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Wednesday, 6 November 2019

Mercury Transit: A rare astronomical event to observe on November 11, 2019

The year 2019 has been rich in observable astronomical phenomena, and it will end on November 11 with a relatively infrequent event: the transit of Mercury. The first planet in the Solar System will travel on a path through the solar disk. The event will be visible from most of the Earth's surface, with the appropriate equipment.

Such a sight is relatively rare seen from Earth. From our point of view, only the transits of Mercury and Venus are visible. This event will be the fourth of the 14 Mercury transits that will take place in the 21st century. In contrast, Venus transits occur in pairs, with each pair spaced more than a century apart.
Mercury will take about 5.5 hours to travel in front of the Sun. Transit will be widely visible from most of the Earth, including from the Americas, the Atlantic and Pacific Oceans, New Zealand, Europe, Africa and West Asia. However, it will not be visible in Central and East Asia, Japan, Indonesia and Australia.

On May 9, 2016, Mercury made a spectacular transit, many astronomers having observed its passage in front of the Sun. The transit of 11 November should be identical. Credits: NASA's Goddard Space Flight Center / SDO / Genna Duberstein

A transit in several stages visible from the Earth

Transit begins before sunrise for observers in western North America. The transit ends after sunset for Europe, Africa, West Asia and the Middle East. The entire transit will be visible from end to end over eastern North America, Central and South America, southern Greenland and a small part of West Africa.

In France , it will be possible to observe the beginning of the passage and the maximum (minimum distance between the center of Mercury and the center of the Sun). However, the rest of the passage, and its end, will not be observable because the Sun will be lying down.

Times of observation of the transit of Mercury according to the different time zones. In Paris (UTC + 1), the hours of passage are those you see under UTC, adding 1h (for example 14h35 for Contact 1). Credits:

The first contact takes place when the disk of Mercury touches for the first time the eastern edge of the Sun. It takes about two minutes for the Mercury disk to move completely to the Sun's disk (second touch). The most important transit occurs when Mercury appears closest to the center of the Sun. The third contact is when the leading edge of Mercury reaches the western edge of the Sun. Two minutes later, Mercury completely leaves the solar disk (fourth touch).

Observation of the transit of Mercury: precautions are necessary

Mercury will appear as a black dot, representing only about 0.5% of the Sun's diameter. It will take a magnifying telescope at least 50 times to see it.

Special precautions should be taken when viewing the solar disk. Be careful never to look directly at the sun with a telescope. The visual requirements are the same as those for observing sunspots and partial solar eclipses - you need to use special sunscreens to protect your eyes.

It is much safer to project the image of the Sun using a telescope on a white card or a screen. If you use a telescope with a large aperture, for example 20 centimeters or more, place a circular mask in front of the lens or mirror to fix the image, reducing the amount of light and heat striking the lens or mirror.

A different transit depending on the time of the year

Since the orbit of Mercury is inclined 7 degrees from the plane of Earth's orbit, most of the time, when Mercury arrives at a lower conjunction (when it is between the Earth and the Sun), it moves above or below the Sun and does not pass through the solar disk from our terrestrial point of view. But in two points of the orbit of Mercury, it crosses the orbital plane of the Earth (called "node").

Astronomical characteristics of the transit of Mercury of November 11, 2019. Credits: F. Espenak

The Earth crosses the knot line every year on May 8th or 9th, then six months later, on November 10th and 11th. Transit may occur when a lower conjunction of Mercury occurs several days after these dates. When a transit occurs in May, Mercury is near the point of aphelion in its orbit - the furthest point from the Sun and closest to the Earth. If Mercury passes in the center of the Sun in May, the transit time can last nearly 8 hours.

When a transit takes place in November, as it will be this month, Mercury is near the point of perihelion of its orbit - the closest point to the Sun and farthest from the Earth, and where its apparent velocity is faster. As such, a central transit in November lasts only 5.5 hours, which is about what we will see on November 11th. And the number of transits in November is twice as large as the number of transits in May.

Predictability of mercurial transits

Mercury transits do not happen at random. At intervals of 13 and 33 years, Mercury and the Earth return almost simultaneously to the same points in their respective orbits, often resulting in repeated transit after this time interval. So, in connection with the coming transit of November 11, we can go back and find transits that happened 13 years ago, November 8, 2006, and 33 years ago, November 13, 1986.

Interestingly, the transits of May 9, 1970 and November 10, 1973, both fell on a Saturday, while the transits of May 9, 2016 and November 11, 2019 are both on a Monday. The next transit of Mercury will not take place before November 13, 2032.


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