Skip to content

The 2016 transit of Mercury

A transit of Mercury is one of a very select group of astronomical events – planetary transits. A planetary transit occurs when one of the two inferior planets, Mercury and Venus passes in front of the solar surface.

While transits of Venus occurs only twice per century, transits of Mercury take place more frequently, i.e. 13 -14 times in a century.

On Monday, May 9 2016 there was a transit of Mercury visible in Denmark for the entire duration of 7½ hour ending about an hour before sundown. Meteorologically, the month of May is quite favourable for astronomical observations as the chances of clear skies is 66% compared to just 44% for November, the other month where transits of Mercury occur.

Mercury is a small planet with a diameter of just 2,440 km. At a distance of 83,379,985 km from earth during this transit, its angular size was just 12″ (12 arcsecs). Thus, the shadow of Mercury on the solar disc would be visible only with the aid of a telescope.

In 1628 Kepler predicted a transit of Mercury for November 6 1631 and he had estimated that the angular size of Mercury would be approx. 2′ (2 arcmin). Thus most of the contemporary astronomers decided to observe the transit with a ‘camera obscura’, a projection method without a telescope and consequently did not observe the transit. Three astronomers, however, chose to use a telescope and did in fact observe the transit. Petrus Gassendi was one of these and his drawing of the transit is shown below.


Gassendi trusting Keplers estimate of Mercury’s diameter at first could not believe that it was Mercury he observed and only after realizing that the ‘sunspot’ he deemed it to be moved faster than sunspots were supposed to, did he relize that it was indeed Mercury he had observed. He estimated the diameter to be 18″.

Now back to the present. As a member of the Astronomical Association for Southern Zealand I set up my equipment at the associations regular observing site at Avnø Naturcenter between Vordingborg and Næstved. In addition to visual observation my second objective was to take photos of Mercury silhoutted against the surface of the sun. For this I used my TEC 140 mm APO fitted with a Herschel wedge and my Nikon D300.

The transit started at 11.12 UTC and shortly after this time it was possible to see a little dark circular spot entering the solar disk. I then proceded to take photos with an interval of approx. 15 minutes. It was fascinating to witness the progress of the tiny shadow passing over the surface of the sun in the process overtaking the sunspots visible at the solar disk.

My best photo during the transit was recorded at 16.57.28 and is shown below


Mercury is visible as the small circular dot in the lower part of the photo somewhat to the right. From this photo I calculated the diameter of Mercury to be 12″ as predicted.

Comparing this photo to Gassendi’s drawing it it is remarkable how close the old astronomer came to the real situation. He overestimated the diameter of Mercury by about 50% but considering the quality of the equipment he had, his achievement is still very impressing. Furthermore, quite a few of his colleagues could not believe the very small size of Mercury. It would, however, be the first step to a new and more realistic picture of the size of the solar system.

At 18.42.UTC the transit was over. Following my successful observations of the transits of Venus in 2004 and 2012 I had been fortunate to observe an entire transit of Mercury under very good conditions.

The photo below is a composite of my photos recorded during the transit.


The next transits of Mercury will occur in November 2019, 2032 and 2039 and these are either not visible in their total duration or are significantly shorter than the transit of 2016. Only in 2049 will there be a Mercury transit in May comparable to the recent. So I am grateful that I was allowed to witness the transit of May 9, 2016.



The total lunar eclipse, September 28 2015

A total lunar eclipse is not a common event. Even so, the total lunar eclipse on September 28 2015 was special and spectacular. The moon was very close to earth and thus larger than normal, a ‘super moon. There was also the possibility to observe a ‘blood moon’ where the light is refracted in the atmosphere predominantly allowing the red light to colour the moon.

I set up my equipment already in the afternoon to make certain that everything would work as expected. I set up my A&M 1056 105 mm f/6.2 APO for visual use and my TEC 140 APO for photography side by side on my AP 900 GTO mount as seen in the photo below.


The set up was completed by adding a 15 mm Nagler ocular to the A&M telescope and my Nikon D300 to the TEC APO. Then it was just to wait for the eclipse to start.

The timetable for the eclipse was as follows (UTC):

Phase of lunar eclipse Time [UTC]
Penumbral eclipse begins 00.11.47
Partial eclipse begins 01.07.13
Full eclipse begins 02.11.12
Maximum eclipse 02.47.09
Full eclipse ends 03.23.05
Partial eclipse ends 04.27.05
Penumbral eclipse ends 05.22.31

The penumbral part of the eclipse is not spectacular and I used this phase to fine tune focus on both telescopes.

I observed the partial eclipse start and at 01.09.59 UTC I made the first photo as seen below.


This was less than 3 minutes into the partial phase  and the shadow of the earth is evident. I continued to observe and take photos as the eclipse progressed. The photo below was recorded at 02.10.55 UTC just two minutes before the start of the total phase of the eclipse.


Only a slight bright section of the moon is still visible and the red colour is already impressive. The bluish tinge in the lower right part of the moon is due to the ozone layer in the upper part of the earth’s atmosphere.

At 02.47.12 I recorded a photo at maximum eclipse.


The moon was now red all over and also an impressive sight with the naked eye.

Since the start of the total phase of the eclipse the sky had become increasingly dark and it was fascinating to witness how more and more stars became visible and eventually the Milky Way stretched its band over the sky. Indeed a breathtaking sight complemented suddenly by a meteor a few minutes after maximum eclipse.

I will conclude with a photo recorded during the last minute of the total phase of the eclipse which was a wonderful experience for me.

DSC_1367-1 The next total lunar eclipses visible in Denmark will take place on January 1 and July 27 1918 and I hope they will be as spectacular as the eclipse on 28 September 2015.


The atmosphere of Venus?

During a survey of my photos from the June 8, 2004 Transit of Venus I took a closer look at those that were  most sharp to see if I could find details that had eluded me. In a few  of these I noticed that there was a bright rim just outside the limb of Venus. I have tried to find similar photos on the internet and did in fact find one that showed the same effect as mine. So maybe I actually did make a photo showing the atmosphere of Venus.

A photo showing the atmosphere of Venus?

A photo showing the atmosphere of Venus?

During the June 6, 2012 Transit of Venus the sun was much at much lower altitudes compared to the 2004 transit. All the 2012 photos showed a high degree of atmospheric refraction and the conditions for capturing a glimpse of Venus’ atmosphere were significantly inferior. Still, a few photos show a white fringe though greatly reduced due to the atmospheric refraction.

A photo from the 2012 Transit of Venus.

A photo from the 2012 Transit of Venus.

I feel that these photos in all probability show the atmosphere of Venus. Also that you may find interesting elements in old data.

A page about Venus transits

A new page about transits of Venus has been added to my blog. This page will contain information about the transits of Venus including the theory of the transits, descriptions of historical transits and their significance and finally records of my own observations of the 2004 and 2012 Venus transits.

At present the page only contains presentations of the observations of the 2004 and 2012 transits of Venus observed and photographed be myself.

Shortly, I will upload a presentation describing the astronomical aspects of the Venus transits, explaining their periodicity and their role in the determination of the astronomical unit.

Currently I am gathering information and documents about the transits of Venus in 1761 and 1769 and I will upload documents relating to these transits.

I hope that some will find this page of interest and I will be delighted to receive your comments.

English version

Da det har vist sig, at flere, der har vist interesse for denne blog ikke behersker dansk, har jeg valgt, at ændre bloggens sprog til engelsk. Men jeg forventer ikke,  at det vil være noget problem for danske læsere, der jo i meget vidt omfang taler (og læser) engelsk.

A photo of the moon.

On March 19, 2011 the moon was closer to the earth than at any time for the previous 18 years. The angular size was about 14 % larger than  normal, and the luminosity 30 % larger.

Obviously, I would observe this ‘super moon” and set up my equipment: My Williams Optics ZS 80D FD APO on the SV/Baader Alt/Az mount. I had also my Nikon D300 ready to make a couple of photos.

The moon was indeed a spectacular sight. Generally I do not pay much attention to differences in the size of the moon but this was something quite special.

Shortly before 20 UTC I recorded the first photo. For photography of the moon, most often I employ auto-exposure, but as there was a thin cloud veil in the sky I decide to use manual exposure. By mistake, the first photo was exposed for ½ second and the result was that the disc of the moon was significantly overexposed. The exposure of the clouds were, however, quite reasonable.

The moon 19:57 UTC.

The sky then became mostly cloudy for the following couple of hours.  Around 23 UTC, however, the sky was almost clear and a got a couple of good shots of the moon. I tried an exposure time of 1/1250 sec., but the photo was still a little too bright so I made another photo with an exposure of 1/1600 sec. This was better.

The moon 23:41 UTC.

At about  midnight I decided to conlude the session, packed up the equipment and went to bed. Some time later when I had downloaded the photos I realised that the first, overexposed photo actually gave a very good impression of the clouds around the moon that I had observed through the eyepiece. The moon obviously was devoid of any surface features due to the overexposure. At the eyepiece, however, I could see both the clouds and details on the lunar surface. So I decided to combine the two photos into one which would provide the same experience as the visual observation.

Previously I have used Adobe Photoshop to combine photos, and I decided to use this program for the task. The first thing to do was to erase the black background of the sky from the photo with the correct exposure of the moon. Secondly, I adjusted the ‘cloud photo’ to render it a little darker, which gave in impression more close to my visual experience.

The dark background removed from the photo of the moon.

In Photoshop I then I selected and copied the moon and pasted it into the photo of the clouds. And voila, here is the result.

The finished photo.

The final photo is actually very close to the visual experience. Of course the moon is behind the clouds but these were so thin that the moon seemed to ‘burn’ through the clouds and appear to be in front of these.

I also worked a little (enhancement of contrast and cropping) on the correctly exposed photo of the moon to enhance the lunar surface details.

Det justerede billede af månen d. 19/3 2011.

Finally I used a printed copy of the moon photo to determine the angular size of the moon, using the pixel scale for the WO ZS80 FD/Nikon D300 combination. The result var 0.571° compared to a nominal value of 0.558°, that is a deviation of no more than  2.2%. Quite reasonable I think.

Down under

In November 2010  Anna and I were on vacation in Australia, where we – among a lot of exciting and interesting activities – participated in an excursion to Ayers Rock. The program included a dinner in the open and viewing of the fantastic starry sky.

We started with champagne and hors d’oeuvres while the sun approached the horizon.

Anna enjoys the refreshment.

Ayers Rock is famous for its changing of colour shades from sunrise until sunset. We admired the purple colour of the immense rock while we waited for darkness and the starry sky.

Ayers Rock just after sunset.

After this breathtaking view of Ayers Rock it was time for dinner. We were fantastically lucky with the weather; there was not even a breeze and the temperature had decreased to a very agreeable level. A splendid dinner with pleasant wines set the stage for a night of viewing of the starry sky.

The moon has appeared over the glow from the setting sun.

While we waited for an astronomer to start her description of the objects to see in the sky, I set up my Nikon on a mini-tripod that I had brought for the occasion. I attached a programmable timer to the camera enabling automatic recording of a series of photos. There were small lights on the tables, so I had to accept that the photos would have some  glare from these.

The night sky at Ayers Rock.

The picture above is composed of 8 photos (@ 16 mm, f/5,6, 60 sec.) aligned and stacked with Maxim DL. In addition to an abundance of stars, the picture shows two small, irregular galaxies, the Large Magellanic Cloud to the left, a little below the middle and the Small Magellanic Cloud up left in the picture. The two galaxies are visible to the naked eye and I picked them out as soon as the sky was sufficiently dark. These galaxies are quite close to our own galaxy, the Milky Way. The distances are 170,000 and 210,000 light years respectively – for comparison, the distance to the Andromeda galaxy is 2,300,000 light years. The Magellanic clouds are probably irregular due to gravitational pull of the Milky way.

The Large and the small Magellanic Clouds.

The picture above shows a section of the preceding picture. More details are visible in this picture, e.g. the Tarantula Nebula at low left of the Large Magellanic Cloud. The bright ‘star’ a little up and to the right of the Small Magellanic Cloud is not a star but a globular star cluster named 47 Tucanæ.

Now it was time for the astronomer’s demonstration of the starry sky. The lights on the tables were extinguished and we were shown different celestial bodies, among those obviously the two Magellanic Clouds, a famous sight of the southern sky. Some telescopes had also been set up, and after the astronomer’s demonstration it was possible to take a closer look at some of the objects, e.g. Jupiter, which was spectacular. With all the participants wandering around the area I did not dare leaving my camera on the ground with the risk of an accidental kick and decided to make no more photos. So Anna and I enjoyed the rest of the event with another glass of Chardonnay under the fascinating canopy of stars.