Kepler predicted that Venus' diameter is almost a quarter of the Sun's diameter, so Mercury should have a tenth of the Sun's diameter. Pierre Gassendi saw the true size of Mercury in 1631 during its transit across the Sun, and was surprised by how small it was as it looked like a sunspot
Stay Tuned for next Transit of Mercury on November 12, 2032. Mercury measures a mere 12 arcseconds across, about 150 times smaller than the Sun, making transits of Mercury significantly tricky to observe
