Transit of Venus in front of the Sun, last time this century. As with a solar eclipse, looking directly at the Sun is dangerous - I am not sure whether there would be much to see, anyway, but specially designed, extremely dark glasses might be used. I am not sure whether welder's glasses are dark enough. And looking through a telescope or binoculars is VERY dangerous - the concentrated light is enough to start a fire. However, if you do have a telescope, an image can be projected on a piece of paper.
Difference in opinions hath cost many millions of lives: for instance, whether flesh be bread, or bread be flesh; whether whistling be a vice or a virtue; whether it be better to kiss a post, or throw it into the fire... (from Gulliver's Travels)
