Here are composite views of Monday's transit (top) and the transit in 2016 (bottom). They were produced by Monica Bobra at Stanford University. What can we learn about Mercury's orbit from these pictures?
First, the images of Mercury (the black dots) are 21 minutes apart in both images. The 2019 dots are further apart than the 2016 dots. This means Mercury was moving faster across the Sun in 2019. If we check the orbit of Mercury at AstroPixels, we find that Mercury was closest to the Sun (at perihelion) on November 16, 2019. A planet's velocity is greatest near perihelion and least near aphelion (the furthest point from the Sun). So Mercury was moving at almost its maximum speed on Monday. The 2016 Mercury ephemeris shows that aphelion was May 20, 2016, only 11 days after the transit that year. That means Mercury was moving at almost its slowest speed during the 2016 transit. The spacing of the dots agrees with the orbit of Mercury.
We can also look at the size of Mercury on the disk. The dots are smaller in 2019, showing that Mercury was further from the Earth during this transit. If we look at Mercury's elliptical orbit from the Earth, Mercury is closest to the Earth when it is furthest from the Sun (aphelion) and farthest from the Earth when it is closest to the Sun (perihelion). Again, this agrees with the transit pictures and orbit.
The different tilts of the two paths show that Mercury's orbit is tilted away from the Earth's orbit (called the ecliptic) and the Sun's equator. Whenever Mercury's orbit crosses the ecliptic we have a chance to see a transit, but most crossings take place when the Earth and Mercury are not aligned. SDO images are usually aligned with the Sun's North Pole at the top of the image. So the interpretation of the tilts in a little more complicated.
We can see sunspots in the 2016 images but not in the 2019 image. The Sun has become very quiet in the last three years. Over the next year or so we should start seeing Solar Cycle 25 activity. But on Monday the Sun was blank in visible light, providing beautiful back-lighting for us to see Mercury.
The sunspots in the 2016 image are nice and sharp. Bobra's program does not use a simple average to make the image. You can check out the actual Python code (with the transit pictures) at her Planetary Transit githib site. Be ready for the 2032 transit!
Check out the 2019 transit in other wavelengths of light at the SDO website.
Many thanks to Dr. Bobra for producing these extraordinary images.