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Mercury Transit

Mercury Image courtesy of ESACelestial transits-where a celestial body is seen to pass across the solar disk from the perspective of the Earth-are relatively rare events. The planet Mercury undergoes around 13 transits a century, and Venus has a pair of transits approximately every 120 years. In 2012 the SWAP EUV imager on the PROBA2 satellite successfully observed a Venus transit of the Sun. On May 9, 2016, it was Mercury's turn! The Mercury transit was seen from Earth starting at 11:13 UT and ending at 18:42 UT. The total transit time was around 7 hours and 30 minutes.


What did we expect to see in SWAP observations? SWAP is an EUV telescope onboard the ESA PROBA2 satelite, it observes the Sun at roughly 1 million degrees from orbit around the Earth. Mercury is expected to be pictured as a small black disk crossing the face of the Sun, which would be seen as roughly 4 pixels in diameter. The PROBA2 team created a simulation of what SWAP was expected to see, which is shown in the video below. This simulation illustrates the path of Mercury as it crosses the Sun from the perspective of SWAP on PROBA2. The path of Mercury appears to 'wobble' as it crosses the Sun, but this is an artificial effect created by parallax from the changing perspective of Mercury in the PROBA2 field-of-view as the spacecraft orbits the Earth.

Seventh Call for PROBA2 Guest Investigator Program - Deadline Extended

Due to a recent surge in interest in the PROBA2 Guest Investigator program the proposal submission deadline has been extended to 2016-April-01. Those who have already submitted a proposal may re-submit their proposal if they wish to work on them more.

PROBA2 views Partial Solar Eclipse - 8 & 9 March 2016

On 2016 March 8 and 9, a solar eclipse took place over the Pacific Ocean. This eclipse was total -that is, the entire solar disk was covered by the Moon- over Indonesia and the central Pacific, starting at sunrise over Sumatra and ending at sunset north of the Hawaiian Islands. Additionally, large parts of South-East Asia, Alaska and Australia witnessed a partial solar eclipse. The path of totality had a maximum width of 155 km and the maximum duration was 4 minutes and 9 seconds at the point of greatest eclipse, which was over the waters of the Pacific Ocean.

SWAP Carrington Rotation Movies

Mini Carrington Rotation ImageWhen observing the Sun for a prolonged period of time, it soon becomes evident that features on its surface, and in its outer atmosphere do not rotate at the same rate. This is because the Sun is not a solid body, but a big ball of magnetised plasma, whose rotation is variable with position and height in the solar atmosphere. One of the most striking observations is that of ‘differential’ rotation, where features on the solar surface and in the solar atmosphere are observed to rotate faster at the equator (rotation period = 25.4 days) when compared to those closer to the poles (rotation period = 36 days). This is evident in observations of sunspots, which have been used as tracers for measuring solar motion for many years. 

SWAP Observes Another Eclipse - and this Time it's Annular

SWAP Annular Eclipse

The total solar eclipse observed in March 2015 caught a lot of people's attention, especially as the path of totality passed over most of Northern Europe. There was a great deal of fan-fair and plans to observe the eclipse from the ground. However, due to heavy cloud cover, a lot of people had to turn to space-based observations, such as those made by the sun watching extreme-ultraviolet imager: SWAP, on board the European Space Agency's PROBA2 satellite, which images the Sun from the vantage point of a polar Earth orbit, away from pesky cloud cover. More information about the March eclipse can be found here and here.

SWAP observes the solar corona in a passband centered on a wavelength of 17.4 nm. The structures seen in SWAP images have a temperature of approximately 1 million degrees. More information about the SWAP instrument is available here.

It may come as some surprise, especially for those in Europe, that there was another eclipse observed on 2015-Sep-13. Whether you are able to observe an eclipse from the ground depends on your geographic location, in contrast to the March eclipse which was seen from Northern Europe and the Arctic regions, the September eclipse was observed in the southern hemisphere from Antarctica and southern Africa. In any given year the Earth will experience at least 2 solar eclipses due to the Earth and Moon's orbit.

PROBA2 Guest Investigator 6th Call Results

We are pleased to announce that the PROBA2 Science Working Team has selected 7 proposals for the 2015 Guest Investigator Program.

Sixth Call for PROBA2 Guest Investigator Program


The PROBA2 PI-team welcomes research proposals for the sixth round of its Guest Investigator program for research based on SWAP and LYRA data analysis by scientists outside the SWAP and LYRA PI-teams. We encourage in particular early-career post-docs and PhD students to apply, although more senior guest investigators' proposals are also welcome. In this round we anticipate funding for around six guest investigators or teams who will visit the PROBA2 Science Center at the Royal Observatory of Belgium, in Brussels, between August 2015 and April 2016. Please note that the deadline for applications has now passed. 

A Deeper Look at the SWAP Movie of the March 2015 Eclipse

To read about the PROBA2 eclipse observation campaign and see images of the eclipse, follow this link.

When ESA posted the video of SWAP’s observations of the March 20 solar eclipse on YouTube a number of viewers shared comments and questions about several unexpected aspects of what they saw. A few viewers were so surprised by what they saw that they even wondered if the images were really authentic. The images were most assuredly real, but nonetheless we on the PROBA2 team were likewise intrigued by similarly unexpected things that we saw in the observations.

Some of the questions commenters asked have straightforward — if not exactly simple — answers, while others required us to dig deeper and do some real research of our own to try to address them.

Among the questions raised in the comments were:

  • Why does the Moon move across the Sun from east to west, the opposite direction of motion from what viewers on the ground observed?
  • Why didn’t the Sun appear to rotate in the movies as it does in many movies from PROBA2 and other Sun-observing spacecraft?
  • Why did the Sun change so little during the movies? Shouldn’t there have been some dynamics visible in the corona?
  • Why did the movie play so quickly? Why were there so few frames showing the eclipse?

Let’s have a look at these questions to see what we can learn about the Sun, solar eclipses, and the PROBA2 spacecraft from them.

PROBA2 Views a Total Solar Eclipse - 2015

Updated (25 March 2015): On 2015 March 20, PROBA2 observed a total solar eclipse — twice! The spacecraft's orbit carried it through the darkest parts of the Moon's shadow two times, first between 08:28 and 08:53 UT and again between 10:24 and 10:50 UT. Eclipse chasers, scientists, media and members of the general public have been following our data closely, so we are collecting all of our results and data products in one place for quick access.

SWAP, an Extreme-Ultraviolet solar telescope, observes the solar corona in a passband centered on 17.4 nm. The structures we see in SWAP images have a temperature of approximately 1 million degrees. LYRA, an X-ray/Ultraviolet radiometer observes the total incoming light levels from the Sun in several wavelength bands.

More information about these instruments is available here: SWAP | LYRA.

Exciting press for SWAP

SWAP was recently featured in two ESA stories! SWAP took this week’s Space Science Image of the Week. The image is from 25 July 2014 and is reproduced below. It shows a large coronal-fan structure on the left side of the Sun.


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