How to explain the eclipse to a child: a torch, some balls, a coin and the golden rule

NASA’s site for children puts it in one sentence: a solar eclipse happens when the Moon gets in the way of the Sun’s light and casts its shadow on the Earth. The hard part is understanding how something so small can cover something so big, and for that a torch, some balls and a coin are all you need. Before the games, the golden rule: you only look at the Sun with eclipse glasses, and without them only during totality, if you are inside the path.

The explanation, in four sentences

For a primary-school child this works, told in your own family’s words:

1. The Moon goes round the Earth, and sometimes it passes right in front of the Sun.
2. The Moon is 400 times smaller than the Sun, but it is also 400 times closer to us: that’s why, seen from here, they look the same size and the Moon can cover the Sun completely.
3. Its shadow is small. Anyone inside it sees the Sun completely covered, and around it you only see a bite. On 2 August 2027 the full shadow will pass over 105 municipalities in the south of Spain; in Madrid, 86.1% of the Sun will be covered.
4. It doesn’t happen every month because the Moon’s path is tilted compared with the Earth’s path around the Sun, and it nearly always passes a little above or below it.

If you want a drawing of the shadow, the big one and the small one, it’s in our learn section.

A torch, some balls and a coin

The eclipse kit from CESAR, the education programme of the European Space Agency (ESA), suggests an experiment of about 50 minutes for children aged 6 to 12: a big ball for the Sun, a medium one for the Earth and a small one for the Moon (or cardboard discs), a powerful torch and a tape measure. At home it goes like this:

• Darkened room. The torch is the Sun and lights up the medium ball, the Earth.
• The child moves the small ball, the Moon, between the torch and the Earth until its shadow falls on it. That little dark spot is the path of totality; from the lighter shadow around it, the eclipse is partial.
• Moving the Moon closer or further away changes the size of the spot, and you can see why the total eclipse only reaches a narrow path.

The coin is for the size trick. With an outstretched arm, the child holds it in front of a lamp switched on across the room until it covers it. The coin is much smaller than the lamp, but it is much closer to their eye, and the Moon does exactly the same with the Sun. With a lamp, never with the Sun.

The same kit suggests an outdoor version, with clay balls and sunlight, in which you only look at the shadows. If you want to do it to scale, the Earth has to be 4 times the size of the Moon, and the distance between them has to be 30 times the Earth’s diameter.

A flipbook of the phases

Another activity in the CESAR kit, also for ages 6 to 12, is a flipbook with the sequence of the eclipse’s phases. You make it with ten or twelve identical cards: on each one you draw the yellow Sun and, on top of it, the circle of the Moon a little further along than on the card before, until it covers the Sun completely (on the middle card, the black Sun with the corona around it) and then uncovers it again. Staple them along one side and flick through them quickly with your thumb. If it’s for school, the facts with their sources are in the school project sheet.

A story for the night before

It was a Monday in August and in Lucía’s village it had been hot since early morning. Mid-morning, the Moon, which had spent the whole month going round the Earth, arrived right in front of the Sun. First it took a small bite, which could only be seen with the special glasses. The bite grew, the light turned strange and the air cooled a little. Then the Moon covered the Sun completely. The grown-ups said “now!”, everyone took off their glasses and they saw a black Sun with a white crown and, very close by, a bright dot: it was Venus. After a while a flash appeared, someone shouted “glasses!” and everyone put them back on before carrying on watching.

The story has a catch: it only happens like that inside the path of totality. Outside it, the Sun never goes black (in Bilbao 74.4% is covered) and there is no moment without glasses.

The paper-plate mask

For the youngest, the glasses can be stuck onto a paper plate that works as a mask: NASA and the American Academy of Pediatrics suggest it, and the step-by-step is in eclipse glasses for children. The plate only holds the glasses; what protects is their filter.

The golden rule

NASA’s site for children puts it like this: never look directly at the Sun, not even for a second! NASA makes a single exception, the brief total phase of a total eclipse, when the Moon completely covers the bright face of the Sun. It is the rule of the IGN (Spain’s National Geographic Institute) and the American Astronomical Society (AAS), and the one we follow here: glasses with the EN ISO 12312-2 filter in all the partial phases, and no filter only during totality itself, from the last flash to the first. With children, also one adult for each young child (in Spanish) while the Sun is in view, and it’s the adult who says when the glasses come off and when they go back on. Practising at home and sizes are covered in the family guide (in Spanish).

On Order your glasses you can ask for ours: cardboard, with the EN ISO 12312-2 certificate, along with the phone filter and packs for groups. To check any pair: our glasses guide.

Sources: Eclipses lunares y solares — NASA Space Place (Spanish edition: what a solar eclipse is, why there isn’t one every month and the “not even for a second” rule); ¿Cómo se bloquea completamente el sol en un eclipse? — NASA Space Place (Spanish edition: 400 times smaller and 400 times closer); What Is an Eclipse? — NASA Space Place (the only exception, the total phase); Kit educativo de eclipses — CESAR, ESA (in Spanish: the balls-and-torch experiment, the scale version with sunlight and the flipbook, with their age ranges); eclipseando data (105 municipalities with totality, from the IGN; the percentage covered in Madrid and Bilbao, skyfield + DE440s engine; Venus next to the Sun during totality).