Wednesday, May 11, 2016

A Few Stories About the Transit of Mercury on Monday

On May 9, 2016, Mercury passed directly between the Sun and Earth, making a transit of the Sun. Mercury transits happen about 13 times each century. NASA’s SDO studies the sun 24/7 and captured the entire nine-hour event. This composite image of Mercury’s journey across the Sun was created with visible-light images from HMI on SDO. Image courtesy of NASA/SDO, the HMI Science Team, and Genna Duberstein.

It's always nice to see stories using SDO images on the web. It was pretty cloudy on the East Coast of the United States on Monday and a cloud-free satellite feed is a great backup. Here are a few examples where SDO images were used to share the Mercury transit to the world.

SDO will now return to its regularly scheduled observations of the Sun — in better than UltraHD!

Tuesday, May 10, 2016

My Favorite View of the Mercury Transit

I am glad so many people could watch the Mercury transit yesterday, with SDO and from the ground. People around the world were watching the transit along with SDO! Here is my favorite view of the transit. An AIA 193 movie of Mercury passing over a small but active prominence as it moves off the disk of the Sun. The mp4 movies of the transit will be archived at, so you can look at the phases and wavelengths you missed the first time.

Many thanks to the people at Stanford University who got up at 3:00 am PT to start the scripts to provide the SDO data. My thanks also to the people at the Goddard Space Flight Center who worked all morning to keep the data flowing and help with the network configuration.

Now I can relax and start looking forward to the next transit of Mercury on November 11, 2019. But first you should watch for the Great American Eclipse, a total eclipse of the Sun on August 21, 2017. The path of totality of this eclipse spans the continental US, so it can be seen by almost everyone in the USA who can drive a few hundred miles or less. SDO won't see this eclipse and I will just be another person jostling for position at some place along the path!

Monday, May 9, 2016

Mercury has left the Sun

Mercury has moved off the Sun but can still be seen against the corona.

Mercury is Approaching 3rd Contact

Mercury is nearing the far edge of the Sun, or 3rd contact. It will pass over a nice prominence!

Half Way Through the Transit

The network seems less overwhelmed now. Here is a movie from the 1600 channel showing Mercury up to mid-transit.

Thanks for Your Interest!

We are being overwhelmed by requests for images of the Mercury transit. We are working to speed up the network access and send the data smoothly to your browser.

Sunday, May 8, 2016

The Mercury Transit is Tomorrow!

Tomorrow at 1030 UTC (6:30 a.m. ET) 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 image at the left shows the predicted positions of Mercury against an AIA 193 Å image from 1414 UTC May 8, 2016. The black circles are about the size of Mercury and are spaced 30 minutes apart. There are no sunspots along the path of Mercury and only a few areas of strong magnetic field (the bright regions in this image). Only active region 12542 has a noticeable area and it is at 11° N, well above the path of Mercury. 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 firsts 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 1730 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 two 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 May 9, 2016, 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 Jupiter to send the Ulysses spacecraft high above the poles of the Sun, or even the incredible accuracy of GPS positions we take for granted as we move about in our everyday lives.

You can safely watch the transit at My thanks to the SDO scientists, engineers, and web programmers that make this SDO Data Event possible.

Never look at the uneclipsed Sun with unprotected eyes!

Always use sun-safe optics to look at the Sun.


Saturday, May 7, 2016

Why is Mercury Bright in Some Images During the Transit?

While watching the transit of Mercury across the Sun on May 9, 2016, at we will sometimes see that Mercury is not black but glows a little. We also saw this during the transit of Venus in 2012, especially when Venus entered and left the disk of the Sun. We use these “bright planets” to study the telescopes on AIA.

Most of the light from the Sun is visible wavelengths. By most I mean sunlight at visible wavelengths is 1 million times brighter than the ultraviolet and extreme ultraviolet wavelengths measured by AIA and EVE. Those instruments use filters on the front of their telescopes to keep the visible light photons from reaching the CCDs.

The filters on the AIA telescopes are thin metal foils that are supported by meshes. The rows and columns of the meshes are rotated about 45° from the top of the Sun and are separated by a rotation of about 10°. Unfortunately, the mesh grid diffracts photons and a point of light is seen as a pattern like the image on the left. This image represents the 171 Å “point spread function,” or PSF. For the 171 Å images, almost 2/3 of photons end up in the “correct” location, at the center of the crosshairs. But 1/3 of the photons are diffracted to other places in the image. The amount of diffraction is wavelength-dependent, so each image has its own corresponding PSF. However, the pattern in each wavelength is similar to the one on the left. Planet transits can be used to calibrate that software by providing a disk that should always be completely black but sometimes isn't.

You might remember seeing the PSF pattern as the eight-spoked pattern can appear over a flaring location (like the AIA 131 Å image from August 2014 on the left). The important thing to remember is that this pattern is not happening on the Sun - it is caused by the telescope. Even better, we can use software to “put” the light back into the right place.

Mercury, which will be roughly 36 million miles from the Sun at the time of the transit, should appear completely black against the solar disk. However, photons diffracted from the filter mesh may make Mercury appear temporarily brighter as it passes through a diffraction pattern from a particularly bright region. The AIA team used observations from the Transit of Venus in 2012 to refine their understanding of the PSF and improve our ability to apply the correction to the science data. It is hoped that we can once again take advantage of a should-be-dark planet to further calibrate our images.

Not to worry, we will get another chance to verify the calibration during the next Mercury transit on November 11, 2019!

You can safely watch the transit at My thanks to the SDO scientists, engineers, and web programmers that make this SDO Data Event possible.

Never look at the uneclipsed Sun with unprotected eyes!

Always use sun-safe optics to look at the Sun.