planets

The last time Jupiter and Saturn came this close was 1623, but that conjunction was too near the sun to be seen on Earth. The most recent visible to humans was 800 years ago.

Mars, Earth and the sun have lined up, a celestial orientation known as opposition. This particular opposition occurs at a time when the orbit of Mars takes it closest to the sun.

Stargazers, ready your (solar-filtered) telescopes: Mercury is passing directly across the sun Monday for more than seven hours. And you can watch — if you do it just right.