Hello!

When another planet passes between the Sun and the Earth, we call it a transit. Transits are very different from the solar eclipses we see on Earth. During a total solar eclipse, the Moon's shadow completely blocks out the Sun because it's apparent size in the sky is about the same as the Sun's. From Earth, we can see transits of Mercury and Venus across the Sun. Because Mercury and Venus are very far away, they appear small in the sky. During a transit of the Sun, Mercury or Venus will appear as a tiny, black dot moving across the Sun. Their apparent sizes in the sky are not large enough to block out the entire disk of the Sun. Because Mercury and Venus are so small compared to the Sun, you need to use a special solar telescope or a telescope fitted with a special solar filter to observe transits. An alternate way to observe transits of Venus or Mercury is to use a telescope or binoculars to project an image of the Sun onto a screen. You should never look directly at the Sun through a telescope unless it has a special solar filter. I have not seen a Mercury transit, but a few years ago I did see a Venus transit by using binoculars to project an image onto a screen.

Kris