Mercury will transit the Sun today between 11:12 GMT (7:12 AM EDT, 12:12 BST) and 18:42 GMT (2:42 PM EDT; 19:42 BST), with NASA TV offering a live feed from its Solar Dynamics Observatory (SDO). The space agency has full details on its coverage of the event here.
The European Space Agency (ESA) will also be catching the action live.
The transit will be visible from Europe, the Americas, Africa and western Asia, but not from eastern Asia, Japan, Indonesia, Australia and New Zealand. Should you wish to eyeball the transit yourself, the British Astronomical Association (BAA) has guidance on the correct kit here.*
We on Earth can only enjoy the sight of Mercury crossing the Sun's fiery face roughly 13 times per century, even though the planet passes between us and the Sun every 116 days in "inferior conjunction". It's elusiveness is because "Mercury’s orbit is inclined at 7° to the Earth-Sun plane, which means it usually passes to one side of the Sun", BAA explains.
BAA elaborates: "Mercury only transits in front of the Sun’s disk when one of these inferior conjunctions coincides with the planet passing through the Earth-Sun plane, which it does on average once every 44 days. These two cycles coincide on average only once every 7-8 years. Each time, a transit can be seen from any location where the Sun is above the horizon at the time."
The next opportunity to catch the phenomenon live will be on 11 November 2019. ®
*Suffice it to say, it's overcast here with light drizzle, so we'll have to make do with a flick through our fave skygazing book while we wait for the online action to kick off:
As owners of this magnificent tome will know, it's "a sumptuously-hardbound coffee table book of 230 full-colour plates celebrating the very best of skygazing disappointments".
The blurb continues: "From crap comets to fog-hidden planetary alignments, this unspectacular guide allows the keen astronomer to fully experience traditional views of heavenly events without the bother of a fruitless trip outside the front door."