It’s interesting to note that with today’s hectic world where most of the people don’t bother do go out and “hang out” in nature, that sometimes – being unfamiliar with the surroundings – they “see things.” Something that might be “ordinary” to astronomers might lead to an “oh wow, what’s that?” moment to someone who is not knowledgeable of the night sky.
Case in point: there have been some beautifully clear nights lately, and with the warm evenings people have actually ventured outside, casting aside the computers and the television. Perhaps it’s to work in the yard; perhaps to take a stroll around the neighborhood. And as the daylight dims through twilight into darkness, the stars come out. Now, if you live in, or close to, a city, you don’t get to see many stars, so if something in the sky catches your eye, especially something bright, it piques the interest.
So I receive a forwarded email from a local television meteorologist, asking me if I had any clue to what someone saw. Apparently the person was outside and saw a “real bright light in the southeast sky, and a ‘much larger’ one in the west. If you know anything about astronomy, you know where this is going.
Yes, someone had stepped outside and saw Jupiter hanging low in the southeast, and when they turned around, there was Venus glowing like a beacon in the western sky. And since these two objects are the brightest things out at night they “pop out” of the twilight sky much earlier than the stars. And they get the attention of the public.
But why did this person see Venus has “much larger?” It’s all in the eyes and the brain. In the same vein as the “moon illusion” but with a twist.
Especially in the night sky, a person’s eyes and brain work together to fool each other into thinking that “brighter is bigger.” This phenomenon is not type-specific to the general public, but also includes members of the astronomical (and other scientific) community.
All you have to do is go out and look at the night sky some evening. You will see a smattering of stars across the canopy of the sky, and you will notice one thing right away: some stars are brighter than others. And even though they are all point light sources, your brain is fooled into thinking that the brighter ones are larger (and sometimes closer). But that is not always the case.
Because of the vast distance involved, stars appear as points of light. All stars are not equal, however: they vary in brightness and size, in addition to being far away. But they are all far enough away to present a pinpoint of light to the human eye. But the combination of distance and brightness lead our brains to think that if a star is brighter, it’s bigger.
So what does that have to do with Jupiter and Venus? Everything. Although Jupiter is much larger than Venus (142,984 km vs. 12,103.6 km), it is much further away from the sun (778,000 km vs. 108,200 km). This also means two other things – Venus is closer to the Earth, and – because it’s closer to the sun – receives a greater intensity of the sunlight. This, combined with the makeup of the Venusian atmosphere, means that more light is reflected from the planet than from Jupiter.
Visually, the apparent diameter of both planets at the current time gives the size advantage (almost double) to Jupiter. So in this case, “size doesn’t matter.” It all has to do with perception. Because Venus is the brightest object in the night sky – apart from the moon – our brain sees it as being larger than anything else up there. Your brain fools your eyes.
Hopefully my explanation to this person (via the meteorologist) will cause them to look up and the night sky more, and even perhaps begin a wondrous astronomical journey through the universe.