Roughly 13–14 times each century, Mercury passes directly in front of the Sun, casting a small black silhouette in front of the solar disk for a few hours. Such a transit will occur on 9 November 1802, lasting from 01:17 until 06:46 LMT.

Between those times, the transit will be visible from anywhere on Earth, providing the Sun is above the horizon, including from Antarctica, Asia, Africa, south-western Russia, Europe, Australia and South America.

Mercury last transited the Sun in May 1799, and its next transit will be in November 1815.

Planetary alignment

Mercury orbits the Sun once every 88 days, completing just over four revolutions around the Sun for every one that the Earth completes. This means that it regularly overtakes the Earth in its orbit, and at such times the two planets lie side-by-side in their respective orbits.

This alignment is called inferior solar conjunction and occurs once every 116 days – a period called Mercury's synodic period. It takes place as Mercury moves from the evening sky into the morning sky. Even though Mercury invariably passes very close to the Sun at these times, it rarely passes exactly in front of the Sun's disk.

Mercury orbits the Sun in a slightly different plane from the Earth: its orbit is tipped up at an angle of 7° relative to Earth's orbit. The diagram below show this inclination of Mercury's orbit (not drawn to scale), with a grid representing the plane of the ecliptic – the plane in which the Earth orbits in Sun.

Mercury spends much of its time significantly above or below the plane of the Earth's orbit, and at such times it cannot pass directly in front of the Sun. If it passes inferior conjunction at such a time, it will appear to pass to the side of the Sun.

Mercury can only pass in front of the Sun if it passes inferior conjunction while it is also very close to the Earth–Sun plane. It crosses through this plane twice on each orbit around the Sun, roughly once every 44 days, at the points on the left and right sides of the diagram above. These points are called the nodes of Mercury's orbit.

The nodes of Mercury's orbit lie alongside the points along the Earth's orbit that we pass on around May 8 and November 10 each year. Consequently, transits of Mercury can only ever occur within a few days of these two possible dates.

Due to the precession of the equinoxes, these two dates advance through the year at a rate of roughly one day every 100 years.

This requirement for Mercury to pass one of the nodes of its orbit at the same time that is also passes inferior conjunction is met on average only once every 7–8 years, though transits do not occur at regular intervals.

Each time, Mercury can be seen to pass in front of the Sun from any location where the Sun is above the horizon at the time.

Further reading

For more information about safely observing the Sun, we recommend Lee Macdonald's excellent book How to Observe the Sun Safely , published by Springer in 2012.

The position of Mercury at the moment of closest approach to the centre of the Sun's disk will be:

The coordinates above are given in J2000.0.

Next/previous transits

The sky on 21 Nov 2024

Warning

Never attempt to point a pair of binoculars or a telescope at an object close to the Sun. Doing so may result in immediate and permanent blindness.

Source

The circumstances of this event were computed using the DE430 planetary ephemeris published by the Jet Propulsion Laboratory (JPL).

This event was automatically generated by searching the ephemeris for planetary alignments which are of interest to amateur astronomers, and the text above was generated based on an estimate of your location.

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Image credit

© edhiker, November 2006.

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