On November 11th, 2019, between 12:35 and 18:04 universal time (UTC), Mercury, the innermost planet, will pass in front of the Sun as seen from Earth: an astronomical spectacle called a “planetary transit”. Planetary transits visible from Earth are relatively rare events: only the inner planets Mercury and Venus can ever pass between the Sun and Earth, and they are only seen to cross its disc when the plane of the planet’s orbit intersects the plane of the Earth’s orbit (the ecliptic) close to the time when the planet is at inferior conjunction with the Sun. On most inferior conjunctions, the orbital planes do not align (or, in other words, are not close to a node crossing) and the planet “misses” the Sun, passing above or below it as seen from Earth.
Mercury’s orbit crosses the ecliptic around May 8 and November 11 at the present epoch, and so transits always occur within a few days of those dates. The most recent transit of Mercury was on May 9th, 2016 (when, despite being clouded out for most of the event, I managed to briefly observe and photograph it through thin clouds), and the next transit will not occur until November 13th, 2032, so if you miss this one, you’ll have a thirteen year wait until the next opportunity.... [Read More]