The Suns Visible Edge Is An Illusion

tte entire Sun is just a giant incandescent, gaseous ball that seems to extend forever, tte gas is compressed at its center, becoming progressively more tenuous further out. And being entirely gaseous, the Sun has no solid surface and no permanent visible features. tte specification of a "surface" that divides the inside of the Sun from the outside is therefore largely a matter of choice, depending on the wavelength that provides the required perspective.

At the wavelengths we see with our eye, the Sun is a bright, round disk technically known as the photosphere. It is the lowest, densest level of the solar atmosphere, the layer that forms the Sun you can watch moving across the sky each day.

tte gases enveloping the photosphere are so rarefied that we can look right through them, so you can't see anything in front of the photosphere, tte overlying material is invisible except during solar eclipses or with special instruments. Indeed, we use the term atmosphere for this tenuous outer part of the Sun because it is relatively transparent at visible wavelengths.

When we look at the visible disk of the Sun, its edge looks sharp, but that is because our eyes can't resolve the details, tte edges of clouds in the Earth's atmosphere look sharp for the same reason. We can't see features narrower than about one minute of arc, or l/30th of the diameter of the Sun, which means that the unaided eye cannot distinguish anything on the Sun smaller than 43,500 kilometers across or smaller than four times the size of the Earth.

So the sharp visible edge of the Sun is an illusion. We can't resolve its details, and there is an extensive, unseen atmosphere that surrounds it. ttis solar atmosphere, located above the photosphere, is an energized realm of violent change, extreme temperatures and powerful eruptions that can strongly affect the Earth's environment.

tte solar gases that reside just above the photosphere are momentarily seen during a total eclipse of the Sun, when the bright solar disk is blocked out, or eclipsed, by our own Moon (Fig. 6.1). It is encircled by a narrow band of light known as the corona, from the Latin word for "crown." tte corona is

FIG. 6.1 Total eclipse of the Sun A multiple-exposure photograph of a total eclipse of the Sun. The circular form eclipsing the Sun is the Moon. Because the Moon and the Sun have nearly the same angular extent, the Moon blocks out most of the Sun's light during a total solar eclipse. Akira Fujii took this photograph on 16 February 1980.

FIG. 6.1 Total eclipse of the Sun A multiple-exposure photograph of a total eclipse of the Sun. The circular form eclipsing the Sun is the Moon. Because the Moon and the Sun have nearly the same angular extent, the Moon blocks out most of the Sun's light during a total solar eclipse. Akira Fujii took this photograph on 16 February 1980.

visible at any total eclipse of the Sun, and has been recorded for more than a thousand years.

But the corona is only seen for a brief interval during an eclipse, for no more than 8 minutes. And it doesn't happen very often, tte Moon only passes directly between the Sun and the Earth once every year and a half, on average, and even then, it's only visible from a narrow track along the Earth's surface, often in remote locations (Table 6.1). At any given point on our planet, you and your ancestors would have to wait about four centuries to witness a total eclipse of the Sun.

But seeing a total solar eclipse is certainly worth the trip, tte world grows dark, as day turns to night, birds go home to roost, fish rise to the ocean's surface to feed, and bright stars become visible in the sky. It's as if the Sun was taken away, abandoning the Earth. Hence the word eclipse, which comes from a Greek word meaning "abandonment" or "to leave."

TABLE 6.1 Total eclipses of the Sun from 2005 to 2024

Date

Maximum Duration (minutes)

Path of Totality

8 April 2005

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