Final answer:
Holding a coin to create an artificial eclipse does not work because it cannot sufficiently block out the Sun's bright photosphere, which overpowers the faint light from the chromosphere and corona.
Step-by-step explanation:
The reason why you cannot simply hold a coin in front of you to cover the solar disk and create an artificial eclipse to study the chromosphere is that the coin does not block out the Sun's overwhelmingly bright surrounding light, known as the photosphere. During a total solar eclipse, the Moon covers the Sun completely, blocking this intense light and allowing the fainter structures of the Sun, like the chromosphere and corona, to be visible. A coin held at arm's length is too close and too small to create the necessary darkness and shadow that would suppress the bright photosphere and reveal the chromosphere.
Astronomers previously relied on natural eclipses because the Sun is an extremely bright source of light. Without the darkness created by a total eclipse, the faint emissions from the Sun's outer layers are outshone by the brilliance of the photosphere, just as city lights can obscure the view of dimmer stars. It is only during an eclipse that the solar features can be observed from Earth; otherwise, space instruments like coronagraphs are needed to study them.