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Mercury Transit 2019

Mercury as it approaches the solar disk

All SWAP Images Available from the Mercury Transit on November 11 2019.

In preparation for the 2019 Mercury transit, the PROBA2 team have made a high cadence image campaign throughout the day. A movie made with all the SWAP data is shown below. All of our SWAP data products for this Mercury transit are now available here.


Movie 1. Latest observations of the 2019 Mercury transit by SWAP. 

Mercury Image courtesy of ESA

How common are celestial transits? Celestial transits-where a celestial body is seen to pass across the solar disk from the perspective of the Earth-are relatively rare events. The planet Mercury undergoes around 13 transits a century, and Venus has a pair of transits approximately every 120 years. In 2016 the SWAP EUV imager on the PROBA2 satellite successfully observed the last Mercury transit of the Sun. This Mercury transit will occur on November 11 2019 and can be observed from Earth starting at 12:34 UT and ending at 19:04 UT, a total transit time of around 6 hours and 30 minutes. Another Mercury transit will then not take place again until 2032!

What do we see in SWAP observations? SWAP is an EUV telescope onboard the ESA PROBA2 satellite, it observes the Sun at roughly 1 million degrees from orbit around the Earth. Mercury can be seen as a small black disk crossing the face of the Sun, roughly 4 pixels in diameter. The PROBA2 team created a simulation of what SWAP is expected to see, which is shown in the video below. This simulation illustrates the path of Mercury as it crosses the Sun from the perspective of SWAP on PROBA2. The path of Mercury appears to 'wobble' as it crosses the Sun, but this is an artificial effect created by parallax from the changing perspective of Mercury in the PROBA2 field-of-view as the spacecraft orbits the Earth.

Movie 2. Simulation of Mercury's path in the SWAP field-of-view. 

The number in the upper right hand corner of Movie 2 is a measure of the 'tangential height', which is the altitude of the point in the optical path that is closest to the Earth’s surface. What does this mean? - As the satellite orbits, there are times when the Earth is closer to the SWAP field of view, and times when it is further away, this number shows that distance. As a comparison imagine taking a video of a distant object when sat on a Ferris wheel, there will be times when the Earth is closer to your camera field of view (near the bottom of the Ferris wheel), and times when it is further away (near the top).

Now, PROBA2 does not orbit in a circle on top of the Earth, but goes around the Earth. This orbit produces periods when the Earth is closer to the SWAP field of view, in fact there are short periods in the orbit when the Earth passes in front of the SWAP field of view, creating eclipses (also known as occultations) of the Sun from the perspective of the satellite. This measure of altitude is an important parameter during PROBA2's occultation period, when the instruments onboard observe the Sun through the Earth's atmosphere. The occultations experienced by PROBA2 will also affect our view of the Mercury transit as there will be short intervals when the Sun is not visible and Mercury appears to "skip forward" in the movie above. 

Due to the relative sizes and brightness of Mercury (4879.4 km) and the Sun (696300 km) it is hard to spot Mercury in SWAPs images. Mercury can be seen as a mere 12 arcsec in diameter (approximately 4 pixels in diameter in our 1024 x 1024 pixel field of view).

PROBA2 carries another solar measuring instrument: LYRA, a radiometer used for measuring the solar irradiance. Due to the fact that Mercury only covered a very small part of the solar surface, it will not block much of the solar radiation. We therefore do not see to see any variation above the noise level in the signal detected by LYRA.