All week, in honor of the summer solstice, I’ve been writing about the sun and about astronomy. Why? Because my son unleashed a torrent of questions, once we started talking about Monday having been the longest day of the year. So far I’ve answered questions about why the sun doesn’t melt, what the hottest star is, and what would happen if the sun turned into a black hole. So now, we’re on to the final question:
“Is the Earth bigger than the Sun?”
This one made me chuckle, just a little. “No,” I told him, “the sun is lots bigger.”
“Well,” he said thoughtfully, “my friend says the Earth is bigger.”
“It isn’t,” I assured him. “It just looks bigger, because we stand on the earth and the sun is really far away.”
He thought about that for a minute. “But it could be bigger!”
No. It really couldn’t.
This is a clear cut answer. The sun has an equatorial diameter of 1,391,400 kilometers and masses 1.988 x 10^30 kg, while the Earth has an equatorial diameter of 12,756.2 kilometers and masses 5.972 x 10^24 kg. Put another way, the Sun’s diameter is about 109 times that of earth, and it is 333,000 times as massive. There is no way the sun could be smaller than the earth.
So why does it look so small in the sky?
I’ll be honest here: this is not something I ever recall stopping to ask. Not until I started writing this article. Like so many things, I just took it for granted that things that are close look bigger than things that are far away. It never occurred to me to ask “why”, before. (Which is one of the cool things about writing these articles – I end up answering questions I never thought to ask.)
The size of the retinal image varies in inverse proportion to the distance of an object. Near objects appear larger than far objects because they occupy more space on the retina. In the perception of real world stimuli, an object 5 feet away casts an image on the retina twice the size as the same object viewed from 10 feet away.
The object on the left is an eye, and the two stick figures are identical. However, the stick figure closer to the eye occupies a greater percentage of that eye’s field of vision, and so will appear larger. Likewise, a small object held close can appear to be the same size (or even larger) than a large object that is far away.
The sun, on average, is 93 million miles away. As a result, even though it’s more than a hundred times as wide as the Earth, it appears small enough that you could make your own solar eclipse by holding a quarter (or similar coin) at arm’s length in front of it.
Don’t try that, though. You could really damage your eyes.