Transit of Mercury, 7 May 2003

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Only the Inferior Planets can transit the Sun. A transit occurs when an Inferior Planet passes directly between the Earth and the Sun appearing as a black spot against the Sun. Transits are rare.

Transits of Mercury occur about 13 times per century. Because of the eccentricity of Mercury's orbit, 9 transits happen in November and only 4 in May. The transit is not visible to the naked eye and must be viewed by projection. This is a list of recent and future transit dates:

There have been three previous transits of Mercury visible in the UK since I was young.

The 7 May 2003 Transit was visible from the UK (including London) during the morning. The map below shows the visibility (the region marked with a V).

Transit Map

7 May 2003

Awoke a 6am to clearish sky. A little cloudy near the horizon but it soon became brighter as the Sun rose above the mist. The garden was sunny so I set up my small 2.5 inch (10cm) telescope on a stool, pointed it at the sun and projected the image onto a card with white paper taped on. A little cardboard covered up the telescope and kept the light off the image of the Sun.

It was 7:30. A huge spot could be seen in the centre of the image. This was not the planet as it was in the wrong place. It was certainly part of the Sun and it turned out to be a huge sunspot. Later, I checked it out and it was a spot system 6 times bigger than the Earth. Near the edge of the Sun another two spots. The sunspots were dark and fuzzy with irregular shapes.

At 7:44, I could see a perfectly round, completely black dot at the bottom of my solar image. Mercury. My first viewing of a transit. I called the wife ("Yes, dear. If you say so, dear"). I thought Mercury would be at the top of my solar image . I remebered that I was projecting - this reverses up and down as well as right and left. It was still early and I was half asleep. After a cup of coffee, I mentally recalled where Mercury should be and flipped the image in my mind - it had to be Mercury.

Confirmation came as I noticed the planet moving slowly across the Sun. The sunspots were still in the same places - they hadn't moved. We sat in the Sun warming our joints. Tha transit was to last over 5 hours. Around 9am it was at maximum - about a third of the way in from the Sun's edge. I took several photos of the projected image by crawling under the chair to get a circular Sun. It was too hot for my anorak but symbolically it was there!

Slowly, the Sun got higher in the sky and Mercury began to move towards the edge of the Sun's disc. At 11:29, three minutes before the actual end of the transit, we lost it - my little telescope couldn't cope any more but it had done its job.


Transit of Mercury
Transit of Mercury (right).

Transit of Mercury
Close up of the Transit and sunspot. Mercury is lower right.

All photos © Bob Snell (2003)