Wednesday, January 8, 2020

Calibration Maneuvers Today

Today from 1315-1915 UTC (8:15 am - 2:15 pm ET) SDO will perform the EVE Field of View and HMI/AIA Flatfield calibration maneuvers. Science data recorded during these maneuvers may be missing, incomplete, or blurry.

Monday, January 6, 2020

Happy Perihelion!

On January 5, 2020, at 0547 UTC (2:47 am ET) the Earth reached perihelion in its orbit around the Sun. At that time the Earth was 147,091,144 km (91,381,199 mi) from the Sun. The Earth's will reach aphelion, the point in our elliptical orbit furthest from the Sun, on June 4, 2020. Enjoy the closest of our favorite star, the Sun!

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


Wednesday, October 23, 2019

Mercury transit, November 11, 2019

Starting around 1200 UTC (7:00 am ET) November 11, 2019, we will begin watching Mercury move into view against the corona of the Sun. This is about 45 minutes before it is visible against the Sun from the ground. We have created to let you watch this transit as short movies in almost real time. The transit lasts a little over 5.5 hours, ending as Mercury leaves the disk of the Sun about 1806 UTC but continues moving out through the corona for another 30 minutes.
The image shows the predicted positions of Mercury against an AIA 1600 Å image from 1803 UTC October 23, 2019. The blue circles are about the size of Mercury and are spaced 30 minutes apart. There are no sunspots along the path of Mercury and the Sun shows very few signs of magnetic activity. The corona is also smaller than it has been, meaning Mercury will come into view in the EUV images less than an hour before 1st contact. Although we will be taking images all the time, the special modes will start 45 minutes before 1st contact and end 45 minutes after 4th contact.

Along with the full-disk view of the Sun we will provide several zoomed views. These views are shown by the boxes that are drawn on the image. You can also see that the EUV telescopes will see Mercury blocking the corona before it moves onto the disk. All of the boxes are built in a 16x9 ratio that nicely fits into an 1080p screen.

  • The Ingress box will show the images from when Mercury moves over the edge of the Sun at the beginning of the transit. This includes 1st and 2nd contact (when Mercury first touches the edge of the Sun and when Mercury moves completely onto the Sun in visible light.) It will also show Mercury against the corona of the Sun before 1st contact in the EUV channels;
  • The Tracking box will follow Mercury as it moves across the disk;
  • The Egress box images will show the data when Mercury moves into that box as it exits the Sun. It will not be available until about 1715 UTC. This box will show 3rd and 4th contact (when Mercury starts leaving the disk of the of Sun and the last instant it touches the edge of the Sun, again in visible light);
  • The Full Passage box will be updated throughout the transit so that you can watch the entire path of Mercury across the Sun.

Each view is available as a self-updating movie, by clicking on the picture shown on the website, and as an mp4 movie, which can be seen by clicking on the View/Download mp4 button below the image. Depending on its settings, your browser will either show the movie or download it. The mp4 movies will also be regularly updated as new images arrive, but are not automatically updated on your browser.

Transits were important for two developments in astronomy, that Kepler's theory using ellipses for planetary orbits was better than the Ptolemaic theory and fixing the size of the solar system. Transits of Mercury and Venus can happen because they are the only planets that orbit between the Earth and the Sun. By using ellipses to describe the orbits of the planets Kepler was able to predict when transits would occur. The Ptolemaic theory that used circular orbits did not have the accuracy needed to do predict these events, even after epicycles and equants were introduced. In 1627, Kepler predicted that a transit of Mercury would occur on November 7, 1631. Pierre Gassendi watched from his Paris observatory and saw a small black dot move across the face of the Sun on that day. A real triumph for the Kepler calculations!

Later transits, especially of Venus, would be used to measure the distance between the Earth and the Sun — the astronomical unit that we now know is 149,597,870,700 meters (or 92.75 million miles).

But on November 11, 2019, we celebrate the success of Kepler's ideas. As we watch Mercury move across the Sun, you could also remember our successes in using Kepler's ideas to slingshot the Voyager spacecraft through a Grand Tour of the outer planets, or to use Venus to send the Parker Solar Probe ever closer to the Sun, or even the incredible accuracy of GPS positions we take for granted as we move about in our everyday lives.

Never look at the uneclipsed Sun with unprotected eyes!

You can safely watch the transit at

My thanks to the SDO scientists, engineers, and web programmers that made this SDO Data Event possible.


Testing of AIA Today, 1800-1900 UTC

Today from 1800-1900 UTC (2-3 pm ET), AIA will record some test images. This will exercise the ability to command AIA to produce partial (cropped) and uncompressed images. Other instruments will be unaffected. The actual testing is scheduled to last approximately 10 minutes. Test images will be in focus but may be missing a part of the Sun.