The moon, when seen in the daytime, looks like a small faint cloud; as it approaches the sun it becomes wholly unseen; and an observer tries to see when this unseen object touches the glowing disc of the sun.
"When we look at any other object than the sun, we stimulate our vision.
A good observer will remain in the dark for a short time before he makes a delicate observation on a faint star, and will then throw a cap over his head to keep out strong lights.
"When we look at the sun, we at once try to deaden its light.

We protect our eyes by dark glasses--the less of sunlight we can get the better.

We calculate exactly at what point the moon will touch the sun, and we watch that point only.

The exact second by the chronometer when the figure of the moon touches that of the sun, is always noted.

It is not only valuable for the determination of longitude, but it is a check on our knowledge of the moon's motions.