Retrograde Motion Of Mercury

The long-term project was imaged from Feb to Aug 2020. Inner planets go through retrograde motion, too. But those are more difficult to follow, as the retrograde motion happens while the planet goes through inferior conjunction, through the Sun’s glare.

One planet that is difficult to follow in retrograde motion is Mercury. The innermost planet does not stray away from the Sun too much, thus seeing and recording is always a problem. The ecliptic needs to have a steep enough angle with the horizon during an evening apparition and the following morning apparition, to have Mercury high enough in a reasonably dark sky, so its position can be photographed together with the background stars. Weather also needs to cooperate, as Mercury needs to be frequently imaged due to its fast motion in the sky.

The opportunities are generally more limited for temperate locations like mine, which is of course Bursa, Turkey, at 40d N latitude. To have a steep enough ecliptic angle on both evening and morning apparitions of Mercury needs the inferior conjunction in late December or early January, or in June or early July. The December-January window is very difficult to work with, because of the winter weather. So, the best timing turns out to be June-July.

2020 was to have a good set of evening and morning apparitions of Mercury, so I decided give it a serious try. I first photographed the background sky of the loop in Gemini in late February. In the following three months until the actual photography of Mercury began, Covid-19 became a pandemic, which needed various methods of serious precautions as severe as curfew and quarantine. In late May, when Mercury’s evening apparition was under way, the rooftop of our apartment building had became my prime (and sometimes only) location where astroimaging was possible. At least, the weather was generally good and we had nice views of late-May grouping of Moon-Mercury-Venus.

From 22nd May on, I tried to photograph Mercury on every other day at least, which went on okay for 2 weeks. After 8th June, we had a stint of more cloudy weather. I was barely able to catch Mercury only twice, on 12th and 16th June, before it disappeared towards inferior conjunction.

Mercury would be out of sight for nearly a month, centered on the inferior conjunction on 1st July. But I had long decided to try using SOHO LASCO C3 coronagraph images, as it would nicely record a 9-day interval of Mercury’s retrograde motion. One surprise was comet Neowise, which crossed LASCO C3 field of view on the same days. Something nice was cooking.

After the inferior conjunction, I was able to catch Mercury on 15th July and then I was able to photograph it every other day until 4th August, when it exited my field of view. July had too many days of lost or divided sleep, thanks to Neowise, Mercury and also Mars photography. Also with Jupiter and Saturn coming to opposition, July 2020 was such an eventful month.

Then I registered and processed these Mercury images I have accumulated on to the master background picture I had got in February. Mercury images may have turned out to be slightly dimmer than they would actually be, if I had used similar settings with the background picture. But that was impossible of course, as I photographed Mercury always in twilight. SOHO LASCO C3 images, on the other hand, probably have Mercury slightly brighter when compared to my master background.

There is another thing worth mentioning about LASCO images. SOHO is positioned at Sun–Earth L1 Lagrangian point, about 1.5 million km closer to the Sun than Earth. From the vantage point of SOHO, Mercury was noticeably further south than it would be seen from here on Earth. But it is nice to see a faint Mercury drift westward in the sky during inferior conjunction, which would otherwise be impossible to follow.

The retrograde motion picture is accompanied by a scene from the evening of 24th May 2020, which shows Mercury with the Moon and Venus shining in the west. This was during the first days of Mercury’s evening elongation.

The second image shows the annotated version of the retrograde, which is accompanied by another scene from the morning of 31st July 2020, as Mercury was rising in the east during its morning elongation.