Sunday, April 10, 2016

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 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.