Clear skies here, chez Oikofuge, for Monday’s transit of Mercury, the first in almost ten years.

Mercury and Venus are the two planets that orbit between Earth and the sun, so they are the only two planets that we can occasionally see passing in front of the sun.

If you look at the orbits of Earth and Mercury below (I’ve removed Venus, for clarity), it would seem there should be many opportunities for that sort of alignment.

Mercury orbits the sun every 88 days, repeatedly lapping Earth on its slower one-year orbit. Each time Mercury draws level with the Earth (on average, at 116-day intervals) that’s an opportunity for a transit.

But the reason we don’t get three transits a year shows up when we look at the orbits edge on. Mercury’s orbit is tilted at seven degrees relative to Earth’s. So most of the times when Mercury is overtaking Earth, it’s either above or below Earth’s orbital plane, and the alignment is imprecise—no transit.

The opportunities for a transit occur only when Mercury is passing through the Earth’s orbital plane. That happens at two precise locations in Earth’s orbit, on directly opposite sides of the sun. If the Earth is at one of those points and Mercury is overtaking at the same time, we have a transit. At any other times, no show.

These two points in Earth’s orbit are (relatively) fixed in space—the orbits of Mercury and Earth do evolve, but only slowly. So Earth reaches them at the same time every year—the start of May, and the start of November. Those are the only times of the year at which transits of Mercury can occur. There’s a little wiggle room, a span of close to a week either side of the exact point, when the alignment is good enough for a transit to occur.

If the transit comes early in one of these two-week “transit seasons”, we see Mercury skim across one edge of the solar disc; in the middle of the season, it crosses the middle of the disc; and in late season it crosses the opposite edge. At the extremes, only part of Mercury may overlap the sun, causing a very brief “partial transit”—a little sector of silhouette that comes and goes very quickly along the edge of the solar disc. Under these conditions, where you are on Earth can change the perspective enough to make a difference to what you see. In November 1999, observers in America saw the entire silhouette of Mercury make a short crossing at the very edge of the solar disc; but parts of Australia and New Zealand saw only a partial transit.

Monday’s alignment was a pretty good one. Mercury crossed reasonably centrally, and so spent a long time (seven-and-a-half hours) in transit. Its orbit is marked in red on the diagram below.

If you look at the May-November diagram above, you’ll see that Mercury is considerably farther from the sun in May. That’s a bit of a two-edged sword. It means that the geometrical alignment has to be a bit tighter in May before Mercury is actually superimposed on the solar disc from our point of view. So May transits are rarer than November ones. But when there is a May transit, Mercury is closer to us, and appears larger—twelve arcseconds across in May, ten arseconds in November.

That’s still tiny, though, as you can see above. It’s well below the resolving power of the human eye, which is conventionally around one arcminute (60 arcseconds). I’ve discussed this issue of optical resolution in a previous post. When Venus is in transit it’s an arcminute across, and so potentially visible to the naked eye for people with good vision and appropriate eye protection, using eclipse-viewing filters. But Mercury needs to be optically enlarged. Usually that means using a telescope with filters in the optical path, which is not something that should be knocked together by an amateur—eye injury is a certainty if you find yourself looking at the unfiltered sun through a telescope.

But there is another way of enlarging the solar image, which keeps your eyes well clear of danger—projecting an image on to a white surface.

So on a half-baked impulse, I trotted out into the garden as the sun was getting low in the sky during the transit, and projected its image on to a piece of paper, using a pair of binoculars. With my back to the sun, I directed the image of the solar disc into the shadow of my own shoulder. You can see the lo-tech (but very safe) set-up below:

Difficult to hold steady, and there were blue and red fringes to the image because I was using the optics to do something they weren’t designed to do. But there was a little fleck of shadow! In the photo below it’s a little red-blue smear. (You might need to click on the image to enlarge it for a proper view.)

Cool, eh? Excitement reigned, together with a certain smug satisfaction at pulling this off with such rudimentary kit. For a while, at least …

A little sober reflection on the size and position of that fleck of shadow (too big, not quite in the right place); a review of proper photographs of the event … Turns out I had rather neatly projected the image of a sunspot. Mercury is just too small to show up with my gimcrack contrivance.

Oh well. The next transit is on November 11, 2019.

Note: All the diagrams and simulations in this post were generated using Celestia, a free (and highly customizable) space simulator. I recommend it, and not just because I appear on the “Authors” list.