Mercury to appear as tiny black dot in rare 'transit' across sun

Yolanda Curtis
November 10, 2019

By the time the Sun rises on the West Coast, Mercury will have been transiting the Sun for almost two hours.

Mercury's orbit around the Sun takes 88 days to complete and it passes between the Earth and Sun every 116 days. This year, part of the 5.5-hour transit will be visible to much of North America starting at 12:36 p.m. UTC.

Slated to start at about 6.35 a.m. Observers in eastern Asia, southern and south-eastern Asia, and Australia will not be able to see the transit.


How can you see it? This could be done by having observers at distant points on Earth look at the variation in a planet's apparent position against the disk of the Sun - a phenomenon known as parallax shift.

WARNING! Looking at the Sun directly or through a telescope without proper protection can lead to serious and permanent vision damage. If you don't have those tools, check if a local astronomy club will be hosting a public viewing. Instead, projections using protectively covered telescopes could work, according to Space.com. From our perspective on Earth, we only ever see two planets transit the Sun: Mercury and Venus.

"Transits are a visible demonstration of how the planets move around the sun, and everyone with access to the right equipment should take a look", Mike Cruise, president of the Royal Astronomical Society, said in a statement from England. Transits of Venus occur in pairs separated by 121.5 or 105.5 years, with eight years separating each transit; the next transit of Venus will occur in the year 2117.


People in Greece will be able to witness a rare celestial show on Monday as Mercury passes between the sun and Earth, a phenomenon that is not expected to occur again until 2032. A transit happens when a planet crosses in front of a star.

Edmund Halley used a transit of Venus in 1761 and 1769 to determine the absolute distance to the Sun. More recently, one team of scientists used a transit to see whether it would produce a discernible decrease in the amount of sunlight (it did not). 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. But remember, don't look directly at Mr. Sun.


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