A trio of planets
Published on Sep 6, 2008 at 2:22 pm.
1 Comment.
Filed under astronomy, skywatching.
This past Thursday night, I took some students out to observe a grouping of three planets. Those planets are still grouped low in the western sky in the early evenings, so you can go look at them yourself. They’ll be in a slightly different arrangement than shown in this image. I created this image using Starry Night software. Click on the image above for a screen sized view. The planets are very low in the sky, and become visible in the evening twilight. Venus is very bright. At magnitude -3.90, it is the brightest natural thing in the sky other than the Sun and Moon. You won’t miss it. However, the other two planets are tougher to see. From campus, in an urban environment with lots of atmospheric pollution, you could see Venus easily with the naked eye, but it was extremely difficult to see Mercury, the planet that was a bit down and to the left of Venus. However, it was quite easy to pick out Mercury in binoculars. And, above Mercury, as twilight deepened, then we were able to pick out Mars. Mercury wasn’t really all that dim. It was magnitude 0.09. That is actually quite bright. However, it was seen against a twilight lit sky. That made it tougher to see. Also, the lower an object is to the horizon, the dimmer it appears due to scattering of light by the atmosphere. We use the term “atmospheric extinction” to refer to the dimming effect of the atmosphere. And, from our viewing location in an urban environment, atmospheric pollution also made seeing the planet tougher. Mars is several times dimmer at magnitude 1.70. That, too, would be plenty bright enough to see if it were higher in the sky. However, as low to the horizon as it is, I doubt that you’d be able to see it at all without binoculars except from very dark and very clear skies, if then.
But, astute readers unfamiliar with astronomy might wonder how we can see Mars, Venus, and Mercury all clumped up in one part of the sky. After all, Mercury and Venus are closer to the Sun than Earth. So, it makes sense that they should occasionally be seen near one another in the sky. Mars, on the other hand, is farther from the Sun than Earth. So, shouldn’t Mars be seen in a totally different part of the sky? As a matter of fact, Mars often is seen far from where Mercury and Venus appear in the sky. However, for a while here, they will all appear close to one another in the sky. How is that possible?
It turns out that this is very simple to explain. Consider the diagram below. It marks the apparent locations of the Sun, Mars, Mercury, Venus, and the Earth. You can see how we can see these planets all in the same direction even though Mercury and Mars are closer to the Sun than Earth, and the distance between Mars and the Sun is greater than the distance between Earth and the Sun. Mars, it turns out, is on the far side of the Solar System at the moment from Earth.
So, we are looking past Mercury and Venus to see Mars along basically the same line of sight. Mercury is closest in this line, at a distance of 1.05 AU. (See this entry for an explanation for the unit AU.) Venus is farther away at a distance of 1.52 AU. However, Venus appears much brighter because it is larger than Mercury and reflects most of the light that shines upon it. Mercury, itself, only about 11% of the sunlight that shines on it. Mars is much farther away, at a distance of 2.42 AU. That is why it appears much dimmer than normal. It is simply almost as far away from us as you can see it.
So, if you get good clear skies towards the west at sunset, go look for this grouping of planets. But, be aware that it is visible very low to the horizon shortly after sunset. So, that means that you need an unobstructed view of the western horizon. The grouping, itself, is just a shade south of due west. Planetary groupings like this are not terribly rare, but they are not everyday occurrences either. That makes them interesting to observe.
-Astroprof
Sky views created using Starry Night Enthusiast
Astroprof’s Page » Pretty planetary grouping on December 1, 2008 at 3:32 pm: 1
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