Solar eclipse: We asked the physics teacher what everyone wants to know

O total eclipse of the Sun on Wednesday, August 12, is the first to be observed in our country since 1912 and even though only the inhabitants of a small region in Portugal can observe the phenomenon with a 100% obstruction of the solar disc will not be less deserving of attention because of this.

It turns out that the phenomenon could be seen with an obstruction of the solar disk above 90% throughout the continental territorywhich means you'll probably want to take advantage to go out on the street during the end of the day and enjoy the moment with family and friends.

Given that this is a special moment, we asked Filipe Ferreira da Silva, a professor at the Physics Department of NOVA FCT, to learn more about this phenomenon.

And if he has not yet been able to get suitable glasses to watch the eclipse safely, Filipe Ferreira da Silva teaches a method that can help him.

Filipe Ferreira da Silva © New FCT  

What is a total eclipse of the Sun? Why is it so rare?

A total eclipse of the sun occurs when the Moon completely covers the Sun, and the moon's shadow is projected on Earth. For this to happen, the moon must be between Earth and the sun. The three stars must be perfectly aligned and the moon's shadow must be projected on Earth. The surface zone of the Earth under shadow experiences the total eclipse of the Sun.

What are the main differences between a solar and lunar eclipse? Are there any other types of eclipses besides these two?

The solar eclipse occurs, as mentioned above, when the Moon hides the Sun, that is, when the Moon is in the New Moon phase. When the Earth hides sunlight on the moon, then it's a lunar eclipse. It happens when the Moon is in full moon phase; the Earth is between the Moon and the Sun. There are other eclipses, although not with that name, such as the transit of Venus, when it passes between the Earth and the Sun, or the hiding of Venus by the Moon; when the Moon passes between Earth and Venus, concealing Venus.

Over the centuries, there have been reports of unusual episodes related to eclipses. Anyone you'd like to highlight?

Christopher Columbus used a total eclipse of the moon to deceive Jamaican Indians. Columbus-led vessels were stranded on the island for over a year. The crew depended on the indigenous to feed and survive, however, at some point the indigenous refused to cooperate.

Columbus knew that on March 1 a total eclipse of the Moon would happen and used this information to “mislead” the indigenous. I told them if they didn't cooperate in helping the crew, they'd have a punishment from God, and as proof of that, the moon would disappear. The Indians terrified of the event, they collaborated with the crew of Columbus.

How has this phenomenon contributed to the development of science?

There are several examples of concrete cases in which eclipse observation contributes to scientific development. In the 17th century, a Danish scientist first estimates the speed of light through eclipses of Jupiter by its moons. The estimate was very close to the exact value, with a discrepancy below 10%.

Another example is the verification of Einstein's theory of general relativity. In 1919, by observing the total eclipse of the Sun on the island of Príncipe and Sobral, in Brazil, it was possible to verify the direction of light from stars near the Sun. The fact that eclipses hide all the sunlight allows, for example, the sun atmosphere to be studied.

What advice do you have for an observation of the eclipse on the 12th?

There will certainly be many places with astronomy lovers that help explain the phenomenon. As the phenomenon will happen at the end of the day, between 6:30 p.m. and 8:30 p.m., you should choose a place with good visibility, without mountains or buildings that can hide the sun.

What care should be applied during observation? Is there a risk of using, for example, the cell phone to observe the eclipse?

The observation of the eclipse implies looking directly at the sun. The risk of eye damage is great and can even lead to blindness. The observation shall be carried out with glasses approved by ISO 12312-2. Any other type of equipment not approved by this standard is not recommended. Cell phone cameras, binoculars or other equipment are not safe because they do not block sunlight.

It is increasingly difficult to find suitable glasses to observe the eclipse. Are there home methods you recommend that are safe?

There are homely ways to watch the eclipse safely. An observation tool can be built in a cardboard box. In a shoe box (for example), you should drill a small hole with the help of a needle or nail where sunlight will enter.

Make a second hole on the same face or on the side of the box, slightly larger, where you can look. Point the smallest hole to the sun and look inside the box through the second hole. You'll see a projection of the eclipse inside the box. For better visualization, line the inside of the face opposite the first hole with a sheet of paper.