Perspectives from Mercury’s Rare May 9 Transit

This is a follow-up piece to my original article on this month’s transit of Mercury

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Tiny Mercury is seen just following second contact in this Solar Dynamics Observatory (SDO) image. The sun is seen here in the Extreme Ultraviolet Region of the spectrum at a wavelength of 171 Angstroms or 17.1 nanometers.

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Composite image of the 9 May transit of Mercury showing the progress of the tiny planet during the 7-1/2 hour transit. Image courtesy of NASA and the Solar Dynamics Observatory (SDO). Visible-light images used in composite from SDO’s Helioseismic and Magnetic Imager.

Many images and videos of the 9 May transit of Mercury were captured with many of them noteworthy for their quality, composition and perspective. Some of them illustrate the 7-1/2 hour transit in a progression of one-hour, 30-minute or shorter-interval individual images compiled into a composite of the entire event. Others illustrate the “black drop” effect, the apparent formation of a “black drop” at second contact or the moment when ingress is complete, the point where the limb of the planet is in “contact” with the larger limb of the sun.


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Tiny Mercury soon after the transit’s second contact. Image from SDO’s Helioseismic and Magnetic Imager.

Solar Dynamics Observatory (SDO)
It is noteworthy that this is the first transit of Mercury observed by the orbiting Solar Dynamics Observatory since its deployment in April, 2010. Since the most recent transit of Venus in June, 2012 also occurred after SDO’s deployment, that was observed with those results available here. The Solar Dynamics Observatory is the first mission to be launched in NASA’s Living With a Star (LWS) Program, a program designed to help determine the causes of solar variability and its impact on Earth. SDO was designed to help us understand the sun’s influence on Earth, the Earth’s biosphere and the near-earth space environment by studying the solar atmosphere on scales small in space and time and in many wavelengths simultaneously. Of particular interest to those studying the sun’s interior dynamics and the poorly-understood link between the solar Chromosphere and solar-Coronal heating is the AIA (Atmospheric Imaging Assembly). Nine out of AIA’s ten channels are devoted to observing the sun in the EUV (Extreme Ultra-Violet) region of the spectrum with a resolutions of one arc-second (a tennis ball at ten kilometers or 6.25 miles).

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The EUV imaging and video channels for SDO’s AIA instrument.

With decreasing wavelength, the nine EUV wave bands, in nanometers, are 170, 160, 33.5, 30.4, 21.1, 19.3, 17.1, 13.1 and 9.4 with each representing an increasingly hotter gas or region of the sun. With decreasing wavelength, the energy of the light increases and thus the hotter the source. In all of the EUV imagery (images and video), the sun does not appear in a manner that we’re used to but presents with many dark regions or areas where there is no emission. If our eyes were sensitive to this waveband, this is what the sun would look like when observed. The dark areas or regions absent of any emission does not mean that these regions have no emission, it means that there is no gas at the temperature that would correspond to the waveband of interest. Just to provide context for the temperatures involved, 9.4 nm emission occurs for a gas at a temperature of 308,300 Kelvin (714,500 F or 396,927 C) and is actually considered soft X-rays with 10 nm representing the upper-wavelength limit to the X-Ray band. The origin of this hot gas, much hotter than the surrounding photosphere at a nominal temperature of 5,780 K, is deep within the sun and provides clues and insights into the electromagnetic dynamics of the solar interior.

To provide “wave length” perspective, our eyes are most sensitive to yellow-green light at about 500 nm (one nanometer or nm is one part in one billion of a meter) and sunburns are the body’s response to skin damage from solar UV between 300 – 380 nm, more than twice the wavelength of AIA’s 170 nm UV continuum band. The one remaining AIA channel is a high-resolution, white-light continuum band for generic, high resolution images and video of the sun. The chart to the upper left illustrates the wavelength expressed in Angstroms, a rarely used unit of length measure that is a factor of 10 smaller than a nanometer or 1 part in 10 billion of a meter. The source column represents the emitter, in every case a hot, ionized gas and in eight out of the ten channels Helium, Carbon and Iron (Fe) in various states of high-ionization are the emitter. The region column provides the location of the source.

Although the videos and imagery are dramatic, none of this materially effects the transit images and video of Mercury. For additional imagery, video and content related to the transit, please visit NASA Goddard’s Mercury Transit page or the SDO Mercury Transit page. The Solar Dynamics Observatory is managed and operated by NASA’s Goddard Space Flight Center.



SDO high resolution video of Mercury as it transits the sun on 9 May. The sun’s surrounding 5,780 K photosphere (the “surface”) appears tranquil compared to the hot gas columns and plumes glowing bright at a temperature of 170,000 K, visible in this 17.1 nm view



SDO full-disk video of Mercury as it transits the sun on 9 May. The sun’s surrounding 5,780 K photosphere (the “surface”) appears tranquil compared to the hot gas columns and plumes glowing bright at a temperature of of 170,000 K, visible in this 17.1 nm view


Imagination is more important than knowledge 585px-Albert_Einstein_signature_1934(invert)
An index of all articles in this blog can be found here.

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