It is remarkable that the Sun and the Moon are roughly the same apparent size so that in most solar eclipses – but not all – the whole solar disk is blocked by the Moon. However, the lunar disk is not a perfect circle, as the moon is not a smooth sphere, and this creates a peculiar effect just minutes before totality.

A series of bright spots will form just off the limb of the Moon. Due to mountains and craters on the Moon, the final slither of the Sun disk will appear fractured in a string of bright spots. DO NOT TRY TO OBSERVE THEM WITHOUT PROPER EYE OR CAMERA PROTECTION. We won’t even apologize for shouting. Eye safety is paramount during eclipses.

The beads are still a portion of the surface of the Sun and they are as bright as the Sun is usually. This is why you shouldn’t look at them without protection. As totality approaches (when it is safe to take off protective glasses), the beads will start disappearing until only one or two are left, producing the spectacular and well-known diamond effect.

The diamond and then the beads will reappear just after the end of totality; be ready to see them on the other side of the Moon as our satellite moves away from the Sun in the sky. Since the geography and the motion of the Moon are well known, it is possible to work out exactly where the beads will appear.

“This phenomenon has very precise parameters,” explains NASA. “[T]he geometry of the observer-Moon-Sun coincidence, and its exact time on sub-second timing scales. The parameters differ systematically for observers located at different places and times within the path of totality.”

So, NASA has developed an app that can focus on the beads at the right time, exactly where they will appear – and they are asking people across the path of totality to download it and let it do its thing. The project, called SunSketcher, will automatically take photos when it needs to and allow astronomers to better understand the Sun, using the light from the beads pre and post-totality.

While the path of totality will stretch from the west coast of Mexico to Newfoundland in Canada, the app is set up only for the US. Still, tens of millions of people will be able to see it there. Just be safe! You can download the SunSketcher app for iPhone, with an Android version coming soon.