observe in the evening hours than during the pre-dawn. Secondly, in the evening it is easier to determine when twilight ends and therefore know when to start looking for it. In the morning it is more likely that confusion between the zodiacal light and the onset of dawn will occur. The final factor, and this is more precisely an issue with observing in our club (that is within the KW and Guelph area) is light pollution. Most of the clubs observing sites including "favorite" sites preferred by some members lie to the west or northwest of the larger urban centers. This means that the worst part of the sky as far as light pollution is concerned is seen in our eastern sky. This will impact seeing the zodiacal light in the eastern morning sky. It may not make it impossible but it can add confusion.

There seem to be three things that confuse people into not recognizing the zodiacal light. As implied earlier two of them are light pollution and twilight. The third is that some novices have assumed they are seeing the Milky Way. The zodiacal light will appear as a large pyramid shaped swath of light. It will be widest at the base near the horizon and reach a point maybe 45 degrees high (although that can vary widely). This point will be situated at or near the ecliptic. This is why it is best to have the steep angle relative to the horizon. We want this point as high above the horizon as possible. It can be seen for quite a while but will quickly fade as the Sun moves further from the horizon in the evening or in morning as twilight takes over.

From a superb site the zodiacal light can be a remarkable display that rivals the brightness of Milky Way. For the most part our observing sites here will not deliver those results but they can be seen quite routinely if you know when to look for it.

Local Conditions

This year (2012) the best dates will be from mid March (11th or 12th) to close to the end of the month (24th or 25th). February’s dates will be similar. I’d point out that the ideal March dates for observing the zodiacal light coincides quite well with the desired dates for the Messier Marathon. This will probably be the case every year so you even have a convenient reminder for the future.

So, as winter starts to give in to more comfortable observing conditions consider adding this to your observing list bucket. It would make a great target and challenge for those who do astrophotography as well.

Clear skies

Clark Muir