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  • Rapt in Awe

    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

Events in February 2012: Look West! Can you see the faintest – and the brightest?

Looking west this month after sunset offers a study in contrast. For one thing, the two brightest planets – in fact, the two brightest objects in our skies after the Sun and Moon – Venus and Jupiter, will be drawing together night-by-night until at the end of the month you could nearly cover the pair with your fist.

Why? Because in a very real sense we live in two worlds. One is the world of the ancients. The world they saw and we still see. In it the planets are wandering stars that  change positions in irregular patterns, while the fixed stars change position in our sky – but hold their positions relative to one another.

That is one world. It is an obvious world, but one many of us have lost touch with because our artificial light and homes hide the night sky from us.  The other world is the one revealed by the past four centuries of science.  In that world the planets are nearby, solid bodies, shining by reflected light and all revolving around a single star – our Sun. How they appear to us is governed by the laws uncovered by the work of Copernicus, Galileo, Kepler, and Newton.

To me the  fun is to live in both worlds – to appreciate the night sky as the ancients saw it – and at the same time to appreciate the night sky as revealed by the great minds of science.  And this February gives us a perfect opportunity to do so. For what really fascinates me is that the Jupiter and Venus show is the brightest part of two western light shows, one involving the largest object in our solar system after the Sun – Jupiter – and the other some of the smallest things we will see – the zillions of dust specs (roughly one millimeter in size) – that make up the Zodaical Light.

So when you are through being dazzled by photons reflecting off of huge bunches of stuff, relax, and see if you can find one of the most delicate reflection features of our solar system – in fact, one of the more subtle things you’ll ever see in the night sky – the Zodaical Light. It, too, is at its prime this month, but you need a genuinely dark sky, especially to the west, to see it.

Much more about that in a moment – but first the easy shot that goes on all month – the Jupiter and Venus Show!

This one you can’t miss, even if you live in a region where most of the stars are washed out by local light pollution. There’s a wonderful symmetry to this show and it’s so simple to see. Just go out about 45 minutes after sunset any night in February and look up in the southwest. Roughly overhead the brightest “star” you will see is Jupiter. And high in the west at this hour will be an even brighter “star,” Venus.

Venus is about six times brighter than Jupiter, but this difference may not be quite that obvious because Jupiter will be seen high in the sky where it has to get through less of our atmosphere – and the sky will be darker near Jupiter. Venus will still be basking against a twilight background.

There are subtle changes in the brightness of each planet as the month goes on. Venus starts the month at magnitude – 4.1. (Remember, the lower the magnitude number, the brighter the object.) By the end of the month it is magnitude – 4.3. Why? Well, in our travels – and it – about the Sun we get closer to it. At the start of the month we’re about 102,858,000 miles apart. At the end of the month this gap has closed to 85,002,000 miles.

That’s really significantly closer,so you might think the change in brightness would be even greater. However, as we draw closer to Venus, Venus is also inserting itself between us and the Sun and so we see less of it – that is, it goes through phases like our Moon and at the start of the month we’re seeing light reflected from 71% of its surface, while at the end of the month we’re seeing just 64% of its surface. (I’m indebted, by the way, to Sky and Telescope magazine which publishes all this data about the planets each month.)

Jupiter, since it’s orbit is well beyond us, doesn’t go through these dramatic phases. We see 99% of it at the start of the month and 99% at the end. But we are drawing apart – essentially the Earth in it’s much smaller orbit is quickly widening the gap between us and Jupiter – and that gap is much larger than the one between us and Venus, so even though Jupiter is much larger, it is so far away, it is still dimmer.

At the start of the month Jupiter shines at magnitude -2.3 and at the end of the month it is magnitude -2.2. That’s a change that will only be noticeable to those with lots of experience at evaluating brightness – to most of us, it will look the same. But in terms of distance Jupiter starts the month about 468,162,000 miles from us and by the end of the month this gap is 507,036,000 miles.

In other words Jupiter is nearly 40 million miles farther away, yet dims in light by just one-tenth of a magnitude.

I know these numbers might just be bouncing off your mind with a sense of irrelevancy, but they fascinate me simply because they reveal some of the inner gurglings of what are the constantly changing dynamics of our solar system – dynamics that result in us seeing in the night sky two very bright lights appearing to approach one another night-by-night.

Think of how that must have looked to people in other times when we didn’t spend so much time indoors, dazzled by artificial lights and we knew nothing about what these bright lights in the night really were, nor how they’re relationships changed as each moved about the Sun at different speeds in orbits of vastly different lengths.

And with that in mind let’s switch gears now and consider some very tiny objects that are orbiting the Sun as well and displaying a soft glow in our western sky as they do so.

Basking in the Zodiacal Light of Almost Spring

The last 10 days or so of February 2011 will be a good time to start looking for the Zodiacal Light.

Planets shine by reflected light, planets are found in the plane of our solar system – and thus in a certain section of sky, marked by the wide band of the zodiac – and so are these minute dust particles. They’re just hard to get your mind around because they don’t consist of the cloud of dust that is anything like our common experience of dust. That is, these dust particles are minute, but they are also far apart. So to get a picture of them, imagine a bunch of them about half the size of a BB shot and each separated from the other by about five miles! Five miles! That’s what we mean by “cloud” in this case.

But, of course, space is so huge and we’re so far from these dust particles that even with that separation, from our distance they look like a cloud and when the sun shines on them, this cloud creates a soft glow in our western sky.

You don’t need a totally clear horizon to see the zodiacal light, or binoculars, but you do need total darkness and that means little-to-no light pollution and no Moon. So you want to wait until a few days after full Moon to begin this quest. I feel I have a good shot at it from my favorite ocean-front observing point where I have a clear horizon to the west with no cities to create light domes there. Evenings in February and March – and mornings in September and October – are the best time for folks at mid-northern latitudes to look for this.

The zodiacal light is roughly the same intensity as the Milky Way, so if you can see the Milky Way from your chosen location, then you should be able to pick up this faint glow. Like the Milky Way, it stretches over a good deal of sky. It is widest near the horizon and gets narrower as it rises towards the zenith. You want to look for this roughly 80 minutes after sunset. You can check for an exact time for your location by getting information from here on when astronomical twilight ends. (The drop-down menu on that page specifies the times for astronomical twilight.) As astronomical twilight ends you want to start looking. As with any faint object, your eyes need to be dark adapted, so I am assuming you have been out for at least 15 minutes with no white light to dazzle you. If you try to look for this earlier, you may confuse it with twilight. Much later and it is not as bright, since what we are seeing is sunlight reflecting off interplanetary dust particles – dust particles that orbit in the same plane as the planets – the area we call the zodiac – and thus the name for this phenomena, zodiacal light.

If you see it, reflect on this more detailed explanation from Wikipedia:

The material producing the zodiacal light is located in a lens-shaped volume of space centered on the Sun and extending well out beyond the orbit of Earth. This material is known as the interplanetary dust cloud. Since most of the material is located near the plane of the Solar System, the zodiacal light is seen along the ecliptic. The amount of material needed to produce the observed zodiacal light is amazingly small. If it were in the form of 1 mm particles, each with the same albedo (reflecting power) as Earth’s Moon, each particle would be 8 km from its neighbors.

For the metrically-challenged (that includes me) that means about one dust particle every five miles! And that causes all that light?! Awesome!

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