Sunday, November 17, 2019

200 Millionth Image from AIA

In all the hype about the Mercury transit last Monday, we overlooked another milestone in the SDO mission. On November 5, 2019, the AIA instrument returned its 200 millionth image. Here it is and it's a beauty in the AIA 171 Å passband!
The Sun looks like it is in solar minimum, with large polar coronal holes over the North and South poles. Hope for the future can be seen in the area with the black line drawn around it. That's active region 12750, a small bi-polar region that has the polarity of Solar Cycle 25 and an appropriately high latitude of 28°S. Other SC25 regions have been seen and they will become more frequent as SC25 starts its rise to maximum.

Mark Cheung, PI of AIA, made a movie to celebrate this milestone.

Congratulations to the AIA team for building a robust instrument that has returned excellent data for almost 10 years.

Wednesday, November 13, 2019

Extraordinary Pictures of the 2019 Mercury Transit

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.

Thursday, November 7, 2019

Mercury Transit this Monday!

We will see a transit of Mercury across the disk of the Sun on Monday, November 11, 2019. You can watch the transit using SDO data at

Here is a movie of the transit (from our flight dynamics team.)

The 2019 Mercury transit will last about 5.5 hours in visible light. Mercury will touch the edge of the Sun in HMI at 1241 UTC (7:41 a.m. ET, called first contact) and leave the HMI disk at 1805 UTC (1:05 p.m. ET, fourth contact). Mercury will be visible in some of the AIA channels throughout the SDO Data Event, which starts at 1200 UTC (7:00 pm ET) and ends at 1845 UTC (1:45 pm ET).

Mercury is smaller and further from the Earth, so, compared to the transit of Venus in 2012, it will be a smaller disk as it passes between SDO and the Sun. Due to its shorter orbital period, transits of Mercury are more common than transits of Venus.

Johannes Kepler predicted a transit of Mercury would occur in November 1631. The first observed transit of Mercury was on November 7, 1631 by Pierre Gassendi. This was the first observed planet transit and showed that Kepler's equations of planetary motion could be used to accurately predict the positions of planets.

From the ground the transit starts at 1235 UTC (7:35 am ET, first contact) and ends at 1804 UTC (1:04 pm ET, fourth contact. Parts of this transit will be visible throughout North America. People in the eastern USA will be able to see most, if not all, of this transit. People in the western USA will see the Sun rise with Mercury already on the disk.

SDO will provide near-realtime pictures of the Mercury Transit at