Mercury will look like a tiny black dot gliding across the Sun's face, according to Nasa.
The transit of Mercury occurs during the inferior conjunction of Mercury, where the centre of three celestial objects (Sun, Earth, and Mercury) will be aligned on a straight line, and Mercury placed in between Sun and Earth.
The smallest planet's eccentric orbit means it doesn't often pass in front of the Sun from Earth's vantage point.
The transit begins at 1235 GMT and ends at 1804 GMT, when Mercury appears to enter on one edge of the Sun and exit on the other. Periodic, fleeting dips of starlight indicate an orbiting planet. It's important to use a Sun filter from a reputable vendor; you'll also need a telescope that is capable of at least a 50x zoom.
Spotting the tiny, swiftly moving planet requires a telescope or binoculars. But the transit isn't just an awe-inspiring spectacle; it has scientific importance, too.
The transit of Mercury is expected to be spotted in Greece shortly after 2.30 p.m. and will last for around five hours, according to experts.
Because of the slight difference in the orbits of Mercury and the Earth, Mercury transits only happen about 13 times per century.
Edmund Halley used a transit of Venus in 1761 and 1769 to determine the absolute distance to the Sun. Because the planet is so tiny and so close to the sun, it doesn't block the sun's light, as the moon does during an eclipse. That knowledge is valuable to astronomers hoping to use the transit method to spot exoplanets around distant stars. If you miss Monday's event, be prepared for a long wait or a long trip. It can lead to serious and permanent vision damage. Do not look directly at the Sun without a certified solar filter.
Venus transits are much rarer.
Mercury's trek across the Sun begins at 4:35 a.m. PST (7:35 a.m. EST), meaning viewers on the East Coast of the USA can experience the entire event, as the Sun will have already risen before the transit begins. Visit the Night Sky Network website to find events near you where amateur astronomers will have viewing opportunities available.
This image of the Mercury transit's final minutes was taken by NASA's Solar Dynamics Observatory on May 9, 2016, with its Atmospheric Imaging Assembly.