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?

November 11, 2019, Courtesy of Monica Bobra

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.

May 9, 2016, Courtesy of Monica Bobra

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.

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 https://mercurytransit.gsfc.nasa.gov/2019/.

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 https://mercurytransit.gsfc.nasa.gov/2019/.

Enjoy!

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 https://mercurytransit.gsfc.nasa.gov 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 https://mercurytransit.gsfc.nasa.gov.

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

Enjoy!

Lunar Transit on September 28, 2019

SDO will experience a lunar transit on September 28, 2019. Here is a movie from the Flight Operations Team that shows SDO, the Sun, and the Moon during the transit. This transit lasts from 2044-2115 UTC (4:24-5:16 pm ET) and covers at most about 7% of SDO's view of the Sun.
The Moon moves from left to right during this transit. That makes it an SDO-overtake transit.

During the transit SDO will be in inertial mode with the fine-guidance system turned off. Images may jiggle around a little bit, but will continue to be observed and recorded.
Although you can see the Moon throughout the movie SDO's instruments cannot see the Moon when it is not covering the Sun. The little white flash seen in the Moon is the word "Moon" being written by the software and then quickly covered. The boxes drawn around and on the Sun help the FOT run the spacecraft. The time is displayed in the lower left corner of the movie. The first seven numbers are the year (2019) and the day of year (271). The six numbers after the period are the hour, minutes, and second of UTC (2 numbers each).
The next SDO Transit will be the Mercury Transit on November 11, 2019. We will have a webpage dedicated to the transit.
Enjoy!

Happy 9th Anniversary SDO!

Nine years ago today, at 10:23 am ET, SDO rose into the sky atop an Atlas V 401 launch vehicle. Since that day SDO has returned over 350 million images of the Sun, produced over 3000 scientific articles, and allowed millions to enjoy the Sun. Solar Cycle 24 was just starting in 2010. SDO watched it grow to maximum in 2014 and now the Sun is often spotless.

Here is a summary slide of what SDO saw. The AIA 193 Å shows the lower corona and the HMI magnetogram shows the surface magnetic field. We try to understand the Sun by watching what happens at the poles. The dark areas near the North and South poles in the October 2010 and February 2019 193 Å images are the polar coronal holes. They have disappeared by solar maximum (the February 2014 images). The magnetograms show how the Sun's magnetic field gets more complicated at maximum and quite simple at minimum. What happens in the poles at solar minimum seems to be one of our best predictors of future solar activity.

SDO data is the basis of accurate models of the solar corona. Machine Learning algorithms are being used to squeeze even more information out of our 12 PB of data. We are looking forward to another lengthy lunar transit next month and the Mercury transit in November.

You can make your own SDO Anniversary movie at our website sdo.gsfc.nasa.gov or the Helioviewer website.

Happy Anniversary SDO!

Welcome to 2019!

The replacement of the computers serving https://sdo.gsfc.nasa.gov have arrived at Goddard! The system will be replaced and the data moved to the new drives over the next few weeks. We will be ready to serve the near-realtime images during the Mercury transit on November 11, 2019.
The highlights of SDO's maneuvers for the first 2/3's of 2019 are:

1. 01/31 @ 0506 UTC (00:06 am ET) - Spring Handover Season Starts
2. 02/06 @ 0723 UTC (02:23 am ET) - Spring Eclipse Season Starts
3. 02/20 @ 2210 UTC (10:10 pm ET) - Station Keeping maneuver #18
4. 03/01 @ 0713 UTC (02:13 am ET) - Spring Eclipse Season Ends
5. 03/06 2200 UTC to 03/07 0206 UTC (03/06, 05:00 to 09:06 pm ET) - Lunar Transit (movie released next week)
6. 03/27 @ 0310 UTC (10:10 pm ET) - Spring Handover Season Ends
7. 05/01 @ TBD - Momentum Management maneuver #34 (Tentative Date)
8. 07/17 @ 0445 UTC (12:45 am ET) – Fall Handover Season Begins
9. 07/24 @ TBD - Station Keeping maneuver #19 (Tentative)
10. 08/08 @ 0709 UTC (03:09 am ET) – Fall Eclipse Season Starts
11. 09/01 @ 0658 UTC (02:58 am ET) – Fall Eclipse Season Ends

Celebrate the Summer Solstice with Cool Pictures of the Mercury Transit!

Today at 2234 UTC (6:34 pm ET) is the summer solstice for 2016. It's also a full moon today, the first time the summer solstice and full moon have coincided since 1967. Enjoy the longest daytime of the year and a bright night as well. Maybe we could celebrate the brightest day of the year?

Today I would like to share another picture from the May 9, 2016, Mercury transit. This is a composite created by Monica Bobra at Stanford University. It's an excellent view of the transit from 2nd contact to 3rd contact, with an image every 20 minutes. Creating these composites takes a good bit of art. You can't just add the images together as that blurs the background image of the Sun. You have to take cutouts around the planet and reinsert them in a selected background image.

It turns out that MDI on SOHO was turned on for the transit. Bobra did the same analysis to produce this image from MDI. The data near 2nd contact is not available.

You can see the paths of Mercury are slightly different in the two images. The MDI path is closer to the solar equator and tilted at an angle to the path in the HMI image. These differences are caused by the different viewpoints of the transit from the two spacecraft. Similar differences are what were measured to determine the size of the astronomical unit (AU) during the Venus transits of the 1700 and 1800's.

Thanks, Monica!

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.

• http://www.nasa.gov/topics/solarsystem/features/eclipse/index.html (Of course!) Lots of stills and movies from SDO, including the composite image at the top of this post.
• https://www.facebook.com/NASA.Little.SDO/?fref=nf
• http://www.slate.com/blogs/bad_astronomy/2016/05/10/nasa_solar_dynamics_observatory_animations_of_the_mercury_transit.html (great headline on the story!)
• https://www.washingtonpost.com/video/national/health-science/mercury-makes-rare-move-across-the-sun/2016/05/09/072ef4aa-15f6-11e6-971a-dadf9ab18869_video.html (link goes to a video series but the Mercury transit video should be at the top of the list)
• http://www.scientificamerican.com/video/mercury-s-rare-road-trip/ (shows a set of short segments, look for the smiley face in the Sun in the HMI full passage video)
• http://www.bbc.com/news/science-environment-36252487
• http://www.nytimes.com/2016/05/09/science/watch-mercury-sail-across-the-sun.html?_r=0
• http://www.engadget.com/2016/05/10/nasa-mercury-sun-transit-video/

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

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 http://mercurytransit.gsfc.nasa.gov, 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!

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 http://mercurytransit.gsfc.nasa.gov 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 http://mercurytransit.gsfc.nasa.gov. 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.

Enjoy!

Why is Mercury Bright in Some Images During the Transit?

While watching the transit of Mercury across the Sun on May 9, 2016, at http://mercurytransit.gsfc.nasa.gov 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 http://mercurytransit.gsfc.nasa.gov. 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.

Enjoy!

Mercury Transit in 8 Days!

For about 9 hours starting at 1030 UTC (6:30 a.m. ET) on May 9, 2016, SDO will watch a small black dot move across the Sun. The black dot won’t be a sunspot, it will be the planet Mercury making a rare transit of the Sun.

We will be providing a near-live feed of the SDO images of the transit at http://mercurytransit.gsfc.nasa.gov. The images are delayed a few minutes by the data delivery method, but our website will display the data as self-updating movies. The movies will include a visible channel and most of the EUV wavelengths. You pick the box and wavelength and watch the transit unfold!

The image at the left shows Mercury’s position during the transit plotted on an AIA 193 image from April 9, 2016. The black circles are about the size of Mercury and are spaced 30 minutes apart.

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 about an hour 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;
• 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;
• The Full Passage box will be updated throughout the transit so that you can watch the entire path of Mercury across the Sun.

Images will be shown as short movies in your browser that update every few minutes to include the latest data from the satellite. The next image is a movie from our testing in AIA 171. Coronal loops and active regions provide an ever-changing background to the circle with a white cross that was drawn to represent Mercury.

Compared to people on the Earth, the orbit of SDO causes the transit to start and end at different times as seen at SDO. The transit starts when SDO is further from the Sun and ahead of the Earth and ends with SDO in front of and closer to the Sun-Earth line.

Even though the Mercury transits were less useful for measuring the size of the solar system, they are fun to watch. You can also compare how the different telescopes on the ground and in space see the black dot of Mercury move across the Sun. Maybe, just maybe, Mercury will pass over a flare!

Never look at the uneclipsed Sun with unprotected eyes!
Always use sun-safe optics to look at the Sun.

You can safely watch the transit at http://mercurytransit.gsfc.nasa.gov. My thanks to the SDO scientists, engineers, and web programmers that make this SDO Data Event possible.

Enjoy!

Maneuvers Today

SDO will perform the EVE Cruciform and HMI/AIA Flatfield maneuvers today starting at 1315 UTC (9:15 am ET) and lasting until 1900 UTC (3:00 pm ET). During the maneuvers the SDO images may be smeared by the motion of the spacecraft. Some partial images may be seen as the AIA team tests the Mercury transit mode in their cameras.

Mercury Transit on May 9, 2016

Between 1120 and 1845 UTC (7:20 am and 2:45 pm ET) on May 9, 2016, SDO will watch a small black dot move across the Sun. The black dot won’t be a sunspot, it’s the planet Mercury making a rare transit of the Sun. The top image shows Mercury’s position during the transit plotted on an HMI continuum image from April 9, 2016. The black circles are about the size of Mercury and are spaced 30 minutes apart.

We will be providing a near-live feed of the SDO images of the transit on the special website http://mercurytransit.gsfc.nasa.gov. Our feed is delayed a few minutes by the data delivery method, but our website will display the data in a self-updating movie format. The stream from the spacecraft will include a visible channel and most of the EUV wavelengths.

Compared to people on the Earth, the orbit of SDO causes the transit to start and end at different times as seen at SDO. The transit starts when SDO is behind and ahead of the Earth and ends with SDO in front of and close to the Sun-Earth line.

While Mercury moves across the face of the Sun we will provide several ways to look at the data. Along with the full-disk view of the Sun we will provide several zoomed views. The second picture shows the boxes where small movies will be published on an AIA 193 image from April 9, 2016. The top part of the image will be cutoff so that no compression is needed to send the images to the ground. You can also see that the EUV telescopes will see Mercury blocking the corona about an hour before it moves onto the disk.

• Full-disk images
• The ingress box will show the data when Mercury moves over the edge of the Sun
• The tracking box will show Mercury moving across the disk
• The egress box images will show the data when Mercury moves into that box as it exits the Sun
• The “Complete Transit box” will be updated throughout the transit so that you can watch the entire path of Mercury across the Sun

Images will be shown as short movies in your browser that update every minute or so to include the latest data from the satellite.

We will see the disk of Mercury against the corona in the EUV channels about 1 hour before Mercury gets to the edge of the Sun.

SDO scientists will use the Mercury transit to study how the Sun is oriented on the cameras. It will also be used as a test of the computer programs used to clean up stray light in the images. This stray light caused ghost images to appear inside of Venus during the 2012 Venus transit. The disk became black when the computer programs were used to analyze the images. Mercury’s disk appears smaller and it is a better test of the programs.

A Mercury transit was the first seen in history. The heliocentric theory of the solar system allowed the orbits of planets to be more accurately calculated and Johannes Kepler determined that a transit of Mercury was possible in 1631. Pierre Gassendi saw that transit on November 7, 1631.

Transits of Mercury across the solar disk are more frequent than transits of Venus. This is because Mercury’s orbital period is 40% of Venus. But Mercury is farther from the Earth during a transit and the shift of the transit path for observers at different latitudes is smaller than for Venus. This made the less frequent Venus transits a better source of data for measuring the distance from the Earth to the Sun (1 AU).

Mercury is smaller in radius than Venus and further away from the Earth. That makes the disk of Mercury smaller on the Sun and more difficult to see as it enters and leaves the solar disk. The times of these contacts are needed for measuring the size of 1 AU. Again the Venus transits are a better measurement of 1 AU.

Even though the Mercury transits were less useful for measuring the size of the solar system, they are fun to watch. You can also compare how the different telescopes on the ground and in space see the black dot of Mercury move across the Sun.

Never look at the uneclipsed Sun with unprotected eyes! Always use sun-safe optics to look at the Sun.

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

Enjoy!

Today's Occultation of Venus and the 2016 Mercury Transit Across the Sun

Today Venus will disappear behind the Moon at varying times across the United States. It will not be visible from SDO as we do not have a lunar transit today. Please see the front page of SpaceWeather.com for details on when to watch this occultation. You will need a pair of binoculars for the best view. Just find the Moon (a little ahead of the Sun in the sky) and watch as the bright dot of Venus goes behind it at about 12:30 p.m. ET.

But that brings up occultations that SDO will see. Next year we will see a transit of Mercury across the disk of the Sun on May 9, 2016. Here is a movie of the transit (using our predicted ephemeris, but it should be fairly close.)

The 2016 Mercury transit will last 7.5 hours. Mercury will touch the edge of the Sun at 1112 UTC (7:12 a.m. ET, called first contact), move completely into the disk at 1115 UTC, cross the center of the Sun at 1457 UTC, touch the far edge of the Sun at 1839 UTC, and leave the disk at 1842 UTC (2:42 p.m. ET, fourth contact). Unlike the 2012 Venus Transit, which was only visible in Alaska and Hawai'i, this transit will be visible throughout North America. People in the USA will be able to see most, if not all, of this transit.

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 the 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 predict the positions of planets.

SDO will provide near-realtime pictures of the Mercury Transit at a dedicated webpage. More details as the time approaches.

While you are waiting, grab your binoculars and go watch Venus disappear behind the Moon!