What part of the sun is visible during an eclipse?
It depends upon the type of Eclipse.
During total solar eclipse: Only the edges of the sun is visible during this type of Eclipse. You can only see the light bleed from the edges.
During partial solar eclipse: In this type of eclipse, the visibility varies with the movement of both moon and sun.
Depending upon the type of solar eclipse, different parts of the Sun are visible.
Viewing the Sun, except during the totality phase of a solar eclipse is only possible using reflected images or special filters. The filters are required to cut down light levels to avoid permanent eye damage and damage to equipment.
Using a mylar filter the photosphere and sunspots are visible. Using a Hydrogen alpha filter on a telescope makes the chromosphere and prominences visible.
There are four types of solar eclipse.
A partial eclipse is where the Sun and Moon are not completely aligned and the Moon doesn't cover all of the Sun's disc. The Sun can only be safely viewed through filters. The other types of eclipse have partial phases before and after the central event.
A total solar eclipse is where the Moon's disc is larger than the Sun's disc and can completely cover it. Only during the total phase is it safe to view with the naked eye. During totality the Sun's corona is visible. It is a spectacular sight indeed. A total eclipse can last over 6 minutes.
For a few seconds just before and after totality, it is possible to see the chromosphere and prominences.
An annular eclipse is where the Moon is too distant to completely cover the Sun's disc. It is possible to see the chromosphere and prominences.
A hybrid eclipse is where the Moon's disc is about the same size as the Sun's disc. Hybrid eclipses are often annular when viewed from some places and total when viewed from others. Totality for a hybrid eclipse is usually less than a minute, but the chromosphere and prominences can be visible.
The images were taken at the 2010 eclipse. The corona is visible. Also, the red chromosphere is just visible on the left of the image, and prominences to the top and bottom of the second image.